Icarus 169 (2004) 3–28
www.elsevier.com/locate/icarus
The final Galileo SSI observations of Io: orbits G28-I33
Elizabeth P. Turtle,a,b,∗ Laszlo P. Keszthelyi,c Alfred S. McEwen,a Jani Radebaugh,a
Moses Milazzo,a Damon P. Simonelli,d Paul Geissler,c David A. Williams,e Jason Perry,a
Windy L. Jaeger,a Kenneth P. Klaasen,d H. Herbert Breneman,d Tilmann Denk,f
Cynthia B. Phillips,g and
the Galileo SSI Team
a Lunar and Planetary Laboratory, University of Arizona, Tucson, AZ 85721-0092, USA
b Planetary Science Institute, 1700 E. Fort Lowell, Suite 106, Tucson, AZ 85719, USA
c Astrogeology Program, U.S. Geological Survey, Flagstaff, AZ 86001, USA
d Jet Propulsion Laboratory, 4800 Oak Grove Dr., Pasadena, CA 91109, USA
e Department of Geological Sciences, Arizona State University, Tempe, AZ 85287, USA
f DLR, Rutherfordstr. 2, 12489 Berlin, Germany
g SETI Institute, Mountain View, CA 94043, USA
Received 21 April 2003; revised 12 October 2003
Abstract
We present the observations of Io acquired by the Solid State Imaging (SSI) experiment during the Galileo Millennium Mission (GMM)
and the strategy we used to plan the exploration of Io. Despite Galileo’s tight restrictions on data volume and downlink capability and several
spacecraft and camera anomalies due to the intense radiation close to Jupiter, there were many successful SSI observations during GMM.
Four giant, high-latitude plumes, including the largest plume ever observed on Io, were documented over a period of eight months; only
faint evidence of such plumes had been seen since the Voyager 2 encounter, despite monitoring by Galileo during the previous five years.
Moreover, the source of one of the plumes was Tvashtar Catena, demonstrating that a single site can exhibit remarkably diverse eruption
styles—from a curtain of lava fountains, to extensive surface flows, and finally a ∼ 400 km high plume—over a relatively short period of
time (∼ 13 months between orbits I25 and G29). Despite this substantial activity, no evidence of any truly new volcanic center was seen
during the six years of Galileo observations. The recent observations also revealed details of mass wasting processes acting on Io. Slumping
and landsliding dominate and occur in close proximity to each other, demonstrating spatial variation in material properties over distances of
several kilometers. However, despite the ubiquitous evidence for mass wasting, the rate of volcanic resurfacing seems to dominate; the floors
of paterae in proximity to mountains are generally free of debris. Finally, the highest resolution observations obtained during Galileo’s final
encounters with Io provided further evidence for a wide diversity of surface processes at work on Io.
 2003 Elsevier Inc. All rights reserved.
Keywords: Io; Surfaces, satellite; Satellites of Jupiter; Volcanism; Tectonics; Geological processes
1. Introduction
The Galileo spacecraft had a long and productive history. It was launched in October 1989, entered orbit around
Jupiter in December 1995 after releasing a jovian atmospheric probe, completed 34 orbits in its nominal and
three extended missions, and ended by plummeting into
* Corresponding author.
E-mail address: [email protected] (E.P. Turtle).
0019-1035/$ – see front matter  2003 Elsevier Inc. All rights reserved.
doi:10.1016/j.icarus.2003.10.014
Jupiter in September 2003. Late in the first mission extension Galileo’s periapse was lowered closer to Jupiter in
order to approach the innermost of the galilean satellites, Io
(e.g., McEwen et al., 2000; Keszthelyi et al., 2001; Turtle et
al., 2001, 2002 [the 2002 correction provides improved reproductions of the images]). Here we describe the imaging
observations of Io made during the final mission extension,
the Galileo Millennium Mission (GMM): orbits G28 through
A34 (Table 1), including three close Io encounters during orbits I31, I32, and I33 (Fig. 1). Observations that are analyzed
in detail elsewhere in this issue (e.g., Radebaugh et al., 2004;
4
E.P. Turtle et al. / Icarus 169 (2004) 3–28
Fig. 1. Map of Io illustrating Galileo’s ground tracks, when within 100,000 km of Io’s surface (within ∼ 3 to 4 hr of closest-approach), for the three close GMM flybys (Table 1): I31 (northern track over
anti-jovian hemisphere), I32 (southern track over anti-jovian hemisphere), and I33 (track over sub-jovian hemisphere). Each track is dashed over Io’s night side and solid over the dayside. Triangles represent the
start (inbound segment) of the track, asterisks indicate closest-approach, and squares represent the end (outbound segment) of the track.
Final Galileo SSI observations of Io
5
Table 1
Galileo’s final orbits
Orbit
G28
G29
C30
I31
I32
I33
A34
J35
Date of Io closest approach
(C/A)
Range to surface of Io at C/A
(km)
21 May 2000
30 December 2000
23 May 2001
6 August 2001
16 October 2001
17 January 2002
5 November 2002
21 September 2003
379,000
963,000
342,000
194
184
102
45,800
–
Geissler et al., 2004; Williams et al., 2004) are described
only briefly here.
Throughout the Galileo mission, the extent of highresolution imaging was tightly constrained by the fleeting
durations of close approaches to the satellites, the restricted
capacity of the onboard tape recorder, and the time available
from the Deep Space Network for downlink via Galileo’s
limited low-gain antenna. Moreover, during Galileo’s final orbits, spacecraft and camera anomalies severely impacted the planned observations. These anomalies were the
inevitable effects of age, accelerated by the intense radiation, detrimental to spacecraft electronics, encountered near
Jupiter (Keszthelyi et al., 2001; Klaasen et al., 2003), and resulted in the loss of numerous observations during two of the
GMM Io flybys. Nonetheless, we have described the unsuccessful observations (Appendix A) as well as the successful
ones because the regions of Io we chose to target with the
limited high-resolution observations reflect our understanding of Io and assumptions we made about its nature.
1.1. The solid state imaging camera
The Solid State Imaging (SSI) camera on the Galileo
spacecraft used an 800 × 800 pixel charge coupled device (CCD) as its detector (Klaasen et al., 1997, 2003).
Galileo was NASA’s first planetary mission to employ such
an instrument. Exposure time, gain state, filter, summation
mode, and compression mode were the primary parameters
that could be adjusted. In the context of this manuscript
the most pertinent parameter was the choice of which of
eight available filters to use. At Io, we primarily used the
violet (413 nm), green (559 nm), red (665 nm), near-IR
(756 nm), 1 µm (> 968 nm), and clear filters. The clear
filter was a broad wavelength filter with an effective wavelength of 652 nm that was used for the majority of the
high-resolution images. The violet filter was most useful
for detecting SO2 -rich plumes and deposits. The green and
red filters, in combination with the violet filter, provided
the best “true-color” data from Io. The 756 nm filter was
often used in place of the red filter to provide enhanced
discrimination of color variations. The 1 µm and clear filters, in combination, provided the best constraints on the
temperatures of active lavas (e.g., McEwen et al., 1998b;
Radebaugh et al., 2004).
Sub-spacecraft point at C/A
(Lat., W. Lon.)
0.9, 285
0.8, 22
−1.2, 85
78, 172
−79, 223
−44, 317
−6, 266
–
Comments
Ganymede flyby
Ganymede flyby; joint with Cassini encounter
Callisto flyby
Io flyby; SSI anomalies
Io flyby
Io flyby; spacecraft anomalies
Amalthea flyby; no remote sensing
Jupiter impact
2. Observations and discussion
2.1. Orbit G28
No SSI observations were made of Io during the distant
G28 encounter in May 2000. The previous encounter, I27 in
February 2000, had been the first Io flyby without any camera or spacecraft anomalies; however, the available downlink
after I27 was severely limited and only a portion of the data
could be returned prior to G28. Therefore, to enable playback of some of the remaining I27 data, during G28 slightly
more than one full track of I27 data on Galileo’s onboard
tape recorder was not overwritten. (The I27 observations
are discussed in Keszthelyi et al., 2001, and Turtle et al.,
2001, 2002.) In order to facilitate fitting the highest priority
G28 data onto the three remaining tape tracks, the SSI and
NIMS (Near-Infrared Mapping Spectrometer) observations
of Io originally planned for G28 were sacrificed. Following the G28 encounter, the SSI camera suffered an anomaly
that was attributed at the time to a malfunction of the camera’s light-flood function, which was consequently disabled
by ground command (Klaasen et al., 2003).
2.2. Orbit G29
This orbit had a distant encounter with Io at the end of December 2000, which coincided with the Cassini spacecraft’s
flyby of Jupiter for a gravity assist on its way to Saturn (e.g.,
Porco et al., 2003). The planned SSI Io imaging sequence
consisted of three global color observations, an observation
of the Prometheus plume on the limb, and eight observations
while Io was in Jupiter’s shadow (Table 2). The geometry of the G29 encounter provided global, low-phase angle
(∼ 9 ◦ –21◦ ) coverage, which is good for monitoring surface
changes. These global color observations were also designed
to compensate for the loss of low-phase imaging during orbits E15 and E18, and to complete the global low-phase coverage needed for photometric mapping, complementing that
at high-phase (∼ 70◦ –80◦ ) which emphasizes topography.
We used four filters: green, red, violet and 756 nm (frames
using SSI’s 889 nm and 1 µm (968 nm) filters were eliminated due to a reduction in sensitivity at longer wavelengths
after the camera’s light-flood capability was disabled). The
6
E.P. Turtle et al. / Icarus 169 (2004) 3–28
Table 2
Galileo SSI observations during orbit G29
Observation
Number of framesa
GLOCOL01
PROMTH01
2 IM8/480 1 × 2 OCM
2 IM8/480 1 1 × 2 OCM
11,000
11,000
Violet, green, red, 756 nm
Clear, violet
GLOCOL02
GLOCOL03
ECLIPS01
ECLIPS02
ECLIPS03
ECLIPS04
ECLIPS05
ECLIPS06
ECLIPS07
ECLIPS08
2 HIM/560 1 × 2 OCM
2 HIM/400 1 × 2 OCM
2 HMA 1 × 2 OCM
2 HMA 1 × 2 OCM
2 HMA 1 × 2 OCM
2 HMA 1 × 2 OCM
2 HMA 1 × 2 OCM
2 HMA 1 × 2 OCM
2 HMA 1 × 2 OCM
2 HMA 1 × 2 OCM
10,000
17,000
24,000
24,000
24,000
25,000
25,000
25,000
25,000
25,000
Violet, green, red, 756 nm
Violet, green, red, 756 nm
Clear, violet, 1 µm
Clear, violet, 1 µm
Clear, violet, 1 µm
Clear, violet, 1 µm
Clear, violet, 1 µm
Clear, violet, 1 µm
Clear, violet, 1 µm
Clear, violet, 1 µm
Resolution
(m pixel−1 )
Filter(s)
Notes
Lost due to camera anomaly
Long-exposure, intentionally overexposed, clear frame
bled profusely. Short-exposure, clear and violet OCM
was lost due to the camera anomaly.
Lost second OCM due to camera anomaly
Lost due to camera anomaly
Lost due to camera anomaly
Lost due to camera anomaly
Lost due to camera anomaly
Lost due to camera anomaly
Lost due to camera anomaly
Lost due to camera anomaly
a In this and all subsequent tables the following abbreviations are used: HMA, half-frame, full resolution mode; IM8 and HIM, full resolution mode; IM4,
full resolution, essentially losslessly compressed to tape; OCM, on chip mosaic (Klaasen et al., 1997). For some observations only partial frames were recorded
to tape. In these cases the frame mode is followed by the number of lines recorded (out of a maximum of 800).
Galileo observations of Io in eclipse were to provide complementary information to the continuous, but lower resolution, movies which were to be acquired simultaneously by
Cassini.
Unfortunately most of these observations were lost due
to a recurrence of the anomaly encountered after the G28
Ganymede flyby (subsequent investigation traced it to an intermittent failure within the SSI circuitry, likely triggered by
radiation, rather than simply a problem with the light-flood
(Klaasen et al., 2003)). PROMTH01 was partially successful. One clear filter image had been intentionally overexposed in the hope that it would reveal the distal reaches
of the Prometheus plume. The long-exposure resulted in
extensive saturation and even bleeding (caused by overfilling of pixels in the CCD) in the image, but part of the
limb was unaffected and the plume was detected rising
above it. Shorter-exposure violet and clear observations of
the plume were lost as a result of the camera anomaly.
The global color frames that were successful revealed yet
another major ionian surprise. GLOCOL02 (top, left image in Fig. 2) showed a giant red plume deposit, similar
to that which has been observed consistently around Pele
by Voyager and Galileo, but centered near the site of the
Tvashtar eruptions (63◦ N) that were witnessed fortuitously
by Galileo during I25 and I27 (Keszthelyi et al., 2001;
McEwen et al., 2003). The red ring had an average radius of 720 km (Geissler et al., 2004), and by comparison
to simultaneous Cassini observations (Porco et al., 2003;
McEwen et al., 2003) it was determined that the plume was
385 ± 30 km high. No plume (of any size) had ever been witnessed at high latitude and, although Voyager 2 had seen surface deposits indicative of higher-latitude plumes (McEwen
and Soderblom, 1983), in its five years at Jupiter before G29,
Galileo had barely detected only one other such deposit (the
“North polar ring” described in Geissler et al. (2004)). However, the Tvashtar plume would prove to be just the first of
four giant plumes observed in Io’s north polar region during
2001, a major surprise in the final orbits of Galileo (McEwen
et al., 2003).
2.3. Orbit C30
Following the G29 encounter it was determined that, although it did not seem possible to prevent SSI from entering the anomalous state, cycling the power to the instrument returned it to its nominal state. Therefore, commands to power-cycle the camera were incorporated into the
C30 encounter command sequences (Klaasen et al., 2003).
Despite these precautions, all of the SSI Io observations
during C30 failed. These had consisted of a 4 km pixel−1
eclipse observation of hotspots on the sub-jovian hemisphere, a 3.4 km pixel−1 color observation of Io’s leading
hemisphere with a half-frame in the violet filter designed
to search for evidence of a continuing plume eruption over
Tvashtar, and a 3.9 km pixel−1 color observation at comparable phase angle to the July 1999, C21 global color observation (Plate 1 in Keszthelyi et al., 2001) to detect surface
changes (Table 3).
2.4. Orbit I31
Orbit I31 included a low altitude (∼ 200 km) flyby over
Io’s high northern latitudes in August 2001 (Table 1; Fig. 1).
If the ∼ 400 km high plume observed over Tvashtar in late
2000 were still active during I31, Galileo would have passed
through it. Indeed, although SSI saw no evidence for plume
activity at Tvashtar, Galileo’s Plasma Subsystem detected
SO2 at this point in the flyby (Frank and Paterson, 2001),
perhaps from an all-vapor “stealth” plume (Johnson et al.,
1995) over Tvashtar or from a dustier neighboring plume
that was detected in the images (see below). In planning
the imaging observations (Table 4; Appendix A.1) for this
Final Galileo SSI observations of Io
7
Fig. 2. Global color observations from G29, I31, and I32. For G29, GLOCOL02 is on the left and GLOCOL03 is on the right. For both I31 and I32, GLOCOL01
is on the left and GLOCOL02 is on the right. Although Io is scaled to the same size in each image, the resolutions of the observations ranged from 5 km pixel−1
for I32 GLOCOL01 to 19.6 km pixel−1 for I31 GLOCOL02 (Tables 2, 4, 5). (Io looks generally yellower in I31 and whiter in I32 due to some variation in
the balance between the three single-filter images that were used to assemble these color images.) Unless otherwise noted, all images in this manuscript are
oriented with North approximately to the top of the figure.
8
E.P. Turtle et al. / Icarus 169 (2004) 3–28
Table 3
Galileo SSI observations during orbit C30
Observation
Number of framesa
Resolution (m pixel−1 )
Filter(s)
HSPOTS01
LEDHEM01
CHANGE01
2 IM8
3.5 IM8
3 IM8
3,900
3,400
3,900
Clear, violet
Violet, green, 756 nm
Violet, green, 756 nm
Notes
Lost due to camera anomaly
Lost due to camera anomaly
Lost due to camera anomaly
a See footnote for Table 2.
flyby we endeavored to achieve a balance between coverage of new territory and re-imaging targets of particular
interest from previous flybys at higher resolution or to document surface changes. We also intended to take advantage
of Galileo’s first opportunity to observe Voyager 1 targets on
the sub-jovian hemisphere of Io at medium resolution. We
designed three observations (MASUBI01, LEIZI_01, and
KANEHE01 (Table 4, Appendix A.1)) that would not only
provide important comparisons to Voyager 1 images to look
for surface changes over the intervening 22 years, but also
facilitate targeting the even higher resolution observations
of these features that would be possible on orbit I33.
The anomalies during C30 had the benefit that they allowed the Galileo engineers to accurately diagnose the cause
as a faulty operational amplifier in the camera’s signal chain
(probably radiation damaged) that tended to go into saturation when triggered by high voltage signals passing through
this circuit. To avoid generating such high voltages, the
SSI detector erase cycle, like the light-flood, was inhibited
(Klaasen et al., 2003). In addition, commands to intentionally power cycle the camera were interspersed throughout
the flyby. Furthermore, the imaging sequence was modified
to include a few distant contingency observations of Io, de-
signed to monitor surface activity at a range from Jupiter that
was hoped to be safe from radiation-induced camera anomalies. This final precaution paid off; due to a recurrence of
the camera anomaly during orbit I31 the distant observations, PLUMES01, GLOCOL01, and GLOCOL02 (Table 4),
were the only successful observations acquired. Nevertheless they revealed yet more surprises about Io.
2.4.1. PLUMES01
This inbound observation at a range of 20 RJ was optimized in phase angle, 133◦ , to observe a plume over
Tvashtar were one still to be active. Instead of a single frame,
three were planned because the spacecraft’s orientation at
this time was such that one of its booms could periodically
obscure Io. Although no plume was detected over Tvashtar
in the images, this observation caught the tallest plume ever
seen on Io, reaching to ∼ 500 km above the surface (Fig. 3),
for which the name Thor* has been provisionally approved
(asterisks are used here and throughout the manuscript to
indicate names that have provisional IAU approval). The
source of the plume, at 41◦ N, 134◦ W, was far from any
previously recognized active volcano or hotspot. This plume
may have been the source of the SO2 that the Galileo Plasma
Table 4
Galileo SSI observations during orbit I31
Observation
Number of framesa
Resolution
(m pixel−1 )
Filter(s)
Notes
PLUMES01
MAXRES01b
TVASHT01
PROMTH01
TVASHT02
PROMTH02b
SAVITR01
3 HMA/320
18,000
Violet
1 × 6 IM4
1 × 6 IM8
1 × 6 IM8
3–5
43–45
50
Clear
Clear
Clear
2 × 4 IM8
130–140
Clear
AMRANI01
SUSANO01
1 × 6 IM8
4-color IM8
140
140
Clear
Violet, green, red, 756 nm
MASUBI01
LEIZI_01
KANEHE01
TERMIN01
TERMIN02
GLOCOL01
GLOCOL02
1 × 5 IM8
1 × 4 IM8
1 × 2 IM4
1 × 5 IM4
1 × 4 IM8
HMA/370 1 × 3 OCM
HMA/370 1 × 3 OCM
390
395
404
405–414
415
19,400
19,600
Clear
Clear
Clear
Clear
Clear
Violet, green, 756 nm
Violet, green, 756 nm
a See footnote for Table 2.
b Observations that were in the original imaging sequence, but deleted from the final sequence.
Added as contingency
Deleted due to data volume constraints
Lost due to camera anomaly
Lost due to camera anomaly
Lost due to camera anomaly
Deleted due to data volume constraints
Last frame was reduced to 320 lines to conserve
data volume
Lost due to camera anomaly
Lost due to camera anomaly
(N.B. actually an observation of Itzamna Patera, see
Appendix A.1.8, SUSANO01)
Lost due to camera anomaly
Lost due to camera anomaly
Lost due to camera anomaly
Lost due to camera anomaly
Lost due to camera anomaly
Lost due to camera anomaly
Added as contingency
Added as contingency
Final Galileo SSI observations of Io
9
Fig. 3. Two images from the 18 km pixel−1 , I31 PLUMES01 observation revealed a bright plume extending 500 km above Io’s sunlit northwestern limb. Io’s
night side can also be seen because of illumination by Jupiter. The contrast in the images has been enhanced to show the plume, therefore background noise
and vertical stripes from bad columns on the CCD can also be seen.
Table 5
Galileo SSI observations during orbit I32
Observation
LOKI_01
PELE_01
MAXRES01b
TELGNS01
EMAKNG01
TELGNS02
TOHIL_01
TUPAN_01
EMAKNG02b
TVASHT01
GSHBAR01
SHMASH01b
TERMIN01
TERMIN02
EAAM_01b
GLOCOL01
GLOCOL02
Number of framesa
Resolution (m pixel−1 )
Filter(s)
Notes
2 IM4
5 IM8
1,100
60–65
Clear
Clear, 1 µm
1 × 4 IM4
1 × 4 IM4
1 × 2 IM8
1 × 5 IM8
3-color IM8
9–11
32–34
41–42
50–52
132
Clear
Clear
Clear
Clear
Violet, green, 756 nm
1 × 2 IM8
1 × 2.4 IM4
200
250
Clear
Clear
1 × 6 IM8
1 × 6 IM8
325
330–335
Clear
Clear
4-color IM8
2 IM8/400 1 × 2 OCMs
5,000
9,800
Violet, green, red, 756 nm
Violet, green, red, 756 nm
Through spacecraft booms
Deleted due to data volume constraints
Deleted due to data volume constraints
Deleted in exchange for contingency observations
Two frames retargeted to observe source of new I31 plume
Deleted in exchange for contingency observations
Added as contingency after C30
Added as contingency after C30
a See footnote for Table 2.
b Observations that were in the original imaging sequence, but deleted from the final sequence.
Subsystem detected near closest-approach (Frank and Paterson, 2001).
2.4.2. GLOCOL01
This outbound observation (middle, left image in Fig. 2)
at a range of 21.7 RJ was also a contingency in case
of camera problems closer to Jupiter. It was a very low
phase angle (3.5◦ ) observation of the sub-jovian hemisphere (sub-spacecraft longitude 336◦ W) at a resolution of
19.4 km pixel−1 . The images revealed another new, large,
red plume deposit, this time around Dazhbog Patera, as well
as remnants of recent red deposits around Surt (discussed
in more detail by Geissler et al., 2004). Thus, over a period
of several months during Galileo’s last orbits we found evidence for four large eruptions at high latitudes, even though
little plume activity had been evident at comparable latitudes
during the first five years of the mission.
2.4.3. GLOCOL02
Another outbound contingency image at a range of 31.2
RJ , this observation (middle, right image in Fig. 2) had
a low phase angle, 27◦ , and a good view of Tvashtar
(sub-spacecraft longitude, 125◦ W) at a resolution of 19.6
km pixel−1 . This observation revealed a new whitish deposit
(Fig. 2) from the 500 km high plume at 41◦ N, 134◦ W
(Fig. 3) superimposed upon the red ring around Tvashtar
(McEwen et al., 2003; Geissler et al., 2004)
2.5. Orbit I32
Orbit I32 included another very close Io flyby, this time at
high southern latitudes (Table 1; Fig. 1). As before, in planning the imaging sequence (Table 5) for this flyby we tried
to balance coverage of new territory against that of previous
targets of particular interest at different resolutions or lighting conditions or to document surface changes.
10
E.P. Turtle et al. / Icarus 169 (2004) 3–28
After the devastating camera anomalies during the I31
flyby, the Galileo engineers revised the spacecraft software
to remove the erase command from the power cycle procedure (which during I31 had brought the camera online with
erase enabled and then subsequently disabled it). As before,
power-cycle sequences were included at several times during
the encounter when there was time between SSI observations, but this time they were to be implemented only if SSI
engineering data indicated that the camera was in the anomalous state (Klaasen et al., 2003). Furthermore, the scan platform was stowed whenever it was not used for a significant
period of time to provide additional radiation shielding to
the SSI camera. These precautionary measures paid off; the
anomaly did not recur and all of the I32 observations were
successfully acquired. This orbit marked Galileo’s fifth close
Io flyby but was only the second trouble-free encounter (I27
had been the first).
2.5.1. LOKI_01
This 1.1 km pixel−1 , near-terminator observation of the
Loki region is one of the highest-resolution sets of images
ever acquired of Loki, the Solar System’s most powerful
volcano. The observation (Fig. 4) revealed that the recent
lavas in Loki and other nearby paterae are strongly forward
scattering, consistent with relatively flat, smooth, glassy lava
surfaces (see Geissler et al., 2004). It has also provided the
best topographic constraints on the patera: the rim of Loki
can be no more than 100 m tall. This result, combined with
Fig. 4. In this 1.1 km pixel−1 , near-terminator (illumination is from the left)
view of Loki Patera the normally dark patera floor looks bright, as do the
floors of other smaller paterae in the image. We have interpreted this brightening as being due to forward scattering off of glassy lava surfaces. Shadow
measurements at the patera rim indicate that its height is no greater than
100 m. Near the bottom of the image, two mountains several kilometers
high (Schenk et al., 2001) and some bright flows extending north from Ra
Patera can be seen.
the observation that the margins of Loki’s dark lavas have not
changed despite the frequent, large-volume, effusive eruptions inferred at Loki, is consistent with the hypothesis that
Loki is a lava lake with an episodically overturning crust
(Rathbun et al., 2002; Lopes et al., 2004).
2.5.2. PELE_01
This five-frame, two-color (clear and 1 µm filters), 60–
65 m pixel−1 , nighttime observation of Pele (Fig. 8 in Radebaugh et al., 2004) finally found the large area of incandescent lava that we had tried to target but had missed with
the I24 and I27 observations (Keszthelyi et al., 2001). (The
long-wavelength filter was used, despite a potential reduction in sensitivity because the camera’s light-flood function
had been disabled, in order to allow lava temperatures to
be estimated. Fortunately, this problem did not impair the
temperature analysis (Radebaugh et al., 2004).) Radebaugh
et al. (2004) used this observation to derive color temperatures of at least 1420 ± 100 K, consistent with compositions ranging from basaltic to ultramafic. The distribution
of glowing lava at Pele is consistent with a region of active fountaining in a lava lake (Radebaugh et al., 2004;
Lopes et al., 2004).
2.5.3. TELGNS01 and TELGNS02
The southeastern scarp of Telegonus Mensae was observed during I32 at 10 m pixel−1 and at 41 m pixel−1 for
context. From a 350 m pixel−1 observation made during I27
(Turtle et al., 2001, 2002), the eastern margin of Telegonus
Mensae had been seen to be scalloped, a morphology that
can result from erosion by sapping. So, it was hoped that
this observation might provide an opportunity to understand whether sapping is an active erosional process on Io
(a goal which had been thwarted on two earlier occasions:
I27 SAPPNG01, a very high resolution, oblique observation, without context, in which the scarp turned out to be
facing away from the camera; and I31 TVASHT02, which
was lost). However, the spectacular I32 images (Fig. 5) primarily revealed evidence for gravity-driven mass wasting in
the form of slumps and landslides. The amphitheaters on the
southeastern margin and the straight scarps along the southern margin exhibit morphologies typical of slumping: flattopped blocks with hummocky textures along their bases.
There are some intriguing valleys in the upper right part of
the context mosaic (Fig. 5a), where it looks like layered material may have been eroded, however, the highest resolution
images (Fig. 5b) did not cover this area.
The very high-resolution images (Fig. 5b) show the
southern scarp of Telegonus Mensae, the dark fracture to
its east, and the plains that lie between them. The southern
scarp faces away from the Sun, nevertheless much of it is illuminated, indicating that its average slope is less than 26◦ .
Interestingly, there are positive relief features (∼ 1 km long)
on the face of this block that suggest some manner of downslope creep. Based on measurements of the shadows that are
cast along the scarp face, we estimate that the total relief
Final Galileo SSI observations of Io
11
(a)
(b)
Fig. 5. (a) Context mosaic (42 m pixel−1 ) of the southeastern margin of Telegonus Mensae, with the high-resolution frames (shown separately in (b)) inset.
(b) High-resolution (9.6 m pixel−1 ) mosaic from the south-facing scarp of Telegonus Mensae across the plains to a dark fracture. Illumination is from the
upper right in both mosaics, so the terrain in the top left corner of each mosaic is elevated above that to the bottom and the right.
12
E.P. Turtle et al. / Icarus 169 (2004) 3–28
is 1.5–2 km. Near the scarp’s eastern edge a succession of
at least three landslides can be seen, the most extensive of
which is 3.8 km long, 2.1 km wide, and less than 100 m thick
at its distal end. This striking difference in erosional style
along the scarp suggests a spatial variation in material properties or composition. There is no unambiguous evidence for
layering in the scarp face (layers a few tens of meters thick
would be resolved directly and thinner, ledge-forming layers
should also be evident); the mass wasting processes dominate its morphology.
Both observations (Figs. 5a and 5b) reveal a surface that
appears to have been blanketed by material with little albedo
contrast. This surface material is markedly different from
that in other very high-resolution images of Io’s surface (e.g.,
Figs. 2 and 9 in Turtle et al. 2001, 2002). Above the scarp
(upper left of Figs. 5a and 5b) the surface is smooth with a
series of gentle undulations (discussed in more detail in Bart
et al., 2004); below it to the southeast (middle two frames
of Fig. 5b) the surface exhibits numerous low mounds a few
tens of meters in diameter. The low hills to the south display a curious faintly geometric pattern on their Sun-facing
slopes (leftmost inset in Fig. 5b).
Finally, the easternmost high-resolution frame includes a
northwest–southeast trending fracture, 100–200 m across.
Near the fracture, the surface exhibits more prominent
albedo variations; dark flows appear to emanate from the
fracture. This fracture has either cut a pre-existing topographic feature or a small (∼ 1.8 km diameter) volcanic
edifice has been constructed (middle inset in Fig. 5b). It is
difficult to discern whether there is any vertical offset across
the fracture; however, at the eastern edge of the frame, debris
from a hill does appear to have covered the fracture (rightmost inset in Fig. 5b). In some places smaller fractures run
parallel to the main one on either side (middle and right insets in Fig. 5b), suggesting some extension, but this could be
localized.
2.5.4. EMAKNG01
Emakong Patera and its surrounding flows were targeted in I25 because it was considered to be a site with a
high potential for sulfur volcanism (Williams et al., 2001b;
Keszthelyi et al., 2001; Lopes et al., 2004). A 140 m pixel−1 ,
I25 mosaic (Fig. 6a) had revealed a dark, sinuous lava channel and extremely complex mixing of dark and light lavas;
however, a higher-resolution observation during I25 had
been lost. The I32 observation (Fig. 6b) was planned to
provide higher-resolution (33 m pixel−1 ) data on the morphology of the channel. Of particular interest was the source
of the channel near the rim of Emakong Patera and the superposition relationships of the dark and bright lavas.
The channel (Fig. 6b) ranges from ∼ 620 m wide at its
head to up to ∼ 1400 m wide further downstream. There
are albedo variations in the channel interior, some of which
are clearly islands of bright material similar to that found
on the channel flanks. The islands are mostly oval in shape,
with long axes ranging from < 150 m to ∼ 1100 m. Albedo
variations in the darker material suggest that segments of the
channel have crusted over; one such segment is ∼ 1 km long.
The new images confirm that the channel originated as an
overflow of lava from Emakong Patera. The channel is very
broad, multi-stranded, and poorly defined near the source;
features that are common for channels formed by the overflow of a lava lake. Very similar channel morphology has
been observed to form at Kilauea as a result of:
(1) overflow across a sector of the crater wall, rather than
focused at a single point;
(2) highly variable overflow rates; and
(3) hot, low-viscosity lavas proximal to their source, which
tend to flow completely away without leaving substantial levees (Keszthelyi, unpublished data).
The dark channel feeds extensive bright (whitish and yellowish) and dark (gray to black) flows (Plate 1 in Williams
et al., 2001b), the largest of which trends north and is at
least 370 km long and ∼ 190 km wide at its widest point.
The bright-colored flows are consistent with the color of
cold sulfur in the ionian environment (Nash, 1987), and
the dark channel had been hypothesized to be molten sulfur feeding these flows (Williams et al., 2001b). However, repeated Galileo SSI imaging of the anti-jovian hemisphere has not detected any obvious changes in the Emakong
flows, suggesting that they have not been active over the
time period of these observations. NIMS temperature estimates of the patera floor indicate that it is warm, 344 ±
60 K, consistent with recent activity (Lopes et al., 2001,
2004).
Parts of the channel flanks and the surrounding flow field
(Fig. 6b) have morphologies that resemble the upper crusts
on terrestrial silicate lava flow fields, except for their color.
The range of albedos may indicate patches of sulfur flows,
in which the young, dark flows brighten with time, similar
to their terrestrial counterparts. Terrestrial, channel-fed sulfur flows have been noted to have pahoehoe- and/or aa-like
upper flow surfaces (Watanabe, 1940; Greeley et al., 1984),
and the patchy morphologies of the Emakong flows resemble
these analogs. The textures of the flow surfaces also appear
to be more consistent with a fractured lava crust than with
pyroclastic mantling; the latter would appear more uniform
over low-relief topography.
The spatial relationships between the dark and bright
lavas (Fig. 6b) defy any simple superposition hypotheses.
There are many examples of bright lavas completely surrounded by dark lava and vice versa. One possible explanation would be that both the bright and the dark lavas
formed during a single extended eruption. A flow of molten
sulfur is capable of producing both bright and dark solid
deposits, depending on the temperature at which the material is quenched (e.g., Sagan, 1979). An alternative is that
silicate lavas emplaced by the channel melted sulfur- or
sulfurdioxide-rich material in the surrounding plain. Once
mobilized, the lower-density sulfur flows could have risen
Final Galileo SSI observations of Io
13
(a)
(b)
Fig. 6. (a) I25 observation of Emakong patera at 140 m pixel−1 mosaicked with the high-resolution (∼ 30 m pixel−1 ) frames from I32. The patera is the source
of numerous dark and light lava flows. (b) High-resolution (∼ 30 m pixel−1 ) I32 mosaic of a lava channel to the east of Emakong Patera.
through cracks to pond in places on top of the silicate flow.
These sulfur flows then brightened over time to form the features seen in the latest images. Such a process was suggested
for the similarly convoluted mixture of dark and bright lavas
seen at ∼ 8 m pixel−1 at Chaac Patera (Keszthelyi et al.,
2001; Williams et al., 2002b). Additional data from future
missions will be necessary to determine the relative propor-
tions of silicate and sulfurous materials here and elsewhere
on Io.
A dark line on the patera floor can be seen to follow
the base of the surrounding wall. One possible interpretation is that it is a shadow cast by the rim of the patera,
in which case the height of the rim is ∼ 230 m (Williams
et al., 2002a). However, comparison to the I25 Emakong
14
E.P. Turtle et al. / Icarus 169 (2004) 3–28
(a)
Fig. 7. (a) High-resolution (50 m pixel−1 ) and (b) context (∼ 325 m pixel−1 ) mosaics of Tohil Mons and Tohil and Radegast Paterae near the terminator.
The illumination is from the right in both mosaics. Tohil Patera (near the upper right edge of the mosaics) has multiple flows with varied albedos on its floor.
Radegast Patera, a small, dark-floored patera between Tohil Patera and the peak of Tohil Mons, is ∼ 20 km across. The peak of Tohil Mons (as determined
from the I24 and I27 stereo observation, Schenk et al., 2001, and in preparation) lies just to the north of the bottom left frame in (a), labeled “peak” in (b).
observation (Fig. 6a) suggests that the dark ring may completely encircle the patera floor, which would be consistent with the broken crust of a lava lake, with a strand
line from a time when the lake was at a higher level, or
with exposed walls composed of dark material. It is difficult to be certain which of these options is correct from
the SSI images; however, I32 NIMS data indicate that the
hottest areas within Emakong Patera coincide with the patera rim (Lopes et al., 2004), supporting the hypothesis
that the dark curve represents the disrupted edge of a lava
lake.
2.5.5. TOHIL_01
Tohil Mons and Tohil and Radegast Paterae had been
observed during both I24 and I27 (at ∼ 190 and ∼ 165
m pixel−1 , respectively) in order to acquire stereo data (Turtle et al., 2001, 2002). These observations revealed that the
mountain is up to 9.4 km high (Schenk et al., 2001, and in
preparation) but the limited vertical resolution and the high
Sun obscured topographic details; Io’s mountains are notoriously difficult to distinguish in low-incidence-angle images
(cf., Gish Bar* Mons in Section 2.5.8 and Tohil Mons in
Fig. 10 in Turtle et al., 2001, 2002). During the I32 encounter
it was possible to acquire a five-frame mosaic across this
Final Galileo SSI observations of Io
15
(b)
Fig. 7. Continued.
region near the terminator at ∼ 50 m pixel−1 to reveal topographic details (Fig. 7a). Furthermore, slightly later another
observation (TERMIN01) captured Tohil, still near the terminator, at ∼ 325 m pixel−1 (Fig. 7b). The combination of
the I24 and I27 high-Sun observations, which provide stereo
views and highlight albedo variations, with the spectacular
I32 low-Sun observations, which reveal topographic details,
has proven quite useful for interpreting Tohil’s geology (e.g.,
Williams et al., 2004).
Tohil Mons, like all other ionian mountains, is being
eroded by mass wasting. A series of large landslide deposits extend west from the mountain (Fig. 7b). Shadow
measurements reveal that their distal scarps are several hundred meters high. The lower left frame of Fig. 7a reveals
a rugged surface with relief of a few hundred meters and
an apparent small landslide (5–13 km long) extending out
from the shadow of the scarp. In the context mosaic (Fig. 7b)
a straight ridge can be seen to extend southwest from the
16
E.P. Turtle et al. / Icarus 169 (2004) 3–28
peak. In the high-resolution mosaic shadows obscure the depression that lies directly to the east of the peak. Although
this depression bears some resemblance to a summit caldera,
there is little evidence for a volcanic origin; no lava flows
are seen within or emanating from it in the context mosaic
(Fig. 7b) or in the earlier high-Sun observations. Another
possibility for the formation of this depression is northeastward slumping of the large block that can be seen in the
I32 context mosaic and the earlier stereo observations (Turtle et al., 2001, 2002) to form the northeastern border of the
depression. However, this interpretation appears to be inconsistent with the lack of significant amounts of debris beneath
the block, indeed this area is instead inscribed by the small,
dark-floored Radegast Patera. This patera also appears to interrupt a series of northwest-southeast trending lineaments
that resemble an imbricate thrust belt along the northeastern
margin of the mountain (Jaeger et al., 2002, and in preparation).
Surprisingly, given the degree of mass wasting evident on
the western and southern flanks of the mountain, the floor
of Radegast Patera is quite clean; collapse of material from
the patera walls is limited to the margins of the patera. This
indicates that the volcanic resurfacing rate in the patera exceeds the rate of mass wasting from the patera walls and the
high ridges immediately to the south. Another possibility is
that this patera, like others on Io (Radebaugh et al., 2004;
Keszthelyi et al., 2004; Williams et al., 2004; Lopes et al.,
2004), is or has been a lava lake capable of consuming debris
that fell into it from the mountain. This interpretation might
explain the observation that Radegast Patera interrupts the
series of scarps along Tohil Mons’ eastern margin and the
lack of debris despite possible northeastward slumping from
the peak. The patera itself is only a few hundred meters deep,
but to its south the mountain rises several hundred meters to
two kilometers above the floor. The darkest lava flow visible on the floor of the patera was observed by NIMS to be
warmer than the surrounding surface, 325 ± 50 K, indicative
of recent activity (Lopes et al., 2004).
The illuminated faces of scarps in the high-resolution
mosaic (which appear uniformly bright in Fig. 7a, second
and third frames from the lower left, because the contrast
has been enhanced to bring out material near the terminator) show no strong evidence for layering. The lack of
obvious stratigraphy is somewhat surprising; however, layers would have to be a few hundred meters thick to be resolved directly in these 50 m pixel−1 images, whereas Io’s
lava flows seem to be ∼ 1–10 m thick (Davies et al., 2000;
Williams et al., 2001a).
Tohil Patera exhibits surprisingly little topographic relief.
From the scarps along the eastern edge of the patera we estimate that it is no more than a few hundred meters deep;
those scarps that are steep enough to cast shadows are generally less than 100 m high. Striking albedo contrasts can be
seen both outside the patera and on its floor (first and second
frames from the upper right in Fig. 7a). Very bright flows
with crenulated margins occur on the southwestern patera
floor. These flows have a NIMS spectral signature of enhanced SO2 content (Lopes et al., 2001), possibly indicative
of SO2 flows like those inferred in Balder Patera (Smythe
et al., 2000; Williams et al., 2002b). A very dark lava flow
more than 26 km long runs southwest across the center of
Tohil Patera. In these cases albedo contrasts are correlated
with identifiable lava flows; however to the east of the patera
some other process appears to cause, or at least contribute to,
albedo variations. The contacts between zones with different
albedos are generally smooth and sometimes they even appear diffuse, unlike the lobate borders of lava flows seen at
this resolution. In some places bright material completely
encloses dark material, elsewhere the opposite relationship
occurs. It is difficult to explain these relationships with successive surface flows. Furthermore, unlike the Emakong observation (Fig. 6b), the Sun was low when these images were
acquired, so topographic relief can be distinguished from
albedo variations of the surface materials. Surprisingly the
albedo variations are not correlated with relief (this is most
striking in the upper right corner of Fig. 7a), another indication that they are not simply due to lava flows of different
ages or compositions (and an observation that has implications for the interpretation of the bright and dark materials
outside of Emakong Patera as well). Perhaps some of the
surface albedo changes are due to heating of material from
below or to preferential deposition of frost on cooler material. From shadow measurements, the topography within the
patera is only on the order of a few tens of meters. Outside of
the patera there is significantly more topographic relief with
scarps ∼ 100 m high.
2.5.6. TUPAN_01
This magnificent 132 m pixel−1 , three-color mosaic of
Tupan Patera (Fig. 8) reveals complex interactions between
different color units in and around the ∼ 900 m deep (determined from shadow measurements) patera. (Originally six
filters were planned, but after the light-flood was disabled
the utility of the 889 nm and 1 µm filters was decreased, so
those images, along with that through the clear filter, were
dropped to save data volume.) Bright red material, probably rich in short-chain sulfur (e.g., McEwen et al., 2003)
or perhaps sulfur dichloride (Schmitt and Rodriguez, 2003)
released by very recent volcanic activity (McEwen et al.,
1998a; Geissler et al., 1999; Lopes-Gautier et al., 1999;
Phillips, 2000), covers much of the patera floor and the
central “island” and there are some diffuse deposits on the
surface above the patera wall to the east. Black, likely silicate, lava dominates the eastern part of the patera floor, but
is limited to isolated patches on the western side. The central region is not covered by dark, i.e., recent (McEwen et
al., 1985, 1997) lava flows, indicating that it stands above the
floor. However, the fact that it is not high enough to cast a noticeable shadow limits its height to less than a few hundred
meters. NIMS data reveal that this central “island” is cold
(Lopes et al., 2004). A relatively uniform black line traces
the edge of the patera floor in the western half of Tupan and
Final Galileo SSI observations of Io
17
Fig. 8. A ∼ 132 m pixel−1 color observation (only slightly enhanced from true color) of Tupan Patera. Illumination is from the right.
the edge of the “island,” and outlines light-colored deposits
on the northern patera floor. Perhaps this represents a strand
line due to withdrawal or degassing of lava after solidification of a crust (as may also be the case at Emakong). Such
deflation has been seen in many terrestrial lava lakes (e.g.,
Kilauea Iki). However, the low albedo of this line suggests
that the edges were recently molten, consistent with interaction of a lava lake’s crust with the confining walls.
The occurrence of bright material on the western and
northern, and to a lesser extent the eastern, patera floor
is consistent with the hypothesis that sulfur-rich materials in the patera walls and the “island” are melting due to
the high temperatures of the silicate lava and then flowing out over warm, but solidified, silicate crust (discussed
in more detail in Keszthelyi et al., 2004). From the variation in the amount of bright material across the patera
floor, we infer that the eastern part of the floor is still warm
enough to vaporize sulfur-rich liquid from the walls, while
the northern and western parts are cool enough for the liquid to persist and solidify on the surface. NIMS data support
this idea; the hottest part of the patera floor is the eastern side, where the dark lava is observed, and the lightercolored western and northern portion of the floor is also
warm, but cooler (Lopes et al., 2004). Several areas on the
eastern side of the patera floor are green, providing strong
support for the hypothesis that a chemical interaction between red sulfur deposits from outgassing and warm silicate
lavas results in green-colored material (McEwen et al., 2003;
Williams et al., 2004).
2.5.7. TVASHT01
This 200 m pixel−1 mosaic (Fig. 9) completed a series of
observations of the Tvashtar Catena region acquired over a
period of just under two years. The intense and unpredictable
eruption witnessed at Tvashtar was first seen as a 50 km
long curtain of fire fountains on 26 November 1999, during
orbit I25 (McEwen et al., 2000; Keszthelyi et al., 2001; Wilson and Head, 2001). Galileo’s next observation of Tvashtar
Catena (22 February 2000, during orbit I27) revealed major activity in the form of surface flows or perhaps a lava
lake in the large patera to the west (Keszthelyi et al., 2001).
Galileo’s orbit G29 on 30 December 2000 coincided with a
Jupiter flyby by the Cassini spacecraft, and both spacecraft
made observations of Io. Quite surprisingly Cassini detected
a plume almost 400 km high (McEwen et al., 2003) and
Galileo revealed a Pele-type plume deposit around Tvashtar
(Fig. 2). At the time of Galileo’s final observation of this
region, the I32 encounter on 16 October 2001, Tvashtar appears to have been quiet (Fig. 9). Bright and dark streaks
thought to be remnants of the plume eruption can be seen radiating away from a shallow patera nested within the large
eastern patera of Tvashtar Catena, the site of the I25 curtain
of fire fountains (labeled I25). To the southwest, emplacement of bright streaks appears to have been controlled to
some extent by the edge of the plateau surrounding the patera. Although we had expected that lava flow margins or
18
E.P. Turtle et al. / Icarus 169 (2004) 3–28
Fig. 9. Tvashtar Catena at ∼ 200 m pixel−1 . Illumination is from the right, revealing three smaller paterae (labeled) set into the floor of the eastern large patera.
Bright and dark streaks can be seen to have emanated from the westernmost of these three paterae, the site of the I25 eruption (indicated by the lines labeled
“I25”) (McEwen et al., 2000). This patera is believed to have been the source of the plume observed in G29. The large, dark-floored, western patera was the
site of extensive surface flows and small breakouts (indicated by the lines labeled “I27”) during I27 (Keszthelyi et al., 2001).
patera boundaries within Tvashtar might have changed following such dramatic activity, the I32 observation revealed
little modification. The final forms of the new dark lavas
match dark patterns observed earlier, suggesting that subtle
topographic confinement is controlling the extent of surface
flows. (For more details of the observed surface changes see
Geissler et al., 2004; Milazzo et al., in preparation.)
2.5.8. GSHBAR01
This 250 m pixel−1 , high-Sun mosaic of Gish Bar Patera,
the 11-km-high mountain it neighbors (Gish Bar* Mons), an
intriguing Y-shaped fracture to its west, and Monan* Mons
(Fig. 10a) was planned as the first part of a stereo observation (the second part was to be acquired during I33, but
was lost due to a spacecraft safing event). Despite this loss,
the I32 observation provided an interesting comparison to
observations made two years earlier (Fig. 10b), during orbits C21 and I24 (Turtle et al., 2001, 2002). Numerous dark,
and presumably recent, lava flows can be seen on the patera floor. Many apparent changes in albedo between the
observations can be attributed to the significant differences
in the illumination and viewing geometries (see figure caption for details) of the observations (e.g., Simonelli et al.,
1997; Phillips, 2000), but others may be the result of recent eruptions. The southeastern corner of the patera ap-
pears to have darkened significantly between July and October 1999 (Fig. 10b), perhaps as a result of an outburst
observed from Earth in August 1999 that has been attributed
to Gish Bar (Howell et al., 2001; Keszthelyi et al., 2001).
If this was a lava flow, the fact that the margins of this region did not change between the two observations implies
that the lava was topographically confined, which suggests
that this part of the patera might consist of ponded lavas
or even a lava lake, as has been hypothesized for other
ionian paterae (Lopes et al., 2004; Keszthelyi et al., 2004;
Radebaugh et al., 2004; Rathbun et al., 2002). A NIMS observation of Gish Bar in I32 revealed that this part of the
patera was warm (Lopes et al., 2004).
The I32 SSI image appears to show a new, dark, 30-kmlong flow covering an area of ∼ 1430 km2 in the northwestern part of the patera (Fig. 10; Perry et al., 2003).
Assuming that this is a flow that started shortly after the
I24 observation in October 1999, we can derive a lower
limit of 22 m2 sec−1 for the coverage rate. However, NIMS
data from I31 (August 2001) showed only low thermal output at this site, while the I32 (October 2001) NIMS data
revealed a significant increase (of two orders of magnitude) in thermal output (Lopes et al., 2004). So, if we assume that this is a flow that began shortly after the I31
NIMS observation, we derive a coverage rate of at least
Final Galileo SSI observations of Io
19
(a)
(b)
Fig. 10. (a) A ∼ 250 m pixel−1 mosaic from Monan* Mons to Gish Bar Patera. Gish Bar Patera is 106 × 115 km (Radebaugh et al., 2001). The Sun was
high (∼ 14◦ incidence angle), so there is little topographic information; 11-km-high (Schenk et al., 2001) Gish Bar* Mons, which abuts Gish Bar Patera to
its northeast, is barely distinguishable here. Brightness variations represent variations in the albedos of the surface material. (b) Surface changes on the floor
of Gish Bar Patera over a period of 27 months from observations made during C21 (July 1999), I24 (October 1999), and I32 (October 2001). Many of the
differences can be attributed to the different illumination geometries of the observations: the phase angles of the observations were ∼ 6◦ , ∼ 20◦ , and ∼ 47◦ ,
respectively; incidence angles were ∼ 46◦ , ∼ 76◦ , and ∼ 14◦ ; and emission angles were ∼ 50◦ , ∼ 58◦ , and ∼ 46◦ ). Furthermore, the C21 observation was
taken through SSI’s green filter, while the I24 and I32 observations were taken through the clear filter. The large, very dark area (labeled “new flow”) in the
western part of the patera may be a recent lava flow.
230 m2 sec−1 , which is similar to that of the 1997 Pillan Patera eruption, ∼ 330 m2 sec−1 (Davies et al., 2001).
For comparison, this coverage rate (230 m2 sec−1 ) is about
twice the peak rate of the current Kilauea eruption and more
than 500 times Kilauea’s typical rate (Mattox et al., 1993;
Heliker and Mattox, 2003). Activity at Gish Bar continued
into at least December 2001, when it was observed from the
Keck Observatory in Hawaii (Marchis et al., 2002).
Another intriguing feature seen in this observation is the
Y-shaped fracture to the west of Gish Bar Patera. This is one
of the few examples of a fracture seen on Io’s surface; probably because features without significant relief are quickly
obscured due to the high resurfacing rate. The morphology is similar to extensional regimes on Earth, with three
faults (one being much longer than the other two) extending from a common point. Extensional faulting is currently
a model for the formation of some ionian paterae, such as
Hi’iaka Patera (McEwen et al., 2000; Jaeger et al., 2000;
Radebaugh et al., 2001). However, there is no evidence for
current volcanic activity along this fracture.
2.5.9. TERMIN01
This ∼ 330 m pixel−1 , near-terminator observation traversed Io from Mycenae Regio at ∼ 40◦ south to Colchis
Regio just north of the equator, and it demonstrates the
wide range of volcanic styles that are found on Io (Fig. 11;
see also Williams et al., 2004). Starting in the south, this
observation revealed that Mycenae Regio consists of thin,
digitate lava flows. As discussed previously (Section 2.5.5
TOHIL_01), part of this observation served as context for
the high-resolution mosaic across Tohil Mons and Tohil and
Radegast Paterae (Fig. 7a). The flows emanating from Culann Patera (McEwen et al., 2000; Williams et al., 2004)
apparently traverse very shallow slopes as no topography is
evident in this low-Sun view. North of Culann Patera lie hundreds of kilometers of layered plains, which in some places
are cut by arcuate, generally north–south trending fractures
(Fig. 11) and in others exhibit low ridges (see Fig. 6 in Bart
et al., 2004).
At the northern end of the mosaic (top frame of Fig. 11)
lies Michabo Patera, which spans almost 100 km from north
to south, is ∼ 1.5 km deep (based on shadow measurements),
20
E.P. Turtle et al. / Icarus 169 (2004) 3–28
sions near its summit. On Io it is uncommon for lavas to
build substantial edifices around vents (Smith et al., 1979;
Schaber, 1982; Jaeger et al., 2003); lava flows are usually long and thin and paterae are incised, sometimes quite
deeply, into the surface (Radebaugh et al., 2001). To the west
of the tholus lies Tsũi Goab Fluctus, a field of high-albedo
lava flows that appear to embay the western margin of the
small shield. These flows, like some seen near Emakong Patera (EMAKNG01, Fig. 6) could consist of sulfur (Williams
et al., 2001b). The bright flow field is the only fresh volcanic
deposit in this area where a low-intensity hotspot was observed by NIMS during I27 (February 2000; hotspot I27D
in Lopes et al., 2001). The relatively low intensity of the I27
hot spot does not rule out a low-temperature eruption, which
together with the bright albedo (Fig. 11) is consistent with
sulfur flows.
Fig. 11. Mosaic of I32 near-terminator images at ∼ 325 m pixel−1 from
the dark lava flows of Mycenae Regio in the south, across Tohil Mons and
Culann Patera, to Michabo Patera and Tsũi Goab Tholus and Fluctus in the
north. The illumination is from the right.
and appears to be set into a low plateau. There is little evidence of lava flows associated with this patera (Williams
et al., 2004). To the east of Michabo is Tsũi Goab Tholus,
which appears to be a small shield volcano with two depres-
2.5.10. TERMIN02
This ∼ 330 m pixel−1 , near-terminator observation
(Fig. 12a) was originally designed to traverse Io from
∼ 15 ◦ N near Zamama northwards to an intriguing group of
yellow-floored paterae near 65◦ N. However, it was rapidly
modified after I31 so that the northernmost two frames covered the site of the new plume eruption detected on that orbit
(see Sections 2.4.1 PLUMES01 and 2.4.3 GLOCOL02).
Starting at Zamama (lower left frame and left inset in
Fig. 12a), the observation reveals that this lava flow field,
which formed sometime between the Voyager flybys in 1979
and the first Galileo observations in 1996, emanates from
one of two small volcanic constructs that are several hundred meters high (based on shadow measurements). The
vent is a radial fissure starting near the summit of the northern small shield and feeding a broader flow field that extends ∼ 160 km across the plains. This situation is precisely
what was predicted from the morphologies of the lavas and
the distribution of the colorful diffuse deposits surrounding Zamama (Keszthelyi et al., 2001). Moving northeast, the
second and third frames of this mosaic contain lava flow
fields and Thomagata* and Reshef* Paterae. It is unclear
whether the broad, shield-like features or plateaus within
which the paterae are incised were created by eruptions from
the paterae, or if they were pre-existing topographic features. Thomagata* Patera is 56 km long, 26 km across and
1.2–1.6 km deep (based on measurements of the shadows
cast by the patera walls). The raised plateau surrounding it is
more than 100 km across and its western margin rises only
∼ 200 m above the plain. Although it looks like the patera
floor sits lower than the surrounding plain, this cannot be determined from the image; the fact that the western flank of
the plateau is completely illuminated only indicates that its
slope is less than 20◦ . If the patera floor and the surrounding plain were at the same level, the slope of the western
flank of the plateau would be just over 2◦ . Reshef* Patera is
∼ 60 km long and ∼ 35 km across. The southern part of the
patera floor is set somewhat deeper than the northern part,
Final Galileo SSI observations of Io
21
(a)
(b)
Fig. 12. (a) Mosaic of I32 near-terminator images at 330–340 m pixel−1 from Zamama to the source of the I31 plume, Thor* , the largest ever seen on Io.
Illumination is from the right. Enlarged and contrast enhanced views of Zamama and Thor* are inset at the bottom of the figure because the illumination
variation across the scene is so great that it is difficult to discern features at the eastern and western edges. The scale also varies, so 200-km scale bars are
included at both edges of the mosaic. (b) The same region seen in July 1999 (C21). The source of the I31 plume is the faint, light-colored flows at the upper
right corner. These flows have the same outline as the new dark flows seen during I32 in (a).
22
E.P. Turtle et al. / Icarus 169 (2004) 3–28
∼ 1.6 and ∼ 1.3 km, respectively (based on shadow measurements).
The northeastern frames of this mosaic (right inset in
Fig. 12a) reveal a series of discontinuous, dark flows within
an irregular network of fractures near the source of the 500km-tall, I31 plume, Thor*. At the regional scale seen during
orbit I31, the large, whitish deposit from the plume appears
generally symmetric (middle, right image of Fig. 2); however, at the local scale seen during orbit I32, the dark diffuse
deposits can be seen to have formed in a crescent shape
following the flows. It is difficult to discern whether the
bright diffuse deposit, which generally lies beyond the dark
one, was symmetric or not at the time of orbit I32. Surprisingly, comparison to earlier (orbit C21) images of this region
(Fig. 12b) indicates that the new, dark lava flows actually
have the same outlines as earlier bright, yellow flows. Therefore, as at Tvashtar, an eruption that at first seemed to be new,
turned out to be a reactivation of an earlier eruptive site.
2.5.11. GLOCOL01
This 5 km pixel−1 , four-color observation (bottom, left
image in Fig. 2) was added as a contingency against loss of
high-resolution imaging due to camera and spacecraft anomalies near Jupiter. Although extreme contrast enhancement
does reveal a hint of something above the limb in this observation, there is no indisputable evidence for continued
eruption of the giant plume that was observed during I31
(PLUMES01), and whose source region was imaged during
I32 (TERMIN02). This may be consistent with the lack of
any obvious blurring of the higher-resolution TERMIN02
frames; such blurring has sometimes been observed when
imaging through plumes (e.g., Amirani in Plate 6 of Keszthelyi et al., 2001; and Zamama in cover image of McEwen et
al., 1998b).
2.5.12. GLOCOL02
This observation (bottom, right image in Fig. 2) consisted of two on-chip-mosaics (cf. Klaasen et al., 1997) at
9.8 km pixel−1 in four colors. Unfortunately due to pointing uncertainties in one of the mosaics, the disks of Io in
the violet and 756 nm filters overlap each other, limiting the
amount of the surface that can be reconstructed in full color.
Nonetheless this final view of Io provided further information on the evolution of the plume deposits around Pele and
Dazhbog (Geissler et al., 2004).
2.6. Orbit I33
I33 was to provide Galileo’s first opportunity to image
the sub-jovian side of Io at moderate to high resolution, covering sites not seen at such resolutions since the Voyager
flybys in 1979 (Fig. 1; Table 6; Appendix A.2). In looking
at targets of Voyager’s highest-resolution, ∼ 500 m pixel−1 ,
imaging (e.g., Apis and Inachus Tholi, Mbali Patera, Pan
Mensa) we were searching for evidence not only of changes
in superficial deposits as have been monitored throughout
the Galileo mission (e.g., Phillips, 2000; Geissler et al.,
2004) but also of morphological changes such as a new
patera or a massive landslide from one of Io’s mountains.
We planned very high-resolution views of Voyager targets,
such as Mbali Patera (20 m pixel−1 ) and the tholi mentioned above (13 m pixel−1 ), which are two of only four
ionian volcanic structures (Jaeger et al., 2003) that actually
resemble volcanoes with summit calderas seen on Earth and
Mars, and therefore may consist of more evolved, higherviscosity lavas than those seen elsewhere on Io. Based on
past experience with very high-resolution observations that
did not have adequate context, we avoided imaging at better
than 10 m pixel−1 , restricting our highest-resolution (10–
20 m pixel−1 ) observations to those for which the geometry of the flyby allowed acquisition of medium resolution (∼ 100 m pixel−1 ) context imaging. These context observations included a three-color view of Mbali Patera at
90 m pixel−1 . The loss of the I31 medium resolution views
of Masubi, Kanehekili, and Lei Zi Flucti made targeting
the I33 imaging sequence significantly more difficult due to
uncertainties of ∼ 1◦ in the basemap of the sub-jovian hemisphere. The geometry of the I33 encounter also provided an
opportunity to acquire:
Table 6
Galileo SSI observations during orbit I33
Observation
Number of framesa
PELE_01
THOLI_01
MBALI_01
KANEHE01
THOLI_02
MBALI_02
KANEHE02
HIIAKA01
PAN_02
GSHBAR01
MASKAN01
GLOCOL01
GLOCOL02
1 × 3.8 IM8
1 × 3 IM4
1 × 4 IM8
1 × 4 IM4
1 × 5 IM4
3 1 × 2 IM8
1 × 4 IM4
1 × 4 IM4
1 × 4 IM4
1 × 4 IM4
1 × 9.5 IM4
2 × 3 3-color IM4
3-color IM4
a See footnote for Table 2.
Resolution (m pixel−1 )
12
13
20
27
85
90–94
98
111
122
130–135
335
1,400
2,400
Filter(s)
Notes
Clear, 1 µm
Clear
Clear
Clear
Clear
Violet, green, 756 nm
Clear
Clear
Clear
Clear
Clear
Violet, green, 756 nm
Violet, green, 756 nm
Lost due to spacecraft anomaly
Lost due to spacecraft anomaly
Lost due to spacecraft anomaly
Lost due to spacecraft anomaly
Lost due to spacecraft anomaly
Lost due to spacecraft anomaly
Lost due to spacecraft anomaly
Lost due to spacecraft anomaly
Lost due to spacecraft anomaly
Lost due to spacecraft anomaly
Lost due to spacecraft anomaly
Lost due to spacecraft anomaly
Lost due to spacecraft anomaly
Final Galileo SSI observations of Io
(1) the second half of the stereo observation of Gish Bar
Patera and Gish Bar* Mons at 130 m pixel−1 (the first
half was successfully acquired during orbit I32);
(2) a 110 m pixel−1 view of Hi’iaka Montes and Patera with
the opposite illumination from the I24 and I25 observations (Figs. 6 and 8b in Turtle et al., 2001, 2002), to test
the hypothesis of massive strike-slip faulting (McEwen
et al., 2000; Jaeger et al., 2000);
(3) a 122 m pixel−1 observation of Pan Mensa and its two
associated paterae; and
(4) a 335 m pixel−1 observation of Masubi Fluctus and its
plume, which had been observed to fluctuate and wander
dramatically throughout the Galileo mission (Phillips,
2000; Geissler et al., 2004).
Furthermore, this encounter allowed for a 1.4 km pixel−1
color mosaic over the portion of Io between 320◦ W and
30◦ W that had not been imaged at resolutions better than
∼ 10 km pixel−1 by either Voyager or Galileo. Finally, we
planned a 2.4 km pixel−1 color view of the anti-jovian features that had been studied so closely during the earlier Io
flybys.
Regrettably Galileo went into safing about 27 minutes
before closest approach to Io. Although the problem was
suspected to be a despun-bus reset, similar to what Galileo
had experienced and handled successfully 20 times in previous encounters (Theilig et al., 2002), it was different enough
that the onboard software patch did not catch it and allow
the spacecraft to ignore it and continue the observation sequence. It was speculated that an unusually long-duration
bus reset, or two occurring in rapid succession, prevented the
patch from working. Radiation levels near perijove peaked
at the highest level seen since I27. The reduced flight team
worked to return the spacecraft to the nominal state and successfully restarted the I33 sequence, but not before the last Io
observation had been lost. Truncated observation sequences
to be uplinked in case of spacecraft problems had been developed ahead of time, as usual, but these were intended to
protect the Io closest-approach data from an anomaly occurring before, rather than during, the encounter. As a result
of the safing event, three tracks of I32 data (everything but
the two GLOCOL observations) were not overwritten during
I33, and in the aftermath we took advantage of this fact to fill
some gaps in I32 images that had been caused by downlink
problems during I32 playback.
2.7. Orbit A34
Orbit A34 included a 160-km-altitude flyby of Amalthea,
one of the small, irregularly shaped satellites located within
Jupiter’s ring system. Unfortunately, remote sensing observations were not supported during the flyby. The lack
of imaging on this orbit was disappointing, not only because high-resolution observations of Amalthea would have
helped greatly in interpreting its intriguing low density
(1.0 ± 0.5 g cm−3 , derived from gravity measurements (An-
23
derson et al., 2002)) and the nature of the objects observed
by Galileo’s star tracker (Fieseler and Ardalan, 2003) during
the flyby, but also because the geometry of this orbit would
have allowed Galileo’s best, moderate-resolution view of
Io’s sub-jovian hemisphere, providing additional context for
regions which would only have been visible at resolutions
better than ∼ 100 m pixel−1 during I33. Furthermore, after
the loss of the I33 data, A34 became Galileo’s only opportunity to view this side of Io at moderate resolution. We had
hoped for two valuable observations to be taken from inside
Io’s orbit. The first was a 500 m pixel−1 daylight view of Pillan and Pele Paterae which would have provided critical tests
of the lava lake hypothesis for Pele (Radebaugh et al., 2004;
Lopes et al., 2004) and allowed us to determine if the eruption at Pillan involved eruptions of lava along the same
faults that dissect the neighboring mountain (Williams et
al., 2001a). The second was a 500 m pixel−1 strip from Loki
to Ra Patera, which would have provided the best view ever
of the Solar System’s most powerful volcano, Loki, and a
much better look at Ra, the most likely site of voluminous
sulfur volcanism on Io.
2.8. J35
Galileo ended its mission by plunging into Jupiter’s atmosphere on 21 September 2003. As Galileo approached
Jupiter it successfully acquired and returned data that are
currently being analyzed at the time of this writing.
3. Conclusions and hopes for future observations
Despite the extremely tight restrictions on data volume
and downlink capacity, during Galileo’s final encounters
with Io, we tried to achieve a balance between exploring
new territory and building upon previous results with new
observations at different resolutions, under different lighting or viewing angles, or to monitor changes over time
(Tables 2–6). Unfortunately camera and spacecraft problems
were encountered frequently in the perilous environment
close to Io. Moreover, until orbit I33, Galileo’s trajectory
restricted us primarily to observing the anti-jovian hemisphere. Regrettably, no images were returned from the only
two encounters that did provide views of the sub-jovian
hemisphere at resolutions comparable to or exceeding those
of Voyager: a close flyby during I33 that suffered a spacecraft safing event, and a distant one during A34 at which
point insufficient resources remained to support acquisition
of remote sensing observations. The loss of these opportunities to search for geologic changes over the 22 years between
the two missions was a keen disappointment.
Nonetheless there were many successful SSI observations during the Galileo Millennium Mission that advanced
our understanding of Io (e.g., Geissler et al., 2004; Radebaugh et al., 2004; Williams et al., 2004). The discovery of
four giant, high latitude plumes during G29 and I31 (Figs. 2
24
E.P. Turtle et al. / Icarus 169 (2004) 3–28
and 3) was surprising because, although Voyager 2 had observed surface deposits from high-latitude plumes in 1979
(McEwen and Soderblom, 1983), in Galileo’s five years in
orbit around Jupiter, it had barely detected one other such deposit (Geissler et al., 2004). No high-latitude plume had ever
been detected directly. The Tvashtar plume eruption (Fig. 2)
was not only a surprise because of its high latitude, but also
because it demonstrated that remarkably diverse eruption
styles were possible at a single location within a relatively
short period of time (13 months between I25 and G29). An
interesting aspect of the new plume observed during I31,
Thor*, was that its source was not actually new (Fig. 12b),
but simply reactivated after a period of dormancy more than
five years long (no evidence for activity had been detected
at this location in Galileo’s first observations of Io in 1996).
In fact, there was no evidence for any truly new volcanic
center seen during six years of Galileo observations; therefore, activity at volcanic centers on Io is likely to persist
for centuries. The I32 TERMIN01 observation (Fig. 11) also
revealed potentially active sulfur flows from an eruption documented by NIMS in February 2000 (Lopes et al., 2001).
Another important result is that many of Io’s paterae appear to be persistent lava lakes (e.g., Lopes et al., 2004; Pele,
Radebaugh et al., 2004; Loki, Rathbun et al., 2002). The paterae containing these lakes may form as heat from intrusions
melts and vaporizes sulfurous volatiles in the crust (cf. Tupan in Fig. 8; Keszthelyi et al., 2004). Some of the biggest
constraints on Io’s subsurface have come from studies of
the interaction of ionian tectonic and volcanic features, for
example our suite of observations of Tohil Mons and neighboring Radegast and Tohil Paterae (Fig. 7; Williams et al.,
2004). Mountains have been demonstrated to have an affinity
for paterae (Jaeger et al., 2003) suggesting a genetic relationship. Mountain building can be driven by the enormous
compressive stresses that accumulate in the lithosphere as
it is buried by Io’s rapid volcanic resurfacing (Schenk and
Bulmer, 1998; Turtle et al., 2001). Observations of paterae
neighboring mountains appear to demonstrate that, once the
compressive stresses are relieved, magma uses the orogenic
faults as conduits to the surface (Jaeger et al., 2003).
The recent observations also revealed details of the mass
wasting processes acting on Io. Slumping and landsliding
dominate at both large (Fig. 7b) and small scales (Fig. 5b)
and can occur in close proximity to each other, demonstrating spatial variation in material properties over distances of
several kilometers. Despite the ubiquitous mass wasting, the
rate of volcanic resurfacing seems to dominate on the small
scale (e.g., the generally clean patera floor in Fig. 7a). However, catastrophic landslides such as those that created the
massive deposits to the west of Tohil (Fig. 7b; Williams et
al., 2004) could rapidly make such drastic alterations to patera boundaries that we would be unable to identify them as
such without actually observing the events themselves.
Finally, the highest resolution observations obtained during Galileo’s final encounters with Io widen the variety of
morphologies observed on Io (Fig. 5b), thus providing fur-
ther evidence for an extremely diverse assortment of surface
processes at work on Io, including volcanism, mass wasting,
possibly tidal kneading (Bart et al., 2004), and undoubtedly
others we have yet to understand (e.g., Fig. 5b; Fig. 9 in Turtle et al., 2001, 2002).
There are also a number of conclusions regarding engineering and mission-planning that we can draw from
Galileo’s second set of close Io flybys. First, a spacecraft can
be built to function in this hostile radiation environment, but
it requires exceptional engineering design and mission support. The latest Io images are testaments to the talents and
dedication of two generations of scientists and engineers.
Second, Io, being a geologically active planet, requires a
different observational strategy from that commonly used
at other planets. Complete spatial coverage is simply not
sufficient; to really understand Io, temporal monitoring at
high spatial resolution is essential. We believe the sequences
of observations we acquired over Prometheus, Pele, and
Tvashtar told us more about Io than we could have learned
from single observations at additional locations. The evolution witnessed at Tvashtar demonstrated that, much like
terrestrial volcanoes, a single site on Io can exhibit quite different eruption styles at different times.
We hope that additional missions will be sent to explore
Io in the near future (Spencer et al., 2002). The next opportunity anticipated at the time of this writing is a Jupiter
flyby by the New Horizons mission on its way to explore
Pluto and the Kuiper Belt. As currently scheduled, New
Horizons is to be launched in January 2006 and would encounter Jupiter at a distance of ∼ 32RJ in January 2007.
Many useful observations could be made of Io during such
an encounter: global color imaging (at ∼ 45 km pixel−1 ,
as well as ∼ 11 km pixel−1 monochrome; J. Spencer, personal communication, 2003) to detect surface changes and
plumes, and eclipse observations to monitor and detect
hotspots. The Jupiter Icy Moons Orbiter that is currently
under discussion would also provide an important opportunity to observe Io (McEwen, 2003; Spencer et al., 2003;
Smythe et al., 2003). Although this mission is not expected
to orbit Io itself, nonetheless imaging of Io at resolutions
better than ∼ 1 km pixel−1 should be possible from orbit
around Ganymede and Europa, depending on instrumentation (McEwen, 2003; Spencer et al., 2003).
There are important imaging goals at Io that remain to be
met: global color coverage at 200 m pixel−1 and relatively
high incidence angle, the conditions we found most useful for interpreting regional geologic relationships, as well
as low-phase-angle coverage, which accentuates albedo and
color variations (sadly a large gap between 320◦ W and
30◦ W longitudes that was not imaged at < 10 km pixel−1 by
either Voyager or Galileo still persists); very high resolution
observations (with context!) of scarps where morphologies
suggestive of sapping or sublimation degradation are observed; high-resolution, dual-filter observations of all of Io’s
hotspots to determine eruption temperatures of Io’s lavas;
more extensive and higher-resolution stereo observations to
Final Galileo SSI observations of Io
reveal topographic relationships; and finally, coverage of regions imaged by both Voyager and Galileo to detect the
changes that will inevitably occur in the intervening years
before another spacecraft arrives at Io.
Acknowledgments
The authors express their gratitude to the Galileo mission and SSI support teams, especially to Bill Cunningham
and Greg Levanas for their exceptional efforts in diagnosing
and resolving the problems that developed with the SSI late
in the Galileo mission. Without their hard work the spectacular I32 images would not have been possible. We also
thank Mike Belton, leader of the SSI team, for his enthusiastic support. We are grateful to Sarah Fagents and John
Stansberry, who provided thorough reviews from which this
manuscript benefited greatly. This material is based upon
work supported by the National Aeronautics and Space Administration under the Galileo Project.
Appendix A
Brief descriptions of the motivation behind each of the
observations from encounters I31 and I33 that were eliminated due to data volume constraints or lost due to camera
or spacecraft problems follow.
A.1. Orbit I31 (Table 4)
25
A.1.3. PROMTH01
This observation was an attempt to observe the structure
of the Prometheus plume above Io’s limb at a resolution
45 m pixel−1 .
A.1.4. TVASHT02
Having learned from previous flybys how difficult it
could be to interpret the highest resolution observations
possible near closest approach, we planned this observation to put TVASHT01 into somewhat lower resolution
(50 m pixel−1 ) context, which, in turn, could be put into the
context of the images acquired during I25 (Fig. 7 in Turtle et al., 2001, 2002) and I27 (Plate 4 in Keszthelyi et al.,
2001). This observation not only included the source region
for the I25 eruption, and potentially the G29 plume, but it
also extended to cover the northern wall of Tvashtar Catena,
which exhibits a very unusual morphology that is suggestive
of erosion by sapping or sublimation degradation (Moore et
al., 2001). Sadly this encounter was Galileo’s last opportunity to observe this intriguing region at high resolution.
A.1.5. PROMTH02
Designed to provide context for PROMTH01, this observation was deleted due to data volume constraints.
A.1.6. SAVITR01
This was a 2 × 3 frame mosaic at 135 m pixel−1 of Savitr
Patera, a large, bright-floored patera, south of Tvashtar, that
is bounded by a plateau.
A.1.7. AMRANI01
The Amirani flow field, the longest active lava flow in
the Solar System, had been observed during both I24 and
I27, from which it had been possible to determine the rate at
which lava was being erupted here (Keszthelyi et al., 2001).
The six-frame, 140 m pixel−1 , I31 mosaic was intended to
detect further changes.
A.1.1. MAXRES01
A single frame that was to be taken at closest-approach
to achieve the highest resolution possible to reveal details
of surface modification processes. Near closest-approach the
spacecraft was moving too fast relative to the surface for
the scan platform to keep up with a mosaic, so a single,
very high-resolution (∼ 1 m pixel−1 ) frame was planned.
The spacecraft trajectory was such that this part of Io was
not visible later in the flyby, so it was not possible to acquire
any context for this observation, calling its utility into question; the very high-resolution SAPPNG01 observation on
I27 (Moore et al., 2001; Turtle et al., 2001, 2002), which also
lacked context, had proven to be very difficult to interpret.
Therefore, when data volume became tight, this observation
was eliminated.
A.1.8. SUSANO01
This observation of Itzamna Patera in four colors and at
140 m pixel−1 was to provide information on the relationship
between the multi-colored materials on the patera floor. (At
the time the observation was planned the name Susanoo had
been proposed, but this name was subsequently used elsewhere by the IAU and the name Itzamna was assigned.)
A.1.2. TVASHT01
The trajectory of the I31 flyby was ideal for observing the
vent of the I25 Tvashtar eruption at exceedingly high resolution (3–5 m pixel−1 ). It was hoped that this observation
would reveal details of the vent morphology that might confirm the interpretation that the eruption observed during I25
was a curtain of fire fountains, and that with luck, it might
also show the source vent for Tvashtar’s giant G29 plume
deposit.
A.1.9. MASUBI01
This was a five-frame, ∼ 400 m pixel−1 mosaic to observe the current status of sinuous Masubi Fluctus which
had been observed by Voyager, and whose plume had been
seen to migrate significantly in global Io monitoring observations during Galileo’s nominal mission (Phillips, 2000;
Geissler et al., 2004). In addition this observation would
have been very useful in planning high-resolution imaging
of Masubi Fluctus during orbit I33.
26
E.P. Turtle et al. / Icarus 169 (2004) 3–28
A.1.10. LEIZI_01
This was a four-frame, ∼ 400 m pixel−1 mosaic of Lei
Zi Fluctus. It, too, would have aided in planning higherresolution imaging of this region in during I33.
A.1.11. KANEHE01
This observation was a two-frame, ∼ 400 m pixel−1 mosaic of Janus Patera and Kanehekili Fluctus, the site of a
number of major eruptions witnessed by Galileo (Geissler
et al., 2004). It was also designed to aid in targeting very
high-resolution imaging of this region during I33.
A.1.12. TERMIN01
This was a five-frame, ∼ 410 m pixel−1 , near-terminator
view that started at Prometheus and ran northward to
∼ 60◦ N including Chaac, Balder, and Surya Paterae, Sobo
and Arinna Flucti, and large unnamed patera at ∼ 50◦ N.
A.1.13. TERMIN02
This was a four-frame, ∼ 415 m pixel−1 , near-terminator
view running from ∼ 45◦ S to 5◦ S, including Shamash, Tohil, Radegast, Culann and some unnamed paterae.
A.2. Orbit I33 (Table 6)
This encounter was Galileo’s first opportunity to observe Io’s sub-jovian hemisphere at resolutions less than
1 km pixel−1 .
A.2.1. PELE_01
This observation consisted of 3.8 frames (looking through
Galileo’s booms, so it was likely that one or more would be
obstructed) in the clear and 1 µm filters at 12 m pixel−1 . Although we had missed the most active area in I24 and I27
(Keszthelyi et al., 2001), these observations and the successful I32 observation (Radebaugh et al., 2004) had helped
to locate the region of interest so we were optimistic that
we could target it successfully with this observation to acquire very high-resolution data on the thermal emissions
from this apparent active lava lake (Lopes et al., 2004;
Radebaugh et al., 2004).
A.2.2. THOLI_01
This observation consisted of a 1 × 3 mosaic at ∼ 13
m pixel−1 across the margin of Inachus Tholus. Inachus
and Apis Tholi, which were observed by Voyager, may be
rare ionian shield volcanoes composed of high-silica lava
flows, as opposed to the mafic lavas that are more commonly
observed on Io (Williams et al., 2001a). This very highresolution observation was specifically targeted to reveal the
morphology of the margin of Inachus Tholus to investigate
whether it was formed by higher viscosity lavas. I33 was
Galileo’s only opportunity to see the tholi at high resolution.
A context observation at ∼ 85 m pixel−1 was also planned.
A.2.3. MBALI_01
This observation was an attempt to observe a flow
vent at 20 m pixel−1 . Previous, very high-resolution (< 50
m pixel−1 ) attempts had not been successful, e.g., Prometheus (Keszthelyi et al., 2001) and Tvashtar, which was lost
during I31.
A.2.4. KANEHE01
This 1 × 4 mosaic at 27 m pixel−1 was designed to
observe Kanehekili, a persistent high-temperature hotspot
which has undergone many changes (Geissler et al., 2004).
I33 was Galileo’s only opportunity to see this volcanic center
at high resolution, which would have provided a comparison
to potential ultramafic flow morphology observed at Pillan
during I24 (Keszthelyi et al., 2001).
A.2.5. THOLI_02
This was a five-frame mosaic at ∼ 85 m pixel−1 containing an oblique view of Apis and Inachus Tholi: their
margins, summit paterae, and the contact between them. Not
only did this observation provide context for the very highresolution view of the margin of Inachus Tholus, but it also
gave the only high-resolution view of their summits, which
appear to resemble other planetary calderas, such as those
at Olympus Mons and Mauna Loa, more than do any of the
other paterae on Io.
A.2.6. MBALI_02
In addition to providing context for the very highresolution observation of Mbali’s vent, this 90 m pixel−1 ,
1 × 2 mosaic was designed in three colors to investigate, in
detail, the relationships of the colorful deposits seen in Voyager images.
A.2.7. KANEHE02
This was a four-frame mosaic of Kanehekili Fluctus at
∼ 100 m pixel−1 to provide context for the earlier very highresolution observation and to give a broader view of Kanehekili’s interaction with nearby mountains.
A.2.8. HIIAKA01
The Hi’iaka Montes–Hi’iaka Patera complex had been
imaged in I24 and I27 at ∼ 570 and ∼ 360 m pixel−1 , respectively (Turtle et al., 2001, 2002). In both of these observations the illumination was from the west, potentially
concealing tectonic evidence that would provide clues to the
nature of the interaction between the patera and the two
mountains. The four-frame, ∼ 110 m pixel−1 , I33 mosaic
had the opposite illumination from the earlier views, so it
provided a complementary observation that was essential to
testing the hypothesis that Hi’iaka Montes had been rifted by
strike-slip faulting (McEwen et al., 2000; Jaeger et al., 2000;
Turtle et al., 2001, 2002).
Final Galileo SSI observations of Io
A.2.9. PAN_02
This four-frame, ∼ 120 m pixel−1 mosaic of Pan Mensa,
a mountain which was shown by Voyager 1 to have paterae
located at either end, provided another opportunity to explore the nature of the relationship between mountains and
paterae.
A.2.10. GSHBAR01
This four-frame mosaic at ∼ 130 m pixel−1 was the second part of a stereo observation (I32 GSHBAR01) to investigate the relationship between Gish Bar Patera and Gish Bar*
Mons, as well as a mysterious Y-shaped crack to its west.
A.2.11. MASKAN01
This was a 9.5-frame mosaic at ∼ 335 m pixel−1 along
Masubi Fluctus and extending northeast past Kanehekili
Fluctus, both sites of persistent high-temperature activity
(Geissler et al., 2004; Phillips, 2000).
A.2.12. GLOCOL01
This 1.4 km pixel−1 , 2 × 3 frame, regional, three-color
mosaic covered part of Io never seen at better than ∼ 10
km pixel−1 by either Voyager or Galileo. It would have filled
a large gap between 320◦ W and 30◦ W longitude, allowing
a global geologic map to finally be constructed for Io.
A.2.13. GLOCOL02
This was a final three-color view of part of Io’s anti-jovian
hemisphere at 2.4 km pixel−1 covering many of the most
dramatic features observed during Galileo’s close encounters with Io, including Prometheus, Amirani, Tvashtar, and
Thor*.
References
Anderson, J.D., Anabtawi, A., Jacobson, R.A., Johnson, T.V., Lau, E.L.,
Moore, W.B., Schubert, G., Taylor, A.H., Thomas, P.C., Weinwurm,
G., 2002. Gravity field, topography, and interior structure of Amalthea.
EOS Trans. AGU Fall Meet. Suppl. 83. Abstract F854.
Bart, G.D., Turtle, E.P., Jaeger, W.L., Keszthelyi, L.P., Greenberg, R., 2004.
Ridges and tidal stress on Io. Icarus 169, 111–126.
Davies, A.G., Lopes-Gautier, R., Smythe, W.D., Carlson, R.W., 2000. Silicate cooling model fits to Galileo NIMS data of volcanism on Io.
Icarus 148, 211–225.
Davies, A.G., Keszthelyi, L.P., Williams, D.A., Phillips, C.B., McEwen,
A.S., Lopes, R.M.C., Smythe, W.D., Kamp, L.W., Soderblom, L.A.,
Carlson, R.W., 2001. Thermal signature, eruption style, and eruption
evolution at Pele and Pillan on Io. J. Geophys. Res. 106, 33079–33104.
Fieseler, P.D., Ardalan, S.M., 2003. Objects near Jupiter V (Amalthea), IAU
Circ. 8107.
Frank, L.A., Paterson, W.R., 2001. Passage through Io’s ionospheric plasmas by the Galileo spacecraft. J. Geophys. Res. 106, 26209–26224.
Geissler, P.E., McEwen, A.S., Keszthelyi, L., Lopes-Gautier, R., Granahan,
J., Simonelli, D.P., 1999. Global color variations on Io. Icarus 140, 265–
282.
Geissler, P., McEwen, A.S., Phillips, C.B., Keszthelyi, L.P., Spencer, J.,
2004. Surface changes on Io during the Galileo mission. Icarus 169,
29–64.
Greeley, R., Theilig, E., Christensen, P., 1984. The Mauna Loa sulfur flow
as an analog to secondary sulfur flows on Io. Icarus 60, 189–199.
27
Heliker, C., Mattox, T.N., 2003. The first two decades of the Pu’u ‘O’oKupaianaha eruption: chronology and selected bibliography. US Geol.
Surv. Prof. Pap. 1676, 1–27.
Howell, R.R., 13 colleagues, 2001. Ground-based observations of volcanism on Io in 1999 and early 2000. J. Geophys. Res. 106, 33129–33139.
Jaeger, W.L., Turtle, E.P., McEwen, A.S., Keszthelyi, L.P., the Galileo SSI
Team, 2000. Possible tectonic origin for Hi’iaka Patera, Io. EOS Trans.
AGU Spring Meet. Suppl. 81. Abstract S291.
Jaeger, W.L., Turtle, E.P., Keszthelyi, L.P., McEwen, A.S., 2002. The effect
of thrust fault geometries on the surface deformation of Io: implications
for mountains and paterae. In: Proc. Lunar Planet. Sci. Conf. 33rd. Abstract #1741.
Jaeger, W.L., Turtle, E.P., Keszthelyi, L.P., Radebaugh, J., McEwen,
A.S., Pappalardo, R.T., 2003. Orogenic tectonism on Io. J. Geophys.
Res. 108 (E8). 10.1029/2002JE001946.
Johnson, T.V., Matson, D.L., Blaney, D.L., Veeder, G.J., Davies, A., 1995.
Stealth plumes on Io. Geophys. Res. Lett. 22, 3293–3296.
Keszthelyi, L., 13 colleagues, 2001. Imaging of volcanic activity on
Jupiter’s moon Io by Galileo during the Galileo Europa Mission and
the Galileo Millennium Mission. J. Geophys. Res. 106, 33025–33052.
Keszthelyi, L.P., Jaeger, W.L., Turtle, E.P., Milazzo, M., Radebaugh, J.,
2004. A post-Galileo view of Io’s interior. Icarus 169, 271–286.
Klaasen, K.P., 15 colleagues, 1997. Inflight performance characteristics,
calibration, and utilization of the Galileo SSI camera. Opt. Eng. 36,
3001–3027.
Klaasen, K.P., Breneman, H.H., Simon-Miller, A.A., Banfield, D., Levanas,
G.C., 2003. Operations and calibration of the solid-state imaging system
during the Galileo extended mission at Jupiter. Opt. Eng. 42, 494–509.
Lopes-Gautier, R., 13 colleagues, 1999. Active volcanism on Io: global distribution and variations in activity. Icarus 140, 243–264.
Lopes, R.M.C., 14 colleagues, 2001. Io in the near infrared: Near-Infrared
Mapping Spectrometer (NIMS) results from the Galileo flybys in 1999
and 2000. J. Geophys. Res. 106, 33053–33078.
Lopes, R.M.C., 10 colleagues, 2004. Lava lakes on Io: observations of Io’s
volcanic activity from Galileo NIMS during the 2001 flybys. Icarus 169,
140–174.
Marchis, F., de Pater, I., Le Mignant, D., Roe, H.G., Fusco, T., Graham,
J.R., Prange, R., Macintosh, B., 2002. Monitoring Io volcanic activity
using the Keck AO system: 2–5 µm sunlit and eclipse observations. EOS
Trans. AGU Fall Meet. Suppl. 83. Abstract F832.
Mattox, T.N., Heliker, C., Kauahikaua, J., Hon, K., 1993. Development
of the 1990 Kalapana flow field, Kilauea volcano, Hawaii. Bull. Volcanol. 55, 407–413.
McEwen, A.S., 2003. High-resolution imaging and topography from JIMO:
the HiRISE model. In: Forum on Jupiter Icy Moons Orbiter, LPI, 12–14
June. Abstract #9007.
McEwen, A.S., Soderblom, L.A., 1983. Two classes of volcanic plumes on
Io. Icarus 55, 191–217.
McEwen, A.S., Matson, D.L., Johnson, T.V., Soderblom, L.A., 1985. Volcanic hot spots on Io: correlation with low-albedo calderas. J. Geophys.
Res. 90, 12345–12379.
McEwen, A.S., Simonelli, D.P., Senske, D.R., Klaasen, K.P., Keszthelyi, L.,
Johnson, T.V., Geissler, P.E., Carr, M.H., Belton, M.J.S., 1997. Hightemperature hot spots on Io as seen by the Galileo Solid State Imaging
(SSI) experiment. Geophys. Res. Lett. 24, 2443–2446.
McEwen, A.S., 13 colleagues, 1998a. Active volcanism on Io as seen by
Galileo SSI. Icarus 135, 181–219.
McEwen, A.S., 14 colleagues, 1998b. High-temperature silicate volcanism
on Jupiter’s moon Io. Science 281, 87–90.
McEwen, A.S., 25 colleagues, 2000. Galileo at Io: results from highresolution imaging. Science 288, 1193–1198.
McEwen, A.S., Keszthelyi, L., Lopes, R., Schenk, P., Spencer, J., 2003. The
lithosphere and surface of Io. In: Bagenal, F., McKinnon, W., Dowling, T. (Eds.), Jupiter: Planet, Satellites and Magnetosphere. Cambridge
Univ. Press, Cambridge. In press.
Moore, J.M., Sullivan, R.J., Chuang, F.C., Head, J.W., McEwen, A.S., Milazzo, M.P., Nixon, B.E., Pappalardo, R.T., Schenk, P.M., Turtle, E.P.,
28
E.P. Turtle et al. / Icarus 169 (2004) 3–28
2001. Landform degradation and slope processes on Io: the Galileo
view. J. Geophys. Res. 106, 33223–33240.
Nash, D.B., 1987. Sulfur in vacuum: sublimation effects on frozen melts,
and applications to Io’s surface and torus. Icarus 72, 1–34.
Perry, J., Radebaugh, J., Lopes, R., McEwen, A., Keszthelyi, L., 2003. Gish
bar patera, Io: geology and volcanic activity, 1996–2001. In: Proc. Lunar Planet. Sci. Conf. 34th. Abstract #1720.
Phillips, C.B., 2000. Voyager and Galileo views of volcanic resurfacing on
Io and the search for geologic activity on Europa. PhD thesis. Univ. of
Arizona, Tucson.
Porco, C.C., 23 colleagues, 2003. Cassini imaging of Jupiter’s atmosphere,
satellites, and rings. Science 299, 1541–1547.
Radebaugh, J., Keszthelyi, L.P., McEwen, A.S., Turtle, E.P., Jaeger, W.,
Milazzo, M., 2001. Paterae on Io: a new type of volcanic caldera?
J. Geophys. Res. 106, 33005–33020.
Radebaugh, J., McEwen, A.S., Milazzo, M.P., Keszthelyi, L.P., Davies,
A.G., Turtle, E.P., Dawson, D.D., 2004. Observations and temperatures of Io’s Pele Patera from Cassini and Galileo spacecraft images.
Icarus 169, 65–79.
Rathbun, J.A., Spencer, J.R., Davies, A.G., Howell, R.R., Wilson, L.,
2002. Loki, Io: a periodic volcano. Geophys. Res. Lett. 29 (10).
10.1029/2002GL014747.
Sagan, C., 1979. Sulphur flows on Io. Nature 280, 750–753.
Schaber, G.G., 1982. The geology of Io. In: Morrison, D. (Ed.), Satellites
of Jupiter. Univ. of Arizona Press, Tucson, pp. 556–597.
Schenk, P.M., Bulmer, M.H., 1998. Origin of mountains on Io by thrust
faulting and large-scale mass movements. Science 279, 1514–1517.
Schenk, P.M., Hargitai, H., Wilson, R., McEwen, A., Thomas, P., 2001. The
mountains of Io: global and geological perspectives from Voyager and
Galileo. J. Geophys. Res. 106, 33201–33222.
Schmitt, B., Rodriguez, S., 2003. Possible identification of local deposits of
Cl2 SO2 on Io from NIMS/Galileo spectra. J. Geophys. Res. 108 (E9).
10.1029/2002JE001988.
Simonelli, D.P., Veverka, J., McEwen, A.S., 1997. Io: Galileo evidence for
major variations in regolith properties. Geophys. Res. Lett. 24, 2475–
2478.
Smith, B.A., 21 colleagues, 1979. The Jupiter system through the eyes of
Voyager 1. Science 204, 951–972.
Smythe, W.D., Lopes-Gautier, R., Douté, S., Kieffer, S.W., Carlson, R.W.,
Kamp, L., Leader, F.E., 2000. Evidence for massive sulfur dioxide deposit on Io. Bull. Am. Astron. Soc. 32, 1047–1048. Abstract.
Smythe, W.D., Lopes, R., Spencer, J.R., 2003. Io in the infrared—science
opportunities with the JIMO mission. EOS Trans. AGU Fall Meet.
Suppl. 84. Abstract #P12A-1048.
Spencer, J.R., 19 colleagues, 2002. The future of Io exploration. Community white paper for the planetary decadal survey. In: Sykes, M. (Ed.),
The Future of Solar System Exploration, 2003–2013. In: Astron. Soc.
Pac. Conf. Ser., vol. 270, pp. 201–216.
Spencer, J.R., Lopes, R., Smythe, W.D., 2003. Io science opportunities from
the JIMO mission. EOS Trans. AGU Fall Meet. Suppl. 84. Abstract
#P12A-1047.
Theilig, E.E., Bindschadler, D.L., Schimmels, K.A., Vandermey, N., 2002.
Project Galileo: farewell to the major moons of Jupiter. 53rd Intl. Astron. Congress. Paper #IAC-02-Q.2.01.
Turtle, E.P., 11 colleagues, 2001. Mountains on Io: high-resolution Galileo
observations, initial interpretations, and formation models. J. Geophys.
Res. 106, 33175–33200.
Turtle, E.P., 11 colleagues, 2002. Correction to: mountains on Io: highresolution Galileo observations, initial interpretations, and formation
models. J. Geophys. Res. 107 (E9). 10.1029/2002JE001928.
Watanabe, T., 1940. Eruptions of molten sulphur from the Siretoko–Iôsan
Volcano, Hokkaidô. Jpn. J. Geol. Geogr. 17, 289–310.
Williams, D.A., Davies, A.G., Keszthelyi, L.P., Greeley, R., 2001a. The
summer 1997 eruption at Pillan Patera on Io: implications for ultrabasic
lava flow emplacement. J. Geophys. Res. 106, 33105–33120.
Williams, D.A., Greeley, R., Lopes, R.M.C., Davies, A.G., 2001b. Evaluation of sulfur flow emplacement on Io from Galileo data and numerical
modeling. J. Geophys. Res. 106, 33161–33174.
Williams, D.A., Keszthelyi, L.P., Davies, A.G., Greeley, R., Head, J.W.,
the Galileo SSI Team, 2002a. High-resolution views of Io’s emakong
Patera: latest Galileo imaging results. In: Proc. Lunar Planet. Sci. Conf.
33rd. Abstract #1339.
Williams, D.A., Radebaugh, J., Keszthelyi, L.P., McEwen, A.S., Lopes,
R.M.C., Douté, S., Greeley, R., 2002b. Geologic mapping of the
Chaac–Camaxtli region of Io from Galileo imaging data. J. Geophys.
Res. 107 (E9). 10.1029/2001JE001821.
Williams, D.A., Schenk, P.M., Moore, J.M., Keszthelyi, L.P., Turtle, E.P.,
Jaeger, W.L., Radebaugh, J., Milazzo, M.P., Lopes, R.M.C., Greeley, R.,
2004. Mapping of the Culann–Tohil region of Io from Galileo imaging
data. Icarus 169, 80–97.
Wilson, L., Head, J.W., 2001. Lava fountains from the 1999 Tvashtar Catena
fissure eruption on Io: implications for dike emplacement mechanisms,
eruption rates, and crustal structure. J. Geophys. Res. 106, 32997–
33004.