JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 110, E08007, doi:10.1029/2005JE002421, 2005
New observations of volcanic features on Mars
from the THEMIS instrument
Peter J. Mouginis-Mark
Hawaii Institute Geophysics and Planetology, University of Hawaii, Honolulu, Hawaii, USA
Philip R. Christensen
Department of Geological Sciences, Arizona State University, Tempe, Arizona, USA
Received 17 February 2005; revised 12 May 2005; accepted 20 May 2005; published 27 August 2005.
[1] Images obtained by the THEMIS instrument on Mars Odyssey permit the
identification of many new volcanic features on Mars. Several new types of vent systems,
including fissure vents within the Tharsis region, flank vents on Olympus Mons and
Ascraeus Mons, and lava shields on the floor of Arsia Mons have been found. The valley
networks on Hecates Tholus are seen to be more complex than previously believed.
Extending from the caldera rim to the base of Hecates Tholus volcano, individual valleys
change width and depth in apparent response to local topography. Valley formation
spanned the time period during which lava flows from Elysium Mons embayed the
southern flank of Hecates Tholus, as demonstrated by the burial of some valleys by the
lava flows and the formation of sediment fans on top of adjacent parts of the flows.
THEMIS images now allow new constraints to be placed on the dynamics of lava flow
emplacement, with the identification of flows with well-defined lava channels on the
northern flank of Ascraeus Mons, as well as an enigmatic braided lava channel. Three
previously unreported collapse events (3.7 to 4.3 km in diameter) are identified within
the caldera of Ascraeus Mons. Details of the mode of formation of parts of the Olympus
Mons aureole can also be identified, supporting the idea that the aureole had a landslide
origin. THEMIS images show that parts of the aureole have been modified by water
discharged to the east of Olympus Mons.
Citation: Mouginis-Mark, P. J., and P. R. Christensen (2005), New observations of volcanic features on Mars from the THEMIS
instrument, J. Geophys. Res., 110, E08007, doi:10.1029/2005JE002421.
1. Introduction
[2] The THermal EMission Imaging System (THEMIS)
instrument on the Mars Odyssey (MO) spacecraft provides
an unprecedented opportunity for imaging the surface of
Mars. With a spatial resolution of 18 m/pixel and a swath
width of 18 km, the camera permits small-scale features to
be identified as well as providing the regional context.
Because of these attributes, analysis of THEMIS visible
(VIS) images obtained during the first 1,260 orbits of MO
has revealed numerous attributes of volcanoes that have not
been previously described in the literature. Here we present
an overview of several new volcanic features in the hope
that other investigators will develop new models, or refine
existing ideas, for the formation of these features.
[3] In this study, THEMIS VIS images allow us to
address four questions in Martian volcanology: (1) What
can we learn about eruption processes on Mars by mapping
individual vent systems when they occur far away from
main volcanic edifices? Not only have we found small
satellite lava shields on the flanks of two Tharsis volcanoes,
Copyright 2005 by the American Geophysical Union.
0148-0227/05/2005JE002421$09.00
but also we have identified a linear fissure system from
which we can map, for the first time, the entire lava flow
field from a well-defined source. (2) How can the geometry
of multiple small (<50 km long) lava flows on Ascraeus
Mons be used to better understand lava flow emplacement
on Mars? New THEMIS data provide a view of the entire
length of some lava flows, thereby enabling the influence
of downslope topography on channel formation to be
observed. In addition to these lava flows, THEMIS images
show that considerable volumes of water were released
high on the southern flank of the volcano, at an elevation
of 6.7 km above datum, providing additional constraints
on the hydrogeology of the summit areas of the Tharsis
volcanoes. (3) How did the Olympus Mons aureole form,
and how has it been modified over geologically recent
times? The embayment of an impact crater supports the
giant landslide model for the formation of this enigmatic
feature [Lopes et al., 1982; Francis and Wadge, 1983].
Furthermore, the aureole may not have been static since its
formation; evidence of remobilization and partial flooding
by water released within the aureole has also been identified.
(4) What are the geomorphic characteristics of the valley
system on Hecates Tholus, and do these valleys support the
earlier idea that the volcano experienced explosive activity
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[Mouginis-Mark et al., 1982]? Inspection of THEMIS data
suggests that water release on the volcano took place over an
extended period of time.
[4] Parametric data collected from the THEMIS images
will undoubtedly aid in the further development of quantitative models to explain the detailed morphology of Martian
volcanic landforms. Such detailed investigations lie beyond
the scope of this work, as our goal here is to show as many
of the new types of volcanic features that can be identified
from THEMIS data; the interpretation of many specific
eruptions will require sophisticated quantitative analysis in
their own right. For instance, the reassessment of the factors
controlling the sizes, shapes and locations of volcanic
intrusions and eruptions on Mars [Scott and Wilson, 1999;
Scott et al., 2002; Wilson and Head, 2002], which are used
to calculate the way that heat is conducted away from
intrusions and convected away by hydrothermal circulation
of meltwater in pore spaces around these intrusions, can
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benefit from the identification of dikes within volcanic
edifices and the existence of effusive activity from fissures.
Our observations with THEMIS will also allow an assessment of the circumstances under which explosive eruptions
may occur. A greater understanding of the temporal and
spatial variations in the eruptive history of volcanoes on
Mars will also be obtained, including the influence of the
volatiles within the top few kilometers of the volcanic
edifice. This relationship in turn pertains to the availability
of volatiles (both juvenile magmatic volatiles and recirculated groundwater contained within the near-surface rocks)
and to magma supply rates at appreciable distances (tens to
hundreds of kilometers) from the centers of volcanoes.
Explosive volcanism on Mars, a major factor in the release
of juvenile water at the surface, may have been driven not
only by volatiles within the parental melt, but also by
magma encountering water or ice at shallow depth within
the volcano [Mouginis-Mark et al., 1982, 1988; Crown and
Greeley, 1993; Robinson et al., 1993].
2. New Vent Systems
[5] While there have been many studies of lava flows on
Mars [cf. Hulme, 1976; Zimbelman, 1985, 1998; MouginisMark and Yoshioka, 1998; Baloga et al., 2003], the spatial
resolution and coverage of Viking Orbiter images (typically
40– 200 m/pixel) have prevented the identification of the
vents for these flows. Conversely, Mars Orbiter Camera
(MOC) images have high spatial resolution (1.5 to 6.0 m/
pixel) but only limited spatial extent so that it is often
difficult to see the volcanic context of the image. Thus the
total length of a Martian lava flow has never been confidently measured. In the past, alternative approaches to locating vents on Mars have been used, such as that employed
by Rowland et al. [2002], who employed the FLOWGO
rheological model to predict the total length of the lava
flows in Elysium Planitia on the basis of the geometry of
the flow front and geometry of the lava channel.
Figure 1. (a) Fissure system to the east of the volcano
Jovis Tholus, centered at 18.1°N, 245.3°E. Lines 1 – 4 mark
the topographic profiles shown in Figure 2, and the
rectangle shows the area of coverage of Figure 1b. Stars
mark the locations where individual MOLA profiles
indicate that the lava flows are only 3 – 4 m thick.
Mosaic of THEMIS images V05484014 and V11238004.
(b) Detailed view of central part of the fissure system,
showing the locations of images in Figure 1c. Arrows point
to ‘‘streamlined blocks’’ that suggest high fluidity of the
lava during its emplacement. THEMIS image V05484014.
(c and d) MOC images of the summit craters along the
fissure system. See Figure 1b for locations. The lack of any
constructional features along the rims of these craters
indicates that the lava welled out of the fracture at low
points (arrowed) rather than being ballistically ejected from
the vent. Note the lack of evidence for a dike on the floor of
any of these craters. Figure 1c is frame number R18-00795,
resolution 4.69 m/pixel. MOLA orbit 19358 indicates that
the fissure is at least 119 m deep. Figure 1d is MOC image
number R19-00384, 5.76 m/pixel. MOLA orbit 11323
indicates that the fissure at right in this image is at least
87 m deep.
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Figure 1. (continued)
[6] Various factors suggest that we should expect that the
sources of many, if not most, lava flows on Mars should be
elongate fissures [Wilson and Head, 1994]. First, the Tharsis Ridge shield volcanoes have topographically-defined rift
zones that extend for more than 500 km down their flanks
[Crumpler and Aubele, 1978], implying that extensive
lateral migration of dikes is common within the edifices.
Second, giant dike swarms more than 2,000 km long are
seen radiating from the major volcanic centers in both
Tharsis and Elysium [Wilson and Head, 2002]. Third,
theoretical arguments [Rubin, 1993] strongly suggest that
the apparently basaltic to intermediate magma forming
volcanoes on Mars should migrate in elongate dikes.
However, until the high-resolution imaging by THEMIS,
no unambiguous example of a fissure eruption had been
detected on Mars. To confidently identify a vent system
requires both the identification of the ground fracture and
one or more lava flows leading away from the fracture.
[7] THEMIS VIS data have enabled us to identify three
new vent systems within the Tharsis region. First, a linear
vent system has been found to the east of the volcano Jovis
Tholus. Lava shields have also been found on the NE flank
of Olympus Mons, and the south flank of Ascraeus Mons
[Sakimoto et al., 2002]. The location of these shields at
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Figure 2. MOLA topographic profiles across fissure, derived from the 128th degree MOLA digital
elevation model. The portion of the profiles that corresponds to the lava flows erupted from the fissure is
indicated. Note that Profile 1 indicates that the depth of the fissure is at least 58 m. See Figure 1a for
profile locations.
distances of several hundred kilometers from the summit of
the nearest volcano supports the idea of volcanic systems
with large dike complexes [McKenzie and Nimmo, 1999;
Wilson and Head, 2002].
2.1. Jovis Tholus Fissure
[8] An elongate fissure has been identified in THEMIS
images 120 km to the east of the volcano Jovis Tholus, at
18.2°N, 245.3°E (Figure 1). This fissure is 26 km in
length, and has lava flows emerging from the vent that head
both to the north and to the south. Mars Orbiter Laser
Altimeter (MOLA) topographic measurements (Figure 2)
indicate that the fissure is a local topographic high, with an
increase in elevation of 75 m over the adjacent area. This
fissure is particularly interesting because the distribution of
many of the lava flows that were erupted can be identified in
the THEMIS data. The maximum flow distance is 18 km
from the fissure, and flows have typical widths in the range
1.3 – 1.8 km. Combined, the surface area and inferred
thickness (75 m) of the entire flow field implies an
erupted volume of 40 km3. Had all of the magma erupted
from this site produced a single lava flow, it would have had
a length of 80 km long (assuming a 50 m thick, 10 km
wide flow). What is different in this case is that the flows
originated at a fissure rather than a point source. MOLA
data from individual orbits indicate that many of the flows
from this fissure are much thinner than 40– 50 m, which is
a typical value for Martian lava flows measured to date
[Mouginis-Mark and Yoshioka, 1998; Baloga et al., 2003].
MOLA orbits 10141, 10820, 13367, and 17165 all show
that individual lava flows have a thickness in the 4 – 6 m
range.
[9] MOC data (Figure 1c) indicate that there is no
preserved evidence for a dike on the floor of the fissure, and
MOLA data (orbit 19358) show that the fissure has a depth
of at least 119 m. This depth is believed to be significant
because it appears that erupting magma from the base of the
fissure, which most likely took the form of a fire fountain
[Wilson and Head, 1994], was only able to flow out of the
fissure by filling the fracture and welling out onto the
surface. The small distributary fan at the western end of
the fracture (Figure 1b) is one of the clearest examples of
this style of eruption. Smooth-edged ‘‘streamlined blocks’’
within the proximal flow units (Figure 1b) attest to the high
fluidity and low gas content of the erupted lava (based on
our observations of terrestrial eruptions such as those that
occur on Mauna Loa and Kilauea volcanoes in Hawaii).
Spatter ramparts may also be an indicator of magma with a
low gas content [Cattermole, 1986], but no examples at a
scale of a few tens of meters are preserved around the rim of
the fissure because the rim of the fissure does not cast any
shadow in the MOC images (Figures 1c and 1d). This
implies that the fire fountains either had a volatile content
that was insufficient to eject material to a height of >120 m
(i.e., more than the depth of the fissure) or that the ejecta were
destroyed by subsequent effusive activity. Furthermore, the
lack of irregular large blocks tens of meters in size (which
would be visible both in THEMIS VIS and MOC images)
that might be fragments of rafted rim material, also suggest that
the volatile content of the magma was always low during the
eruption.
2.2. Olympus Mons Flank Vent
[10] Relatively little is known about the internal structure
of the Martian shield volcanoes. Zuber and Mouginis-Mark
[1992] used the distribution of extensional and compressional features within the caldera of Olympus Mons to infer
the size and depth of the magma chamber at the time that
the volcano was active. Wilson et al. [2001] also made
inferences about the size and depth of the magma chamber
on the basis of the plausible repose periods between
eruptions, and the need to prevent the conduit system from
freezing between eruptions. Deformation studies of the
volcanoes, using the distribution of large-scale terraces on
the flanks of the edifice [McGovern and Solomon, 1993],
and the comparison between the Hawaiian islands and
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flows) from the summit or from local vents that are now too
degraded to be recognizable. There are many short lava
flows close to the summit caldera, and longer flows on the
middle and lower flanks [Mouginis-Mark, 1981], but further
analysis of the THEMIS data set should help to resolve the
relative importance of intrusive activity compared to extrusive activity in building the Olympus Mons edifice.
2.3. Ascraeus Mons Flank
[12] The lack of near-vent constructional features on Mars
is surprising, as one would expect spatter ramparts to have
formed if there was even a low (<1 wt%) volatile content in
the magma [Wilson and Head, 1994; Fagents and Wilson,
1996]. THEMIS data provide an excellent opportunity to
search large areas on Mars to identify the rare examples of
cones that do exist. One of the best candidate examples
that we have found of a spatter or cinder cone is shown in
Figure 4, which is located on the southern flank of
Ascraeus Mons. This cone is 540 600 m in diameter,
and lies at the upslope end of a 4.3 km long lava channel
that has its distal end buried by younger lava flows. This
cone is located astride a linear structural feature, which
may be the surface expression of a dike or a small spatter
ridge, that extends 3.3 km upslope from the cone.
[13] The Ascraeus Mons cone most likely formed as a
spatter cone or cinder cones during the eruption of the
associated flow. Possible spatter ramparts have already been
identified on Alba Patera volcano [Cattermole, 1986], but
this Ascraeus Mons cone is much larger and has a lava
channel originating from it. When volatile-rich magmas
reach the surface, it has been predicted that cinder cones
or spatter cones might develop around the vent [Head and
Figure 3. Isolated vent complex (arrowed) on the NE
flank of Olympus Mons, at 21.2°N, 230.0°E. The regional
downslope direction is toward the top right of this image.
THEMIS image V11251006.
Olympus Mons [McGovern et al., 2004], suggest that the
structure of the volcano was affected by large slip faults that
most likely affected the pathways along which magma may
have reached the surface.
[11] A vent system on the NE flank of Olympus Mons
volcano (Figure 3), located at 21.2°N, 230.0°E, indicates
that subsurface magma transport took place within the
edifice to radial distances of at least 235 km from the
summit caldera. It has long been hypothesized that dike
systems might exist within the Tharsis volcanoes, with
eruptions occurring down slope away from the summit
calderas [Mouginis-Mark, 1981]. Our discovery of satellite
vents at the 2.0 km elevation level on the lower flanks of
Olympus Mons strongly supports the idea that a dike reached
the surface at this point and fed short (<10 km long) surface
lava flows. While THEMIS coverage of the lower flanks of
Olympus Mons is currently incomplete, the example identified here raises the question of how the volcano grew to its
current size, either by large eruptions (and hence long lava
Figure 4. Dissected vent (arrowed) on the southern flank
of Ascraeus Mons, located at 9.97°N, 258.14°E. THEMIS
image V11250004.
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Figure 5a. Mosaic of THEMIS daytime IR images of
the summit caldera of Arsia Mons volcano. Data
acquisitions at different times of day mean that the dust
on the caldera floor produces temperature differences that
cannot be removed at some scene boundaries. Rectangle
marks the location of Figure 5b. Mosaic of THEMIS images
I01116001, I01428001, I01453001, I02564001, I04399001,
I05510001, I08506001, and I09441024.
Wilson, 1989; Wilson and Head, 1994]. The magma volatile
content and mass flux control the pyroclast size distribution
and acceleration of eruptive products out of the vent [Wilson
and Head, 1994; Fagents and Wilson, 1995, 1996], which
in turn dictate the structure of the lava fountain and the
dispersal of pyroclasts around the vent [Head and Wilson,
1989]. The rate of pyroclast accumulation together with the
local pyroclast temperature (both a function of clast size
and fountain structure) determine whether lava flows
(produced by coalescence of magma clots on landing),
spatter, or cinder deposits are produced. The cone identified here therefore allows more complex models of the
Figure 5b. Lava shields on the floor of Arsia Mons
caldera at 8.77°S, 239.89°E. Arrows indicate the summit
craters of these small shields, which MOLA data indicate
are 20 –30 m high. Note that alignment of this line of
shields is not the same as the orientation of the individual
summit craters of the shields. THEMIS image number
V04399002.
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that produced this line of small shields. Typically, it would
be expected that the vents should be aligned with the axis of
the rift zone through the summit because this marks the line
of weakness across the caldera floor that was exploited by
the dikes that formed the vents; but it appears as if the
formation of each shield was controlled by a very localized
stress field that differed from the caldera-wide stress
regime. Because Arsia Mons has the largest single
caldera on Mars [Crumpler et al., 1996], it is possible
Figure 6a. THEMIS mosaic of the summit of Ascraeus
Mons volcano, showing the locations of Figures 6b and 6c.
Mosaic of frame numbers V01464013, V01826008,
V05209010, V06320025, V07793019, V08155019,
V08155019, V10077005, V10389011, V11612005, and
V11924010.
Martian eruption conditions, and magma volatile content
in particular, to be developed.
2.4. Small Shields Within Arsia Mons Caldera
[14] One of the earliest features that were identified on
volcanoes on Mars from the Viking mission was the set of
low hills that cross the caldera floor of Arsia Mons [Carr et
al., 1977; Crumpler and Aubele, 1978]. These small shields
lie along a structural trend that passes through the summit
of the volcano (Figure 5a), and aligns with the inferred
rift zone between Ascraeus, Pavonis, and Arsia Montes.
THEMIS images (Figure 5b) show that the summit craters
for these small shields are typically elongate, measuring
225 – 520 m wide and 880– 1,350 m long. THEMIS data
also reveal that there is a nonalignment of the summit
craters of the shields compared to the structural trend of the
caldera floor. Instead of the NNE – SSW trend of the
fissure, the summit craters are generally oriented NNW to
SSE. Thus a still-to-be-answered question is focused on
understanding the underlying structure of the caldera floor
Figure 6b. Channelized lava flows on the northern rim of
Ascraeus Mons caldera at 12.17°N, 255.41°E. The caldera
rim is 20 km to the south (bottom) of this image. See
Figure 6a for location. THEMIS image V10389011.
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Figure 6c. Braided lava channel (arrowed) on the northern
rim of Ascraeus Mons caldera at 11.78°N, 255.56°E. See
Figure 6a for location. THEMIS image V11612005.
that the caldera has experienced multiple collapse events
that have been subsequently buried by the last, and
largest, collapse and flooding event. The orientation of
the small shields suggests that an earlier structural fabric
still influences the evolution of the present caldera floor.
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on the basis of a steady state Newtonian flow rate. Their
approach featured a new length scale that described the
transfer of lava from the active advancing component to
the passive margins of the flow. Thickening and widening of
the flow with distance are predicted in this model for a
single, coherent, isothermal, viscous flow. Baloga et al.
[2003] studied a single lava flow to the north of Pavonis
Mons volcano to explore the motion of the flow, particularly
with respect to the formation and evolution of lava channels
and the thickening of the flow. It was discovered that none of
the physical processes that cause large viscosity increases in
terrestrial flows [Moore, 1987; Crisp et al., 1994; Crisp and
Baloga, 1994] seem to have occurred with the Pavonis flow.
[17] Detailed information on the geometry of entire lava
flows has so far been missing against which the numerical
models of lava flow rheology can be tested. THEMIS VIS
images of the summit of Ascraeus Mons (Figure 6a) now
permit such models to be tested. In these images it is
possible to identify more than a dozen different flows with
lava channels, and show where the channel occurs with
respect to the full length of the flow (Figure 6b). Individual
flows are 0.3– 2.0 km wide and 20– 36 km long. Lava
channels occur along parts of the length of several of these
flows, but their occurrence is not constrained to either the
proximal or distal portion of the flow. In general the lava
channel is seen along 20– 40% of the length of the flow,
and starts 5 – 20% along the length of the flow going away
from the summit. Part of the explanation for the occurrence
of the lava channels may be the role of local topography;
3. Summit Features of Ascraeus Mons
[15] Because of their ease of comparison with terrestrial
calderas [Mouginis-Mark and Rowland, 2001] the summit
calderas of Martian volcanoes are some of the most well
studied volcanic features on the planet. The THEMIS VIS
instrument has obtained almost complete coverage of all of
the calderas on Mars. While all of the calderas have
interesting features, many have already been identified from
Viking Orbiter images and MOLA data. Here we focus on
three previously unreported landforms on Ascraeus Mons
(Figures 6a – 6c) that may help constrain rheologic or
structural models.
3.1. Lava Channels
[16] During the emplacement of a lava flow, three
physical processes can be important: (1) changes in
viscosity, (2) the formation of channels, levees, or stationary margins that divide the flow into active and inactive
components, and (3) a loss of volatiles that cause a change
in the density of the lava [cf. Walker, 1973; Crisp and
Baloga, 1994; Zimbelman, 1998]. It is particularly difficult
to determine the relative roles of these processes because
they can occur simultaneously in active flows. Furthermore, for Martian lava flows, the application of numerical
models is best achieved when the entire flow length is
considered, rather than just the distal portion. Dimensions
of lava flows can be used to unravel the relative roles of
viscosity changes, the formation of lateral levees, stationary
margins and stagnant zones. Baloga et al. [2003] developed
a new formula for the relative change in viscosity of the flow
Figure 7. Oblique view of Ascraeus Mons caldera,
looking toward the northeast. This image was created by
merging the THEMIS VIS images with the MOLA 1/128th
degree digital elevation model. Previously unrecognized
collapse craters (letters) can be seen in this view. These
craters range in diameter from 3.7 km (‘‘A’’) to 7.3 km
(‘‘C’’). Vertical exaggeration 8x. THEMIS images
V01464013 and V01826008.
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Figure 8. Channels on southern flank of Ascraeus Mons, located at 6.06°N, 254.48°E, which may be
water-carved. Arrows point to streamlined islands in the channels that are not seen in lava channels, but
are very similar to the landforms identified at Cerberus Fossae [Burr et al., 2002; Head et al., 2003].
Downslope direction is toward the lower left. North is toward the right in this image. THEMIS image
V08155020.
there is also good topographic control from MOLA, which
indicates that the average local slopes within 10 km of the
caldera rim are <0.1°. Slopes increase to 4° at a radial
distance of 35 km from the rim. Most lava channels are
found in areas with higher slopes, but additional studies
need to identify specific slope/channel geometry relationships. In addition, in this region of very low local slopes
very close to the caldera rim, we have also identified the
first braided lava channel on Mars, just to the north of the
caldera rim of Ascraeus Mons (Figure 6c). While common
on terrestrial flows near the vent [Lipman and Banks, 1987],
this is the first time that braiding has been seen on a Martian
flow. Such channels are believed to be indicative of highly
fluid flow, and may therefore indicate unusual eruptions at
Ascraeus Mons. We note that the vent for this presumed
lava channel has now been destroyed by a more recent
episode of caldera collapse.
3.2. Episodes of Caldera Collapse
[18] Additional degraded collapse structures within the
caldera of Ascraeus Mons can be identified using a combination of THEMIS VIS images and MOLA topographic
data (Figure 7). Previously, eight collapse events, ranging in
diameter from 7 to 40 km, were believed to have created the
summit caldera complex [Mouginis-Mark, 1981]. Using
THEMIS images, we have identified three additional
collapse events that produced subsidence features measuring 3.7 to 4.3 km in diameter. The dimensions of these
collapse events have implications for the size of magma
chambers that existed at the summit, as well as the volume
of magma erupted at the surface from these chambers
[Wilson et al., 2001]. The fact that these new collapse
events are smaller than any of the previously identified
examples implies that the chamber underwent smaller
episodes of magma withdrawal early in the history of the
present caldera, and that not all of the episodes of caldera
collapse were accompanied by intracaldera eruptions of
lava.
3.3. Evidence of Water Discharge
[19] New evidence from the THEMIS VIS data set
supports the idea that water once existed within the edifice
at high elevations. We have found examples of braided
channels at an elevation of 6,690 m on the axis of the
southern rift zone of Ascraeus Mons (Figure 8) that look
Figure 9. Summary map of the THEMIS VIS coverage
for the Olympus Mons aureole, including all data up until
orbit number 1260 of the Mars Odyssey spacecraft. Arrows
point to the locations of Figures 10, 11, 12a, and 13.
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Figure 10. A 4.3 km diameter crater has been partially
buried by radial flow of the Olympus Mons aureole
material. Notice that the aureole material is as thick as the
crater rim on the western (left-hand) side of the crater but
that there is a prominent rim on the eastern side. This crater
is located at 10.5°N, 234.2°E. THEMIS image V13597012.
Figure 11. Part of an aureole block that may have been
remobilized to partially cover lava flows beyond the
perimeter of the aureole at 25.80°N, 232.75°E. Arrow
points to a lobate margin of what is interpreted to be a lava
flow that has been partially buried by the aureole moving
from lower left to top right of this image. See Figure 9 for
location. THEMIS image V05834011.
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Figure 12a. Part of the aureole to the NE of a segment
called the Sulci Gordii region of the Olympus Mons aureole
materials, showing flooding of the aureole. See Figure 9 for
location. Arrows point to stream channels between the
aureole blocks. Mosaic of THEMIS images V02738003 and
V11151066. Box shows area of coverage of MOC image
shown in Figure 12b.
very similar to water channels elsewhere within Tharsis
[Mouginis-Mark, 1990]. The channels on Ascraeus Mons
have many streamlined islands, originate from a fracture
that is more than 12 km long, and have multiple terraces in
the channel walls; all features that are very rare for lava
channels. In many respects, the source area for these
channels appears to be very similar to the fractures that
acted as the source for the Cerberus Fossae flood waters
[Burr et al., 2002; Head et al., 2003]. In addition, THEMIS
data (Figure 8) show that there is no discernable margin to
the channel that could represent the edge of a lava flow.
Thus these braided channels on Ascraeus Mons are quite
different from the channel at the summit that is shown in
Figure 6c. Scott and Wilson [1999] have discussed the
possibility that the summit areas and rift zones of Martian
volcanoes may have accumulated bodies of ice, and that
water was retained below a sealing cap of ice. They
proposed that the source of this water and carbon dioxide,
which would most likely be juvenile water, should be the
cooling magma chambers as the magma becomes supersaturated in volatiles. These volatiles would then become
trapped elsewhere within the near-surface layers of the
volcano, providing a volatile source for subsequent activity.
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crater that has been buried. There is a ‘‘zone of avoidance’’ on
the eastern, distal, side of the crater, and it is clear that the
aureole material did not flow uniformly around the crater rim
as the lobes of material do not wrap around the crater rim.
[21] A second landform supporting the landslide origin of
segments of the aureole is shown in Figure 11, which shows
the perimeter of the aureole NE of Olympus Mons. At this
site, lava flows from the central portion of Tharsis can be
seen along the margin of the segment of the aureole called
Cyane Sulci and the lower SW flanks of Alba Patera
volcano. Close inspection of the edge of the aureole shows
that some of these lava flows have been partially covered by
aureole material. Had this aureole material been created by
tectonic uplift [Borgia et al., 1990], we believe that parts of
the surface would have been preserved, enabling the flow to
be identified within the aureole; instead, we prefer the
interpretation that the aureole moved toward the northeast.
This suggests that this part of the aureole is either very
young (postdating the formation of the lava flows, which
seems unlikely as most of the aureole is embayed by lava
flows) or, more likely, that the aureole has become reactivated in the relatively recent past.
Figure 12b. High-resolution view of the channel within
the aureole material. Direction of water flow was toward the
top of image. Note the bench that surrounds all of the hills
in this image, suggesting a possible highstand of the water.
MOC image R20-001328, 3.69 m/pixel, located at 21.36°N,
235.69°E.
Our THEMIS observations show that large volumes of
water were first trapped high on the flanks of the volcano,
and were released without any significant magma/groundwater
interactions because no products of possible explosive origin
(e.g., ash deposits or cinder cones) can be found.
4. Olympus Mons Aureole
[20] McGovern et al. [2004] have recently reviewed the
origin of the rough-textured aureole materials that surround
much of Olympus Mons, and concluded that it formed by
collapse of the Olympus Mons basal escarpment and subsequent lateral movement of the material. We have searched the
THEMIS VIS data set for all of the aureole material, which
comprises 260 images (Figure 9) up until orbit 1260, and
found two places where lateral flow has clearly taken place.
The first example lies to the southeast of Olympus Mons at
10.5°N, 234.2°E. Here a preexisting 4.3 km diameter crater
lies at the perimeter of a lobe of the aureole (Figure 10).
Radial flow of the aureole material is evident because it is
only the western, proximal (to Olympus Mons), side of the
Figure 13. Fracture system within the eastern segment of
the aureole materials from which water has been released.
Direction of flow was toward the bottom of this image. See
Figure 9 for location. THEMIS image V10265009, located
at 16.17°N, 232.92°E.
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Figure 14. THEMIS daytime IR mosaic of Hecates Tholus at 100 m/pixel resolution. The summit
caldera of the volcano is centered at 32.59°N, 151.69°E. Boxes indicate the locations of Figures 15, 16,
17, and 18. THEMIS images I02017005, I02404005, I02429005, I02716006, I02741004, I03128002,
I04289002, I04988015, I08683018, and I09644004.
[22] THEMIS VIS images reveal that the aureole material
has also been the site of multiple episodes of water release
and flooding. Figure 12a illustrates a part of the aureole to
the NE of the segment called Sulci Gordii, where a water
channel has flooded the area between individual blocks
within the aureole. Raised benches, perhaps formed by the
highstand levels of the water that flooded the aureole, can
be seen in the THEMIS VIS and MOC images (Figure 12b).
The aureole is also seen to be the site of recent water release
(Figure 13). While fractures on the plains to the east of
Olympus Mons have been previously identified as the
source for recent water release [Mouginis-Mark, 1990],
the observations presented here provide the first evidence
for water flow within the aureole materials.
5. Valley Networks on Hecates Tholus
[23] The volcano Hecates Tholus (Figure 14) provides
some of the best evidence for recent, explosive volcanism
on Mars by virtue of the absence of impact craters on the
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Figure 15. The eastern flank of Hecates Tholus at 32.59°N, 151.69°E shows a much more developed
valley system than had been identified from Viking images. Previously identified valleys correspond to
deep segments incised into volcano where slopes appear to be greatest. Shallow slopes appear to be
characterized by braided deltas. See Figure 14 for location. Mosaic of THEMIS images V02716007 and
V11441007.
upper western flanks of the volcano [Mouginis-Mark et al.,
1982]. Another enigmatic aspect of Hecates Tholus, which
may be connected to the style of volcanism on the volcano,
is that it has a very well developed system of valleys on its
flanks [Reimers and Komar, 1979; Gulick and Baker,
1990]. The existence of these valleys is consistent with
the idea that the flanks comprise ash deposits that were
more easily eroded than the lava flows on other volcanoes.
The Hecates Tholus valleys raise several important questions about the earlier climate on Mars. For example, was
the source of the water that carved the valleys rainfall, snow
accumulation, or groundwater sapping?
[24] Knowledge of the distribution and source of water in
space and time on Mars remains a tantalizing goal [Carr,
1996; Baker, 2001]. The identification of the relative timing
of valley formation on Hecates Tholus, evidence for the
duration of water flow, and the source areas for the water are
important steps in building local knowledge of the former
climate of Mars. Inspection of the THEMIS VIS images
reveals several attributes of the valley system that help
refine this understanding of the climatic conditions. The
individual valleys observed in THEMIS images (Figure 15)
are much longer than previously believed, and have a
variable depth of incision along their length that appears
to be related either to local topography or to strength
differences in the country rock.
[25] THEMIS VIS data show that the valleys are much
more interconnected than inferred from Viking images
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(Figure 15). In many instances, a single valley extends for
many tens of kilometers, repeatedly changing its depth and
width in response to the local topographic gradient and
forming braided streams where the gradient is low. MOLA
data indicate that slopes on the volcano range from 8° at the
summit, to 10° on the middle flanks, and 7° to 12° on the
lower flanks. The existence of long valleys supports the idea
that the surface flow of water was widespread and easy at
some stage during the history of Hecates Tholus. In addition, the THEMIS VIS images show that the source of water
that carved the valleys extended all the way to caldera rim
(Figure 16), raising the potential of either snowmelt [Clow,
1987; Christensen, 2003; Fassett and Head, 2004] or rain as
the source of the water.
[26] Information on the timing of valley formation is also
available. On the southern flank of Hecates Tholus, we have
found some valleys that have their distal ends buried
beneath lava flows that were erupted from Elysium Mons,
while at least three other valleys have built sediment fans on
top of these same lava flows (Figure 17). Valley morphology also indicates that some valleys are older than others,
because of the variable level of degradation of the sides of
the valleys. These observations imply a relatively long-lived
period of valley formation on Hecates Tholus, suggesting
that the source of the water was not a transient event.
Further information regarding the timing of valley formation can be seen in Figure 18, which shows the eastern flank
of the volcano. Several valleys on this part of the volcano
have been cut by a low-relief wrinkle ridge formed, presumably, by local subsidence of the volcano flank. We see
no evidence for the diversion of any of these valleys by the
topography of the ridge, indicating that the valleys on this
part of the volcano formed prior to the deformation event
that produced the ridge.
[27] Finally we note that we have also studied all of the
THEMIS VIS images for Ceraunius Tholus, which is the
other relatively young Martian volcano with extensive
valley networks on its flanks [Reimers and Komar, 1979].
We find no evidence of protracted valley formation on this
volcano that spans the time interval over which the adjacent
lava plains were emplaced. Indeed, valley systems on
Ceraunius Tholus are more degraded than those seen on
Hecates Tholus and Alba Patera, suggesting that they are
significantly older and predate the emplacement of the
younger lava flows in central Tharsis.
6. Discussion
Figure 16. The summit area of the volcano Hecates
Tholus illustrates that the fluvial valley system extends
within 2 km of the rim of the caldera. The caldera (only the
eastern side is shown here) is shown at the top left of this
image and is centered at 31.73°N, 150.08°E. See Figure 14
for location. THEMIS image V11466010.
[Mouginis-Mark et al., 1982]. Deep segments of valley
systems were carved where local topographic slopes are
relatively steep, and a shallow braiding system of channels developed where the slopes are relatively shallow
[28] From our analysis of the THEMIS VIS images of
volcanoes on Mars it appears that one of the most important
observations is the frequent occurrence of features formed
by flowing water in close proximity to eruption sites. The
identification of water discharge at high elevations (>6 km)
on Ascraeus Mons is particularly noteworthy, but there also
appears to have been abundant water on the flanks of
Hecates and Ceraunius Tholi as well as within segments
of the Olympus Mons aureole materials. No morphologic
evidence has been found in the THEMIS images to identify
a source for this water, but snowmelt [Fassett and Head,
2004] or the remobilization of volatiles released from the
degassing volcano [Scott and Wilson, 1999] appear to be the
most likely. Water close to the summit of a volcano has
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Figure 17. (a) Along the southern flank of the volcano Hecates Tholus (at 30.80°N, 149.20°E), some
distal parts of the valley system have been buried by lava flows from Elysium Mons, while other valleys
have created alluvial fans that lie on top of the lava flows. See Figure 14 for location. Mosaic of THEMIS
images V02404006 and V13625007. Location of Figure 17c is indicated by white box. (b) Sketch map of
the prominent valleys that have been active after the lava flows at bottom of image were emplaced. Area
of coverage is the same as Figure 17a. (c) MOC image showing the details of the fan identified in
Figure 17a. MOC image number R19-00073, 4.08 m/pixel.
important implications for affecting the type of eruption,
and has been proposed for both Ascraeus and Arsia Montes
[Scott and Wilson, 1999; Head and Wilson, 2002; MouginisMark, 2002].
[29] The abundance of water at the summit of Hecates
Tholus may also be relevant to the possible explosive
eruption style of this volcano. As described by Mouginis-
Mark et al. [1982], this volcano has some of the best
evidence for recent explosive volcanism on Mars in the
form of an asymmetric super-posed impact crater distribution. The area immediately to the west of the summit caldera
is devoid of impact craters larger than 10 m (i.e., the
resolution limit of MOC images). The Viking-era interpretation was that this area is mantled by an air-fall deposit that
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system such as the one proposed by Gulick [1998] could
have remobilized the volatiles and explain the nonuniform
distribution of small impact craters on the flanks for the
volcano. Explosive volcanism is therefore still a possible
style of activity for Hecates Tholus but alternative models
including, for instance, glacial erosion or the rotting of
surface lava flows by an active hydrothermal system, need
to be investigated.
[31] If a nonexplosive model for the volatile history of
Hecates Tholus were to be found correct, it would raise
additional questions about the diversity of eruption style on
Mars. Previous discussions of Martian volcanism [e.g.,
Francis and Wood, 1982; Greeley and Spudis, 1981; Wilson
and Head, 1994] have included the possibility of explosive
volcanism because the flanks of certain volcanoes (notably
Alba Patera, Hecates Tholus, and Ceraunius Tholus) have
easily eroded flanks that may comprise ash deposits. Even
the flanks of the highland paterae (Hadriaca Patera and
Tyrrhena Patera) may be formed from ash [Greeley and
Crown, 1990; Crown and Greeley, 1993]. If this easily
eroded material is in fact composed of hydrothermally
altered lava flows, then the range of magmatic compositions
on Mars does not need to be as diverse, nor is there the need
to include primary melts with high volatile contents.
[32] As of May 2005, the collection of images from
THEMIS is continuing. Future data acquisitions will specifically be targeted to create complete high-resolution
mosaics of the main volcanic edifices, thereby allowing
additional details of the volcanic history and styles of
eruption to be investigated.
Figure 18. Several valleys (arrowed) on the lower eastern
flank of Hecates Tholus have been deformed by a wrinkle
ridge at 31.9°N, 151.7°E. The direction of flow is from left
to right. See Figure 14 for location. No diversion of the
water flow can be found, implying that the volcano
deformed after the period of valley formation. THEMIS
image V11129005.
was generated by either a plinian or subplinian eruption at
the summit of Hecates Tholus. An important, but worrying,
component of the model is the cause of the asymmetry,
which relies on either a directed blast or strong local winds
that preferentially deposit material on only one flank of the
volcano. Weathering and erosion, rather than explosive
volcanism, may be a way to explain the unusual distribution
of subkilometer sized impact craters on the volcano.
[30] Alternatively, Neukum et al. [2004] have suggested
that glacial deposits may also exist on this part of the
volcano, providing a nonjuvenile source for the volatiles.
Snowmelt has also been proposed as a mechanism for
valley formation on Martian volcanoes [Fassett and Head,
2004], which could explain the protracted period of valley
formation that can be inferred from the relative timing of the
valleys and the lava flows from Elysium Mons that surround the southern base of Hecates Tholus. We see from the
THEMIS VIS images that water was most likely released to
the surface close to the summit caldera rim. This implies
that a large volatile reservoir existed at the summit, and this
reservoir could have been derived from out-gassing of the
juvenile magma or from snowmelt. In either the glacial/
snowmelt model or the degassing model, a hydrothermal
[33] Acknowledgments. We thank the THEMIS Science Team for
their support of this work, especially Laurel Cherednik, Kelly Bender, and
Andreas Dombovari for the targeting of THEMIS, and Noel Gorelick, Jim
Torson, and the THEMIS Software development Team at Arizona State
University and the U.S. Geological Survey in Flagstaff for processing and
mosaicking of THEMIS images. Harold Garbeil created the software that
enabled us to co-locate the MOLA profiles with the THEMIS images.
Helpful comments were provided by Jim Head and an anonymous reviewer.
We also thank Mike Malin and Ken Edgett of Malin Space Science Systems
for specifically tasking the MOC instrument under the MOC Public Target
Request Program to obtain the MOC images of selected features seen in the
THEMIS data. This is HIGP publication 1400 and SOEST Contribution
6631.
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P. R. Christensen, Department of Geological Sciences, Arizona State
University, Campus Box 871404, Tempe, AZ 85287-6305, USA.
P. J. Mouginis-Mark, Hawaii Institute Geophysics and Planetology,
University of Hawaii, 1680 East-West Road, POST 504, Honolulu, HI
96822, USA. ([email protected])
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