Volcanoes on Io
Volcanism on Io: the
Io, Jupiter’s innermost Galilean satellite, is the most volcanically active
body in the solar system. Ashley Gerard Davies reviews the wealth of data
returned by NASA’s veteran spacecraft Galileo, that has led to a better
understanding of the volcanic processes wracking Io.
T
o a volcanologist, Io is a paradise. The
discovery of active volcanism on Io by
the Voyager spacecraft (Smith et al.
1979) was the first extraterrestrial case of a
process that constantly reshapes the surface of
the Earth. Io is the most volcanically active
body in the solar system, as a result of tidal
heating: Io is caught in a gravitational tug-ofwar between Jupiter and Europa (Peale et al.
1979), and the intense heating manifests itself
as widespread volcanism.
Since it began making observations of the
Galilean satellites in June 1996, the spacecraft
Galileo has produced extraordinary images of
volcanism on Io, collected a wealth of data concerning thermal emission and composition, and
greatly advanced what is known about this
highly volcanic satellite. Observing Io’s volcanism, studying thermal output and eruption evolution, determining lava composition and measuring the resulting geomorphology were major
objectives of the Galileo mission, building on
the data collected by the Voyager spacecraft
and continuing the monitoring of volcanic
activity from ground-based telescopes (see, for
example, Veeder et al. 1994) augmented by
Hubble Space Telescope observations (e.g.
Spencer et al. 2000a). In this paper the extraordinary discoveries pertaining to Io’s volcanism
made during the Galileo epoch are reviewed.
Galileo
Launched in 1989, the Galileo spacecraft
arrived at Jupiter in December 1995 and
opened a new era of monitoring volcanism on
Io, providing regular observations at higher
spatial, spectral and temporal resolution than
ever before. That Galileo arrived at all as a
functioning spacecraft was a tribute to the skill
of the mission engineers and scientists who had
to overcome many obstacles (including the failure of the main antenna, necessitating in-flight
reprogramming of the spacecraft to use a secondary system, as well as unexpected radiation
effects on the spacecraft and instruments)
before data from Io finally arrived. Due to
problems with the tape recorder on which data
are stored prior to transmission to Earth, no
2.10
imaging data of Io were collected during insertion into orbit around Jupiter in December
1995, when Galileo passed within 180 km of
Io. Because of the high radiation environment,
this was the only close flyby of the nominal 11orbit mission; it would be another four years
(October 1999) before a close flyby.
Galileo carries three instruments that have
been used to examine Io: the Solid State Imaging
experiment (SSI), the Near Infrared Mapping
Spectrometer (NIMS) and the Photo-Polarimeter Radiometer (PPR), all described in the box
“Galileo’s instruments”. Together, these three
instruments produce mutually constraining
datasets over a wide wavelength range, allowing
detailed study of surface composition and the
thermal behaviour of Io’s volcanic processes.
The first images of Io by Galileo were obtained
during orbit G1 (June 1996) and continued at
low-to-moderate spatial resolution, at more or
less bimonthly intervals, until orbit I21 (August
1999). These data provided the best opportunity to date to study the distribution and temporal
behaviour of volcanism both globally and locally. From SSI data obtained during these orbits,
about 30 large-scale (tens of kilometres across)
surface changes from Voyager (1979) images
were apparent, all volcanic in origin. These
included new pyroclastic deposits of several
colours, and bright and dark flows (McEwen et
al. 1998a). The beautiful flows at Ra Patera had
been obliterated by an ongoing eruption. Figure
1 shows views of the anti-jovian hemisphere as
seen by both SSI and NIMS. During the lowresolution phase of the mission several large surface changes occurred, including a 400 km
diameter “black-eye” formed during the 1997
Pillan eruption (see figure 2a). Although impressive, the changes were not as extensive as expected, based on changes that occurred in the four
months between Voyager encounters.
It appears that widespread plume deposits are
ephemeral, disappearing within a few years
unless replenished, and most volcanic activity is
restricted to resurfacing of calderas (called
paterae on Io), and emplacement of flow fields
that cover a small fraction of Io’s surface
(McEwen et al. 1998b). In some cases where
upiter’s moon Io is the only other
body in the solar system known
to have active, high-temperature
volcanism like that found on Earth.
The Galileo spacecraft has been
observing Io regularly since June
1996, and the data that it has
returned have led to many new
insights into the volcanic processes
that have shaped not only Io, but
Earth in its distant past.
J
large changes were expected (for example, at
Loki, Io’s most powerful volcano and the site of
numerous eruptions) there was little change, the
activity confined within the margins of Loki Patera. NIMS obtained spectra that showed that
sulphur dioxide was ubiquitous on the surface,
although with varying concentrations and grain
sizes (Carlson et al. 1997), and that there were
many thermal anomalies in the infrared (LopesGautier et al. 1997).
The close flybys of Io during orbits I24
(October 1999), I25 (December 1999) and I27
(February 2000) provided the highest resolution (SSI 5 m/pixel) images ever seen of Io’s
surface, revealing multicoloured, irregularshaped volcanic paterae and volcanogenic
deposits, and spectacular eruptions.
Magma temperature and composition
One of the most spectacular and significant
discoveries by Galileo was that of very-hightemperature volcanism on Io. Many of Io’s volcanoes glow in the dark at short SSI wavelengths, due to thermal emission from surfaces
in excess of 700 K, with most hotter than
1000 K (McEwen et al. 1997, 1998b, Davies et
al. 1997, 2001). After the 1979 discovery of
active volcanism on Io, the composition of the
dominant volcanic lava was unknown for
years: the debate swayed between relatively
cool sulphur and hotter silicate melts. Data
from the Voyager Infrared Imaging System
were inconclusive, although at Pele the IRIS
data could be fitted with temperatures as high
as 854 K (Pearl and Sinton 1982), too high for
sulphur volcanism. In the years preceding the
Galileo mission, although ground-based observations (e.g. Veeder et al. 1994, Stansberry et
al. 1997) revealed high-temperature eruptions
April 2001 Vol 42
Volcanoes on Io
view from Galileo
Fo Patera
Zamama
Amirani
Isum
Patera
Prometheus
Reiden
Patera
/Pele
Tupan Patera
Malik Patera
Marduk
that had to be silicate, it was not until Galileo
started observing Io that the full extent of hightemperature volcanism became known. SSI
observed a hemisphere of Io in eclipse through
the instrument’s clear filter during Galileo’s
first orbit (G1) and imaged areas glowing in
the darkness, glowing with volcanic heat
(McEwen et al. 1997). These areas had to be at
a temperature of at least 700 K, assuming the
source filled the entire SSI pixel (4.5 km2).
They were more likely to be smaller areas at
higher temperatures, but how much hotter?
One constraint on the temperature/area relationship came from a two-temperature fit to a
volcano thermal spectrum obtained by NIMS
during the same orbit, which showed that part
of the Zamama volcano was at a temperature
of 1100 K (Davies et al. 1997). These data provided an unambiguous identification of molten
silicates from Galileo data and, with other
identifications during the subsequent years,
showed that silicate volcanism was widespread
on Io (McEwen et al. 1998a,b). To date, over
100 active sources have been identified.
Throughout the low-resolution orbits, NIMS
and SSI regularly observed thermal emission
from Io in eclipse and night-side observations.
Some of the most spectacular and valuable
data were collected during Orbit C9 (June
1997) when SSI imaged Io in darkness through
two filters, closely followed by a NIMS observation of the same hemisphere. These images
revealed, firstly, that the volcano Pillan was
undergoing a huge eruption, and secondly, that
the magma temperature at Pillan was hotter
April 2001 Vol 42
Aidne
Culann Patera
than any observed terrestrial lavas, well in
excess of 1500 K, suggesting not basaltic but
ultramafic (magnesium rich) composition
(McEwen et al. 1998a). The best constraint on
magma temperature comes from a joint analysis of the NIMS and SSI Pillan data (Davies et
al. 2001) and shows that the lava at Pillan is at
a temperature of at least 1870 K. Subsequently, other locations were also found to have
these very-high-temperature lavas.
Early Earth and Io
The implications of the presence of ultramafic
magmas on the interior structure of Io are discussed in McEwen et al. (1998a). Ultramafic
magmas may be produced by large degrees of
partial melting of an undifferentiated mantle,
after separation of an iron-rich core. This
process is analogous to the formation of
komatiites on Earth during the Archean, suggesting Io’s interior is similar to that of the
early Earth (Matson et al. 1998). This primitive composition could have been preserved by
rapid, continuous, recycling. Resurfacing of Io
is certainly rapid. Not one single impact crater
has been identified, even at high resolution
(McEwen et al. 2000a), which implies the surface is of the order of only 1 million years old.
The high rate of resurfacing of Io (>1 cm/year)
may lead to such rapid recycling of both
volatiles and lavas that primitive melts are
reproduced. A fast recycling of the surface layers may also explain how some magmas, feeding plumes like Pele, have a high volatile content (Davies et al. 2000a): surface or
1: Io as seen by SSI (left) and
NIMS (right) during orbit G2, 6
September 1996. The SSI image
shows a multicoloured surface.
Colours are interpreted as
follows: red, short-chain sulphur
molecules, produced at sites
with active silicate volcanism;
black, silicate lavas and silicaterich volcanogenic deposits;
yellow, sulphur-rich deposits;
white, sulphur dioxide-rich
deposits. Other higherresolution images revealed
green deposits that may be
sulphur-rich (Kargel et al. 1999)
or silicate in composition
(Geissler et al. 1999). The NIMS
image shows Io at 4.8 µm, with
the most intense thermal
sources labelled.
near-surface volatiles can be sealed under
newer deposits, forming reservoirs of volatiles
in the crust (Johnson et al. 1995). Additionally,
there has been no evidence of highly viscous
lavas or domes, although the enigmatic tholii
seen by Voyager have yet to be imaged at high
resolution (Keszthelyi et al. 2001). The absence
of high-silica-content, highly evolved magmas
is another indication of a highly efficient
degree of recycling of the crust.
Alternatively, volcanic activity may have
highly differentiated Io’s mantle, resulting in a
low-density crust that is rich in alkali elements,
aluminium and silica, and a magnesium, iron
and calcium-rich mantle (Keszthelyi and
McEwen 1997a). Melts from such an upper
mantle may have temperatures up to ~2100 K,
even hotter than komatiites.
These models are end members, and the truth
may lie somewhere between them. Whatever
the case, these high-temperature, high-massflux eruptions are being witnessed for the first
time. It is not yet known how widespread ultramafic magma eruptions are on Io. From
remotely sensed data the best indicator of
magma composition is the eruption temperature of the lava. While differentiating between
sulphur- and silicate-dominated volcanism is
straightforward when high-temperature
(>700 K) components are identified, differentiating between basaltic magmas (up to ~1500 K)
and ultramafic lavas (up to 1900 K) is more
problematic, given the rapid cooling of newly
erupted lava. Identification of the highest temperature components is difficult in all but the
2.11
Volcanoes on Io
Galileo’s instruments
Three instruments on Galileo have been
monitoring Io’s volcanism. The Solid State
Imaging experiment (SSI) is a digital camera containing a CCD and an eight-position
filter wheel, covering effective wavelengths
from 990 to 418 nm. SSI is particularly sensitive to thermal radiation from the highest
temperature areas of active volcanism,
especially in long-exposure eclipse and
nightside observations. At Io, observations
were made at high illumination angles to
accentuate topography. Bright limb observations were made to detect volcanic
plumes to compile a systematic plume
inventory. Full-disk images were collected
in four colours to monitor surface changes,
and six-colour global images obtained for
compositional mapping. Resolutions during
orbits from June 1996 (orbit G1) to June
1999 (C21) were 2.5 km per pixel at best
(McEwen et al. 1998a), and were as high as
5 m per pixel during the close flybys in late
1999 (orbits I24 and I25) and early 2000
(orbit I27) (McEwen et al. 2000a).
The Near Infrared Mapping Spectrometer
(NIMS) is a moving-grating imaging spectrometer that can image a target at up to
408 wavelengths between 0.7 and 5.2 µm.
NIMS is theoretically capable of detecting
temperatures as low as 180 K (Smythe et al.
1995) but the source at this temperature
would have to fill the entire NIMS field of
vision to be detected. As the highest spatial
resolution obtained by NIMS during the
first 23 orbits was 135km/pixel, and volcanic thermal sources are unlikely to fill an
area this size, the actual lower detection
limit is higher, around 220 K. Nevertheless,
NIMS is an ideal instrument to observe
most vigorous eruptions, where there is, firstly,
a sufficiently large area of exposed lava surface
at high enough temperatures that the thermal
emission is visible from a remote sensing platform, and secondly, the thermal signal is not
swamped by emission from larger, cooler components. Happily, Pillan fits these criteria.
Eruption style, evolution and thermal
signature
The extraordinary temporal coverage by
Galileo has allowed not only the identification
of volcanoes but also the charting of the evolution of individual eruptive episodes. The
Galileo data complement data from Earthbased and Earth-orbiting telescopes obtained
during a highly successful Io observing programme, under the auspices of the International Jupiter Watch Io Working Group, coordi2.12
thermal emission from active volcanism,
typically at temperatures from 1200 to in
excess of 1600 K for silicates. With no
absorption caused by a significant atmosphere (as would be the case on Earth),
these NIMS observations are the best highspectral resolution data yet obtained of volcanic thermal emission. NIMS is also sensitive to active volcanism at lower
temperatures, such as eruptions of liquid
sulphur. An active, stable, liquid sulphur
lake has been shown to have a surface temperature as high as 500 K (Lunine and
Stevenson 1985), although sulphur can be
heated to higher temperatures before eruption. Spatial resolutions of NIMS observations ranged from 600 to 135 km/pixel during the distant encounters, but were as high
as 0.5 km/pixel during the close flybys.
Many of the distant observations had high
spectral resolution but, unfortunately, prior
to the I24 Io encounter in October 1999
the movable grating on the instrument
stuck, and resisted any effort to free it.
Observations were therefore restricted to
the wavelengths at the stuck position, providing 12 wavelengths spaced across the
full NIMS wavelength range. The highest
resolution NIMS images were about 200 m
per pixel.
The Photo-Polarimeter Radiometer (PPR)
is capable of observation of long-infrared
wavelength observations from 17 to 30 µm,
and can map the surface temperature of Io
both at and away from volcanic centres.
Problems with the instrument filter wheel
during the distant observations of Io
restricted data collection, but multiwavelength observations were obtained during
the close encounters at resolutions as high
as 2.2 km per pixel (Spencer et al. 2000b).
nated by John Spencer at Lowell Observatory,
Tucson, Arizona.
Monitoring of the volcanoes showed that they
can persist over months to years (perhaps even
longer), or erupt in more short-lived or sporadic
events, detected only a few times (Lopes-Gautier et al. 1999). The two types may represent different styles of magma supply or eruption.
Active and recently active volcanic centres,
especially paterae, the dark-floored calderas,
are distributed uniformly on Io’s surface (Carr
et al. 1998). However, volcanic plumes (the
most spectacular visual manifestation of Io’s
volcanism) and the persistent volcanoes are
concentrated in equatorial regions (Lopes-Gautier et al. 1999), a distribution of volcanism
consistent with a tidal heating model where
heating occurs mainly in an asthenosphere
about 100 km thick (Segatz et al. 1998). The
exception to this predicted heat-flow distribution is Loki Patera, which may be the result of a
mantle plume, very much like Hawaii on Earth.
Models of thermal emission from active
volcanism (e.g. Carr 1986, Davies 1996, Howell 1997, Keszthelyi and McEwen 1997b) have
been used to determine eruption physical parameters such as ages and rates of emplacement
of lava, and to classify styles of eruption from
the shape of the thermal spectrum in the
infrared. During orbit G1, NIMS obtained
spectra, uncontaminated by sunlight, of 14 hot
spots (see Davies et al. 2000b) which proved to
be typical of many volcanoes on Io, exhibiting
a definitive thermal ramp towards longer wavelengths (Lopes-Gautier et al. 1997). This particular spectral signature is characteristic of active
lava flows with ages typically ranging from
days to months. Analysis of NIMS and SSI
observations obtained throughout the Galileo
prime mission showed that the level of activity
seen during G1, determined by the number and
distribution of volcanoes, is fairly typical for Io.
This represents the background (non-outburst)
level of activity, what most of Io looks like,
most of the time (Davies et al. 2000b).
Study of Galileo data allowed determination
of areal coverage rates by lava flows (Davies et
al. 2000b, McEwen et al. 2001), and the first
measurements of active lava-flow thicknesses
(an important eruption physical parameter)
allowed mass eruption rates to be calculated
(Williams et al. 2001a, Davies et al. 2001).
Estimates of magma physical properties can be
made from a knowledge of flow thickness and
mass eruption rate (e.g. Pinkerton and Wilson
1994). The thickness of flows can be inferred
for several volcanoes in NIMS data obtained
during orbit G1 in June 1996 (Davies et al.
2000b). From calculating the volume of lava
erupted (which solidifies and cools, yielding
latent and sensible heats) necessary to produce
the observed thermal emission, and dividing
this figure by the rate of areal coverage of the
volcanoes, yields an average flow thickness of
about 1 m. Eruption volume fluxes are therefore approximately tens of m3 s–1. While
greater than the average eruption flux at
Kilauea (~0.2 m3 s–1), these fluxes are within
the range of terrestrial eruption volume fluxes.
Actual measurements of flow thickness were
made from high-resolution SSI images of
extensive lava flows emplaced during the Pillan
1997 eruption. These data are discussed below.
A volcanic tour of Io
Several volcanic centres on Io have been studied in detail. Each reveals more about a particular facet of Io’s volcanism. One, shown in figure 2b, is called Prometheus. Prometheus has
been likened to Old Faithful: it is the site of a
volcanic plume that has been seen in every
appropriate image obtained by Galileo and
April 2001 Vol 42
Volcanoes on Io
Voyager. A new ~80 km long lava flow field
was emplaced in the years between Voyager
(1979) and Galileo (1996) and the annular
plume shifted the same distance (McEwen et al.
1998b). Analysis of NIMS data shows areas at
molten silicate temperatures (Davies 2001).
Mapping of the deposits laid down by the
plumes reveals asymmetries that indicate two
sources of plume material (Doute et al. 2001,
Keszthelyi et al. 2001). The lesser source
emanates from the most intense thermal area of
the volcano, close to the Prometheus caldera.
The most likely explanation for this material is
sulphur dioxide escaping from newly erupted
magma. The second source is from the ends of
active flow lobes at the other end of the flow
field, 80 km to the west. The most likely explanation is the plume-generating mechanism is
from interaction between silicate lava and a
sulphur dioxide-rich icy substrate (Kieffer et al.
2000, Milazzo et al. 2001). The Kieffer et al.
model has a silicate flow generating a reservoir
of SO2 beneath by mobilizing the frozen substrate. The plume erupts from a rootless vent.
The process is analogous to lava flows over
permafrost on Earth. Milazzo et al. also propose that the generation of the Prometheus
plume is caused by individual flow lobes interacting with the frozen substrate, without necessarily a single main vent. High-resolution SSI
images (McEwen et al. 2000) show areas of
new flow emplacement that imply that the
plume is formed from multiple sources.
a
b
5 km
I27 high (10 m/pixel) and medium (180 m/pixel)
resolution in C21 context (1.3 km/pixel)
Lava tubes and lava lakes
Whatever the actual plume-generation model,
study of the Prometheus flow field in high-resolution NIMS and SSI data shows that there is
thermal emission along most of the length of
the flow, from the Prometheus caldera to the
end of the flow field (Lopes et al. 2001): the
closest terrestrial analogue is that of magma
flowing through tubes and erupting onto the
surface, the typical eruption mechanism at
Kilauea, Hawaii. Galileo imaged Prometheus
at high resolution during close-encounter
orbits I24 and I27 and this has allowed
changes between orbits to be measured. New
flows on Io appear very dark and fade with
time as they cool and are covered with pyroclastic debris and condensing volatiles. Comparisons between images taken from different
orbits revealed new flows, emplaced in the
time between orbits. This allowed rates of
areal coverage to be determined for the area of
the flow field covered in both orbits. At
Prometheus the flows are apparently being
emplaced through an extensive lava tube system, erupting onto the surface at a rate of
between 5 and 35 m2 s–1, determined from SSI
and NIMS (McEwen et al. 2000, Davies et al.
2000b). It appears that a long-lived magma
supply and unconstrained surface flows are
April 2001 Vol 42
c
d
2: Volcanic features as seen by Galileo.
(a) Galileo SSI images of Pele and Pillan Patera (NASA image PIA00744, McEwen et al. 1998b). The
figure on the left was obtained on 4 April 1997. When imaged again (right) in September 1997, the
dark deposit (400 km across) is probably pyroclastic material from the Pillan summer 1997 eruption.
At the centre of the black area are extensive lava flows emplaced during the Pillan eruption. The red
deposits around Pele are probably rich in sulphur, and appear to come from an active lava lake.
(b) Prometheus high-resolution comparison showing new flows (NASA image PIA02564 Keszthelyi et
al. 2001). The Prometheus plume source moved 80 km in 17 years, matching the emplacement of new
surface flows. Images at higher resolution show the emplacement of newly erupted material that
appears dark, too hot for sulphur and sulphur dioxide to condense on the surface.
(c) Culann Patera, as seen by SSI (NASA image PIA02535 McEwen et al. 2000a). Culann is a small
green depression surrounded by radial lava flows, with active flows appearing black. Bright red
deposits surrounding Culann indicate the presence of a plume rich in sulphur.
(d) Tvashtar Catena (NASA image PIA02550 McEwen et al. 2000a). The SSI experiment imaged a rare
fire-fountain episode during December 1999 (I25). Emission was so intense that the detector became
overloaded and bled. An interpretation of the eruption by Laszlo Keszthelyi, of fire-fountains erupting
from a long fissure, was confirmed in February 2000 (I27) images, when flows emplaced during the
short eruption were imaged.
2.13
Volcanoes on Io
both needed to generate the Prometheus
plume. If an eruption is confined to a caldera
floor (and there is a strong correlation between
paterae and active volcanism on Io) a supply of
volatiles for a plume will be limited to outgassing of the erupting lava, and from interaction with volatile deposits in the caldera.
These will soon be either buried or exhausted.
Qualitatively, a diminished supply of volatiles
would result in a more diffuse plume deposit as
seen, for example, around the thermal sources
at Amirani/Maui. At Prometheus a rich source
of volatiles can be tapped for as long as lava
keeps erupting, and flows keep extending over
the surface.
The plains to the north of Prometheus are
found to be ridged as SO2 emanating from the
flow margins has condensed on the flow-facing
sides. The formation process of the ridges is as
yet unknown. Even as questions are answered
by Galileo, new ones appear.
Pele’s sulphur plume
A very different style of eruption is taking place
at Pele, a persistent hot spot. Pele is the source
of a large, diffuse, intermittently observed
plume that has laid down red deposits
1500 km across. Pele yielded the highest temperature derived from Voyager IRIS data,
654 K (Pearl and Sinton 1982), with temperatures as high as 850 K fitted within the noise of
the instrument. The discovery of sulphur in the
Pele plume from Hubble Space Telescope data
(Spencer et al. 2000a) was a triumph of Earthorbit astronomy, and implies that the red
deposits observed at Pele and many other sites
of active silicate volcanism (Culann is a particularly fine example: figure 2c) are rich in shortchain (S4) sulphur molecules (Geissler et al.
1999). An analysis of NIMS data of thermal
emission from Pele throughout the Galileo mission showed that the thermal output from Pele
is both large (240 ± 40 GW) and relatively constant, especially when compared to Pillan (see
figure 2). The peak of Pele’s thermal emission is
at much shorter wavelengths than almost every
other thermal source on Io seen by NIMS, the
exception being Pillan at the height of the 1997
eruption. Such a thermal signature is indicative
of a high mass eruption flux, but the evolution
of the thermal signal from orbit to orbit does
not show the extensive flows expected from
such an eruption. The simplest explanation of
Pele’s thermal signature is that it is an active
lava lake (Davies et al. 2001). Pele is the only
volcano on Io seen, so far, to exhibit this thermal behaviour.
The lava lake hypothesis was reinforced by
high-resolution NIMS and SSI observations of
Pele (McEwen et al. 2000a, Kezthelyi et al.
2001, Davies et al. 2001) which showed that the
Pele thermal source was confined between two
grabens at the end of a rift in Danube Planum,
2.14
the grabens acting as a dam. Long-lived lava
lakes are also rare volcanic phenomena on
Earth. The lava lake at Erte Ale in Ethiopia is in
a region of tectonic extension, allowing magma
a path to the surface. Like most other volcanic
features on Io, the Pele lava lake is orders of
magnitude larger than its terrestrial counterparts. The areal control on the extent of Pele’s
lavas is particularly significant because it
implies that the volatiles making up the Pele
plume are degassing from the magma and are
not caused by the mobilization of volatiles by
magma flowing over a frozen, volatile-rich surface as is observed at Prometheus.
Less than 300 km from Pele is Pillan, the site
of the largest eruption yet witnessed by Galileo
and the most closely studied (Davies et al.
2001, Williams et al. 2001). The eruption laid
down a deposit of dark, diffuse material
400 km across (McEwen et al. 1998) (see figure 2a) and provided the best evidence so far
for very-high-temperature lavas on Io. NIMS
collected thermal data from Pillan over several
orbits, allowing the start of the eruption to be
constrained to just before orbit G8 (7 May
1997). The eruption reached its peak around
orbit C9 (18 June 1997), and declined during
the following years (Davies et al. 2001). SSI
provided direct observations of the style of
eruption at Pillan (Williams et al. 2001), from
the start thought to be rapid emplacement of
possibly turbulent flows. In the space of a few
months, over 5300 km2 of Io’s surface was
covered with lava to a depth of about 10 m
(Williams et al. 2001a), an eruption volume of
53 km3. The largest terrestrial eruption ever
witnessed (in terms of the eruption volume
flux) was Laki in 1783 (Thordarson and Self
1993), which emplaced 15 km3 of lava in a few
months. The average areal coverage and mass
eruption rates at Pillan 1997 were 330 m2 s–1
and 3300 m3 s–1, although the peak eruption
rates were most likely much greater. The close
Io flybys in later 1999 and 2000 yielded highresolution images of Pillan and also night-time
observations by PPR at 17 microns. The Pillan
flows had cooled down to about 200 K by I24
in October 1999 (Spencer et al. 2000),
although some “hot spots” at temperatures
above 380 K were seen by NIMS, most likely
hot material showing through cracks in the
lava flows (Davies et al. 2001). High-resolution SSI images of the Pillan flows (up to 19 m
per pixel) revealed a surface composed of a
complex mix of pits, domes, channels and possibly rafted plates, the latter features associated with initial rapid emplacement (McEwen et
al. 2000a). The pits and domes, analogous
with terrestrial rootless cones, may result from
interactions between hot lava and a volatilerich substrate.
One of the most spectacular images obtained
by Galileo was of the onset of a new eruption
at Tvashtar Catena during I25 in December
1999 (figure 2d). Incandescent lava was seen
rising over a kilometre above the surface: a
“curtain of fire” emanating from a 25 km long
rift. This eruption was also observed from
Earth (Acton et al. 2001) from which magma
temperatures were derived in the range 1300–
1900 K. Terrestrial fire-fountain episodes typically last only hours, controlled by the supply
of volatile-rich magma. Ionian fire-fountain
episodes may last for similar periods of time,
explaining transient events such as those witnessed by Stansberry et al. (1997). Modelling
of the dynamics of the Tvashtar eruption by
Wilson and Head (2001) and using SO2 as a
driving volatile, yielded mass eruption rates
(per unit length of the fissure) of 0.7 to
7 m3 s–1, similar to Hawaiian rift eruptions.
However, the Tvashtar total volume flux of
1.75 × 104 to 1.75 × 105 m3 s–1 is very much larger, similar in fact to implied “outburst” volume
fluxes (Blaney et al. 1995, Davies 1996).
Temperature maps of Loki
Loki Patera has the appearance of a large
(150 km diameter), dark-floored caldera, with
a higher albedo “island” in the centre. Outputting 25% of Io’s thermal emission, Loki is
Io’s most powerful volcano. Large eruptions at
Loki that are observable from Earth (called
brightenings) occur approximately once a year
and last for several months. Surprisingly, given
the scale of the eruptions that are taking place
to produce the observed thermal emission,
there were no changes seen in the appearance
of the caldera when Galileo and Voyager
images were compared. High spatial resolution
images of Loki were obtained by NIMS and
PPR during I24, and again by PPR during I27,
allowing temperature maps to be constructed
(Lopes-Gautier et al. 2000, Spencer et al.
2000b). The PPR data identified a hot source
in the southwest corner of the patera during
I24 and more or less uniform temperatures
elsewhere (as does NIMS). By I27, most of
Loki’s floor had been resurfaced and floor temperatures had increased by 40 K.
However, the nature of the eruption mechanism at Loki remains enigmatic, even with
high-resolution data. What exactly happens
during a Loki brightening, and what is the
nature of an outburst event? The temperatures
derived from two-temperature fits to NIMS
Loki data are somewhat lower than at many
other volcanoes (Davies et al. 2000a, Davies
2001) and the Loki Patera thermal signature
(see figure 3) lacks the extensive high-temperature component seen at other volcanoes (e.g.
Pillan Patera, Pele and Prometheus). This is
indicative of a relatively quiescent eruption
style at Loki. Two mechanisms are proposed
for the emplacement of lava (“eruption style”)
at Loki. The first is that Loki is a huge lava
April 2001 Vol 42
Volcanoes on Io
thermal output (GW/ micron)
300
Pillan C9
250
Loki C9
200
150
100
Prometheus
50
Pele
Pillan E16
0
1
3
2
4
wavelength (microns)
lake, the crust of which periodically founders
and overturns. If so, then orderly overturning
can explain (thermally) Loki’s brightenings:
violent disruption of the crust may result in an
outburst. The time between brightenings may
be the time necessary for the crust on the lake
to thicken and become unstable. The second
eruption mechanism has the floor of Loki periodically resurfaced with flows, in a relatively
quiescent emplacement style (a laminar flow
regime). A larger, more vigorous eruption may
produce an outburst. The extent of the activity
is topographically controlled by the margins of
the patera. In both eruption cases, the period
between eruptions may be the time taken for
the magma supply system to recharge. Detailed
modelling of the existing data from groundbased, Hubble, Voyager and Galileo observations may reveal the true nature of the eruption
processes at Loki.
Eruption size, frequency and
classification
As a result of Galileo’s regular surveillance, it is
possible to separate volcanic eruptions into
some loose classes. Generally, the larger the
eruption, the less frequent the event; and as
observation resolution improves, more and
more hot spots are seen. Galileo has yet to witness the largest and rarest eruption type, in
terms of thermal output, the “outbursts” first
witnessed by Witteborn and colleagues in
1974, and characterized by a doubling of Io’s
5 µm thermal output. The exact style of eruption is not known. Next down on the power
output scale are “pillanian” eruptions
(Keszthelyi et al. 2001) characterized by large
pyroclastic deposits and extensive but shortlived outpourings of lava from fissures feeding
open channel or open sheet flows (Pillan 1997
and the Tvashtar eruption fall into this category). Below these are “promethean” eruptions
(Keszthelyi et al. 2000), typified by the
Prometheus volcano, that are characterized by
long-lived, steady eruptions producing a compound flow field emplaced over a period of
years to decades. This is the most common
eruption type on Io as observed by NIMS, and
may make up at least 10% of Io’s total thermal
output (Davies et al. 2000).
The promethean and pillanian eruptions can
April 2001 Vol 42
5
3: A comparison of volcanic thermal emission spectra as seen by NIMS. These spectra are from lowresolution observations that allow study of each volcano in its entirety. The spectra are best-fit twotemperature fits to NIMS data of several volcanoes on Io, each showing a different eruption style and
resulting thermal signature, each corrected for emission angle. The Pillan 1997 eruption shows a
preponderance of thermal emission at short wavelengths indicative of a violent, vigorous eruption, but
emission decays from C9 (June 1997) to E16 (21 July) as the emplaced lava cools. Pele shows a
constant thermal emission at short wavelengths, indicative of an active lava lake or areally confined
active flows. Prometheus shows a consistent thermal ramp towards longer wavelengths,
characteristic of lava fields emplaced over days and months. Loki exhibits a changeable thermal
signature, but in NIMS data always exhibits a thermal ramp similar to that seen at Prometheus, on a
much larger scale.
also be identified by different thermal signatures (thermal emission as a function of wavelength), as seen by NIMS (figure 3).
Promethean eruptions exhibit a thermal ramp
towards longer wavelengths (as shown in
Davies et al. 1997, 2000) with thermal emission peaking close to or beyond 5 µm. Pillanian eruptions have a preponderance of thermal
emission (at the height of eruption) at shorter
wavelengths, around 3 µm (Davies et al. 2001),
and then decay to a promethean-type thermal
signature. The final class of thermal anomalies
consists of very small eruptions, possibly shortlived, identified in low-resolution NIMS dayside observations by identifying excess thermal
emission above that seen in an averaged Io
spectrum (Blaney et al. 2000) and seen at long
wavelengths in high-resolution NIMS observations (Lopes et al. 2001). Approximately 100
promethean and pillanian sites have been identified from NIMS and SSI observations (LopesGautier et al. 1999) and it has been estimated
that there may be hundreds of these very small
eruptions (Blaney et al. 2001, Lopes et al.
2001). These small eruptions may be silicate in
nature (all of Io’s thermal output can be
explained by active or cooling silicate flows
[Blaney et al. 1995]) but in all probability this
class includes eruptions of remobilized volcanogenic sulphur deposits, for example, at
Emakong Patera (Williams et al. 2001b).
There are often exceptions to even broad
classifications. Pele is a long-lived volcano and
plume site and, as explained earlier, is most
likely an active lava lake (Davies et al. 2001b).
Loki exhibits a promethean thermal signature
(Davies et al. 2000c) but on a much larger spatial scale than any other promethean volcano
(see figure 3). Loki is the site of regular, large
eruptions on a timescale of once or twice a
year, but apparently without a pillanian thermal signature and with no discernible pyroclastic deposits.
Conclusion
Although Galileo has already revealed much
about the volcanic wonders of Io, there should
be more to come from this durable spacecraft:
close flybys of Io by Galileo are planned during
2001 and 2002. But eventually, the Galileo
mission will end. Before it runs out of fuel and
the mission engineers lose control of the craft,
it will probably be sent into a controlled dive
into the jovian atmosphere in 2003, to safeguard Europa – where many people hope to
find signs of extraterrestrial life – from possible
contamination. ●
Ashley G Davies is a volcanologist and member of
the Galileo NIMS team at the Jet Propulsion Laboratory, California ([email protected]). Many of
the images obtained by SSI and NIMS are available
from the Galileo Web site at galileo.jpl.nasa.gov.
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