Io's Sodium Cloud
This image of Io and its surrounding sky is shown
in false color. North is at the top, and east is to the right.
Most of Io's visible surface is in shadow, though one can see part
of a white crescent on its western side. This crescent is illuminated
mostly by Jupitershine (i.e. sunlight reflected off Jupiter).
The striking burst of white light near Io's eastern equatorial edge is sunlight being scattered by the plume of the volcano Prometheus. Prometheus lies just beyond the visible edge of the moon on Io's far side. Its plume extends about 100 kilometers above the surface, and is being hit by sunlight just a little east of Io's eastern edge.
Scattered light from Prometheus' plume and Io's lit crescent also contribute to the diffuse yellowish emission which appears throughout much of the sky. However, much of this emission comes from Io's Sodium Cloud: sodium atoms within Io's extensive material halo are scattering sunlight at the yellow wavelength of about 589 nanometers.
This image was taken at 5 hours 30 minutes Universal Time on November 9, 1996
through the green-yellow filter of the solid state imaging (CCD) system
aboard NASA's Galileo spacecraft. Galileo was then in Jupiter's shadow,
and located about 2.3 million kilometers (about 32 Jovian radii)
from both Jupiter and Io.
(Courtesy NASA/JPL)
Galileo Image of Io
The mottled face of Jupiter's volcanically active moon Io,
viewed by the Galileo spacecraft, shows
dramatic changes since it was seen 17 years ago by the exploratory NASA
spacecraft Voyagers 1 and 2.
This image, taken on June 25, 1996 at a
range of 2.24 million kilometers (1.4 million miles), is centered on the Media
Regio area and shows details of the volcanic regions and colored deposits that
characterize Io. North is at the top of the picture and the Sun illuminates the
surface from the east (right). The smallest features that can be discerned here
are approximately 23 kilometers (14 miles) in size.
Io's surface is covered with volcanic deposits that are thought to contain
ordinary silicate rock, along with various sulfur-rich compounds that give the
satellite its distinctive color. In the brighter areas the surface is coated
with frosts of sulfur dioxide. Dark areas are regions of current or recent
volcanic activity. Planetary scientists say many changes are evident in the
surface markings since this region of Io was imaged 17 years ago by the Voyager
spacecraft. The bright regions near the eastern limb of the moon are much more
prominent in the Galileo images than they were previously. Surface details have
also changed dramatically in the vicinity of the eruptive volcano Masubi (the
large, predominantly white feature seen near the 6 o'clock position in this
view). Masubi was discovered as an active volcano during the Voyager
encounters of Io in 1979.
(Courtesy NASA/JPL)
Io Triplet
Three full-disk color views of Jupiter's volcanic moon Io as seen by
NASA's Galileo spacecraft camera are shown
in enhanced color to highlight details of the surface. Comparisons
of these images to those taken by the Voyager
spacecraft 17 years ago has revealed many changes have
occurred on Io. Since that time, about a dozen areas at least as large
as the state of Connecticut have been resurfaced.
These three views, taken by Galileo in late June 1996, show about 75 percent of Io's surface. The images reveal that some areas on Io are truly red, whereas much of the surface is yellow or light greenish. The major red areas shown here appear to be closely associated with very recent fragmental volcanic deposits (pyroclastics) erupted in the form of volcanic plumes. The most prominent red oval surrounds the volcano Pele (far right), as previously discovered by Hubble Space Telescope images. An intense red spot lies near the active plume Marduk east of Pele. Other reddish areas are associated with known hot spots or regions that have changed substantially since the Voyager spacecraft flybys of 1979. The reddish deposits may be the products of high-temperature explosive volcanism.
There are some curious differences in the overlap region between the images at left and center. There are several especially bright areas in the image at left that appear much darker in the image at center. These may represent transient eruptions or surface materials with unusual light-scattering properties.
Several volcanic plumes active during the Voyager flybys in 1979
occurred near the bright limbs or terminator regions of these images,
where airborne materials should be detectable. Loki and Amirani appear
to be inactive, Volund is active, and Pele may be active but is
extremely faint. The plume Marduk also seems to be active, and dark
jets of erupting materials can be seen against the disk. Several
previously unknown mountains can be seen near the terminators.
(Courtesy NASA/JPL)
Eruption on Io
This image, taken by NASA's Galileo spacecraft, shows a new
blue-colored volcanic plume extending about 100 kilometers (about 60
miles) into space from Jupiter's moon Io (see inset at lower left).
The blue color of the plume is consistent with the presence of sulfur
dioxide gas and "snow" condensing from the gas as the plume expands and
cools. Galileo images have also shown that the Ra Patera plume glows
in the dark, perhaps due to the fluorescence of sulfur and oxygen ions
created by the breaking apart of sulfur dioxide molecules by energetic
particles in the Jovian magnetosphere. The images at right show a
comparison of changes seen near the volcano Ra Patera since the Voyager
spacecraft flybys of 1979 (windows at right show Voyager image at top
and Galileo image at bottom). This eruptive plume is an example of a
new type of volcanic activity discovered during Voyager's flyby in
1979, believed to be geyser-like eruptions driven by sulfur dioxide or
sulfur gas erupting and freezing in Io's extremely tenuous atmosphere.
Volcanic eruptions on Earth cannot throw materials to such high
altitudes.
Ra Patera is the site of dramatic surface changes. An area around the
volcano of about 40,000 square kilometers, area about the size of New
Jersey, has been covered by new volcanic deposits. The image was taken
in late June 28, 1996 from a distance of 972,000 kilometers (604,000
miles).
(Courtesy NASA/JPL)
Io Glowing in the Dark
Volcanic hot spots and auroral emissions glow on the
dark side of Jupiter's moon Io in the image at left. The image was taken
by the camera onboard NASA's Galileo spacecraft on 29 June, 1996 UT while
Io was in Jupiter's shadow. It is the best and highest-resolution image
ever acquired of hot spots or auroral features on Io. The mosaic at right
of 1979 Voyager images is shown with an identical scale and projection to
identify the locations of the hot spots seen in the Galileo image. The
grid marks are at 30 degree intervals of latitude and longitude. North is
to the top.
In the nighttime Galileo image, small red ovals and perhaps
some small green areas are from volcanic hot spots with temperatures of
more than about 700 kelvin (about 1000 degrees Fahrenheit). Greenish areas
seen near the limb, or edge of the moon, are probably the result of
auroral or airglow emissions of neutral oxygen or sulfur atoms in volcanic
plumes and in Io's patchy atmosphere. The image was taken from a range of
1,035,000 kilometers (about 643,000 miles).
(Courtesy NASA/JPL)
Io's Full Disk
Io, the most volcanic body in the solar system, is
seen in front of Jupiter's cloudy atmosphere in this image from NASA's
Galileo spacecraft, now orbiting the giant planet. This newly processed
image is the best and highest resolution view of Io produced thus far by
Galileo. Galileo was about 487,000 kilometers (about 302,000 miles) from
Io when this was taken on September 7, 1996, and Jupiter was about 908,000
kilometers (about 564,000 miles) away. The image is centered on the side
of Io that always faces away from Jupiter. The color in the image is
composed of data taken in the near-infrared, green and violet filters of
Galileo's solid-state imaging camera, and has been enhanced to emphasize
the extraordinary variations in color and brightness that characterize
Io's volcano-pocked face. The black and bright red materials correspond to
the most recent volcanic deposits, probably no more than a few years old.
The near-infrared filter makes Jupiter's atmosphere look blue. The active
volcano Prometheus is seen near the right-center of the disk. Scientists
are noting many changes that have occurred on Io's surface since the
Voyager flybys 17 years ago, and even a few changes in the two months
since Galileo's imaging of Io this summer.
(Courtesy NASA/JPL)
Changing Volcanic Activity on Io
Volcanoes on Jupiter's moon Io are compared in these
images from NASA's Galileo spacecraft (right) taken in early September of
this year, and from the Voyager spacecraft (left) taken in 1979.
Prometheus (bright ring in upper right) was first seen as an erupting
volcano by the Voyager spacecraft and still features an active plume. A
smaller active plume was discovered at the volcano Culann Patera (dark
feature at lower left) by the Galileo spacecraft.
Prometheus has
displayed similar characteristics such as size, shape and brightness to
Galileo's cameras as it did to Voyager's. However, several intriguing
differences are also apparent. There appears to be a new dark lava flow
emanating from the vent of Prometheus, and the plume is now erupting from
a position about 75 kilometers (46.5 miles) west from where the hot spot
resided in 1979. It is not known if the plume source is the same or if the
plume is now emanating from a new source. Overall, scientists studying
Galileo images of Io see that a wide variety of surface changes have
occurred on Io since 1979. The Galileo image was taken at a range of about
487,000 kilometers (about 302,000 miles) from Io. The Voyager image was
taken from about 800,000 kilometers (about 500,000 miles).
(Courtesy NASA/JPL)
NIMS Image of Io - First Encounter
Io has been imaged by the Near Infrared Mapping
Spectrometer (NIMS) on Galileo. The image on the right shows for the first
time the distribution of volcanic hotspots on the surface of Io, as seen
by NIMS. Three of these hotspots are new discoveries, only detectable with
the NIMS instrument. This image was taken during the First Ganymede encounter on June
29, 1996. The image on the left shows the same view of Io as seen by the
Voyager spacecraft in 1979. At least one dozen hotspots have been
identified from this NIMS image. Most of the hotspot locations can be
matched with volcanic features on the surface of Io, including the vent
area of the active Prometheus plume.
(Courtesy NASA/JPL)
NIMS Image of Io - Second Encounter
The Near Infrared Mapping Spectrometer (NIMS) on the
Galileo spacecraft imaged Io at high spectral resolution at a range of
439,000 km (275,000 miles) during the second Ganymede encounter on 6 September 1996.
This image shows, on the right, Io as seen by NIMS, centered on 220 W
longitude. The image on the left shows the same view point from Voyager
data (from the encounters in 1979 and 1980). The NIMS image can be
compared to the above NIMS hotspot image (from the First orbit on June 1996) to
monitor changes on Io. The most dramatic feature of this image is the
hotspot at Malik Patera. Preliminary analysis of the data yields a
temperature of at least 1000 K (727 C) for this hotspot, an increase of
more than 300 K from the first encounter. In the overlap area of the first and
second images all the hotspots seen during the first encounter are also seen in
the second image. Other hotspots were seen, including one at the Pele plume
origin site. This image is at the 4 micron band to best view the Malik
hotspot. Most of the other hotspots are best seen at longer wavelengths.
NIMS is continuing to observe Io to monitor volcanic activity throughout
the Galileo mission.
(Courtesy NASA/JPL)
Distribution of Sulfur Dioxide Frost on Io
Sulfur dioxide, normally a gas at room temperatures, is known to
exist on Io's surface as a frost, condensing there from the hot gases
emanating from the Io volcanoes. However, the deposition patterns and
relation of the frost distribution to the volcanic activity is
unknown, since prior measurements lacked the spatial resolution to
accurately map the surface frost.
The Galileo Near Infrared Mapping Spectrometer (NIMS) obtained relatively high spatial and spectral resolution images during the C3 orbit, and the characteristic infrared absorptions of sulfur dioxide frost appearing in the spectra were used to produce the SO2 frost map shown on the right. The comparison image on the left (from 1979 Voyager measurements) shows the same view and indicates the surface brightness as seen in visible light.
The frost map shows maximum SO2 concentration as white, lesser amounts as blue coloration, and areas with little or no SO2 as black. The resolution of this map is about 120 km (75 miles), which spans the latitude range 120 W to 270 W.
It is interesting to compare this frost distribution with regions of volcanic activity. Volcanic hotspots identified from NIMS and SSI images occur in many of the dark - low SO2 - areas, a reasonable finding since sulfur dioxide would not condense on such hot regions. The Pele region (to the lower left), N. Colchis hot spots (upper center) and S. Volund (upper right) are good examples of hot spot areas depleted in sulfur dioxide. Much of the rest of this hemisphere of Io has varying amounts of sulfur dioxide present. The most sulfur dioxide-rich area is Colchis Regio, the white area to the right of center.
Of particular interest is the dark area to the south of Colchis Regio. From
the study of other NIMS images, it is seen that this region does not have any
large, obvious hotspots. However, it is depleted in sulfur dioxide.
(Courtesy NASA/JPL)
Image of Ionian Volcano Pele
These frames detail the changes around Pele on
Io, as seen by Voyager 1 (left), Voyager 2 (middle), and
Galileo (right). The Voyager frames were taken in 1979 when the two
spacecraft flew past Jupiter and it's moon Io. The Galileo view was
obtained in June, 1996. Note the changes in the shape of the deposits
further from the vent while the radial dark features closer to the vent
show little change. The Voyager images use orange, blue, and violet
filters. The Galileo image uses the green and violet filters of the Solid
State Imaging system aboard the Galileo spacecraft and a synthetic blue.
All three images are in a simple cylindrical projection and are
approximately 1700 km x 1500 km. North is to the top. A high-resolution
image of Pele can be found on the
Io page.
Geologic Landforms on Io
Shown here is one of the highest-resolution images of
Io (Latitude: -60 to +20 degrees, Longitude: 150 to 230 degrees) acquired
by the Galileo spacecraft, revealing a great variety of landforms. There
are rugged mountains several miles high, layered materials forming
plateaus, and many irregular depressions called volcanic calderas.
Similar landforms were seen near Io's south pole by the Voyager
spacecraft, but Galileo has revealed that such landforms are ubiquitous.
Several of the dark, flow-like features correspond to hot spots, and may
be active lava flows. There are no landforms resembling impact craters,
as the volcanism covers the surface with new deposits much more rapidly
than the flux of comets and asteroids can create large impact craters.
North is to the top of the picture and the sun illuminates the surface
from the left. The image covers an area 2000 kilometers wide and the
smallest features that can be discerned are 2.5 kilometers in size. This
image was taken on November 6th, 1996, at a range of 245,719 kilometers
by the Solid State Imaging (CCD) system on the Galileo Spacecraft.
Volcanically Active Regions on Io
Shown here is a portion of one of the highest-resolution
images of Io (Latitude: +10 to +60 degrees, Longitude: 180 to
225 degrees) acquired by the Galileo spacecraft, revealing immense lava
flows and other volcanic landforms. Several high-temperature volcanic hot
spots have been detected in this region by both the Near Infrared Mapping
Spectrometer and the imaging system of Galileo. The temperatures are
consistent with active silicate volcanism in lava flows or lava lakes
(which reside inside irregular depressions called calderas). The large
dark lava flow in the upper left region of the image is more than 400 km
long, similar to ancient flood basalts on Earth and mare lavas on the
Moon.
North is to the top of the picture and the sun illuminates the
surface from the left. The image covers an area 21230 kilometers wide and
the smallest features that can be discerned are 2.5 kilometers in size.
This image was taken on November 6th, 1996, at a range of 245,719
kilometers by the Solid State Imaging (CCD) system on the Galileo
Spacecraft.
Jupiter