Callisto

Callisto is the most heavily cratered object in the solar system. It is thought to be a long dead world, with a nearly complete absence of any geologic activity on its surface. In fact, Callisto is the only body greater than 1000 km in diameter in the solar system that has shown no signs of undergoing any extensive resurfacing since impacts have molded its surface. With a surface age of about 4 billion years, Callisto has the oldest landscape in the solar system.

Callisto is the outermost of Jupiter's four planet-sized moons and is dominated by impact craters. Despite this, a few more interesting features are also visible, including gigantic impact basins, suggestions of ancient volcanism, and a few ancient tectonic features. Among the most interesting features on Callisto are impact scars from tidally disrupted comets. Callisto is nearly as large as the planet Mercury. Its bulk density, however, is only ~1.8 grams per cubic centimeter, or only twice that of water ice. This indicates that the interior is approximately half water ice as well. Callisto is very similar in bulk properties to Ganymede, but apparently had a much simpler geologic history. The different geologic histories of these two bodies has been an important problem for planetary scientists, which may be related to the orbital and tidal evolution of Ganymede.

At a global distance, Callisto is dominated by bright spots. At high resolution, these spots are revealed to be fresh, young impact craters and their ejecta patterns. Callisto is relatively brownish in color, possibly due to the contamination of the icy surface by meteoritic material.

Compared to Ganymede

Both Callisto and Ganymede are covered by huge tracts of relatively dark, heavily cratered terrains, shown here at similar image resolution and solar illumination. Ancient dark terrain on Ganymede is complex. Numerous parallel troughs (furrows) are probably part of a vast impact structure, similar to the rings observed at Valhalla (see below). Numerous scarps, smooth dark features, and thick crater-floor domes occur throughout dark terrain on Ganymede and indicate a complex geologic history. Ancient dark terrain on Callisto appears to have had a simpler geologic history. Scarps, furrows, and domes are rarely seen.

Callisto Quick-Look Statistics

Discovery: Jan 7, 1610 by Galileo Galilei
Diameter (km): 4,806
Mass (kg): 1.077e23
Mass (Earth = 1) 1.807e-02
Surface Gravity (Earth = 1): 0.127
Mean Distance from Jupiter (km): 1,883,000
Mean Distance From Jupiter (Rj): 26.6
Mean Distance from Sun (AU): 5.203
Orbital period (days): 16.68902
Rotational period (days): 16.68902
Density (gm/cm?3) 1.86
Orbit Eccentricity: 0.007
Orbit Inclination (degrees): 0.281
Orbit Speed (km/sec): 8.21
Escape velocity (km/sec): 2.45
Visual Albedo: 0.19
Subsolar Temperature (K) 168
Equatorial Subsurface Temperature (K) 126
Surface Composition: Dirty Ice

Comet Impacts

There are eight prominent crater chains on Callisto. They probably formed by the impact of comets that were tidally disrupted by the planet Jupiter, like the split comet Shoemaker-Levy 9 in 1992-1994. The observed number of chains suggests that comets are disrupted by Jupiter an average of once per 275 years or so. Gomul Catena is ~350 kilometers long, with craters up to 25 kilometers wide (scale bar is 100 kilometers long). The arcuate scarps are part of the vast Valhalla multiring impact basin.

Gipul Catena

This and other unusual crater chains on Callisto formed by the impact of tidally disrupted comets. This chain is ~620 kilometers long, with craters up to 40 kilometers across (scale bar is 100 km long). Gipul Catena is located near the north pole of Callisto.

Several similar crater chains have also been identified on Ganymede.

Ancient History

Dominating the scene is the impact structure, Asgard, centered on the smooth,
bright region near the middle of the picture and surrounded by concentric rings up to 1,700 kilometers (about 1,050 miles) in diameter. A second ringed structure with a diameter of about 500 kilometers (310 miles) can be seen to the north of Asgard,
partially obscured by the more recent, bright-rayed crater, Burr. The icy materials excavated by the younger craters contrast sharply with the darker and redder coatings on older surfaces of Callisto.

Landslides

Recent Galileo images of the surface of Jupiter's moon Callisto have revealed large landslide deposits within two large impact craters seen in the right side of this image. The two landslides are about 3 to 3.5 kilometers (1.8 to 2.1 miles) in length. They occurred when material from the crater wall failed under the influence of gravity, perhaps aided by seismic disturbances from nearby impacts. These deposits are interesting because they traveled several kilometers from the crater wall in the absence of an atmosphere or other fluids which might have lubricated the flow. This could indicate that the surface material on Callisto is very fine-grained, and perhaps is being "fluffed" by electrostatic forces which allowed the landslide debris to flow extended distances in the absence of an atmosphere.

NIMS

The Near Infrared Mapping Spectrometer (NIMS) acquired this global mosaic (right) at a spatial resolution of 100 km during Galileo's third orbit on November 4, 1996, roughly 7.5 hours prior to Callisto closest approach.

The lighter bluish area in the upper latitudes is the Asgard multi-ring structure (the second largest surface feature on Callisto) with crater Burr to the north and Tornasuk to the east. The bluish color indicates regions with more exposed water ice while the reddish/rusty color indicates surface areas rich in non-ice minerals.

....
exhibits a greater abundance of water ice compared with the surrounding region. This may be due to impact excavation revealing a more ice-rich subsurface and suggesting that the darker material is a relatively thin surface covering. This covering could be either impact debris material or a lag deposit of existing material from which the ice has evaporated away.

SSI & NIMS

The data from the two instruments has been mosaicked to produce this unique view. Related releases and detailed captions are available for the NIMS and SSI products.

These false color images show surface compositional differences, red = more ice,
blue = less ice.

The upper left view contains Buri, a crater with a diameter of about 60 km. In the infrared spectrum, Buri and the rays that extend from the crater have high abundance of water ice compared to the surrounding region. The center view, a large (200 km or 120 mile diameter) unnamed impact crater with a distinct ring or circle around it reveals a complex mix of ice and non-ice materials. This is possibly due to impact excavation of the ice-rich subsurface which suggests that the darker material is just a thin surface covering caused by impact debris or a lag deposit from which the ice has evaporated away. The infrared data shows spectral signatures for both sulfur and carbon as two potential materials which could play a part in the complicated make-up of Callisto's surface.

Crater chain blowout

On a global scale, Callisto is heavily cratered, indicating the great age of its surface. At the scale of this image, it was anticipated that the surface would be heavily cratered as well; however, there is a surprising lack of small craters, suggesting that one or more processes have obliterated these and other small-scale features. For example, downslope movement of ice-rich debris could bury small craters. The bright slopes visible in this picture represent places where downslope movement has taken place, exposing fresh ice surfaces.

Crater pic

The 105 kilometer (64 mile) double ring crater in the center of the image is named Har. Har displays an unusual rounded mound on its floor. The origin of the mound is unclear but probably involves uplift of ice-rich materials from below, either as a "rebound" immediately following the impact that formed the crater or as a later process. Har is older than the prominent 41 kilometer (25 mile) crater superposed on its western rim. The large crater partially visible in the northeast corner of the image is called Tindr. Chains of secondary craters (craters formed from the impact of materials thrown out of the main crater during an impact) originating from Tindr crosscut the eastern rim of Har.

Smooth Craters

The smoothness of the plains appears to increase toward the south pole, approximately 480 kilometers (293 miles) south of the bottom of the image. This smoothness of Callisto's surface was not evident in images taken during the 1979 flyby of NASA's Voyager spacecraft because the resolution was insufficient to show the effect. This smooth surface, and the process(es) that cause it, are among the most intriguing aspects of Callisto. Although not fully understood, the process(es) responsible for this smoothing could include erosion by tiny meteorites and energetic ions. Some craters, such as Keelut, the 47 kilometer (29 mile) crater in the lower right corner, have sharp, well defined rims. Keelut contains an inner ring surrounding a central depression about 17 kilometers (11 miles) in diameter. Keelut, and the more irregularly shaped, degraded Reginleif, the 32 kilometer (19.5 mile) crater in the top center of the image, are very shallow and have flat floors.

Fields and Particles

Of the four Galilean satellites, Callisto appears to have the weakest interaction with the Jovian magnetosphere. This is perhaps due to its relatively large distance from the planet (about 25 Jovian radii), or perhaps due to its surface characteristics.

Electric Field Spectrogram

The Callisto interaction appears to consist of a number of broadband, bursty electrostatic emissions, although one cannot rule out the possibility that these bursts are associated with the ambient magnetosphere. The brief high-frequency bursts are thought to be upper hybrid resonance bands indicating a plasma density of about 100 per cubic centimeter. Since this is much larger than the surrounding Jovian plasma density, this feature suggests that Callisto is a source of plasma, although not nearly as prolific as Io!

Series

The top left frame is scaled to 10 kilometers (km) per picture element (pixel) and covers an area about 4400 by 2500 km. The moon Callisto, which has a diameter of 4806 km, appears to be peppered with many bright spots. Images at this resolution of other cratered moons in the Solar System indicate that the bright spots could be impact craters. The ring structure of Valhalla, the largest impact structure on Callisto, is visible in the center of the frame. This color view combines images obtained in November 1997 taken through the green, violet, and 1 micrometer filters of the SSI system.

The top right frame is ten times higher resolution (about 1 km per pixel) and covers an area approximately 440 by 250 km. Craters, which are clearly recognizable, appear to be the dominant landform on Callisto. The crater rims appear bright, while the adjacent area and the crater interiors are dark. This resolution is comparable to the best data available from the 1979 flyby's of NASA's two Voyager spacecraft; it reflects the understanding of Callisto prior to new data from Galileo. This Galileo image was taken in November 1996.

The resolution of the bottom right image is again ten times better (100 meters per pixel) and covering an area of about 44 by 25 km. This resolution reveals that some crater rims are not complete rings, but are composed of bright isolated segments. Steep slopes near crater rims reveal dark material that appears to have slid down to reveal bright material. The thickness of the dark layer could be tens of meters. The image was taken in June 1997.

The bottom left image at about 29 meters per pixel is the highest resolution available for Callisto. It covers an area about 4.4 by 2.5 km and is somewhat oblique. Craters are visible but no longer dominate the surface. The image was taken in November 1996.