Solar System, technical/Callisto



At right is a complete global color image of Callisto.

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Planetary science
Bright scars on a darker surface testify to a long history of impacts on Jupiter's moon Callisto. The picture, taken in May 2001, is the only complete global color image of Callisto obtained by Galileo, which has been orbiting Jupiter since December 1995. Of Jupiter's four largest moons, Callisto orbits farthest from the giant planet. Callisto's surface is uniformly cratered but is not uniform in color or brightness. Scientists believe the brighter areas are mainly ice and the darker areas are highly eroded, ice-poor material.

"Callisto rotates synchronously with its orbital period, so the same hemisphere always faces (is tidally locked to) Jupiter. Callisto's surface is less affected by Jupiter's magnetosphere than the other inner satellites because it orbits farther away. "

"Compounds detected spectroscopically on the surface include water ice, carbon dioxide, silicates, and organic compounds."

Electron astronomy
"Callisto's ionosphere was first detected during Galileo flybys; its high electron density of 7–17 x 104 cm&minus;3 cannot be explained by the photoionization of the atmospheric carbon dioxide alone."

Optical astronomy
"The analysis of high-resolution, near-infrared and UV spectra obtained by the Galileo spacecraft and from the ground has revealed various non-ice materials: magnesium- and iron-bearing hydrated silicates, carbon dioxide, sulfur dioxide, and possibly ammonia and various organic compounds. Spectral data indicate that the moon's surface is extremely heterogeneous at the small scale. Small, bright patches of pure water ice are intermixed with patches of a rock–ice mixture and extended dark areas made of a non-ice material. "

Visual astronomy
"Callisto's surface has an albedo of about 20%. ... The Callistoan surface is asymmetric: the leading hemisphere [The leading hemisphere is the hemisphere facing the direction of the orbital motion; the trailing hemisphere faces the reverse direction.] is darker than the trailing one. This is different from other Galilean satellites, where the reverse is true. "

Infrared astronomy
"Near-infrared spectroscopy has revealed the presence of water ice absorption bands at wavelengths of 1.04, 1.25, 1.5, 2.0 and 3.0 micrometers. "

"Callisto has a very tenuous atmosphere composed of carbon dioxide. It was detected by the Galileo Near Infrared Mapping Spectrometer (NIMS) from its absorption feature near the wavelength 4.2 micrometers. The surface pressure is estimated to be 7.5 x 10-12 bar (0.75 µPa) and particle density 4 x 108 cm&minus;3. Because such a thin atmosphere would be lost in only about 4 days (see atmospheric escape), it must be constantly replenished, possibly by slow sublimation of carbon dioxide ice from the satellite's icy crust, which would be compatible with the sublimation–degradation hypothesis for the formation of the surface knobs."

Crater astronomy
"The surface of Callisto is heavily cratered and extremely old. It does not show any signatures of subsurface processes such as plate tectonics or volcanism, and is thought to have evolved predominantly under the influence of impacts. Prominent surface features include multi-ring structures, variously shaped impact craters, and chains of craters (catenae) and associated scarps, ridges and deposits. At a small scale, the surface is varied and consists of small, bright frost deposits at the tops of elevations, surrounded by a low-lying, smooth blanket of dark material. "

"Many fresh impact craters like Lofn also show enrichment in carbon dioxide. "

"The ancient surface of Callisto is one of the most heavily cratered in the Solar System. In fact, the crater density is close to saturation: any new crater will tend to erase an older one. ... The impact craters and multi-ring structures—together with associated fractures, scarps and deposits—are the only large features to be found on the surface. "

"Callisto's surface can be divided into several geologically different parts: cratered plains, light plains, bright and dark smooth plains, and various units associated with particular multi-ring structures and impact craters. The cratered plains constitute most of the surface area and represent the ancient lithosphere, a mixture of ice and rocky material. The light plains include bright impact craters like Burr and Lofn, as well as the effaced remnants of old large craters called palimpsests, [In the case of icy satellites, palimpsests are defined as bright circular surface features, probably old impact craters; see Greeley et al. 2000. ] the central parts of multi-ring structures, and isolated patches in the cratered plains. These light plains are thought to be icy impact deposits. The bright, smooth plains constitute a small fraction of the Callistoan surface and are found in the ridge and trough zones of the Valhalla and Asgard formations and as isolated spots in the cratered plains."

"Impact crater diameters seen range from 0.1 km&mdash;a limit defined by the imaging resolution&mdash;to over 100 km, not counting the multi-ring structures. Small craters, with diameters less than 5 km, have simple bowl or flat-floored shapes. Those 5–40 km across usually have a central peak. Larger impact features, with diameters in the range 25–100 km, have central pits instead of peaks, such as Tindr crater. The largest craters with diameters over 60 km can have central domes, which are thought to result from central tectonic uplift after an impact; examples include Doh and Hár craters. A small number of very large&mdash;more 100 km in diameter&mdash;and bright impact craters show anomalous dome geometry. These are unusually shallow and may be a transitional landform to the multi-ring structures, as with the Lofn impact feature. Callistoan craters are generally shallower than those on the Moon."

"The largest impact features on the Callistoan surface are multi-ring basins. Two are enormous. Valhalla is the largest, with a bright central region 600 kilometers in diameter, and rings extending as far as 1,800 kilometers from the center (see figure). The second largest is Asgard, measuring about 1,600 kilometers in diameter. Multi-ring structures probably originated as a result of a post-impact concentric fracturing of the lithosphere lying on a layer of soft or liquid material, possibly an ocean. The catenae&mdash;for example Gomul Catena&mdash;are long chains of impact craters lined up in straight lines across the surface. They were probably created by objects that were tidally disrupted as they passed close to Jupiter prior to the impact on Callisto, or by very oblique impacts. "

Astrogeology
"The Galileo images also revealed small, dark, smooth areas with overall coverage less than 10,000 km2, which appear to embay [To embay means to shut in, or shelter, as in a bay.] the surrounding terrain. They are possible cryovolcanic deposits. Both the light and the various smooth plains are somewhat younger and less cratered than the background cratered plains. "

"[S]mall patches of pure water ice with an albedo as high as 80% are found on the surface of Callisto, surrounded by much darker material. High-resolution Galileo images showed the bright patches to be predominately located on elevated surface features: crater rims, scarps, ridges and knobs. They are likely to be thin water frost deposits. Dark material usually lies in the lowlands surrounding and mantling bright features and appears to be smooth. It often forms patches up to 5 km across within the crater floors and in the intercrater depressions. "

"On a sub-kilometer scale the surface of Callisto is more degraded than the surfaces of other icy Galilean moons. Typically there is a deficit of small impact craters with diameters less than 1 km as compared with, for instance, the dark plains on Ganymede. Instead of small craters, the almost ubiquitous surface features are small knobs and pits. The knobs are thought to represent remnants of crater rims degraded by an as-yet uncertain process. "

"The most likely candidate process is the slow sublimation of ice, which is enabled by a temperature of up to 165 K, reached at a subsolar point. Such sublimation of water or other volatiles from the dirty ice that is the bedrock causes its decomposition. The non-ice remnants form debris avalanches descending from the slopes of the crater walls. Such avalanches are often observed near and inside impact craters and termed "debris aprons".  Sometimes crater walls are cut by sinuous valley-like incisions called "gullies", which resemble certain Martian surface features. In the ice sublimation hypothesis, the low-lying dark material is interpreted as a blanket of primarily non-ice debris, which originated from the degraded rims of craters and has covered a predominantly icy bedrock."

"Recent Galileo images [top left] 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."

"This image was acquired on September 16th, 1997 by the Solid State Imaging (CCD) system on NASA's Galileo spacecraft, during the spacecraft's tenth orbit around Jupiter. North is to the top of the image, with the sun illuminating the scene from the right. The center of this image is located near 25.3 degrees north latitude, 141.3 degrees west longitude. The image, which is 55 kilometers (33 miles) by 44 kilometers (26 miles) across, was acquired at a resolution of 100 meters per picture element."

Views of eroding (top) and mostly eroded (bottom) ice knobs (~100 m high), possibly formed from the ejecta of an ancient impact are shown in the second images at left.

"The highest-resolution views ever obtained of any of Jupiter's moons, taken by NASA's Galileo spacecraft in May 2001, reveal numerous bright, sharp knobs covering a portion of Jupiter's moon Callisto."

"The knobby terrain seen throughout the top inset is unlike any seen before on Jupiter's moons. The spires are very icy but also contain some darker dust. As the ice erodes, the dark material apparently slides down and collects in low-lying areas. Over time, as the surface continues to erode, the icy knobs will likely disappear, producing a scene similar to the bottom inset. The number of impact craters in the bottom image indicates that erosion has essentially ceased in the dark plains shown in that image, allowing impact craters to persist and accumulate."

"The knobs are about 80 to 100 meters (260 to 330 feet) tall, and they may consist of material thrown outward from a major impact billions of years ago. The areas captured in the images lie south of Callisto's large Asgard impact basin."

"The smallest features discernable in the images are about 3 meters (10 feet) across."

Atmospheric astronomy
"Callisto is surrounded by an extremely thin atmosphere composed of carbon dioxide and probably molecular oxygen, as well as by a rather intense ionosphere. "

"Callisto has a very tenuous atmosphere composed of carbon dioxide. It was detected by the Galileo Near Infrared Mapping Spectrometer (NIMS) from its absorption feature near the wavelength 4.2 micrometers. The surface pressure is estimated to be 7.5 x 10-12 bar (0.75 µPa) and particle density 4 x 108 cm&minus;3. Because such a thin atmosphere would be lost in only about 4 days (see atmospheric escape), it must be constantly replenished, possibly by slow sublimation of carbon dioxide ice from the satellite's icy crust, which would be compatible with the sublimation–degradation hypothesis for the formation of the surface knobs."

"[O]xygen has not yet been directly detected in the atmosphere of Callisto. Observations with the Hubble Space Telescope (HST) placed an upper limit on its possible concentration in the atmosphere, based on lack of detection, which is still compatible with the ionospheric measurements. At the same time HST was able to detect condensed oxygen trapped on the surface of Callisto. "

"Callisto's ionosphere was first detected during Galileo flybys; its high electron density of 7–17 x 104 cm&minus;3 cannot be explained by the photoionization of the atmospheric carbon dioxide alone. Hence, it is suspected that the atmosphere of Callisto is actually dominated by molecular oxygen (in amounts 10–100 times greater than CO2). "