Deborah & William Hillyard
Deborah & William Hillyard
Deborah & William Hillyard
Deborah & William Hillyard
Deborah & William Hillyard
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Solar System -
Jupiter's Moons
Callisto
Callisto's composition is approximately 49% to 55% ices, with the rest as rock. It is thought to have formed over a period of up to 10 million years from the accretion disk that surrounded Jupiter. Because of its slow formation, and the lack of tidal heating, not enough heat was generated to melt the ice and allow Callisto to have a differentiated interior. The slow convection in the interior did permit some partial differentiation, so there may be an ocean beneath the crust as well as a small, rocky core, though the bulk of the interior is a mixture of rock and ices, with the density of the rock increasing with increasing depth. Once formed, Callisto evolved by the balance of the radioactive heating in the interior, cooling through thermal conduction near the surface, and convection in the interior. The convection effect is a very slow process, but extremely effective over timescales of hundreds of millions to billions of years.
While lacking any real mountains, there are two huge impact craters on Callisto. Valhalla is about 3,800 km across and Asgard is 1,600 km.
Spectroscopic analysis of the surface has revealed water ice, carbon dioxide, silicates, sulfur dioxide, and possibly ammonia, together with organic compounds. The lack of tidal heating means there has been little volcanism or plate tectonics, so the surface is very old, and shows evidence of heavy cratering. Unlike the other Galilean moons, Callisto's leading hemisphere (the one facing the direction of its orbit) is darker than the trailing hemisphere. The leading hemisphere appears high in sulfur dioxide, while the trailing one is high in carbon dioxide.
Callisto supports a very tenuous atmosphere of carbon dioxide and oxygen. The low gravity of Callisto means the atmosphere would be lost in a matter of days if it were not continually replenished by slow sublimation of carbon dioxide ice from the crust. While oxygen has yet to be detected in the atmosphere, the Hubble Space Telescope has found oxygen trapped within the surface.
Callisto orbits Jupiter at a distance of between 1,869,000 km and 1,897,000 km once every 16.689 days, which is also its rotational period. It is 4,820 km in diameter which is only about 60 km less than the planet Mercury, but because of its low density, it only weighs a little less than one third of Mercury's weight. Of the Jovian moons, only Ganymede is larger.
While lacking any real mountains, there are two huge impact craters on Callisto. Valhalla is about 3,800 km across and Asgard is 1,600 km.
Spectroscopic analysis of the surface has revealed water ice, carbon dioxide, silicates, sulfur dioxide, and possibly ammonia, together with organic compounds. The lack of tidal heating means there has been little volcanism or plate tectonics, so the surface is very old, and shows evidence of heavy cratering. Unlike the other Galilean moons, Callisto's leading hemisphere (the one facing the direction of its orbit) is darker than the trailing hemisphere. The leading hemisphere appears high in sulfur dioxide, while the trailing one is high in carbon dioxide.
Callisto supports a very tenuous atmosphere of carbon dioxide and oxygen. The low gravity of Callisto means the atmosphere would be lost in a matter of days if it were not continually replenished by slow sublimation of carbon dioxide ice from the crust. While oxygen has yet to be detected in the atmosphere, the Hubble Space Telescope has found oxygen trapped within the surface.
Callisto orbits Jupiter at a distance of between 1,869,000 km and 1,897,000 km once every 16.689 days, which is also its rotational period. It is 4,820 km in diameter which is only about 60 km less than the planet Mercury, but because of its low density, it only weighs a little less than one third of Mercury's weight. Of the Jovian moons, only Ganymede is larger.
A view of the hemisphere pointing away from Jupiter taken in May 2001 by the Galileo probe. The bright areas are probably surface ice. Asgard is the large impact structure is just visible on the upper right; it is about 1,600 km across. The prominent crater with rays coming out about a third of the way up and just right of center is called Bran.
The Valhalla crater. The central bright region is about 600 km across, while at its maximum across the surrounding ring structure, it is about 3,800 km across. The image was taken by Voyager 1 in 1979.
The upper of these two images shows the crater Har on the left of, and below, center. It is 50 km (30 miles) across with an unusual central rounded mound probably caused by material from below being forced upwards. On its left edge is a newer crater about 20 km (12 miles) across.
Partially visible in the top right corner of the image is the crater Tindr. This is shown in more detail in the lower picture that was also taken by Galileo.
Partially visible in the top right corner of the image is the crater Tindr. This is shown in more detail in the lower picture that was also taken by Galileo.
Below Callisto's surface there is an ice layer, shown here in white. It is about 200 kilometers (124 miles) thick. The next thinner blue layer is a possible ocean. Based on information obtained from Galileo, the liquid layer must be at least 10 km (6 miles) thick. The bulk of the interior is a rock/ice mixture.
Images
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The Asgard impact basin. The center is the bright region near the middle of the picture, and it is surrounded by concentric rings to about 1,600 km across, with many small craters superimposed on it. The very bright areas (top & right) are more recent craters where the ejected material is ice. A Galileo image from 1997. Credit: NASA/JPL/University of Arizona