Jupiter's Moons
Callisto
Callisto is the outermost satellite of
Jupiter's four largest moons.It revolves around Jupiter in 16.67 days at a distance of 15
planetary radii. Callisto's diameter is 4800 km, almost the size of the planet, Mercury.
Its density is 1.8 gm/cc. This low d ensity rules out the possibility of it this satellite
to be made of rock. Callisto formed in a frozen environment far away from the sun, which
allows us to believe that water ice may have been a major component of the material that
condensed to form this body. The Surface of Callisto Voyager cameras have revealed a
surface that is dark but filtered images have shown that the dark regions were more red
than the lighter ones. The most unique feature of the surface was a large bulls eye
structure, with a bri ght center about 600 km in diameter. This central region was
surrounded by rings separated by about 50-200 km and extending 2000 km.
Callisto's crust reacts differently to large impacts
than Mercury or our moon. Just as in all cases, kinetic energy must be deposited
instantaneously at an impact site, but the post impact recoveries differ.
Europa:
Europa's diameter is 3130 km and
compared to our moon, it is much brighter with a 64 percent reflectivity compared
to the moon's 7 percent. The ground based infrared spectra has shown that there is a lot
of water ice present on the surface.The Surface of Europa: A detailed inspection has shown
that there are fewer than twelve circular craters that have been formed from meteoric
impacts. This information leads to the belief that Europa's surface is mobile and can
erase all but the most recent impacts. There are
several types of terrain on the surface of Europa. THese include plains cut by grooves and
ridges. The mottled terrains are either brown or gray. The brown tint is due to angular
structures and vertical ridges. The gray regions show less distin ction.
There are many linea (linear grooves) and macula
(brown regions). This satellite consists of a rock core covered by a crust of ice.
Fractures that have formed may be the result of tidal stresses.
Ganymede
Ganymede is the largest satellite in the solar system. It
has a diameter of 5260 km and is larger than Mercury. It revolves around Jupiter at a
distance of 9.5 planetary radii, with a period of 7.16 days. A large amount of water ice
is present of the sur face. Ganymede can reflect about 44 percent of the light on its
surface which indicates that it is covered by some dark material. The average density of
Ganymede is 1.9 gn/cc.
Ganymede is heavily cratered and has regions with
ridges or faults with a structure similar to a bulls eye. This region is names the Galieo
Regio. There are many trends among the craters. The larger the diameter, the flatter is
the appearance. Small crat ers are often times bowl like and similar to those of our own
moon. The other most common terrain of Ganymede is less cratered and more reflective.
Other features include impact basins, ray craters, etc.
Io
Io has many structures of all sizes, colors, and textures. When
viewed by the Voyager, none of these structures were identified as an impact crater.
Instead, features were seen that appeared to have been formed by local fluid motion.
Io has many volcanic eruptions, some of which extend
300 km above the surface. When viewed by the spacecraft there were 9 erupting volcanoes
all concentrated towards the equator. A detection of water ice or even steam on Io was not
found. There were spectral lines of doubly ionized sulphur. Sulphur was abundant on the
surface, and because of this the surface of Io seems white.