GALILLEAN MOONS
This composite includes the four largest moons of Jupiter which are known as the Galilean satellites. From left to right, the moons shown are Ganymede, Callisto, Io, and Europa. The Galilean satellites were first seen by the Italian astronomer Galileo Galilei in 1610. In order of increasing distance from Jupiter, Io is closest, followed by Europa, Ganymede, and Callisto.
The order of these satellites from the planet Jupiter helps to explain some of the visible differences among the moons. Io is subject to the strongest tidal stresses from the massive planet. These stresses generate internal heating which is released at the surface and makes Io the most volcanically active body in our solar system. Europa appears to be strongly differentiated with a rock/iron core, an ice layer at its surface, and the potential for local or global zones of water between these layers. Tectonic resurfacing brightens terrain on the less active and partially differentiated moon Ganymede. Callisto, furthest from Jupiter, appears heavily cratered at low resolutions and shows no evidence of internal activity.
The Solid State Imaging (CCD) system aboard NASA's Galileo spacecraft obtained the Io and Ganymede images in June 1996, while the Europa images were obtained in September 1996. Because Galileo focusses on high resolution imaging of regional areas on Callisto rather than global coverage, the portrait of Callisto is from the 1979 flyby of NASA's Voyager spacecraft.
Launched in October 1989, the spacecraft's mission is to conduct detailed studies of the giant planet, its largest moons and the Jovian magnetic environment. The Jet Propulsion Laboratory, Pasadena, CA, manages the mission for NASA's Office of Space Science, Washington, DC.
Astronomers say they have found even more evidence that Jupiter's moon Europa, and perhaps a second moon, Callisto, have liquid oceans under their icycrusts.
They said the Galileo spacecraft that has been orbiting Jupiter since 1995 had picked up data showing Europa and Callisto disturbed the planet's magnetic fields. Since neither moon has a magnetic field of its own, the effect is puzzling. One good explanation, Krishan Khurana and colleagues from the University of California in Los Angeles said in the journal Nature, is that liquid salty oceans are sloshing around under the ice, causing the formation of electromagnetic fields.
Scientists believe there is a chance life may exist or may once have existed in such subsurface oceans. ``One of the conditions which must be fulfilled for the origin of life is fulfilled on these satellites,'' Fritz Neubauer, a geophysicist of the University of Koln in Germany, said in a telephone interview. In an accompanying commentary in Nature, Neubauer agreed with Khurana that the likeliest explanation for the electromagnetic induction was liquid water. The Galileo data supports geological evidence of underground water on Europa. Electromagnetic induction techniques are important tools for geophysicists. Neubauer said an underground ocean on Europa would not be unexpected, but one on Callisto would be a completesurprise.
One of the key findings of Galileo's mission was the discovery of volcanic ice flows and melting of ice on the surface of Europa that supported the premise of liquid oceans underneath, according to NASA. It also discovered an atmosphere of hydrogen and carbondioxide on Callisto.
OUR MOON
Explanation:
Locked in synchronous rotation, the Moon always presents its well-known near side to Earth. But from lunar orbit, Apollo astronauts also grew to know the Moon's far side. This sharp picture from Apollo 16's mapping camera shows the eastern edge of the familiar near side (left) and the strange and heavily cratered far side of the Moon. Surprisingly, the rough and battered surface of the far side looks very different from the near side which is covered with smooth dark lunar maria. The likely explanation is that the far side crust is thicker, making it harder for molten material from the interior to flow to the surface and form the smooth maria.
MOONS OF MARS
Phobos [FOH-bohs] (fear) is a moon of Mars and was named after an attendant of the Roman war god Mars. Phobos is a dark body that appears to be composed of C-type surface materials. It is similar to the C-type (blackish carbonaceous chondrite) asteroids that exist in the outer asteroid belt. Some scientists speculate that Phobos and Mars' other moon, Deimos,are captured asteroids,this is what I believe. However, other scientists point to evidence that contradicts this theory. Phobos shows striated patterns which are probably cracks caused by the impact event of the largest crater on the moon.
