urcht
und Schreckenurcht
und Schrecken
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Der erste der beiden Marssatelliten ist Phobos. Der
kartoffelförmige Körper
ist überseht von tiefen Kratern. Der Größte, Stickney,
ist sogar 10 km breit. Darauf folgen die Krater Hall
und Roch, welche 5 km lang sind. Die scheinbare Helligkeit
beträgt 11, 5 m und ist damit erst in großen Teleskopen
zu sehen.
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Der kleinere Deimos hat etwa die gleichen Strukturen
wie Phobos, nur sind sie nicht ganz so stark ausgeprägt
(Krater, Furchen).
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Mars I
Phobos (FOH bus) is the larger and innermost
of Mars' two moons. Phobos is closer to its primary than any
other moon in the solar system, less than 6000 km above the surface
of Mars. It is also one of the smallest
moons in the solar system.
orbit:
9378 km from the center of Mars diameter:
22.2 km (27 x 21.6 x 18.8) mass:
1.08e16 kg
In Greek mythology, Phobos is one of the sons
of Ares (Mars) and Aphrodite (Venus). phobos is Greek for fear
(the root of phobia). Discovered 1877 August 12 by Hall;
photographed by Mariner
9 in 1971, Viking
1 in 1977, and Phobos
in 1988. 
Phobos orbits Mars below the synchronous
orbit radius. Thus it rises in the west, moves very rapidly
across the sky and sets in the east, usually twice a day. It
is so close to the surface that it cannot be seen above the horizon
from all points on the surface of Mars. And Phobos is doomed:
because its orbit is below synchronous altitude tidal forces
are lowering its orbit (current rate: about 1.8 meters per century).
In about 50 million years it will either crash onto the surface
of Mars or (more likely) break up into a ring. (This is the opposite
effect to that operating to raise the orbit of the Moon.)
Phobos and Deimos
may be composed of carbon-rich rock like C-type
asteroids. But their densities are so low that they cannot be
pure rock. They are more likely composed of a mixture of rock
and ice. Both are heavily cratered. New images from Mars Global
Surveyor indicate the Phobos is covered with a layer of fine
dust about a meter thick, similar to the regolith
on the Earth's Moon. 
The Soviet spacecraft Phobos
2 detected a faint but steady outgassing from Phobos. Unfortunately,
Phobos 2 died before it could determine the nature of the material;
water is the best bet. Phobos 2 also returned a few images (right).
The most prominent feature on Phobos is the large crater named
Stickney, the maiden name of Hall's wife (above). Like Mimas'
crater Herschel (on a smaller scale) the impact that created
Stickney must have almost shattered Phobos. The grooves and streaks
on the surface were probably also caused by the Stickney impact.
Phobos and Deimos are widely believed to be captured asteroids.
There is some speculation that they originated in the outer solar
system rather than in the main asteroid belt. Phobos and Deimos
may someday be useful as space stations from which to study Mars
or as intermediate stops to and from the Martian surface; especially
if the presence of ice is confirmed.
more
images
from LANL
© Bill
Arnett; last updated: 1998 Sep 11
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