Asteroids
and the Mars Satellites
Asteroids
The asteroids are bodies that are predominantly located between the
orbits of Mars and Jupiter. However, there are
asteroids that orbit the Sun inside the Earth's orbit (the Atens).
Origin
Apparently, the asteroids and their ancestors should be planetesimals similar to those ones that
formed the other planets of the solar system.
However, these bodies were gravitationally disturbed until they assumed
tilted and noticeably elliptical orbits. Two processes could have caused this
disturbance:
- Jupiter could have scattered big
planetesimals into eccentric orbits, making them penetrate the region of
the asteroids before they were ejected out from the solar system. The
asteroids would have been accelerated and could have had their orbits
changed by the close transit of these bodies. During the impacts, the high velocity favoured
the mutual destruction, instead of favouring the accretion. The proto-planets or planetesimals would have been then
destroyed in mutual collisions or in collisions with the bodies that were
ejected by Jupiter.
- In another scenario, the Jupiter's gravity itself would accelerate the
planetesimals of the asteroids' regions, namely those occupying the
so-called Kirkwood gaps (resonant orbits).
Morphological Diversity
The biggest asteroid, Ceres, reaches a diameter of about 930 km and
represents more than 25% of the combined mass of all the asteroids. The shapes
of the asteroids unveil their violent history and vary between nearly spherical
forms and irregular or elongated forms. The rotation periods vary between some
few hours and several days. Satellites orbit some asteroids, as it happens with
Ida and Dactyl.
Composition and Geological Differentiation
There are several types of asteroids according to their compositions.
The chondrites are asteroids whose content of non-volatile elements reflects
the proportions that should prevail in the primordial nebula. Most chondrites
are rich in carbon. Others are clearly enriched with determined elements.
The presence of these elements at the surface of the asteroids would
have been manifested through the geological modification of the materials,
occurred during the planetary evolution processes. One of those processes would
have been the differentiation, which consisted in the melting and physical
segregation of the materials. Just as in the inner planets, the heavier
materials would have been sunk to form the core, while the lighter ones emerged
to form the crust. It remains a mystery the fact that some asteroids seem to
have been kept unchanged, while other asteroids of similar sizes and in nearby
orbits clearly seem to have been modified.
The asteroids that are composed by geologically modified minerals are
divided in classes like the irons, the stony-irons and the achondrites. The
composition of the asteroids would have been determined by their positions in
the primitive solar nebula. The asteroids that are rich in
high temperature minerals are essentially placed in zones close to the Sun,
while the asteroids that have a composition more similar to the comets' are
essentially placed at the outer regions.
The asteroid Ida and, to
the right, its satellite Dactyl (NASA - JPL)
One of the last pictures taken from the asteroid
Eros, in February 2001. The image is only 12 metres across (NEAR Project -
JHU/APL)
The Mars
Satellites
The satellites of Mars have tiny
dimensions are irregularly shaped. Phobos, the biggest among them, displays
perpendicular axis of 27 x 21 x 19 km. It is thought that, in remote times,
Phobos and Deimos belonged to the asteroid belt, both displaying similar
characteristics with the asteroids classified as carbonic chondrites.
It's possible that Mars further captured them, but it is also possible
that they were members of the planetesimals cluster that ended up
forming the red planet.
The Phobos satellite (NASA)
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