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The early planetary
accretion phase last only a few million years. It is succeeded
by an even more violent period when growing bodies encounter each
other at increasingly high velocities, boosted by mutual
gravitational interactions. This phase lasts for another 100-200 million years, until
all remaining bodies have been swept up by the new planets. Collisions
occur in a random fashion and involve objects of different masses,
structures and composition and moving at different speeds. Thus, this phase of planet formation must not be viewed as a
monotonous process by which material is incrementally added to a
growing planet. Instead, accretion should rather be understood as a
long chain of random ("stochastic") events in which violent
disruptions eventually are exceeded by non-disruptive infall
(accretion) of matter onto the largest planetary bodies in the
system. The so-called giant impacts in which proto-planets of
comparable size collide represent the ultimate in violence during
planetary accretion. While they can lead to the total destruction of
the planets involved, they can also leave big scars, still visible on
some of the planets and moons in the solar system. They constitute the
best evidence we now have of such a violent past. The Earth's moon,
for example, is believed to originate from the debris that
was ejected after such a giant impact on the Earth. This material was
subsequently "re-assembled" in an orbit around the Earth to become the
Moon we see today. Computer simulations of both impact and re-accumulation have not
only shown that such a scenario is possible, but have also made it
today's favorite theory of lunar origin. Studies of the lead and tungsten
isotopic composition of the silicate Earth have even allowed the dating of
that giant impact to about 50 million years after the beginning of
the Solar System! Mercury's anomalous composition can also be explained in
terms of a giant impact which ejected most of the mantle of the
planet, essentially leaving the iron core behind. A similar event
might have caused the large obliquity of Uranus; compared to
the other major planets, its rotation axis is tilted by nearly
90°. Giant impacts may explain many individual planetary characteristics
as the outcome of a general process, rather than as the result of
unique and special local conditions. This mechanism has now become a
central characteristic of modern theories of planetary formation.
Life in the Universe |
Last updated September 3, 2001
The Next 100 Million Years