8.6. FORMATION OF THE COMETS AND ASTEROIDS 117
an ancient surface bear the obvious scars of this period of intense bombardment. e surface
features of Earth, for example, are dominated by the remnants of internal processes; the crust of
Earth is constantly altered by plate tectonics (continental drift) and volcanoes. And so most of
the original impact features have been long covered over; only the hundred or so most recent
ones are left. Smaller bodies, such as the Moon, did not have enough internal heat to greatly
alter their surfaces with plate tectonics, because they cooled off faster. And so the Moon still
bears the scars of nearly its entire history.
8.6 FORMATION OF THE COMETS AND ASTEROIDS
Small bodies in orbit about the Sun are highly susceptible to having their orbits greatly altered
(perturbed) by the gravity of the large planets. Jupiter, by far the most massive of the planets,
is the most common cause of these perturbations. In the region between Mars and Jupiter, no
large rocky planet could form; the orbits there are constantly perturbed by Jupiter’s gravity such
that they cross over each other. ese crossed orbits in the asteroid belt region result in violent
collisions that break bodies apart more often than allowing them to join together to make larger
bodies.
Beyond the asteroid belt, icy bodies formed in the early solar system. It is hypothesized
that repeated encounters with the enormous gravity of Jupiter deflected most of these small icy
bodies out of the part of the solar system occupied by the Jovian planets. Some of them would
have been deflected to the inner solar system, adding icy bodies to the mix of impacts suffered
by Earth, the Moon and the other terrestrial planets. is is considered a likely origin for the
water on Earth; our planet formed too close to the Sun for water to have condensed out directly.
But models suggest that many of the small icy proto-bodies that condensed out of the solar
nebula in the region of the Jovian planets would have been deflected to much larger distances,
to form the Kuiper belt and the Oort cloud (see Sections 8.1.7 and 8.1.4).
..................Content has been hidden....................
You can't read the all page of ebook, please click here login for view all page.