Geoscience Reference
In-Depth Information
onthewaytoJupiter,wherewemightexpect tomeet aplanet, wefindinstead only
a belt of asteroids.
In their 1975 paper, Bill Hartmann and Donald Davis used a set of questions to
show how giant impact has enough variations to explain the genealogy of our pe-
culiar planetary family:
Does the second-largest planetesimal in each system hit the planet after 10
7
years or l0
8
years?
Is it large or small? Does it hit the planet dead center? Retrograde [backward]? A glancing
blow prograde? Or is it captured? Or is it destroyed by a planetesimal-planetesimal collision
so that it has no appreciable effect on the planet other than to produce many small craters? Or
does it hit a pre-existing satellite of the planet, perhaps converting it to several small satellites?
Only one of these kinds of fates can befall the second-largest planetesimal. And this fate, the
product of small-number statistical chance encounters, may determine whether the planet ac-
quires a tilted axis, a massive circumplanetary swarm of dust, a captured satellite, or perhaps
loses a larger satellite, gaining small fragmentary satellites.
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Such processes would leave each planet with a different speed of rotation and a
different axial tilt, as we find them. A giant impact could have blasted away most
of Mercury's mantle and atmosphere, leaving it with an iron core, remnants of its
original mantle, and no moon, crust, or clouds. Venus may have been struck head-
on and stopped dead in its tracks, or it may have escaped being hit, maintaining the
spin of the primordial dust cloud. One unsolved puzzle is why Venus, so close to
the Earth, has no moon. Perhaps Venus suffered a second giant impact that offset
the angular momentum produced by the first, removing the “kick” in the system
and leaving Venus spinning too slowly to spawn a moon. Alternatively, perhaps
the tidal pull of the Sun destroyed a proto-Venusian moon before it got going.
But how to account for the asteroid belt between Mars and Jupiter? Until re-
cently, scientists thought that the asteroids were remnants of an exploded planet.
Today we know that their total mass amounts to only about 5 percent of the mass
of our Moon. So small a body could never have been a true planet. But why did a
planet not form at the location of the asteroid belt? The answer may be: Jupiter.
Jupiter was born at just the right distance from the Sun to grow rapidly. The
faster it grew, the more gravitational pull it exerted on nearby objects, and the
faster still it grew. Any planetesimals that had begun to aggregate between Mars
and the young giant found themselves first ripped to pieces by Jupiter's massive
gravity, then sucked into its greedy maw, colliding like the “string-of-pearl” frag-
ments of Comet Shoemaker-Levy 9 that vanished into Jupiter in July 1994. After
the early giant impact, Mars had to get by on the scraps from Jupiter's table, stunt-
ing its growth. Had Jupiter been smaller, a planet might now exist where we find
