[meteorite-list] Venus May Have Once Had A Moon

From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Tue Oct 10 19:06:26 2006
Message-ID: <200610102306.QAA12505_at_zagami.jpl.nasa.gov>

http://skytonight.com/news/4353026.html

Why Doesn't Venus Have a Moon?
by David Tytell
Sky & Telescope
October 10, 2006

Back when Earth was very young, our home world was steadily pummeled by
large solar system debris. While Earth withstood the barrage of hits
like a prizefighter that wouldn't fall down, one blow nearly destroyed
the world. A Mars-size body plowed into us, completely disrupting both
bodies and splashing massive amounts of debris into orbit which, most
astronomers agree, coalesced to form our Moon.

But if something that large hit us, how did our nearest-neighbor planet,
Venus, dodge the same fate? According to a new study, it didn't.
Billions of years ago, according to work announced yesterday, Venus once
had a moon that formed the same way Earth's did.

On Monday at the American Astronomical Society's Division of Planetary
Sciences meeting in Pasadena, California, Caltech undergraduate Alex
Alemi presented models created with David Stevenson of Caltech that
suggest Venus was not only slammed with a rock large enough to form the
Moon, the event happened at least twice.

According to Alemi and Stevenson, in models of the early solar system it
is nearly impossible for Venus to avoid a big hit. Most likely, Venus
was slammed early on and gained a moon from the resulting debris. The
satellite slowly spiraled away from the planet, due to tidal
interactions, much the way our Moon is still slowly creeping away from
Earth.

However, after only about another million years Venus suffered another
tremendous blow, according to the models. The second impact was opposite
from the first in that it "reversed the planet's spin," says Alemi.
Venus's new direction of rotation caused the body of the planet to
absorb the moon's orbital energy via tides, rather than adding to the
moon's orbital energy as before. So the moon spiraled inward until it
collided and merged with Venus in a dramatic, fatal encounter.

"Not only have we gotten rid of the moon, but we've also done well to
explain Venus's current slow rotation rate [and direction]," says Alemi.
If a second moon formed from the second collision, it too would have
been absorbed the way the first one was.

The models do allow for more than two impacts, but the probability of
Venus enduring several massive collisions is low. "You can do this with
multiple collisions, but the hypothesis is that [the net result] adds up
to a negligible contribution" to the planet's final state, says Alemi.
Received on Tue 10 Oct 2006 07:06:24 PM PDT