[meteorite-list] Venus May Have Once Had A Moon
From: Sterling K. Webb <sterling_k_webb_at_meteoritecentral.com>
Date: Wed Oct 11 03:23:30 2006 Message-ID: <02b201c6ed05$fd12b980$674ce146_at_ATARIENGINE> Hi, All, Wow! Radical new theory... Not! This simulation takes two Big Hits to get rid of Venus' Moon(s) and slow the planet to a rotational crawl, but this theory... Malcuit, R. J., and Winters, R. R., 1995, Numerical simulation of retrograde gravitational capture of a satellite by Venus: Implications for the thermal history of the planet: Abstracts Volume, 26th Lunar and Planetary Science Conference, Lunar and Planetary Science Institute (Houston), p. 829-830. ...takes only one satellite and no hits to achieve the same result. See Robert Malcuit's webpage at: http://www.denison.edu/geology/malcuit/_malcuit.html Essentially, his computer simulation/theory is this: Venus captures a retrograde satellite (like Neptune does with Triton) early in its life. The retrograde satellite slows the planet's axial rotation as it spirals in for about 3.5 billion years. This is the opposite of what happens with a prograde satellite, which speeds up the planet and causes the Moon to move out, like the Earth Moon system. When it gets down to a very close orbit, it heats and melts the crust and some of the mantle, then breaks up at the Roche limit and pummels the planet, completely re-surfacing it and de-gassing all the carbonates to produce the CO2 atmosphere we see there today. This scenario matches (roughly) the crater dated age of the Venusian surface (480 million to 560 million years old). Malcuit has apparently been running successful simulations of satellite captures for over twenty years, including a Venus satellite. He doesn't care for the "satellite by impact" theory to account for everything. One reads that "capture" can't happen, but that may be wrong since he has a successful computer model of it. I don't know. But, of course, a giant impact could form a retrograde satellite instead of a prograde one if it hits just right. And then everything would follow as above. There is lots of evidence of some major impact event in the inner system about the time Venus gets re-surfaced. You could certainly call the Fall of a Moon a major impact! The flux of meteorites to the Earth increases manyfold for a short time period around 420 million years ago. All tektites show an original formation age of 440 +/- 50 million years. The Earth suffered its worst ice age ever 535-565 million years ago. It also seems to have made some rapid changes in its obliquity (axial tilt) during that time (from the weight of the ice). In other words, something big was going on between 420 and 560 million years ago. Not on the scale of the Late Bombardment, but involving many substantial objects. Perhaps that event, whatever it was, created a population of big impactors in the inner system for the Earth to deal with, like the Ordovician terminal event ~440 mya, the Permian terminal event ~250 mya, the Cretaceous terminal event ~65 mya, not to mention the 3 to 5 major extinction events in the Cambrian. A Moon makes a big mess when it breaks up and falls to its death. In the gravitational disruption of a satellite, many objects would escape the planet. The "leftovers" from such a breakup of a Moon, though a small percentage of its mass, could be both numerous and contain some sizeable objects, as big or bigger than any inner system debris in billions of years. The size of a (presumed) Permian impactor, the biggest thing to hit the Earth in 100's of millions of years, would have been about 30 miles, a tiny tiny fragment of a Moon! And like so many essentially similar ideas, I'll bet that Alemi and Stevenson never heard of Malcuit and Winters. Sterling K. Webb ------------------------------------------------------- ----- Original Message ----- From: "Ron Baalke" <baalke_at_zagami.jpl.nasa.gov> To: "Meteorite Mailing List" <meteorite-list_at_meteoritecentral.com> Sent: Tuesday, October 10, 2006 6:06 PM Subject: [meteorite-list] Venus May Have Once Had A Moon > > 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 Wed 11 Oct 2006 03:22:19 AM PDT |
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