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Old 02-15-2013, 04:19 PM   #117
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Join Date: Jan 2004
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Originally Posted by mhgaffney View Post
When Sagan made this pronouncement the vulcanism on Venus was not yet known. Which today makes Sagan look silly. No runaway greenhouse can cause vulcanism over 100% of a planetary surface.
No one claimed that the greenhouse effect *causes* Venus' vulcanism. That's a mistake you've made for ages now. It's a strawman.

Originally Posted by mhgaffney
FYI I was correct when I stated that Venus' retrograde rotation is indeed opposite the other planets. The assertion that this was caused by a collision is laughable. Face the fact -- it is an anomaly and has never been explained.
We don't have a known correct explanation, but a collision with a large protoplanet has the ability to result in Venus' spin axis being flipped N -> S and its very slow rate. That it doesn't satisfy you doesn't make it laughable.

Originally Posted by mhgaffney
There is another unexplained anomaly as well regarding Venus' rotation: Venus is in resonance with the earth. Venus always shows the same face to earth at their nearest approach. Again - this has never been explained.

See Nonresonance rotation of Venus

Radar observations accumulated over the past 14 yr are used to make a precise estimate of the spin vector of Venus. The results obtained show that the spin vector of Venus may be adequately described in the standard 1950.0 coordinate system by a period of 243.01 + or - 0.03 days (retrograde) and a north pole direction corresponding to alpha = 272.8 + or - 0.5 deg and delta = 67.2 + or - 0.3 deg; the quoted errors represent estimates of 70% confidence intervals. The angular separations between the spin vector and those vectors representing the unit normals to the invariable plane of the solar system and the orbital plane of Venus are found to be 0.5 deg and 2.6 deg, respectively. It is concluded that Venus is not rotating with a resonance spin period relative to the orbit of earth and that the spin of Venus may be in a generalized Cassini state.
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