Did Venus Have A Moon? Options

[nprev]

...or two?

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

[nprev]

Argh...my apologies for misunderstanding you, RN.

Well, the central point I was trying to make was that Mars' rotation period does not seem to be an artifact of primordial major impacts, although I'm sure that some such events certainly had an influence. Venus' case is entirely different...and just to throw a bit more churn into the mix, what in blazes must've happened to Uranus???

I am willing to accept chaotic early System events like major collisions as likely explanations for some current dynamic oddities like the Venusian rotation period & direction. However, Sagan's Law must still apply: extraordinary theories require extraordinary evidence. Using this heuristic, the "two-moon Venus" (or even one-moon) scenario seems difficult to defend.

(Did someone just mutter "Hyperion"??? Quiet, you! )

[JRehling]

what in blazes must've happened to Uranus???

This is solidly off-Venus-topic, but I was wondering recently: What if Uranus once orbited in the plane of its rotation and the tugs of other giant planets pulled it towards the ecliptic while the axis remained gyroscopically (near) fixed?

[ugordan]

What if Uranus once orbited in the plane of its rotation

That would be one very inclined orbit. Which brings up another question: why Uranus? why not Neptune too? What made Uranus form in such a weird orbit?

[helvick]

That was my thinking too.

To nudge this slightly back on topic. There are plenty of theories out there (Martian polar wander, the long term stability of the earth's inclination resulting from the moon etc) that seem to imply that the obliquity of the planets should be fairly widely distrubuted. In that case no really odd mechanics is required to explain the Uranian tilt. However if it really is that "simple" then how come it's the only really extreme one?

edited: replaced inclination with obliquity. Doh!

[JRehling]

That was my thinking too.

To nudge this slightly back on topic. There are plenty of theories out there (Martian polar wander, the long term stability of the earth's inclination resulting from the moon etc) that seem to imply that the inclination of the planets should be fairly widely distrubuted. In that case no really odd mechanics is required to explain the Uranian tilt. However if it really is that "simple" then how come it's the only really extreme one?

We can probably exclude Mercury and Venus as being solar-locked, and we can perhaps exclude Earth on the anthropic principle (would we be here as an advanced species to discuss the issue if Earth had Uranus's inclination?).

That leaves us with Mars and four giants, plus choose-as-you-like for other worthy bodies to include in the discussion. Pluto is tidally locked to its satellite, which probably excludes that one from consideration (Mars and the giants are NOT locked to their satellites, to say the least). Eris is unknown.

Jupiter and Ceres are very low inclination. Earth, Mars, Saturn, and Neptune are eerily similar in their modest inclinations; Vesta is also in that general vicinity. Then Uranus is the big outlier. Mars, at least, is prone to swings over geological time. Earth and probably Saturn seem to be stabilized and I'm quite sure that the other three giants are too big/too far from anything to get tossed around.

It's important to note that this is a very small n. You can't call any trends you spot statistically significant. I wonder if the "expected" distribution is flat or clustered in Gaussian fashion around 0. The data leads me to the wild guess that it is the latter, and it's a fluke that the "midrange" instances are clustered around 25 degrees with nothing between 6 and 21 or between 30 and 90. If one standard deviation is 30 degrees, we would expect about 2/9 to be in each of these bins: 0-10, 11-20, 21-30. The first bin got its quota and it just happened that the third bin ended up with "both" of the planets that "should have" been in the second bin, and one that "should" have been in a higher-number bin. Three little flukes. A friend of mine had a phone number with "666" in it -- things like this happen.

[helvick]

It's important to note that this is a very small n...

All true and very good points.
However some further digging has reminded me that the really extreme one is actually Venus since it's not tidally locked to the sun and it's obliquity is 178deg so we really should be binning 6 samples into the 0-180 range. If obliquity is chaotic in the long term then surely neither Venus or Uranus are in any way in need of an extraordinary explanation.

I haven't done the numbers at all but adding the above to the Solar tidal drag explanation certainly seems more convincing to me - Venus "reverse" obliquity is just an extreme outlier by chance and it's dense atmosphere\high surface temp\proximity to the sun have allowed solar tidal drag to slow it down nearly (but not quite) to a stop.

[JRehling]

All true and very good points.
However some further digging has reminded me that the really extreme one is actually Venus since it's not tidally locked to the sun and it's obliquity is 178deg so we really should be binning 6 samples into the 0-180 range. If obliquity is chaotic in the long term then surely neither Venus or Uranus are in any way in need of an extraordinary explanation.

I haven't done the numbers at all but adding the above to the Solar tidal drag explanation certainly seems more convincing to me - Venus "reverse" obliquity is just an extreme outlier by chance and it's dense atmosphere\high surface temp\proximity to the sun have allowed solar tidal drag to slow it down nearly (but not quite) to a stop.

But if Venus's rotation is being altered by the Sun, per the paper Don cited, it isn't part of the distribution the gas giants are in. It says in a nutshell that Venus could have had a rotational speed of +12 mph or -4 mph. Presumably, then, we can say that Venus has historically undergone a change towards one of these attractors and the angular momentum of its (slight) inclination away from the ideal attractor is so slight as to be a rounding error. Whatever accounts for this: either a deviation from the dynamics that would draw it to the attractor (which could be atmospheric) or residual from the original rotation, are assuredly not among the important phenomena influencing the giants' rotation (which have not settled on solar-tide-induced attractors), so it's not really part of the same distribution.

Put another way, if we looked at large numbers of planets (using extrasolar ones as our data), you are always free to call whatever set you like a distribution, but you may find multiple independent modes caused by meaningful subgroups. For example, "height" of adults is bimodal, because men and women have different modes and the heights between the female mode and the male mode are cumulatively rarer than the female mode or the male mode. It looks to me like Mercury and Venus either represent a "sun-altered" mode of their own or even two different modes (tidally locked vs. solar-thermal tidally locked) which would both be closer to low-inclination than everything else.