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Did Venus Have A Moon?
nprev
post Oct 11 2006, 07:06 PM
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...or two?

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


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Guest_Myran_*
post Oct 11 2006, 08:01 PM
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I read it and agree that slow rotation of Venus are most likely due to one huge impact some time in the past.
I grant the author that much.
But to assume that a satellite have to be involved at one point just make the theory unneccesarily complicated. So I maintain my 'keep it simple stupid' attitude towards complex theorys, also called Occams Razor and use it to shave off the extra step of one temporary "moon".
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Jyril
post Oct 11 2006, 08:08 PM
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I agree, the theory sounds a bit too complicated and my KISS alert went off. Still, the claim that Venus was almost certainly hit by a large impactor may be good news since that would mean large satellites around terrestrial planets could be common.


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Rob Pinnegar
post Oct 11 2006, 08:09 PM
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Cute, but it's just speculation. Nobody can prove or disprove that any such moons ever existed.

The presenter is listed as being an undergraduate... I'm guessing this is his fourth year thesis. Presumably it's being reported in the news because it "sounds exciting".

As far as the model's concerned, I'd been under the impression that Venus' slow rotation rate can be explained away by despinning due to solar tides acting on its atmosphere, because the atmosphere is so massive. (This is a dim recollection from years ago -- correct me if my memory is inaccurate here.) If that's correct, it's an important point, because it would make a second impact unnecessary, which would really simplify the model. *Any* moon, whether primordial, captured, or giant-impact-derived, would have spiralled into Venus after the planet's rotation was tidally slowed.

Guess the same goes for Mercury.
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JRehling
post Oct 11 2006, 09:07 PM
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Keep in mind that the rate of Venus's rotation, at its equator, is just a fast walk by a person. It is incredibly close to zero -- though not quite there. It's more likely to signify an attractor AT zero (eg, solar tides) than cosmic billiards which would have no such attractor.
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tasp
post Oct 12 2006, 03:20 AM
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Whew, heady stuff.

I think the odds for the Saturnian Trojanettes (Telesto, et al) having started off orbiting Dione and Tethys are a bit higher than for the Venusian siblings. And at least the Trojanettes are still around for closer examination in this regard.

I had considered Valhalla and Asgard crater formations on Callisto as possibly caused by 'spun off' Callistan subsatellites, but it occurs to me they might have started off around Ganymede too.

Much harder slogging through these ideas without the corpus delecti . . . .
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nprev
post Oct 12 2006, 03:53 AM
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I gotta agree with the majority opinion so far; the scenario outlined in the article seems a lot less likely than one good smack (which may or may not have produced a short-lived moon) that pretty much nullified most of Venus' rotational momentum. KISS usually works, eh, Jyril?smile.gif


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AndyG
post Oct 12 2006, 08:41 AM
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QUOTE (Rob Pinnegar @ Oct 11 2006, 09:09 PM) *
I'd been under the impression that Venus' slow rotation rate can be explained away by despinning due to solar tides acting on its atmosphere, because the atmosphere is so massive.

My initial thought was "but that requires dumping a massive load of energy".

However, a quick look at Excel, and I see (assuming an early Venus rotated at terrestrial kind-of-rates) some 1.9*10^29 joules of rotational energy needs to have been absorbed. That's four hundred million joules per kilo of CO2 in the current Venusian atmosphere...but then there's been billions of years to do it. I have to say that the per-second wattage of tidal slowing is not at all, I think, unlikely.

Andy
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edstrick
post Oct 12 2006, 10:18 AM
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Venus's solar tidal drag on the solid body is enough to have modified the planet's spin rate, but as I recall, models have indicated despinning from solid body tide during the age of the solar system is unlikely.

There are also significant thermally driven and gravitational tides in the atmosphere, some models have indicated these have contributed to the non-zero spin rate of the planet.

The really BIZARRE and unexplanable thing is that Venus is close to being tidally locked with the same hemisphere facing Earth every conjunction. It's not exact, but it is very close. No tidal model involving Earth comes anywhere near able to explain that and it may be pure cooincidence.

Note: All this info is off the top of my head from old studies. Take with a grain of iron sulfide <venusian salt>
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Rob Pinnegar
post Oct 12 2006, 02:20 PM
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QUOTE (edstrick @ Oct 12 2006, 04:18 AM) *
There are also significant thermally driven and gravitational tides in the atmosphere, some models have indicated these have contributed to the non-zero spin rate of the planet.

The really BIZARRE and unexplanable thing is that Venus is close to being tidally locked with the same hemisphere facing Earth every conjunction...


I also remember these points now that you mention them. First guess: they were probably reported in an old issue of Astronomy or Sky&Tel. It would have to have been a long time ago, as my subscriptions to those magazines ran out around 1990 or thereabouts. (By then, I was at U o'Toronto and had discovered journals.)

These sorts of observational coincidences, i.e. the same side of the planet facing us at optimal observation times, led to astronomers thinking that both Mercury and Venus were locked in synchronous rotation around the Sun for a long time. (I know you know that, Ed, but am mentioning it for other readers who might not.)

As for tidal forces, keep in mind that they go by the inverse *cube* of the distance, so Venus' solar tides are about as strong as Earth's solar and lunar tides combined.
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RNeuhaus
post Oct 12 2006, 02:49 PM
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QUOTE (nprev @ Oct 11 2006, 02:06 PM) *

So that it would be probed, Venus must show any impact marks of two moons.

Does the Magallen's radars show any indication of these impacts? I haven't heard of anyone speculating about this. However, I seems that it would be very hard to prove since at about 500 millions years ago Venus might have undergone a big volcan eruption that have resurfaced most of the planet.

Anyway, due to its odd direction of its polar spin, there must have some kind of impact that have changed its polar position and not due to the influence of Sun tidal and the Venusian's massive atmosphere in which they are, most probably, be impossible to alter the polar positions.

On the other hand, I think that the changing the rotation way by reversing the poles is lower than by reversing the rotation direction which would be twice force than the polar's one. So, this kind of mystery would continue...and hope to attract more spacecraft to visit to Venus. smile.gif

Rodolfo
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Bill Harris
post Oct 12 2006, 03:42 PM
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QUOTE
My initial thought was "but that requires dumping a massive load of energy".

However, a quick look at Excel, and I see (assuming an early Venus rotated at terrestrial kind-of-rates) some 1.9*10^29 joules of rotational energy needs to have been absorbed.

Just a thought... perhaps this atmospheric despinning added to Venus' hot surface temperature. The despinning could have created very high surface temperatures for the first couple of billion years and the greenhouse effect would add to the temperature and keep it from cooling as fast. Or vice versa. Very odd planet, but then, aren't they all...

--Bill


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JRehling
post Oct 12 2006, 04:08 PM
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QUOTE (RNeuhaus @ Oct 12 2006, 07:49 AM) *
So that it would be probed, Venus must show any impact marks of two moons.

Does the Magallen's radars show any indication of these impacts? I haven't heard of anyone speculating about this. However, I seems that it would be very hard to prove since at about 500 millions years ago Venus might have undergone a big volcan eruption that have resurfaced most of the planet.

On the other hand, I think that the changing the rotation way by reversing the poles is lower than by reversing the rotation direction which would be twice force than the polar's one. So, this kind of mystery would continue...and hope to attract more spacecraft to visit to Venus. smile.gif

Rodolfo


Venus does not show any such features, although you correctly note that it might have been covered by volcanism -- we aren't seeing very much of Venus's full history on its surface.

Note, though, just how slow Venus is rotating: 4 miles per hour = 6.5 km/hour. It doesn't take much of an input to that system to make whatever change you would like.

The low inclination is another argument for a solar-tide basis to Venus's rotation.

I suggested a few years ago that Mercury may bear the impact scar of a satellite that "augured in". There is a large radar feature called "Feature C" that is near the equator and 240 West longitude -- an area not imaged by Mariner 10. That area shows nothing especially compelling in albedo maps made with Earth-based telescopy, but neither does the estimable Caloris basin, which simply isn't much of an albedo feature. There is another proposed near-equatorial basin that has perhaps been imaged as a *relief* feature casting shadows of a double-rimmed basin, around 270 West longitude. We will find out more about the anti-Mariner hemisphere of Mercury when Messenger makes its initial flybys and then enters orbit.
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Rob Pinnegar
post Oct 12 2006, 04:58 PM
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Hmmm. Would a satellite small enough to avoid disintegrating inside the Roche limit be large enough to create a double ringed basin? I suppose a rubble pile might do the trick -- it wouldn't get dispersed by atmospheric friction at Mercury the way it would at Venus, Earth or Mars.

This is getting off the topic of Venus and onto that of Mercury, but it seems to me that an inspinning rubble-pile moon might leave a pretty impressive linear debris trail rather than a crater. A solid body would lose its forward momentum very rapidly upon contacting Mercurcy's surface, but a less consolidated rubble pile might be able to keep rolling forward for quite a long distance. Hard to predict intuitively exactly what it would do. An interesting problem for someone good at computer simulations.
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RNeuhaus
post Oct 12 2006, 06:53 PM
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QUOTE (JRehling @ Oct 12 2006, 11:08 AM) *
I suggested a few years ago that Mercury may bear the impact scar of a satellite that "augured in". There is a large radar feature called "Feature C" that is near the equator and 240 West longitude -- an area not imaged by Mariner 10. That area shows nothing especially compelling in albedo maps made with Earth-based telescopy, but neither does the estimable Caloris basin, which simply isn't much of an albedo feature. There is another proposed near-equatorial basin that has perhaps been imaged as a *relief* feature casting shadows of a double-rimmed basin, around 270 West longitude. We will find out more about the anti-Mariner hemisphere of Mercury when Messenger makes its initial flybys and then enters orbit.

The longitudinal 270 degrees West of Mercury are still not shown in maps except from 0 to 180 degrees West. Mariner 10's three fly-by only took half planet picture and hope that Messenger would success its mission and by able to take pictures to all round Mercury planet.

On the other hand, about the two theories which leads to explain to the Venusian's oddy rotation (opposite and slow) are:
  1. Impact by at least two Moons
  2. Solar tidal influence
However, the only true is that there are similiarities of slow axis rotation behavior of Mercury and Venus comparing to the planet orbit around the Sun. The Earth would be an exception to that in fact that it was hit by a Mars' size that leaded in the creation of Moon. The hard hit on Earth might have implicated to accelerate the Earth spin axis rotation. Doesn't it?

However, the slow axis rotation of Mercury and Venus would be the best goal to explain the enigma.

Rodolfo
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