Printable Version of Topic

Posted by: nprev Oct 11 2006, 07:06 PM

...or two?

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

Posted by: Myran Oct 11 2006, 08:01 PM

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".

Posted by: Jyril Oct 11 2006, 08:08 PM

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.

Posted by: Rob Pinnegar Oct 11 2006, 08:09 PM

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.

Posted by: JRehling Oct 11 2006, 09:07 PM

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.

Posted by: tasp Oct 12 2006, 03:20 AM

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 . . . .

Posted by: nprev Oct 12 2006, 03:53 AM

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?

Posted by: AndyG Oct 12 2006, 08:41 AM

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

Posted by: edstrick Oct 12 2006, 10:18 AM

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>

Posted by: Rob Pinnegar Oct 12 2006, 02:20 PM

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.

Posted by: RNeuhaus Oct 12 2006, 02:49 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.

Rodolfo

Posted by: Bill Harris Oct 12 2006, 03:42 PM

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

Posted by: JRehling Oct 12 2006, 04:08 PM

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.

Posted by: Rob Pinnegar Oct 12 2006, 04:58 PM

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.

Posted by: RNeuhaus Oct 12 2006, 06:53 PM

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

Posted by: DonPMitchell Oct 12 2006, 07:36 PM

According to dynamics calculations by Alexandre Correia and Jacques Laskar, thermal tides and other forces create two stable rotational states for Venus: a retrograde period of -243.02 days, or a prograde rotation of 76.83 days. Random chance determined which of the two states the planet ended up in. If their work is correct, then a catastrophic theory seems unnecessary.

Posted by: JRehling Oct 12 2006, 08:25 PM

Mercury has been mapped almost fully by Earth-based means, with amateurs contributing to the best imagery in some areas, professionals contributing quite a bit, and radar having mapped yet other areas VERY well, although some of the radar mapping is not public.

Here are six maps of the entire planet:

http://img203.echo.cx/img203/64/mercurysix1vc.jpg

See

http://www.unmannedspaceflight.com/index.php?s=&showtopic=946&view=findpost&p=10557

...for explantion.

In any event, the rotational period of Mercury holds little mystery: It is obviously synchronized to the orbital period, with the same axis aligned sunward at each perihelion.

Posted by: nprev Oct 13 2006, 04:55 AM

Interesting...but how about Mars' relatively rapid rotation? Would the Hellas and Argyre impacts be enough to induce it?

Posted by: RNeuhaus Oct 13 2006, 06:59 PM

The other big ones is Sirtis Isidis. The impact on Hella and Argyre haven't induced to accelerate the Mars' axis rotation but they, probably, have created the Tharsis mountains and Ellisium mountains which are just on the opposite side of the Hella and Argyre creaters.

Rodolfo
P.D. Corrected.

Posted by: nprev Oct 14 2006, 12:14 AM

Well, I'll be...I actually didn't know that Syrtis was an impact basin...guess I need to really look at the topo maps sometime (mostly the north polar region captures my attention). Thanks, RN.

Posted by: JRehling Oct 14 2006, 02:24 AM

Syrtis isn't an impact basin -- it's volcanic, with a large crater remnant in the middle.

Beagle 2's crashing site, Isidis, is an impact basin, just east of Syrtis.

Posted by: nprev Oct 14 2006, 03:45 AM

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! )

Posted by: JRehling Oct 14 2006, 04:59 AM

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?

Posted by: ugordan Oct 14 2006, 09:21 AM

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?

Posted by: helvick Oct 14 2006, 11:17 AM

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!

Posted by: RNeuhaus Oct 15 2006, 12:00 AM

Below picture corresponds to Syrtis Major Plantia. I am referring to the East of Syrtis in which JRehling told you that is right: Isidis. I am sorry for that confusion. But, anyway, very close!!!! just on the East

Below shows the Syrtis Major Plantia (black part) and the East is the Isidis.




Rodolfo

Posted by: JRehling Oct 15 2006, 04:59 AM

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.

Posted by: helvick Oct 15 2006, 06:07 AM

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.

Posted by: JRehling Oct 15 2006, 03:32 PM

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.

Posted by: Myran Oct 16 2006, 11:48 AM

If one such Earth had the same orbit as it have now, evolution would of course have taken another path and Homo Sapiens are very unlikely to be around. Yet I see no reason why one such alterative Earth not would be host to advanced life.
Half year long periods with sunlight around the clock followed by equally long dark winters are after all the natural state of things!

Posted by: tty Oct 16 2006, 06:19 PM

In earlier warmer times when there were forests as far north as Ellesmere land and in inland Antarctica lots of plants and animals apparently did quite well under conditions that approached these, so I can't see any strong reason that "Uranian" climate should prevent advanced life forms from developing.

tty

Posted by: JRehling Oct 16 2006, 06:35 PM

Earth is a biased point whether we can prove that life couldn't have gotten sophisticated on a tilted Earth or not -- the burden of proof is on data being unbiased, not vice versa.

But to get into a bit of detail, if we want to talk about a technological civilization, then there is a need for high-yield food production, which in turn requires plants and animals that can be domesticated. And it's a very small fraction of all plants and animals that can be domesticated -- if you cut the number of available species, you presumably cut the ratio of those that can be domesticated.

For example, there are no plant species native to Canada that could be domesticated for food production, even though Canada has plenty of forests.

This may be like standing farther away from a dartboard: as a matter of principle, there's no reason why you can't hit a bullseye, but it gets less likely.

Posted by: nprev Oct 19 2006, 09:27 AM

Delinquent thanks for the great "pocket" Mars map, Rodolfo!

Here's a thought: What if any relationship does internal volcanic activity/mass redistribution have on planetary inclination & rotation period? I know that the Tharsis bulge is thought to have had a significant influence on Mars' obliquity history, but how to explain Earth's almost identical inclination in the present era? Not to mention the fact that Venus has both an obviously active volcanic history and an anomalous rotation period, yet virtually no axial inclination! And finally, who knows what's happening now or in the distant past on the "surface" (to say nothing of the interior) of Uranus?

The circumstances influencing these planetary characteristics must be fearsomely complex.

Posted by: diane Oct 20 2006, 10:42 PM

Wincing at the thought of a "Lonely Planet Travel Guide to Meridiani Planum"

Posted by: dvandorn Oct 21 2006, 02:11 AM

Here's a question:

If this model is correct and Venus originally had a moon that escaped -- what happened to it?

I don't suppose it's possible that this original Venusian moon that escaped is now called Mercury?

-the other Doug

Posted by: tasp Oct 21 2006, 02:24 AM

Orpheus?

Solar tidal effects pumped up it's eccentricity and it womped earth . . . .

Posted by: Rob Pinnegar Oct 24 2006, 08:57 PM

That's a nice idea -- but it would run into the same problems that plagued the "coaccretion" hypothesis for the formation of Earth's Moon: namely, the compositional dissimilarities between the two bodies.

One idea for why Mercury is so small and has such a big core is that it got hit by something huge late in its formation that blew most of the planet's mantle away. Assuming that to be true, you could address the above point in a couple of ways:

1. Postulate that Mercury escaped from Venus by some unknown mechanism (uh oh!), and *then* got whacked by something big that blew off its mantle and left mostly core behind. However, this would require proto-Venus to effectively be a double planet with similarly sized components, since Mercury would have been so much bigger in the first place.

2. Alternatively, I guess you could start with the double-Earth idea, and then hypothesize that the impact that blew off Mercury's mantle also ejected it from Venusian orbit into solar orbit. This might work better.

I don't know if either of these ideas would ever get taken seriously. Probably not. The whole idea is most likely just not dynamically workable. Neat to speculate about, though.