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Posted by: Antdoghalo Oct 31 2009, 10:42 PM

I think i know why Mercury and Venus have nothing orbiting them.

You know how our moon and Deimos are slowly getting farther from Earth and Mars.
And Phobos is getting closer to Mars and will impact it eventually.

Phobos orbits in less than a Martian day.
Deimos and our moon orbit slower than there parents rotation.

Maybe Mercury and Venus consumed there moons because the slow rotation of those planets allowed tidal forces with any moon that orbited them to spiral inward and eventually impact them.

Posted by: scalbers Oct 31 2009, 10:57 PM

Sounds like a pretty good theory. I'm curious though why Phobos would noticeably spiral in towards Mars when its gravity should give a weak tidal effect on the planet?

Posted by: Greg Hullender Nov 1 2009, 12:14 AM

That effect is very weak for slowly-rotating planets, though. A much better explanation is that the closer you are to the sun, the smaller the Hill Sphere around a planet. Likewise, the smaller the mass, the smaller the Hill Sphere. Outside the Hill Sphere, it's difficult for a planet to have a satellite. (Earth's moon is a very special case.) Neither Earth, Venus, nor Mercury have any satellites inside their (tiny) Hill Spheres.

So it should come as no surprise that Mercury and Venus have no moons, and that Mars has just two very tiny ones. The anomaly is that Earth has a planetary-sized moon!

--Greg

Posted by: Hungry4info Nov 1 2009, 08:06 AM

I'm pretty sure Earth's moon is within the planet's hill sphere. Is that wrong?

Posted by: As old as Voyager Nov 1 2009, 10:19 AM

Here's a couple of links related to this topic:

http://www.skyandtelescope.com/news/home/4353026.html

http://www.agu.org/pubs/crossref/1992/92GL01067.shtml

Posted by: scalbers Nov 1 2009, 03:14 PM

Yes, I recall even our moon spiraled away from about 10000 miles to the present 240000. This spiraling will stop when Earth's rotation syncs up with the lunar orbit (whenever that would be).

Posted by: Phil Stooke Nov 1 2009, 05:56 PM

Hypothesis is the word to use for this, not theory.

"I'm curious though why Phobos would noticeably spiral in towards Mars when its gravity should give a weak tidal effect on the planet?"

Phobos spirals in because it orbits faster than Mars rotates. The tidal interaction works in the opposite sense. Mars exerts a drag on Phobos. In our case, Earth speeds the Moon up, causing it to move to a higher orbit. (As a simplified way of looking at things, not hassling with angular momentum etc.)

Phil

Posted by: scalbers Nov 1 2009, 06:24 PM

Thanks - though I think I understand the basic mechanism. I'm asking a sort of "second order" question on whether these tidal effects apply just as much to low mass satellites like Phobos. Phobos would hardly excite much of a tidal bulge on Mars. How would the mechanism work in that case? Is the rate of spiraling out independent of the satellite mass?

Posted by: Greg Hullender Nov 1 2009, 07:52 PM

You're correct. I made the mistake of mindlessly repeating something I'd read (or thought I'd read) as a teenager. Always a hazard.

Sorry about that.

--Greg

Posted by: Phil Stooke Nov 2 2009, 03:12 PM

I would think that the Tharsis bulge is as effective as a tidally induced bulge when it comes to having an effect on Phobos.

Phil

Posted by: Greg Hullender Nov 2 2009, 09:03 PM

I don't see how that would work. I'd expect the effects of it to cancel out. The tidal bulge (tiny as it is) lags slightly behind Phobos and so slows it down all the time. The bulge on the far side of Mars has the opposite effect, but, being on the far side, has a smaller effect and so doesn't completely cancel the slowing effect of the near-side bulge. However, I don't pretend to fully understand the math involved.

As it turns out, this effect is well-studied and measured:

http://www-geodyn.mit.edu/bills_phobos05.pdf

Phobos only has about 30 to 50 million years left, which took me be surprise; that suggests it hasn't belonged to Mars for very long at all.

--Greg

Posted by: ngunn Nov 2 2009, 10:17 PM

Maybe there was originally a moon in a synchronous orbit for who knows how long, then an impact disrupted it leaving two major fragments, one outside and one inside the synchronous orbit. One would then start spiralling outward and the other inward.

Posted by: Greg Hullender Nov 3 2009, 04:27 PM

Seems unlikely. They certainly don't LOOK like two halves of the same whole.

--Greg

Posted by: ngunn Nov 3 2009, 04:56 PM

Being inside the (zero strength) Roche limit loose stuff will tend to fall off Phobos, whereas Deimos can accumulate dust, so they might look different for that reason. I have to admit, though, that my suggestion was fairly whimsical. I just don't like the idea that Phobos was nowhere near Mars until a few million years ago. Also, two small moons both pretty close in is a bit of a coincidence if they have completely independent origins. What happens if you rewind back in time the opposite spirallings of Phobos and Deimos? These are such obvious questions that they must have been asked and answered many times before - probably on this forum!

EDIT - Yep, here's a post from 2005 (para 6): http://www.unmannedspaceflight.com/index.php?s=&showtopic=1305&view=findpost&p=33612

Posted by: ngunn Nov 3 2009, 09:02 PM

Following the Joe Burns trail even further back I found these titles (not links, unfortunately):

J. A. Burns, 1972. The dynamical characteristics of Phobos and Deimos. Reviews of Geophysics and Space Physics 10, 462-483.

and to swing this discussion right back to it's starting point:

J. A. Burns, 1973. Where are the satellites of the inner planets? Nature 242, 23-25

Posted by: Greg Hullender Nov 3 2009, 09:55 PM

Those are all really old though, and predate decent computer simulations of solar system dynamics. I found some promising-sounding papers in the UW library catalog, but none available online. I may drop in and take a look later.

One suggestion I ran across for the origin of Phobos that sounded plausible was that it might originally have been a double asteroid which was split up during a close encounter with Mars. The other piece would have escaped Mars, of course (it's not Deimos), but Phobos' orbit would relatively quickly have been circularized and equatorialized.

Given Phobos' short life, it's quite a coincidence for us to be seeing it, unless Mars captures asteroids this way fairly frequently. But if it happens frequently, why is Deimos the only other satellite of Mars? Is it possible that the widest-separated binary asteroid cannot be captured into an orbit much smaller than Deimos, and hence ALL of them have been, like Phobos, doomed to impact Mars? I can't find any evidence that anyone has tried to work this out.

--Greg