Possible theory on why Mercury and Venus have no moon, Clues from Earth and Mars moon orbits |
Possible theory on why Mercury and Venus have no moon, Clues from Earth and Mars moon orbits |
Oct 31 2009, 10:42 PM
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#1
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Member Group: Members Posts: 223 Joined: 13-October 09 From: Olympus Mons Member No.: 4972 |
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. -------------------- "Thats no moon... IT'S A TRAP!"
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Oct 31 2009, 10:57 PM
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#2
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Senior Member Group: Members Posts: 1625 Joined: 5-March 05 From: Boulder, CO Member No.: 184 |
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?
-------------------- Steve [ my home page and planetary maps page ]
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Nov 1 2009, 12:14 AM
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#3
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Senior Member Group: Members Posts: 1018 Joined: 29-November 05 From: Seattle, WA, USA Member No.: 590 |
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 |
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Nov 1 2009, 08:06 AM
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#4
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Senior Member Group: Members Posts: 1417 Joined: 26-July 08 Member No.: 4270 |
I'm pretty sure Earth's moon is within the planet's hill sphere. Is that wrong?
-------------------- -- Hungry4info (Sirius_Alpha)
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Nov 1 2009, 10:19 AM
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#5
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Member Group: Members Posts: 117 Joined: 7-December 06 From: Sheffield UK Member No.: 1462 |
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. 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 -------------------- It's a funny old world - A man's lucky if he gets out of it alive. - W.C. Fields.
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Nov 1 2009, 03:14 PM
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#6
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Senior Member Group: Members Posts: 1625 Joined: 5-March 05 From: Boulder, CO Member No.: 184 |
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).
-------------------- Steve [ my home page and planetary maps page ]
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Nov 1 2009, 05:56 PM
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#7
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Solar System Cartographer Group: Members Posts: 10146 Joined: 5-April 05 From: Canada Member No.: 227 |
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 -------------------- ... because the Solar System ain't gonna map itself.
Also to be found posting similar content on https://mastodon.social/@PhilStooke NOTE: everything created by me which I post on UMSF is considered to be in the public domain (NOT CC, public domain) |
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Nov 1 2009, 06:24 PM
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#8
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Senior Member Group: Members Posts: 1625 Joined: 5-March 05 From: Boulder, CO Member No.: 184 |
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?
-------------------- Steve [ my home page and planetary maps page ]
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Nov 1 2009, 07:52 PM
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#9
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Senior Member Group: Members Posts: 1018 Joined: 29-November 05 From: Seattle, WA, USA Member No.: 590 |
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Nov 2 2009, 03:12 PM
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#10
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Solar System Cartographer Group: Members Posts: 10146 Joined: 5-April 05 From: Canada Member No.: 227 |
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 -------------------- ... because the Solar System ain't gonna map itself.
Also to be found posting similar content on https://mastodon.social/@PhilStooke NOTE: everything created by me which I post on UMSF is considered to be in the public domain (NOT CC, public domain) |
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Nov 2 2009, 09:03 PM
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#11
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Senior Member Group: Members Posts: 1018 Joined: 29-November 05 From: Seattle, WA, USA Member No.: 590 |
I would think that the Tharsis bulge is as effective as a tidally induced bulge when it comes to having an effect on Phobos. 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 |
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Nov 2 2009, 10:17 PM
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#12
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Senior Member Group: Members Posts: 3516 Joined: 4-November 05 From: North Wales Member No.: 542 |
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.
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Nov 3 2009, 04:27 PM
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#13
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Senior Member Group: Members Posts: 1018 Joined: 29-November 05 From: Seattle, WA, USA Member No.: 590 |
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Nov 3 2009, 04:56 PM
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#14
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Senior Member Group: Members Posts: 3516 Joined: 4-November 05 From: North Wales Member No.: 542 |
Seems unlikely. They certainly don't LOOK like two halves of the same whole. --Greg 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.p...ost&p=33612 |
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Nov 3 2009, 09:02 PM
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#15
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Senior Member Group: Members Posts: 3516 Joined: 4-November 05 From: North Wales Member No.: 542 |
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 |
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