Origin of Phobos and Deimos, Where did these guys come from? |
Origin of Phobos and Deimos, Where did these guys come from? |
Mar 25 2006, 02:49 PM
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Member Group: Members Posts: 154 Joined: 17-March 05 Member No.: 206 |
So what is everyone's thoughts on the origin of Mars' moons Phobos and Deimos? They are a bit of a mystery.
Here are the different theories: 1. They formed along with Mars when it accreted out of the plantary nebula. Pros: explains how both are in the same circular, equatorial orbit around Mars. Cons: Seems a strange coincidence that we are around to witness Phobos in such a low orbit that it is about (in a couple million years) to crash out of orbit. Also this would be the only case in the solar system where such small "asteroid-like" moons formed around such a large body. 2. They were captured into orbit around Mars. Pros: This would explain their similarity to asteroids out in the Belt. Cons: The probability that they would be both be captured into circular and equatorial orbits is virtually zero. Also, there is no know mechanism for asteroids to be captured by such a small body like Mars (after all the moons didn’t do perigee burns to brake them into orbit) 3. They were once part of a larger moon that that broke up into several pieces. Phobos and Deimos are the last remnants of it. Pros: This would explain how both moons have circular and equaltorial orbits (since they started from the same body). Theoretically, there would have been many more moons at one time, but they have crashed into Mars one by one, as Phobos is on course to do. Cons: Phobos and Deimos do not appear to be very similar compositionally, which is strange if they came from the same moon. Of course it was large enough, the large proto-moon may have been differentiated. 4. The moons were formed from a large impact early in Mars history, perhaps from the impact that created the Hellas basin or the northern lowlands. This impact formed a small debris field around Mars which accreted into the moons. Pros: Explains the circular orbits of the moons and Moons created from early gigantic impacts seems to be a re-occurring theme we see in the rest of the solar system (i.e. Earth's Moon and likely Pluto's moons) Cons: While it explains the circular orbits, it does not explain how they are equatorial. I believe the favored theory this decade is number 3, where a large body was present, but was broken up. What is everyone's thoughts? |
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Jul 6 2016, 03:42 PM
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Solar System Cartographer Group: Members Posts: 10229 Joined: 5-April 05 From: Canada Member No.: 227 |
" I wonder if Phobos' orbital history can be traced backwards to approximate the age of the purported impact event that created it & perhaps identify a candidate crater."
No. The system is far too chaotic and unpredictable for that. Among other factors, mass redistribution via volcanism and polar volatiles cycling would alter the gravity field of Mars enough to introduce small orbital changes we can't model. Also the suggested existence of other moons in the past makes for interactions which can't be modelled. Also,"There are sufficient craters on Mars large enough" - no, there are not lots of craters big enough, only a few huge basins, and they are very old. Craters like Lyot or Lowell produce lots of ejecta, but only a small fraction of it could end up in a debris ring (if it's even possible for impacts that size). For those craters we might look at sources of SNC meteorites, not moons. Another point to consider. Phobos and Deimos are in orbits very close to the equator. But the obliquity of the axis of Mars swings wildly between about 0 and 60 degrees - where ours varies very little. So either the present low inclination orbits are a coincidence or the orbit planes also vary significantly over relatively short periods. Phil -------------------- ... because the Solar System ain't gonna map itself.
Also to be found posting similar content on https://mastodon.social/@PhilStooke Maps for download (free PDF: https://upload.wikimedia.org/wikipedia/comm...Cartography.pdf 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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