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
Post
#1
|
|
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? |
|
|
Mar 25 2006, 06:16 PM
Post
#2
|
|
Solar System Cartographer Group: Members Posts: 10227 Joined: 5-April 05 From: Canada Member No.: 227 |
Well... this is an interesting subject, but not one we can yet resolve.
First, we don't know the compositions at all well. The old statement that they are carbonaceous asteroids is not well supported with evidence. They are too faint and too close to Mars for good spectral studies from Earth (I might not be up to date on that, though, my attention has been elsewhere). Very little compositional work has been done by spacecraft. Phobos 2 IR spectroscopy is probably best. But contamination by Mars ejecta and also by light scattered off Mars complicates the issue. In truth we can't say yet whether these moons are the same composition or different, or if they are asteroidal, bulk Mars or more like Mars regolith in composition. Actually there could be a mixture of all these components on the surfaces of the moons. Second - only Phobos is in a decaying orbit. But that fact does suggest it might not have been there for 4.5 Ga. I don't see the equatorial circular orbit as too serious a problem, since close orbits of oblate planets ought to evolve in that direction if they have time. But capture is VERY difficult to make work. Ejecta seems more likely to me than capture. Maybe we can try to put this together: a very early large impact (Hellas or northern plains) puts a lot of debris into orbit. Actually most would not stay in orbit, but complex dynamics may allow a bit to end up in an equatorial ring which can then accrete into a moon. This would be very early, and maybe would have to be out near the synchronous orbit area. It sits there for a while. Then it is fragmented by a large impact. Phobos and Deimos are the only two remaining fragments of that disruption. One was thrown just inside the synchronous orbit and has slowly drifted inwards - Phobos. One was pushed out a bit and has slowly evolved outwards - Deimos. This disruption had to happen quite a long time ago, so Phobos has had time to acquire its dense population of craters. Oh - and Stickney is probably not the cause of the grooves. That set of internal fractures is more likely to date to the disruption, and I'm assuming that the parent body was quite well consolidated in order for that to be true. Stickney might have shaken them open a bit. We really need a Phobos and/or Deimos sample return mission. I believe it is now the highest priority sample return mission for which we actually have the technology today. 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) |
|
|
Lo-Fi Version | Time is now: 24th September 2024 - 10:12 PM |
RULES AND GUIDELINES Please read the Forum Rules and Guidelines before posting. IMAGE COPYRIGHT |
OPINIONS AND MODERATION Opinions expressed on UnmannedSpaceflight.com are those of the individual posters and do not necessarily reflect the opinions of UnmannedSpaceflight.com or The Planetary Society. The all-volunteer UnmannedSpaceflight.com moderation team is wholly independent of The Planetary Society. The Planetary Society has no influence over decisions made by the UnmannedSpaceflight.com moderators. |
SUPPORT THE FORUM Unmannedspaceflight.com is funded by the Planetary Society. Please consider supporting our work and many other projects by donating to the Society or becoming a member. |