[X] My Assistant

[Antdoghalo]

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.

[Greg Hullender]

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