The Lost Martian Moon.

[Krupin]

Phobos, the Martian moon, quickly approaches to the surface of the planet due to tidal effect (because Phobos's orbital period is shorter than a Martian day). Only a few hundred million years ago Phobos had to housed almost in a stationary orbit. The probability of formation of Phobos in accuracy in a stationary orbit is negligible. Therefore we must explain how Phobos could be in this orbit.

The author offers the following explanation. After its formation the system of Martian moons consisted of three bodies, like modern Plutonian system. In addition to the tiny moons Phobos and Deimos Mars had another large moon (moon X). Phobos was in orbital resonance with the moon X, and this resonant system was stable and self correcting. Phobos was, as it is now, located below the stationary (areostationary) orbit. The moon X was located above it and tidal influence of Mars pushed the moon X away from the Martian surface. In an effort to keep the orbital resonance, resonant influence of the moon X pulled Phobos up too, in spite of the tidal effect of Mars.

When Phobos had reached stationary orbit, new factor had appeared. This factor was an asymmetry of the Martian surface. Because Phobos's orbital period became equal to Martian day, Phobos located stationary above the surface on Mars, herewith some mountain area was behind him. The force of gravity of huge stone rocks permanently pulling Phobos ago, preventing resonance effect of the moon X. As a result, the moon X continued to rise, and Phobos had stopped at a stationary orbit. Orbital resonance was be broken. From this point Phobos was lowered under the influence of tidal friction of Mars.

The moon X escaped from Mars because of tidal effect. Or it was broken accidentally flown asteroid. Perhaps some meteorites are fragments of the moon X.

[CraigD]

How both of Mars’s moons got to be in their present orbits is a interesting mystery. A greater mystery, I think, than how Phobos came to have an orbit lower than synchronous, is how either it or Deimos came to have such low (Phobos is 9,377 km from Mars’s center, Deimos 23,460 km, just slightly above its 20,400 km synchronous orbit), and more mysterious still, nearly circular, orbits.

 

I think your idea of a third, large moon – “Moon X” is intriguing, Krupin, but has some serious problems.

 

First, how was Moon X formed, captured, or ejected by Mars? The best guess, I think, is that, as best current theory (the “giant impact hypothesis”) proposes in the case of Earth’s moon, it would have resulted from a collision of Mars with a large (10²² kg +) body. But extensive computer simulation modeling of Earth’s hypothetical giant impact, indicate, that though it would have resulted in many Deimos and Phobos size-bodies (10¹⁵ to 10¹⁶ kg), none of them being stabilized in nearly circular orbits, nearly all coalescing into the Earth and the Moon, a few remaining at Lagrangeian points for 100,000,000 years or so before being perturbed out of them and falling to Earth.

 

Moon X being captured by Mars seems to me unlikely, as capture models rely on aerobraking with Mars’s atmosphere. The effectiveness of Aerobraking is proportional to surface area / mass, so is roughly inversely proportional to mass. A 10²² kg body, then, would be 1,000,000 times less likely to be captured by Mars than a 10¹⁶ kg, Phobos-size.

 

Second (and last I’ll mention), what happened to Moon X? By no model I’ve personally run, or read about, can tidal effects eject a moon – although they do, very slowly, increase outer moons’ orbits. That leaves your explanation of an impact with another large body – an “broken [by an] accidentally flown asteroid”. But such an impact would not destroy Moon X, only fragment it in its orbit. It would eventually re-coalesce (models show this happening mostly in less than 1 year, entirely in less than 100).

 

This leaves the only explanation for the disappearance of Moon X that it was perturbed by a large body passing near it. While this seems plausible to me, how would such an event have not perturbed Deimos and Phobos from their nearly circular orbits?

 

What a hypothesis like yours needs to be credible and convincing, answering questions like the above, is computer simulations. Have you done any? :QuestionM: Would you like to? It could be fun! :)