The Moons of Mars

Mars has two satellites and they are so small they were not discovered until 1877.

Phobos and Deimos, Mars’ little warrior twin children are named after Greek god of war Ares’ sons.  So, we use the Roman name Mars but the Greek names for his kids.

They are quite small; Deimos is 7.5 miles in diameter and Phobos is a whopping 13.7 miles.  Their orbits are close; Phobos is 5828 miles and Deimos 14,577 miles – so observers on Mars, when there are observers on Mars, will get a show.  Phobos appears around 1/3 the size of our Moon and zips across the Martian sky in about 4 hours.  Deimos looks slightly bigger than Venus does from Earth and takes nearly three Martian days to rise and set.  Their masses indicate they are not solid, likely aggregated material.  Phobos is showing signs of gravitational/tidal stress from Mars in the form of long faults.

How these little satellites came to be is still under investigation.

One hypothesis is they are captured asteroids. Phobos and Deimos have similar spectral characteristics with ancient carbonaceous-chondrite bodies.  But their orbits are nearly circular.  Captured asteroids have their own momentum and would enter eccentric elliptical orbits.  They either would need more time or outside forces to circularize their orbits.  Also, basaltic rock in orbit can, over time produce similar spectra.  So, the captured asteroid hypothesis, while still on the table, is losing supporters.

A Giant Impact, the theory of how Earth’s Moon formed is considered a possibility, but not without questions, such as why such a discrepancy in their distances?  After many simulation scenarios were tried one seems to address the situation.  The impactor slammed into Mars, forming the Borealis basin over 4 billion years ago.  According to this model the mass of debris sent into orbit was way more than needed to form Phobos and Demos.  However, much of the material could not coalesce because it was below the Roche limit, where Mars’ gravitational/tidal forces kept the material perturbed.  This material eventually fell back to Mars.

The material just outside the Roche limit coalesced into Phobos and material further out formed Deimos.  The kicker, in this simulation Phobos is still 1000 times its current mass.

Other investigators looking at the simulation have proposed the possibility of a recurring cycle of Phobos breaking-up and reforming since its orbit is so close to the Roche limit. Mars’ tidal force is dragging Phobos closer and eventually will cause it to break up.  They propose that each time it occurs, Phobos loses mass as some material falls back to Mars, and the remaining material reforms after going beyond the Roche limit.  Deimos on the other hand is drifting away, like our Moon.

Future missions to Phobos and Deimos will help sort this out.

What’s in the Sky?

April 30; 20 minutes after sunset; west-northwest: See Venus, Mercury, and the Pleiades form a vertical line up from the horizon.