Moons in the muse

Over the years, the kids have been taught that the moons of Mars are likely asteroids (due to their composition) that wandered through the inner solar system and subsequently were captured by the planet's gravitational pull.

The larger of two, Phobos, is doomed! It has an extremely low orbit -- the nearest revolution of any satellite to its primary in the solar system -- and it is slowly lowering, getting closer to Mars and eventually will either break up in the atmosphere or crash onto the surface!

But Phobos as a captured asteroid in such a near orbit is an unlikely scenario as the asteroid's necessary angle of approach should have caused it to break up in the Martian atmosphere.

Because of this incongruence and general curiosity, astronomers have been measuring the gravitational perturbations in the orbit of the Mars Express caused by Phobos. What they found was Phobos was much less dense than it likely would be if it was a captured asteroid. However, spectral analysis also revealed that Phobos' composition was similar to that of an asteroid and not the surface of Mars -- as it would be if the moon formed from ejected debris, the result of an asteroid crashing onto the surface.

So what's the deal?

One hypothesis is that an asteroid did crash onto the surface of Mars and the ejected debris, over time, began to accrete in near planetary orbit. That correlates with the density.

The astroid-al composition, it is supposed, came then from another large asteroid that collided with the proto-moon and was either captured by that or became part of the debris that re-accreted to form Phobos.

That's awfully complicated for such a dinky little moon.

Elsewhere in our fleet of spaceships, extraordinary Cassini dipped behind Saturn's moons Titan and Enceladus to measure the gravitational perturbations on Cassini by the former and to perform a spectral analysis of Enceladus' volcanic plumes, using the Sun as a back light.

Enceladus is unquestionably the rock star of moons!

The Big Picture has another marvelous spread of Cassini photos, Universe Today has a few words and The Planetary Blog has a great animation of Cassini approaching the Plumes of Enceladus.