Mars’s “Death Star” Moon Just a Pile of Rubble

There’s always a twinkle in a science writer’s eye when real life imitates art.

In 2005 we had a snapshot of gases and dust around a star that seemed to be auditioning for the next Lord of the Rings film.

Then in 2007 there came the news that the universe could be packed with double-sunned planets like Star Wars’ Tatooine.

Earlier this year a Mars orbiter sent in high-resolution shots of a body called Phobos, highlighting its massive Stickney Crater and its uncanny resemblance to the Empire’s ultimate weapon.


With apologies to Sir Alec Guinness, this time that is a moon—Phobos is the larger of the two known natural satellites orbiting Mars.

—Image courtesy NASA/JPL-Caltech/University of Arizona

Although it was discovered way back in 1877, Phobos has remained fairly enigmatic.

In the late 1950s, its odd orbit inspired Russian astronomers to suggest that the moon is a hollow shell, and an artificial one at that.

It took almost a decade to silence that offbeat theory, based on better calculations of the moon’s orbit combined with new density measurements and eventually images from the Viking mission.

But Phobos still boasts some unusual characteristics, prompting much speculation about what the moon is made of and how it took up residence around Mars.

This week the European Space Agency released its newest images of Phobos taken by its Mars Express orbiter over a series of eight flybys.


—Image courtesy ESA/ DLR/ FU Berlin (G. Neukum)

The probe’s data on density and composition have ESA scientists “almost certain” that Phobos is a rubble pile, a loose collection of debris held together by gravity—and not a single, solid object.

If this is the case, it would support theories that Phobos and possibly also its “twin brother” moon Deimos are asteroids that got caught by Mars’s gravity. Another possibility is that they are lumped collections of debris that flew off Mars during an asteroid impact.

Oddly, one of the best pieces of supporting evidence for Phobos as rubble pile is its distinctively massive Stickney Crater.

On the surface it would seen reasonable to assume that an impact big enough to create a depression that size would have broken up a loose pile of rocks and scattered the debris to the cosmic wind.

But physics is phunny, and it turns out that such huge craters can exist only on weakly bound piles of stuff. That’s because the loose jumble actually dampens the propagation of shock waves after an impact, creating an indentation but allowing the overall structure to hold its shape.

It’s like the difference between shooting a gun at a rock versus firing at a sand dune. The bullet would shatter the solid rock into bits, but would merely put a dent in the sand.

With any luck, scientists should be able to put the story of Phobos’ origin to rest in the next few years. A planned Russian mission set to launch in 2009 aims to put a lander on the Martian moon and collect a soil sample to bring back to Earth for study.

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