Where Do They Come From?
Not everything in the solar system is as solid as it looks at first glance. Take Phobos, the larger moon that orbits Mars. It looks solid, but it may well be an orbiting pile of rubble. Now where would that rubble come from? Most likely a collision of some kind.
The European Space Agency’s Mars Express mission passed by Phobos this past summer and took a series of high-definition stereo images and data. That information got fed into a 3D modeling program that is letting astronomers measure this moon’s characteristics, including its volume and the interacting gravitational tugs between Mars and Phobos.
The analysis suggests that Phobos may be more of an asteroid than a body that evolved as a single piece.
The closest match that scientists can make for Phobos is with D-class asteroids, which are highly fractured and riddled with caverns. They are really more like pieces of rock that stick together by gravity. Scientists refer to these loosely grouped rock piles as “rubble piles.”
So, if it’s likely that Phobos didn’t form around Mars, how did this orbiting rocky junkyard get into its current equatorial orbit around Mars? There are two ideas. First, Mars gravitationally “captured” a passing rubble pile, which settled into orbit as Phobos. Deimos, the other moon, was likely captured the same way.
The other possibility is that a meteorite (a chunk of rock from an asteroid) smashed into early Mars and pieces of it got blasted back into space. Eventually, they clumped together into a rubble pile of rocks bound together by gravity, forming what we know today as Phobos.
In the near future the Russian space agency will send a probe called Grunt to Phobos to do study this moon and collect samples of rock for further study. Studies of rocks and continuing studies of Phobos’s subsurface structure (done using radars onboard the Mars Express spacecraft) will help astronomers figure out at least some of Phobos’s past and possibly even its origin.
As they say: stay tuned!
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