New observations of Mars' moon Phobos show the object is more like a pile of rubble than a single solid body.
New data from the European Space Agency's Mars Express spacecraft currently in orbit around the red planet are helping to shed light on the moon's origins.
Scientists have created a 3-D map of Phobos from high-resolution images taken during Mars Express's summer 2008 flybys of the moon, and used this data to calculate its volume.
Another team of researchers used radio signals from the spacecraft that vary based on the tug of Phobo's gravity to calculate the object's precise mass, and found that it weighs 1.072 x 10^16 kg (2.36 x 10^16 pounds), or about 1 billionth the mass of the Earth.
"We can be ten times more precise in our frequency shift measurements today" than previous estimates based on data from earlier satellite missions, said Pascal Rosenblatt of the Royal Observatory of Belgium, a member of the Mars Express Radio Science team.
The mass and volume information allowed researchers to reckon Phobos' density, and the scientists found that the moon is not solid, but probably filled with giant caverns.
Researchers call this kind of body, which is basically a clump of rocks held together by gravity, a rubble pile.
Phobos' density — 1.85 grams per cubic centimeter (0.067 pounds per cubic inch) — is lower than the density of Martian surface rocks, which are 2.7 to 3.3 grams per cubic centimeter (0.098 to 0.119 pounds per cubic inch).
In fact, its density is similar to that of some asteroids, suggesting that Phobos and its sister moon could be captured asteroids, rather than satellites created out of material from Mars itself.
To get to the bottom of Phobos' origins, samples of the moon must be brought back to Earth for analysis, scientists say.
A Russian mission planned to launch next year, called Phobos-Grunt (meaning "Phobos-soil"), could do exactly that, and will need Mars Express's precise measurements of Phobos' mass to land on the moon.
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