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Martian moon Phobos a rubble pile?

Posted: October 20, 2008

New Mars Express observations of Martian moon Phobos suggest it could be a rubble pile rather than a single solid object, but questions remain as to where the material actually came from in the first place.

This image was taken with the HRSC from a distance of 352 kilometres from the moon's centre. The origin of the tiny moon is much debated, since its density places it in the class of D-type asteroids rather than with rocks found on the surface of Mars. Image: ESA/ DLR/ FU Berlin (G. Neukum).

Mars plays host to two small moons, Phobos and Deimos, with the larger moon Phobos measuring 27 x 22 x 19 kilometres. During the summer, ESA’s Mars Express spacecraft made several close flybys of Phobos, and data gleaned from new High Resolution Stereo Camera (HRSC) images are being used to determine the moon’s volume more precisely and to generate a three dimensional map of the moon’s surface. The spacecraft’s radio signals were also carefully tracked, since any change in frequency allows scientists to determine the gravity of the moon pulling the spacecraft in, and can be used to calculate the mass of the moon. Previous radio tracking from the Soviet Phobos 88 mission and from other spacecraft orbiting Mars in the past decades had provided the most accurate mass to date of 1.072 x 10^16 kilograms, or about one billionth the mass of Earth.

"We can be ten times more precise in our frequency shift measurements today," says Pascal Rosenblatt of the Royal Observatory of Belgium and member of the Mars Express Radio Science (MaRS) team. By combining the mass and volume data, planetary scientists will be able to calculate the density of Phobos. Preliminary calculations put the density at just 1.85 grams per cubic centimetre, which is much lower than the density of typical Martian surface rocks at 2.7-3.3 grams per cubic centimetre.

Artist's concept of the Russian Phobos-Grunt spacecraft that has been designed to collect samples of soil on Phobos and to bring them back to Earth for analysis, in order to learn more about the evolution of the Martian system. Image: Babakin Science and Research Space Center.

The density, however, is very similar to that of the D-class asteroids, which are believed to be highly fractured and porous bodies made up of different pieces of rock, hence their nickname of rubble piles. Previous observations have already suggested that the Martian moons’ composition closely resemble that of the asteroids, and theories have proposed that the moons could be captured asteroids.

But there is one problem with that idea. Usually asteroids are flung into random orbits around their host planet after they are captured by the planet’s gravity, but Phobos orbits around Mars' equator – a very precise location that is giving scientists a headache as to how this came to be. One solution is that Phobos could have resulted from Martian rocks that were blasted into space during a large meteorite impact, fusing together in orbit around the Red Planet in a scaled down version of the formation of our own Moon, which was born from material thrown out in a giant impact event of a Mars-sized body with the young Earth. Alternatively, the moon could have formed in its present position, and is therefore a remnant from the planetary formation period.

The answer to the mystery of the tiny moon’s origin may have to wait until samples are returned from the moon’s surface for analysis back on Earth. But we may not have to wait too long; the Russians will be attempting a sample return mission – the Phobos-Grunt mission – that is scheduled to launch in 2009. The mission will rely on the precise knowledge of mass as measured by the Mars Express radio science experiment in order to navigate correctly, and will use the HRSC images to select a suitable landing site. This, combined with other new Mars Express data, such as that from the Mars Advanced Radar for Subsurface and Ionosphere Sounding experiment (MARSIS) to determine information on Phobos’ subsurface, will clearly help place constraints on the tiny moon’s origin.

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