Paris, Oct 17 : Scientists at the European Space Agency (ESA) are getting closer to unraveling the origin of Mars' larger moon, Phobos, thanks to a series of close encounters by ESA's Mars Express spacecraft.
So far, observations by the Mars Express have found that the moon looks almost certain to be a 'rubble pile', rather than a single solid object.
However, mysteries remain about where the rubble came from.
Unlike Earth, with its single large moon, Mars plays host to two small moons. The larger one is Phobos, an irregularly sized lump of space rock measuring just 27 km x 22 km x 19 km.
During the Summer, Mars Express made a series of close passes to Phobos. It captured images at almost all flybys with the High Resolution Stereo Camera (HRSC).
A team led by Gerhard Neukum, Freie Universitaet Berlin, also involving scientists from the German Aerospace Centre (DLR), is now using these and previously collected data to construct a more accurate 3D model of Phobos, so that its volume can be determined with more precision.
In addition, during one of the nearest flybys, the Mars Express Radio Science (MaRS) Experiment team led by Martin Paetzold, Rheinisches Institut fuer Umweltforschung at the University of Cologne, carefully monitored the spacecraft's radio signals.
They recorded the changes in frequency brought about by Phobos' gravity pulling Mars Express.
This data is being used by Tom Andert, Universitaet der Bundeswehr Muenchen and Pascal Rosenblatt, Royal Observatory of Belgium, both members of the MaRS team, to calculate the precise mass of the Martian moon.
Putting the mass and volume data together, the teams will be able to calculate the density. Eventually, this will be a new important clue to how the moon formed.
Previously, radio tracking from the Soviet Phobos 88 mission and from the spacecraft orbiting Mars in the past decades had provided the most accurate mass.
"We can be ten times more precise in our frequency shift measurements today," said Rosenblatt.
The team's current mass estimate for Phobos is 1.072 1016 kg, or about one billionth the mass of the Earth.
The MARSIS radar on board Mars Express has also collected historic data about Phobos' subsurface.
This data, together with that from the moon's surface and surroundings gathered by the other Mars Express instruments, will also help put constraints on the origin.
It's clear though that the whole truth will only be known when samples of the moon are brought back to Earth for analysis in laboratories.
This exciting possibility might soon become reality because the Russians will attempt to do this with the Phobos-Grunt mission, to be launched next year.