Catastrophic blast created ‘Martian moon Phobos’

Tuesday, September 21, 2010

WASHINGTON - Scientists have found evidence that Mars’s biggest moon, Phobos, is made from rocks blasted off the Martian surface in a catastrophic event.

Two independent approaches of compositional analyses of thermal infrared spectra, from ESA’s Mars Express and NASA’s Mars Global Surveyor missions, yield very similar conclusions.

The re-accretion scenario is further strengthened by the measurements of Phobos’s high porosity from the Mars Radio Science Experiment (MaRS) on board Mars Express.

The origin of the Martian satellites Phobos and Deimos is a long standing puzzle. It has been proposed that both moons may be asteroids formed in the outer part of the main asteroid belt (between Mars and Jupiter) and were subsequently captured by Mars’ gravity.

Previous observations of Phobos at visible and near-infrared wavelengths have been interpreted to suggest the possible presence of carbonaceous chondritic meteorites, carbon-rich “ultra primitive” materials, commonly associated with asteroids dominant in the middle part of the asteroid belt.

This finding has supported the early asteroid capture scenario.

However recent thermal infrared observations from the Mars Express Planetary Fourier Spectrometer, show poor agreement with any class of chondritic meteorite. They instead argue in favor of the in-situ scenarios.

“We detected for the first time a type of mineral called phyllosilicates on the surface of Phobos, particularly in the areas northeast of Stickney, its largest impact crater,” said Giuranna of the Istituto Nazionale di Astrofisica in Rome.

“This is very intriguing as it implies the interaction of silicate materials with liquid water on the parent body prior to incorporation into Phobos.

“Alternatively phyllosilicates may have formed in situ, but this would mean that Phobos required sufficient internal heating to enable liquid water to remain stable,” he added.

“The asteroid capture scenarios also have difficulties in explaining the current near-circular and near-equatorial orbit of both Martian moons,” said Rosenblatt of the Royal Observatory of Belgium.

The MaRS team, led by Martin Patzold has used the frequency variations of the radio-link between the spacecraft and the Earth-based tracking stations, in order to precisely reconstruct the motion of the spacecraft when it is perturbed by the gravitational attraction of Phobos. From this the team was able to reduce Phobos’s mass.

“We obtained the best measurement of its mass to date, with a precision of 0.3 percent,” said Rosenblatt.

A highly porous asteroid would have probably not survived if captured by Mars.

Alternatively, such a highly porous Phobos can result from the re-accretion of rocky-blocks in Mars’ orbit.

Finally, a relatively smooth surface masks the space of voids inside the body, which then can only be indirectly detected. Thus, a highly porous interior of Phobos, as proposed by the MaRS team, supports the re-accretion formation scenarios.

The findings were published in the journal Planetary and Space Science. (ANI)

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