Martian moon Phobos created by huge blast

Mars’ largest moon, Phobos, may have been blasted off into space by some catastrophe on the planet’s surface, say two different scientific studies.

Compositional analyses of thermal infrared spectra from ESA’s Mars Express and NASA’s Mars Global Surveyor missions, may solve the long-standing mystery of the origin of the Martian satellites Phobos and Deimos.

Some have suggested that both moons could be asteroids formed in the outer part of the main asteroid belt and subsequently captured by Mars’ gravity. Another possibility is that both moons were formed in situ by the re-accretion of rocky debris blasted into Mars’s orbit after a large impact, or by re-accretion of remnants of a former moon which was destroyed by Mars’s tidal force.

Previous observations of Phobos have suggested the possible presence of carbonaceous chondritic meteorites, carbon-rich materials commonly associated with asteroids.

However, new thermal infrared observations from the Mars Express Planetary Fourier Spectrometer don’t support this, but instead back up the ‘in-situ’ theory.

“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”, says Dr Marco Giuranna of Italy’s Istituto Nazionale di Astrofisica.

“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.”

Other observations appear to match the types of minerals identified on the surface of Mars.

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

Meanwhile, a team let by led by Dr Martin Pätzold of the Rheinisches Institut für Umweltforschungh an der Universität zu Kölnalso has found that Phobos is highly porous – and an asteroid with this structure would probably not have survived the process of capture by Mars.

The future Russian Phobos-Grunt mission, to be launched in 2011, may deliver more information.