[LINK] "Massive blast 'created Mars moon'"

[rfmcdonald]

I had thought it practically established that Mars' moon Phobos--its companion Deimos too--was an asteroid captured by Mars' gravity. I had no idea that there was a possibility the moons might have been formed of debris thrown into Mars orbit by a massive impact, in a way similar to how our moon was formed save of smaller scale, but such may be the case.

Recent observations as thermal infrared wavelengths using the Planetary Fourier Spectrometer (PFS) instrument on Mars Express show a poor match between the rocks on Phobos and any class of chondritic meteorite known from Earth.

These would seem to support the "re-accretion" models for the formation of Phobos, in which rocks from the surface of the Red Planet are blasted into Martian orbit to later clump and form Phobos.

"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 co-author Dr Marco Giuranna, from the Italian National Institute for Astrophysics in Rome.

These phyllosilicate rocks are thought to form in the presence of water, and have been found previously on Mars.

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

[. . . S]aid Pascal Rosenblatt of the Royal Observatory of Belgium, "the asteroid capture scenarios also have difficulties in explaining the current near-circular and near-equatorial orbit of both Martian moons (Phobos and Deimos)".

The researchers also used Mars Express to obtain the most precise measurement yet of Phobos' density.

"This number is significantly lower than the density of meteoritic material associated with asteroids. It implies a sponge-like structure with voids making up 25%-45% in Phobos's interior," said Dr Rosenblatt.

A highly porous asteroid would have probably not survived if captured by Mars. Alternatively, such a highly porous structure on Phobos could have resulted from the re-accretion of rocky blocks in Mars' orbit.

Sky and Telescope goes into much greater technical detail.