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Phobos born
from Mars impact

Posted: 21 September 2010

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New evidence from ESA's Mars Express and NASA's Mars Global Surveyor missions supports the idea that martian moon Phobos was born from a catastrophic impact on the surface of the red planet.

Mars has two irregularly shaped moons, Phobos and Deimos, and their origin has long been debated, falling into three main camps: were they captured as asteroids from the main Asteroid Belt, or did they form in situ from impact debris from a catastrophic impact on the martian surface, or re-accretion of a shattered moon?

Phobos (lower right) and Deimos (lower left) compared with asteroid 951 Gaspra (top). Although they appear similar in appearance, the martian moons likely underwent a very different evolution to asteroids. Image: NASA/JPL.

New evidence from two independent surveys of the thermal infrared spectra of Phobos point to the impact theory, but, says, Dr Giuranna of the Istituto Nazionale di Astrofisica in Rome, “Understanding the composition of the Martian moons is the key to constrain these formation theories.”

Past observations at visible and near infrared wavelengths had implied the presence of primitive carbon-rich materials on the moons, which are usually associated with asteroids that populate the central part of the Asteroid Belt. But recent thermal infrared observations from Mars Express' Planetary Fourier Spectrometer did not find any such evidence, instead finding signatures that match types of minerals identified on Mars' surface.

“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 Giuranna. “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. More detailed mapping, in-situ measurements from a lander, or sample return would ideally help to settle this issue unambiguously.”

Phobos, with its large 9 km wide Stickney crater (Phobos has dimensions of roughly 27 x 22 x 18 km). Scientists say that the presence of Stickney is evidence that Phobos is highly porous; a less porous body would like have shattered after an impact of this energy. Image: NASA/JPL-Caltech/University of Arizona.

Adding in data from Mars Express' Mars Radio Science Experiment (MaRS), which uses the frequency variations of the radio-link between the spacecraft and the Earth-based tracking stations to reconstruct the motion of the spacecraft when it is perturbed by the gravitational attraction of Phobos, has provided high-precision information on its mass with an accuracy of 0.3 percent. The MaRS team, led by Martin Patzold of the Rheinisches Institut fur Umweltforschungh at the University of Koln, also arrived at a density estimate of Phobos of 1.86±0.02 g/cm3.

“This number is significantly lower than the density of meteoritic material associated with asteroids,” reports Dr Rosenblatt of the Royal Observatory of Belgium. “It implies a sponge-like structure with voids making up 25-45 percent in Phobos’ interior.”

The high porosity supports the re-accretion models, with large blocks accreting first then smaller blocks not quite infilling all of the gaps, and a relatively smooth veneer masking the internal void spaces. A high-porosity also permits the formation of Phobos' large Stickney crater by absorbing much of the impact energy, which might have otherwise caused the tiny moon to shatter.

The final piece of evidence comes from considering the moons' near-circular and near-equatorial orbits – a much more likely result of re-accretion around Mars than by the capture scenario. But these pieces of evidence still cannot conclusively determine the moons' origin. More vital evidence will be provided by the Russian Phobos-Grunt mission which, launching in late 2011, is a sample return mission that will enable scientists to directly analyse the moon's composition and hopefully arrive at a sound conclusion as to these tiny moons' intriguing evolution.