Martian moon likely forged by ancient impact, study finds

Sept. 24 (UPI) -- New research suggest Mars' largest moon, Phobos, was formed from Martian debris excised by an ancient impact.

Because Phobos and Deimos are dark in color, some planetary scientists believe the two Martian moons are asteroids pulled into orbit by Mars' gravity. Other researchers argue the shape of the moons' orbits don't support an asteroid-capture origin.

"They say that, given the inclination and the details of Phobos' orbit, it's almost impossible that it was captured," Tim Glotch, a geoscientist at Stony Brook University in New York, said in a news release.

In an effort to solve the mystery of the moons' beginnings, scientists returned to a dataset collected in 1998 by NASA's Mars Global Surveyor mission.

Spectrographic analysis of the Martian moons typically relies on visible and infrared light, both of which reveal signatures similar to the radiation reflected by the surfaces of D-class asteroids. The Mars Global Surveyor data, however, features mid-infrared observations of Phobos, the darker of the two moons.

Mid-infrared light includes body heat. Just like visible light, thermal energy can be broken down into spectral bands, revealing a unique signature.

The mid-infrared analysis -- detailed Monday in the Journal of Geophysical Research: Planets -- showed Phobos heat signature is most similar to Martian surface features.

"At these wavelength ranges, the Tagish Lake meteorite doesn't look anything like Phobos, and in fact what matches Phobos most closely, or at least one of the features in the spectrum, is ground-up basalt, which is a common volcanic rock, and it's what most of the Martian crust is made out of," Glotch said. "That leads us to believe that perhaps Phobos might be a remnant of an impact that occurred early on in Martian history."

The match isn't perfect, but Phobos' heat signature suggests the moon is at least partially composed of Martian crust -- perhaps mixed with remnants of the impacting body.

The new research isn't sufficient to settle the debate once and for all, authors of the study acknowledged.

"But it will help keep the discussion moving forward," said Marc Fries, a planetary scientist and curator of cosmic dust at NASA's Johnson Space Center.

"The really cool thing is that this is a testable hypothesis, because the Japanese are developing a mission called MMX that is going to go to Phobos, collect a sample and bring it back to Earth for us to analyze," Glotch said.