The Martian moons Phobos and Deimos have been suggested to be captured asteroids based on the similarities between the dark, red, nearly featureless spectra of these bodies and D-class asteroids. However, the capture hypothesis suffers from difficulties associated with the shapes and inclinations of the Martian moons’ orbits. A new look at old data from NASA’s Mars Global Surveyor lends support to the idea Phobos (and likely Deimos) formed after a large impact on the planet threw a lot of rock into orbit. The dataset held unplumbed clues to the stuff Phobos is made of, which may be more similar to the Martian crust than it appears.

Planetary researchers study the mineral composition of objects by breaking the light they reflect into component colors with a spectrophotometer, creating distinctive visual ‘fingerprints.’

By comparing the spectral fingerprints of planetary surfaces to a library of spectra for known materials, they can infer the composition of these distant objects.

Most of the research into the composition of asteroids has examined their spectra in visible light and in near-infrared light, which is just beyond human vision on the red side of the visible spectrum.

Phobos and D-class asteroids look much the same — that is, both their spectra are nearly featureless because they are so dark.

D-class asteroids are nearly black as coal because, like coal, they contain carbon. This dark aspect of Phobos led to the hypothesis that the moon is a captive asteroid that flew a little too close to Mars.

But planetary scientists looking at the orbits of the Martian moons argued they could not have been captured. They believe the moons must have formed at the same time as Mars, or resulted from a massive impact on the planet during its formative millennia.

“If you talk to the people who are really good at orbital dynamics and figuring out why certain bodies orbit the way they do, they say that, given the inclination and the details of Phobos’ orbit, it’s almost impossible that it was captured,” said Dr. Tim Glotch, a geoscientist at Stony Brook University in New York.

“So you have the spectroscopists saying one thing and the dynamicists saying something else.”

Dr. Glotch and colleagues decided to look at the problem in a different light: the mid-infrared, which is in the same range as body temperature.

They compared the mid-infrared spectra of Phobos — collected in 1998 by the Thermal Emission Spectrometer carried on the Mars Global Surveyor — to samples of a meteorite that fell to Earth near Tagish Lake, British Columbia, which some scientists have suggested is a fragment of a D-class asteroid, and other rock types.

In the lab, they subjected their samples to Phobos-like conditions of cold vacuum, heating them from above and below to simulate the extreme changes in temperature from the sunny to the shady sides of airless objects in space.

“We found, 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,” Dr. 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 study does not argue Phobos is made entirely of material from Mars, but the new results are consistent with the moon containing a portion of the planet’s crust, perhaps as an amalgamation of debris from the planet and the remnants of the impacting object.

“The Tagish Lake meteorite is unusual, and perhaps not the best example of a D-class asteroid available for a compelling comparison with Phobos,” said Dr. Marc Fries, a planetary scientist and curator of cosmic dust at NASA’s Johnson Space Center, who was not involved in the study.

“The new study was unlikely to be able to produce a definitive answer because Phobos is subject to space weathering, which affects its reflectance spectrum and is difficult to replicate in the lab.”

The findings were published online this week in the Journal of Geophysical Research: Planets.

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Timothy D. Glotch et al. MGS-TES spectra suggest a basaltic component in the regolith of Phobos. Journal of Geophysical Research: Planets, published online September 24, 2018; doi: 10.1029/2018JE005647