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Martian moon may have come from impact on home planet

Spectral fingerprints of Phobos' surface support an ancient big crash origin for the Martian moon

Phobos, the larger of Mars' two tiny satellites, pictured near the limb of Mars by the robot spacecraft Mars Express in 2010.


The weird shapes and colors of the tiny Martian moons Phobos and Deimos have inspired a long-standing debate about their origins. The dark faces of the moons resemble the primitive asteroids of the outer solar system, suggesting the moons might be asteroids caught long ago in Mars' gravitational pull. But the shapes and angles of the moons' orbits do not fit this capture scenario. A fresh look at 20-year-old data from the Mars Global Surveyor mission lends support to the idea the moons of Mars formed after a large impact on the planet threw a lot of rock into orbit, according to a new study in the Journal of Geophysical Research: Planets, a publication of the American Geophysical Union.

The dataset held unplumbed clues to the stuff Phobos is made of, which may be more similar to the crust of the Red Planet than it appears, according to the study's authors. "The fun part for me has been taking a poke at some of the ideas out there using an old dataset that's has been underutilized," said Tim Glotch, a geoscientist at Stony Brook University in New York and the lead author of the new study. Marc Fries, a planetary scientists and curator of cosmic dust at NASA's Johnson Space Center, who was not involved in the new study, said the inability to explain the genesis of two moons around a neighboring planet is a glaring shortcoming in scientists' understanding of moon formation. Clearing it up will help with interpretations of how other moons and planets formed in our solar system and beyond. The new study does not clinch the mystery, but it is a step in the right direction, he said. "The issue of the origins of Phobos and Deimos is a fun sort of mystery, because we have two competing hypotheses that cannot both be true," Fries said. "I would not consider this to be a final solution to the mystery of the moons' origin, but it will help keep the discussion moving forward."

The debate over the origin of Mars' moons has split scientists for decades, since the early days of planetary science. In visible light, Phobos and Deimos look much darker than Mars, lending weight to the adoption hypothesis. Scientists 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. In visible and near-infrared light, 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 scientists looking at the orbits of Mars' moons argued they could not have been captured. These scientists 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.

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