Tiny, lumpy, and dark, the Martian moons Phobos and Deimos have long sparked hot scientific debate over their origins. Some researchers argue that the moons are former asteroids, lassoed in by Mars’s gravity. Others hold that the moons formed when a massive impact knocked chunks of Martian rock into orbit. Now, a new examination of infrared data captured decades ago by the Mars Global Surveyor supports the latter hypothesis—at least Phobos, it suggests, is a chip off the old Martian block.
Launched in 1996, NASA’s Mars Global Surveyor finally lost touch with Earth on 2 November 2006, likely because of a battery failure. In the meantime, in addition to a wealth of data on Mars’s rocks, soil, ice, and atmospheric dust, it captured a few glimpses of the Red Planet’s two moons, Phobos and Deimos. Images captured in visible light reveal that the moons are dark as coal—much darker than Mars itself. Scientists have long interpreted this striking difference in appearance as evidence that the moons could not have come from Mars but were once carbon-rich asteroids. Others argue that the moons’ orbits, shapes, and inclinations do not fit the asteroid scenario.
In their new study, Glotch et al. looked at a different data set collected by Mars Global Surveyor: the thermal infrared energy, or heat, emitted by Phobos, the larger of the two moons. Using the Thermal Emission Spectrometer instrument, which can determine the mineral and chemical composition of a material on the basis of such emissions, the team compared Phobos’s infrared signature to that of a carbon-rich meteorite that fell near Tagish Lake in British Columbia, which some researchers have suggested is similar to the type of asteroid that could have formed Phobos. First, however, the team exposed the meteorite chunks to conditions similar to those on Phobos: a cold, airless vacuum with extreme temperature swings.
The infrared signature from the meteorite did not resemble that of Phobos, the analysis revealed. Instead, Phobos’s infrared spectrum looks more like those produced by volcanic rocks. Mars’s crust is made up mostly of finely ground basalt, a common volcanic rock, so the new study supports the hypothesis that Phobos—and possibly Deimos—was indeed formed from an early impact with Mars.
Settling the debate for good will require going to Phobos. Luckily, plans are in motion to do precisely that: In the early 2020s, the Japanese plan to launch the Martian Moons Exploration, which will collect a sample of rock from one of the moons. (Journal of Geophysical Research: Planets, https://doi.org/10.1029/2018JE005647, 2018)
—Emily Underwood, Freelance Writer