In astronomy news this week: Dust storms are seen blowing around Titan for the first time, and new clues from old data suggest that an impact on Mars gave birth to Phobos.

Dust storms on Titan

Saturn’s largest moon, Titan, now joins Earth and Mars among the worlds in our solar system known to whip up dust into massive, roaming storms.

In 2009 and 2010, the Cassini spacecraft spied three distinct, short-lived, relatively bright patches of infrared light near the moon’s equator. The timescale and spectra of these regions appear to be most consistent with dust storms whipped up by gusts blowing across dunes.

Sébastien Rodriguez (Paris Institute of Earth Physics) and colleagues report their findings September 24th in Nature Geoscience.

Infrared maps showing dust storms on Titan
Three alleged dust storms on Titan, marked by white arrows in the middle panels, can be seen in these composite infrared images from Cassini. White arrows in top and bottom rows mark locations of storms before and after they appeared.
Credit: NASA / JPL-Caltech / University of Arizona / University Paris Diderot / IPGP /S. Rodriguez et al. 2018

The researchers considered several explanations, including cryovolcanoes, lava flows, and methane rain storms. But none of these were a good match for the infrared spectra recorded by Cassini or the observed timescales, ranging from 11 hours to 5 weeks.

Instead, simulations of the atmosphere implicate surface-hugging clouds of dust particles, roughly 5 micrometers in diameter. The largest of these clouds spanned about 420,000 square kilometers.

Dust on Titan isn’t too surprising. When the Huygens probe landed on Titan in 2005, it kicked up a small cloud of dust that persisted for a few seconds. However, the dust storms do point to fairly strong surface winds—or at least, strong for Titan. The roving dust requires gusts of at least 3 mph, which is about five times the estimated ambient wind speed. The winds could have accompanied methane squalls that are expected at the equator during Titan’s equinox.

Impact Origin for Phobos

The origin of Mars’s moons, Phobos and Deimos, is the subject of ongoing debate. Some researchers say the moons are captured asteroids, others insist they are chunks of Mars’s surface blown off in an asteroid impact.

A team led by Timothy Glotch (Stony Brook University) makes an argument in favor of the asteroid impact in the September 24th Journal of Geophysics Research: Planets.

Phobos over Mars
Phobos hovers over the Martian landscape in this image from ESA's Mars Express orbiter taken in November 2010. Irregularly shaped and only 27 km long, Phobos is actually much darker (due to its carbon-rich surface) than is apparent in this contrast-enhanced view.
ESA / DLR / G. neukum

The researchers combed through twenty-year-old data from NASA’s Mars Global Surveyor, a global mapping mission that lasted from 1997 to 2007. They looked at mid-infrared spectra of Phobos and compared it with laboratory spectra of a variety of materials studied in a simulated, airless environment. The spectra of Phobos most closely matched those of basalts, one of the main ingredients of the Martian surface.

The team also compared Phobos’ spectra to that of the Tagish Lake meteorite—a chunk of a meteoroid that exploded over British Columbia, Canada, in 2000—and found that the two spectra were a poor match. These two comparisons suggest that Phobos is at least partly composed of material from the Red Planet’s surface, Glotch and colleagues argue.

This debate is far from over, however. In visible and near-infrared light, Phobos looks very similar to D-class asteroids, which the Tagish Lake meteorite is thought to represent. Both are nearly black with featureless spectra. Phobos is also subject to extensive space weathering, which is difficult to replicate in a lab, and the Tagish Lake meteorite might not even be the best example of a D-class asteroid.

Stay tuned!


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