Mission to Phobos Should Yield Soil from Mars and its Moon

The Martian moon Phobos is likely riddled with dust, soil and rock blown off the Mars surface by over millions of years worth of large projectile impacts, and sending a probe to the moon on a sample-return mission could yield a cosmic two-for-one, providing samples from the Red Planet and its largest moon, according to new research ahead of a planned mission to the distant world.

Russian space agency Roscosmos plans to send a probe to Phobos to collect soil samples in 2020, a second attempt after the launch of its Phobos-Grunt probe in 2011 suffered a technical setback which prevented the craft from leaving low Earth orbit.

James Head, professor of geological sciences at Brown University, led the study suggesting Martian matter could be found on Phobos and returned to Earth. The works appears in the latest issue of Space and Planetary Science.

"This work shows that samples from Mars can indeed be found in the soil of Phobos, and how their concentration might change with depth. That will be critical in the design of the drills other equipment," he said.

The new research grew out of preparation for the original Phobos-Grunt mission, which would still be en route to the moon had it not fallen out of low Earth orbit in an uncontrolled crashed into the Pacific Ocean in January 2012.

It has long been suspected that Phobos will contain bits of Martian soil and dust, but Russian mission planners wanted to get a better idea of what to expect and prepare for, turning to Head and his colleague Ken Ramsley for expert advice, specifically on how much Martian matter is on Phobos and where it might be found.

To answer those questions, the scientists began with a model based on our own Moon to estimate how much of Phobos' regolith (loose rock and dust on the surface) would come from projectiles like meteorites. Then they calculated how much the ejected matter would have resettled on Phobos' surface using gravitational and orbit data.

"When an impactor hits Mars, only a certain of proportion of ejecta will have enough velocity to reach the altitude of Phobos, and Phobos' orbital path intersects only a certain proportion of that," Ramsley said. "So we can crunch those numbers and find out what proportion of material on the surface of Phobos comes from Mars."

According to their calculations, Phobos should contain Martian material at a rate of about 250 parts per million, distributed fairly evenly across the surface and mostly on upper layers.

"Only recently - in the last several 100 million years or so - has Phobos orbited so close to Mars," Ramsley said. "In the distant past it orbited much higher up. So that's why you're going to see probably 10 to 100 times higher concentration in the upper regolith as opposed to deeper down."