A sample-return mission planned by Russia to Mars' moon Phobos could sample two celestial bodies for the price of one, a U.S. study suggests.

A study by Brown University confirms a suggestion that the surface of Phobos contains tons of dust, soil and rock blown off Mars' surface by large projectile impacts, and collected by the moon as its orbital path passed through occasional plumes of martian debris.

"The mission is scheduled to be flown early in the next decade, so the question is not academic," James Head, professor of geological sciences and an author on the study, said.

Phobos has been gathering martian castoff material for millions or years, he said.

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

The Russian mission will be the Roscosmos space agency's second attempt to return a sample from Phobos; a first try was launched in 2011 but an engine failure doomed the spacecraft before it could leave Earth orbit.