Billions of years ago, Mars suffered from numerous big impacts, and the resulting backwash ultimately scarred the surface of Phobos, one of the Red Planet's two tiny moons, researchers say.
In 1976, images from NASA's Viking orbiter revealed that the surface of Phobos is covered in numerous parallel, channel-like grooves. Over the years, researchers have come up with many hypotheses to explain the odd features, but the origin of the satellite's grooves are still heavily debated today.
In the new study, a pair of researchers reviewed the evidence for the major hypotheses and concluded that only one holds water: The grooves are chains of secondary impacts, the landing sites of material blasted to the Mars moon by impacts on the Red Planet. [Moons of Mars: Amazing Photos of Phobos and Deimos]
Using new data and images from the European Space Agency's Mars Express orbiter, the scientists also mapped the grooves in much greater detail than ever before, and calculated that the amount of Mars material needed to form all of Phobos' grooves is about two orders of magnitude lower than the total ejecta from Mars' craters.
"Everything fits in with this hypothesis," said John Murray, a planetary scientist at Open University in the U.K., and lead author of the new study, which was published in April in the journal Planetary and Space Science. "We can even trace the ejecta that produced the grooves back to [source areas] on Mars."
Most scientists think Phobos and Deimos, Mars' two minuscule moons, are former asteroids captured by the Red Planet's gravity long ago. Phobos is 14 miles (22 kilometers) wide, while Deimos has a diameter of just 7.7 miles (12.4 km).
The 3-billion-year-old grooves on Phoboscan be divided into different "families," with each groove plane running parallel to the other grooves within the same family, Murray told Space.com.
The widths of the grooves vary greatly, from 76 feet (23 meters) to 1,558 feet (475 m). Similarly, they come in a large range of lengths — at least one groove stretches for 18.5 miles (30 km) without any breaks, while others are only a little more than 1 mile (1.6 km) long.
The channels cover almost the entire surface of the moon, except for a relatively small area on the hemisphere facing away from Mars, Murray said. (Phobos is tidally locked with Mars, meaning that one side of the moon faces the planet at all times.)
Some scientists have previously speculated that the grooves are fractures resulting from tidal forces, the impact that created Phobos' prominent Stickney Crater or other sources.
"It hasn't really been a generally accepted idea, or one that has gained universal approval," Murray said, adding that there are several issues with all fracture hypotheses for the origin of the grooves. For instance, the near-perfect alignment of the grooves within each family doesn't fit with other fracture fields throughout the solar system. [7 Biggest Mysteries of Mars]
Other hypotheses posit that the grooves on Phobos are the result of local impacts. According to one idea, the meteor that created Stickney Crater kicked up ejecta that showered Phobos, creating the grooves; a related hypothesis proposes that rolling boulders from the crater scarred Phobos. Or, the grooves may have developed when Phobos was hammered by orbiting debris, according to some researchers.
But none of these ideas can explain all of the observed characteristics and patterns of the grooves, Murray said.