The Grooves of Phobos: A Mars Moon Mystery in Pictures

Meet Phobos

Phobos is one of the two moons that revolve around Mars. It's a lumpy, crater-filled pile of rubble that measures only 10 by 14 by 11 miles (17 by 22 by 18 km). Phobos also orbits closer to Mars than its smaller counterpart, Deimos, at an altitude of only 3,700 miles (6,000 kilometers). This photo was taken by the High-Resolution Stereo Camera on board ESA's Mars Express in Jan. 2008. Read our full story here: Weird Grooves On Mars' Moon Phobos Traced to Asteroids

First Photos Reveal Grooves

NASA's Viking 1 orbiter, one of the first spacecraft to go to Mars, took this photo of Phobos on June 10, 1977. Images from the Vikings revealed streaks and craters all over the surface of Phobos. Astronomers believed these long, parallel grooves were stress fractures, or 'stretch marks' caused by Mars' gravity. ['Stretch Marks' on Phobos Show Martian Moon Is Falling Apart]
Read our full story here: Weird Grooves On Mars' Moon Phobos Traced to Asteroids

The Surface Gets Stranger

Later images of the Martian moon revealed a second family of grooves. On July 23 2008, the High Resolution Stereo Camera on board the ESA’s Mars Express took the highest-resolution full-disc image yet of the surface of the moon Phobos. These images revealed a smaller and less noticeable set of grooves that didn't line up parallel to the tidal grooves. Read our full story here: Weird Grooves On Mars' Moon Phobos Traced to Asteroids

Dominated by a Crater

The massive Stickney Crater is the largest feature on the surface of Phobos. It stretches almost 6 miles (9.5 kilometers) at one end of the elongated moon, giving it the shape of a partially-deflated football. This color-enhanced photo was taken by the High Resolution Imaging Science Experiment (HiRISE) on NASA's Mars Reconaissance Orbiter. Read our full story here: Weird Grooves On Mars' Moon Phobos Traced to Asteroids

Grooves on Phobos

Researchers suggested that the grooves were not caused by gravity at all. Instead, they thought they might be chains of little impact craters. But these impact craters aren't tiny asteroids or comets. Rather, the craters came from Mars' rubble striking the moon when large impacts happened on the planet itself. Read our full story here: Weird Grooves On Mars' Moon Phobos Traced to Asteroids

Modeling Moon Dirt In Orbit

More recently, researchers at the University of California in Santa Cruz decided to test a different theory: What if the rubble that created these little crater chains wasn't from Mars, but from Phobos' own surface? This illustration demonstrates the sequence of events that they believe formed these craters on the Martian moon. (Orbital illustrations not drawn to scale)
Read our full story here: Weird Grooves On Mars' Moon Phobos Traced to Asteroids

Predicting Patterns of Crater Chains

Michael Nayak and Erik Asphaug at the University of California, Santa Cruz performed some computer simulations to test their hypothesis about the origins of Phobos' grooves. A computer simulation modeled primary impacts, or initial strikes that create major craters and send stuff flying into space, followed by the secondary and sesquinary impacts of the moon's rubble returning back to the surface. The crater chain pattern on the left matches the chain of craters their simulation predicted (right). Read our full story here: Weird Grooves On Mars' Moon Phobos Traced to Asteroids