Peculiar chains of craters located on the Martian moon Phobos are likely to have been created from material ejected during previous impacts on the moon. This material re-impacted the moon after some time in orbit, according to a study published in Nature Communications this week. These findings provide new insights into the geological history of satellites in our Solar System.

As a result of its close orbit to Mars, Phobos is affected by massive tidal influences from Mars’ gravity, which create large bulges and striations on its surface. However, many of these linear grooves and crater chains do not align to the direction of tidally predicted stress features, suggesting another origin must also be responsible.

Through modelling of the trajectory of ejecta material generated from a primary impact strike, Michael Nayak and Erik Asphaug found that crater chains on Phobos were created by specific impacts known as sesquinary impacts, whereby material ejected during a crater forming event on Phobos escapes the gravity of the satellite and spends a period of time orbiting Mars before re-impacting Phobos. The authors propose that these impacts create a chain of craters (sesquinary catenae) as the process is repeated.

Although sesquinary impacts are likely to be the cause of some of the crater chains, the massive tidal forces acting on Phobos also have a large influence on its surface features, note the authors.