Today, Mars has two tiny moons. But early in its history, the Red Planet may have had a much larger moon, which might be responsible for Mars' weird shape and extreme terrain, Michael Efroimsky, an astronomer at the U.S. Naval Observatory in Washington, D.C., proposes in a paper that has been submitted to the Journal of Geophysical Research: Planets and is available as a preprint via arXiv.

Mars hosts some of the most extreme terrain in the solar system, including the largest canyon, the tallest mountain and the greatest highland region. This highland region, known as the Tharsis bulge or Tharsis rise, dominates Mars' western hemisphere near its equator. The Tharsis region is about 3,100 miles (5,000 kilometers) wide and rises up to 4.4 miles 7 (km) high, excluding its massive shield volcanoes, which rise even higher. Almost exactly on the opposite side of the planet from Tharsis sits Terra Sabaea and Syrtis Major, another highland region and massive shield volcano, respectively.