The Red Planet has never been a particularly spherical one. Indeed, its shape has changed numerous times over its history and its polar axis has wandered significantly. New findings have revealed the locations of Mars's ancient poles, a finding that could shed light on the amount of underground water on the planet.

Jafar Arkani-Hamed of McGill University discovered that five impact basins--dubbed Argyre, Hellas, Isidis, Thaumasia and Utopia--form an arclike pattern on the Martian surface. Three of the basins are well-preserved and remain visible today. The locations of the other two, in contrast, were inferred from measurements of anomalies in the planet's gravitational field. His calculations, which will be published in the April 23 issue of the Journal of Geophysical Research (Planets), indicate that a single source--most likely an asteroid that was initially circling the sun in the same plane as Mars--created all five craters. At one point the asteroid passed close to the Red Planet, Arkani-Hamed posits, and was broken apart by the force of the planet's gravity. The resulting five pieces subsequently slammed into Mars along its then equator.

The center of the circle inscribed by the five craters, which marks the planet's ancient south pole, lies at present day latitude -30 and longitude 175. If corroborated, the new findings could have implications for assessing how much underground water may exist on the Red Planet. "The region near the present equator was at the pole when running water most likely existed [on the planet]," Arkani-Hamed explains. "As surface water diminished, the polar caps remained the main source of water that most likely penetrated to deeper strata and has remained as permafrost, underlain by a thick groundwater reservoir. This is important for future manned missions to Mars."