14 April 2014
Nature: Although Mars’s topography indicates that water probably flowed on the planet's surface at one time, the atmosphere has generally been too thin to allow surface temperatures to rise above freezing. To find out how Mars’s layered sediments and rugged canyons could have formed, Edwin Kite of Princeton University and colleagues examined crater images taken by the Mars Reconnaissance Orbiter and ran computer simulations of meteors striking the planet’s surface. Because of the number and distribution of small craters, they determined that ancient Mars’s atmosphere was not thick enough to break up small meteors and therefore too thin to warm the planet enough to melt ice. Instead, Mars may have experienced periodic bouts of warming caused by variations in the tilt of its axis, greenhouse gases from volcanic activity, or gases released when meteors struck its surface. Such phenomena could have temporarily thickened the atmosphere for decades or centuries, long enough to melt surface ice and allow water to flow.