Space missions to Mars in the 1970s first brought back the surprising news that the Red Planet had once been awash in water. Ever since then, scientists have been working to determine the source of the flowing water that left telltale tracks on Mars's surface. Now findings published in the current issue of the journal Science suggest that collisions with comets and asteroids during the planet's early years may have played a role.

Teresa Segura of the University of Colorado and her colleagues used a mathematical model to investigate the potential relationship between markings on the Red Planet that indicate the presence of water and those created by collisions with asteroids and comets. "The river valleys and large craters of Mars may both be about the same age geologically," Segura explains. "We think that the two must be related, and our paper describes one possible connection." Specifically, the researchers modeled the impact of 25 bodies between 100 and 250 kilometers in diameter that bombarded the planet around four billion years ago. According to the team's calculations, the impact of even the smallest asteroid would deliver an amount of energy equivalent to millions of megatons of TNT to the planet. This influx of energy, the team posits, could have melted nearly three meters of polar ice and injected up to 16 meters of water into the atmosphere. Torrential rainfall--two meters a year--would have ensued.

These periods of rain, the scientists say, lasted from months to decades. "This happened dozens of times, maybe more, but after it rained, Mars would go dry," study co-author Kevin Zahnle of NASA's Ames Research Center comments. This hypothesis does not bode well for the existence of life on the Red Planet. The authors conclude that "only during the brief years or decades after the impact events would Mars have been temperate, and only then might it have bloomed with life as we know it."