After studying geologic and topographic information from Mars collected over the past decade, scientists are suggesting there was once an ancient drainage basin the size of the U.S. located in Tharsis, a region in the Red Planet's western hemisphere. James Dohm, a hydrologist at the University of Arizona, and his colleagues describe their theory in a paper set to be published in the Journal of Geophysical ResearchPlanets.

Tharsis is currently home to towering volcanoes, lava fields, fault systems and extensive canyons. The researchers reconstructed its evolution over the past three billion years using both recent data collected by the Mars Global Surveyor Spacecraft and decade-old Viking data. The findings imply that there were five geologic stages, which the researchers have reconstructed in a 3-D visualization that starts with the ancient basin and ends with the present-day landscape.

The scientists estimate the basin ranged in depth from two to seven kilometers and, had it been filled, would have had a capacity of 12 billion billion gallons. Early in the planet's history the basin was transformed into a vast aquifer, the researchers say, as lavas, sediments and volatiles, among them water, partly filled the chasm. Volcanic activity later fractured and deformed the aquifer and gave rise to the present-day landscape.

The authors note that their basin hypothesis provides the essential source of water necessary to shape the large outflow channel systems that are a hallmark of Tharsis and its surrounding regions. What's more, according to Dohm, "the unmodified parts of the basin/aquifer system appear to still contain near-surface water reservoirs that may one day be sampled and analyzed by astronauts."