A study of martian terrain in the northern lowlands suggests that water discharged from underground reservoirs slowly over time, rather than in catastrophic flooding events.

The formation of lakes and seas on Mars is widely debated: were they formed from the sudden release of huge volumes of water and sediments when large areas of crust collapsed, or was the process a lot calmer, with lakes being filled and maintained by groundwater over long periods of time?

J Alexis Palmero Rodriguez of the Planetary Science Institute found that a region in the northern lowlands of Mars, in an area known as Planum Boreum, contains sedimentary deposits that appear similar to Earth's ocean floors. He suggests that widespread fractures in the floor of large basins could have allowed groundwater to emerge, as well as promote the formation of river systems, and ultimately, the erosional features observed.

“With the loss over time of water from the subsurface aquifer, areas of the northern plains ultimately collapsed, creating the rough hilly surfaces we see today,” explains Rodriguez. “Some plateaus may have avoided this fate and preserved sedimentary plains containing an immense record of hydrologic activity whereas the geologic record in the collapsed hilly regions would have been jumbled and largely lost.”

Rodriguez says that his model allows for frequent groundwater discharge to create many lakes and seas, and that the discharge could have occurred any point in Mars' history. “There could have been many oceans on Mars over time,” he says.

Furthermore, if life existed in subsurface Mars, it could have been brought to the surface during these periods of outflow, with fossil remnants preserved in some of the sedimentary deposits observed on the Martian surface or in crater walls today.

Nick Warner of Imperial College London, not involved in this study, agrees that multiple flooding events in the lowlands seems likely. "At Ares Vallis we see evidence for at least six unique flood surfaces spanning a period between 3.6 and 2.9 billion years ago," he tells Astronomy Now. "The question is whether these floods, as well as those in other regions, were of significant volume to create an ocean in the northern lowlands. If each individual flood was separated by a significant amount of time then it's possible that the water may have only ponded in the lowlands temporarily, creating transient seas or oceans that were subject to rapid evaporation."

Warner adds that it is also possible that the water infiltrated into the subsurface or froze before it could create a significantly large body of liquid water, and that there might have been hundreds of smaller floods in these channels that did little to no erosive work, but contributed to a standing body water. "But it seems likely that Rodriguez's observations require water to have been re-charged into these terrains, likely through a globally connected aquifer," he says.