Los Angeles: Oceans on Mars formed several hundred million years earlier and were not as deep as once thought, according to a study.
Researchers at The University of California, Berkeley in the US have linked the existence of oceans early in the history of Mars to the rise of the solar system's largest volcanic system, Tharsis.
They also highlight the key role played by global warming in allowing liquid water to exist on Mars.
"Volcanoes may be important in creating the conditions for Mars to be wet, said Michael Manga, a professor at UC Berkeley.
Those claiming that Mars never had oceans of liquid water often point to the fact that estimates of the size of the oceans do not jibe with estimates of how much water could be hidden today as permafrost underground and how much could have escaped into space.
These are the main options, given that the polar ice caps do not contain enough water to fill an ocean.
The new model proposes that the oceans formed before or at the same time as Mars' largest volcanic feature, Tharsis, instead of after Tharsis formed 3.7 billion years ago.
Since Tharsis was smaller at that time, it did not distort the planet as much as it did later, in particular the plains that cover most of the northern hemisphere and are the presumed ancient seabed.
The absence of crustal deformation from Tharsis means the seas would have been shallower, holding about half the water of earlier estimates, researchers said.
The assumption was that Tharsis formed quickly and early, rather than gradually, and that the oceans came later," said Manga, senior author of the study published in the journal Nature.
"We are saying that the oceans predate and accompany the lava outpourings that made Tharsis," he said.
It is likely, he added, that Tharsis spewed gases into the atmosphere that created a global warming or greenhouse effect that allowed liquid water to exist on the planet.
It is also possible that volcanic eruptions created channels that allowed underground water to reach the surface and fill the northern plains.
The model also counters another argument against oceans: that the proposed shorelines are very irregular, varying in height by as much as a kilometre, when they should be level, like shorelines on Earth.
This irregularity could be explained if the first ocean, called Arabia, started forming about four billion years ago and existed, if intermittently, during as much as the first 20 per cent of Tharsis's growth, researchers said.
The growing volcano would have depressed the land and deformed the shoreline over time, which could explain the irregular heights of the Arabia shoreline.