Twenty years ago, the question of whether Mars ever had large reserves of liquid water was still open to debate. Today, the discussion has shifted to an evaluation of the nature and size of those reserves, where they existed, and what this implies for any effort to find life (or the remains of it) on the Red Planet. Scientists with the European Space Agency have released the results of Mars Express’ observation of some 24 deep, enclosed craters in the northern hemisphere of Mars.
“Early Mars was a watery world, but as the planet’s climate changed this water retreated below the surface to form pools and ‘groundwater,’” says lead author Francesco Sales of Utrecht University in the Netherlands. “We traced this water in our study, as its scale and role is a matter of debate, and we found the first geological evidence of a planet-wide groundwater system on Mars.”
The floor of the impact craters showed features that could only have formed in the presence of water, at depths ranging from 4,000 to 4,500 meters. The various depth findings show that the water level changed and receded over time.
The water level in the craters aligns well with the proposed water level in the Mars ocean hypothesis, which argues that a global ocean situated in the Vastitas Borealis (pictured above) once covered the northern third of Mars. It’s also possible that at least two global oceans existed on Mars at different points — one vast and persistent, during the earliest days of the planet, and a smaller, shallower, and less-persistent ocean (or system of rivers and lakes) that may have temporarily existed when vast amounts of volcanism or other geothermal activity heated ice trapped below ground.
The existence of a groundwater system on Mars is compatible with these hypotheses. Crater lakes would have attached to each other through the same types of groundwater systems we see on Earth. Such commonalities are a major component of why we believe Mars supported significant amounts of liquid water for long periods of time. We’ve found rocks whose formation on Earth depends on the presence of liquid water and large-scale features of the terrain that indicate water-driven erosion once played a significant role in weathering the Martian landscape.
The incidence of such activity drops off sharply after Mars’ Noachian period transitions into the Hesperian, which is also when Mars is believed to have become much drier. The Amazonian period that followed the Noachian is characterized by the cold, arid Martian conditions that still dominate the planet today.
Substantial amounts of ice are known to still exist at the planet’s north pole, and a lake is believed to still remain beneath the south polar ice cap, similar to the liquid lakes under miles of ice in Antarctica on Earth. Any liquid water still existing on Mars might be better described as being in Mars at this point, given the depths at which it would be located. Traces of liquid are occasionally found at the surface, but no significant amount of free-running water has ever been observed.
The same team also spotted five specific craters where mineral ingredients believed to be common to the rise of life on Earth also exist, including various clays, carbonates, and silicates. Such basins could be prime locations for searching for the life that may have existed on Mars.