Observations by ESA’s Mars Express orbiter show evidence of an ancient planet-wide groundwater system on the Red Planet.

“Early Mars was a watery world, but as the planet’s climate changed this water retreated below the surface to form pools and groundwater,” said Dr. Francesco Salese, a researcher at 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.”

Dr. Salese and co-authors explored 24 deep, enclosed craters in the Martian northern hemisphere, with floors lying roughly 2.5 miles (4 km) below ‘sea level.’

The scientists found features on the floors of these craters that could only have formed in the presence of water.

Many craters contain multiple features, all at depths of 2.5 to 2.8 miles (4-4.5 km) — indicating that these craters once contained pools and flows of water that changed and receded over time.

Features include channels etched into crater walls, valleys carved out by sapping groundwater, dark, curved deltas thought to have formed as water levels rose and fell, ridged terraces within crater walls formed by standing water, and fan-shaped deposits of sediment associated with flowing water.

The water level aligns with the proposed shorelines of a putative ocean thought to have existed on the planet 3-4 billion years ago.

“We think that this ocean may have connected to a system of underground lakes that spread across the entire planet,” said co-author Dr. Gian Gabriele Ori, director of the Università D’Annunzio’s International Research School of Planetary Sciences in Italy.

“These lakes would have existed around 3.5 billion years ago, so may have been contemporaries of a Martian ocean.”

The team also spotted signs of minerals within five of the craters that are linked to the emergence of life on Earth: various clays, carbonates, and silicates.

The finding adds weight to the idea that these basins on Mars may once have had the ingredients to host life.

Moreover, they were the only basins deep enough to intersect with the water-saturated part of Mars’ crust for long periods of time, with evidence perhaps still buried in the sediments today.

“Findings like this are hugely important; they help us to identify the regions of Mars that are the most promising for finding signs of past life,” said Mars Express project scientist Dr. Dmitri Titov, who was not involved in the study.

The findings were published in the Journal of Geophysical Research: Planets.

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Francesco Salese et al. Geological Evidence of Planet-Wide Groundwater System on Mars. Journal of Geophysical Research: Planets, published online January 21, 2019; doi: 10.1029/2018JE005802