When NASA launched the Mars projects of the early 2000s, its missions were guided by a central theme known as “Follow the Water.” The basic idea is that if humans are to colonize Mars, they’ll need to have water to drink. Ideally, that water would come from Mars itself, which overflowed with rivers three billion years ago. But now, researchers report in Nature Geoscience that water remains on the Red Planet, hidden in unexpected locations deep underground.

The body of water discovered by scientists nearly a mile under the planet’s southern pole in 2018 was the only confirmed reservoir of water on Mars, together with the ice that covers the poles. Scientists from the University of Southern California Arid Climate and Water Research Center, however, report in their paper that there’s also water deep underground in some of the planet’s near-equatorial areas. Furthermore, this underground water system is likely still active and occasionally surfaces.

The team reports that this active system extends as deep as 750 meters, and groundwater emerges through cracks on surface craters. The fractures enable water springs to rise to the surface as a result of the deep pressure below. When these springs leak onto the topsoil, they create distinct linear features called slope lineae, shown below.

Recurrent slope lineae on the Palikir Crater on Mars. NASA/JPL/University of Arizona

The USC researchers examined high-resolution optical images of these slope lineae, which look like dried, short streams on crater walls, to examine how the dry stream imprints might relate to the fractures on the craters. The team used their experience studying subsurface and groundwater flow movement on Earth in desert environments to draw a parallel.

“We have seen the same mechanisms in the North African Sahara and in the Arabian Peninsula, and it helped us explore the same mechanism on Mars,” first author Abotalib Z. Abotalib, a postdoctoral research associate at USC, explained.

The mechanism Abotalib refers to is a deep pressurized groundwater source that he and collaborator Essam Heggy, Ph.D. believe is causing these slope lineae to appear.

The slope lineae themselves have been subject to scientific debate: In 2015, NASA announced that the dark streaks were strong evidence of liquid water. In 2017, scientists from the U.S. Geological Survey’s Astrogeology Science Center made their own appeal, stating that the lineae are not evidence of liquid water but instead streaks of sand slipping downhill.

This study argues that the slope lineae are evidence of liquid water and states that their features seem to fluctuate because they are rare leaks onto the surface.

An ancient landslide on Mars' surface. Mars has been known as a dry, dusty planet. NASA/JPL-Caltech/Univ. of Arizona

Abotalib and Heggy further reason that the crater fractures where the water seemingly emerges should be considered as primary location candidates for future Mars habitats. In NASA’s plan for creating a sustained human presence on Mars, Phase 1 involves a “landing site selection and water extraction go-head.” Phase 2 is when the initial colonists come in.

Understanding how groundwater systems work on Mars also helps scientists figure out what parallels there are between Mars and Earth — and could help us predict the future.

“It helps us to understand the similarities to our own planet and if we are going through the same climate evolution and the same path that Mars is going,” says Heggy. “Understanding Mars’ evolution is crucial for understanding our own Earth’s long-term evolution and groundwater is a key element in this process.”

Abstract: The recurring slope lineae on Mars have been hypothesized to originate from snow melting, deliquescence, dry flow or shallow groundwater. Except for the dry flow origin, these hypotheses imply the presence of surficial or near-surface volatiles, placing the exploration and characterization of potential habitable environments within the reach of existing technology. Here we present observations from the High Resolution Imaging Science Experiment, heat-flow modelling and terrestrial analogues, which indicate that the source of recurring slope lineae could be natural discharge along geological structures from briny aquifers within the cryosphere, at depths of approximately 750 m. Spatial correlation between recurring slope lineae source regions and multi-scale fractures (such as joints and faults) in the southern mid-latitudes and in Valles Marineris suggests that recurring slope lineae preferably emanate from tectonic and impact-related fractures. We suggest that deep groundwater occasionally surfaces on Mars in present-day conditions.