Mars is dry and cold today, but there could still be liquid water deep beneath its surface: analyses of seismic data from NASA’s Mars InSight lander suggest that the crustal rock at depths of 11 to 20 kilometers is porous and saturated with liquid water. According to calculations, there could be enough water down there to cover the entire surface of the Red Planet with a layer of water up to two kilometers high. The researchers conclude that the water from the ancient Martian oceans did not escape into space, but seeped into the depths. In theory, this deep water reservoir could even be a refuge for microbial life, as the team explains. However, this water is not accessible or sampleable: no drill can penetrate to these depths.
In its early days, Mars was much warmer and wetter than it is today, and its surface contained lakes, rivers and probably even an extensive, shallow ocean. Until about 3.5 billion years ago, this covered large parts of the northern
Lowlands. This Martian northern sea may have covered around 20 percent of the planet’s surface, but then disappeared relatively quickly when the climate on the Red Planet became cold and dry. What happened to the water in the Martian ocean is not clear: “The ancient surface water could have been bound in minerals, buried as water ice, seeped away as liquid water in deep aquifers, or been lost into space,” explain Vashan Wright from the University of California in San Diego and his colleagues. Data from Mars probes suggest that the water of the former Martian ocean cannot have been preserved in the polar ice caps of Mars – their volume is not sufficient for that.
Seismic view into the middle crust
Wright and his team have therefore pursued another possibility: They investigated whether the water of the former Martian ocean might be hidden deep in the Martian crust. To do this, they evaluated seismic data from NASA’s Mars Insight lander, which is located just north of the Martian equator on the western edge of the Elysium Planitia plain. Using the Marsquakes recorded by their seismometer and physical models, they can determine what structure the subsurface has – whether porous or solid – and whether it contains water or water-containing minerals. As early as 2022, such analyses had shown that the Martian crust is rather dry, at least to a depth of around 300 meters, and contains little water ice. However, Wright and his colleagues have now looked to greater depths. Their focus was on the middle crust of Mars at a depth of 11.5 to 20 kilometers. “The temperatures on Mars today are warm enough to allow stable liquid water at the upper edge of the middle crust,” the researchers report. Previous analyses had also shown that this crustal area could be porous and riddled with abundant rock pores.
For their study, the scientists therefore specifically analyzed the seismic data that provide information about the nature of the middle crust of Mars. They used a mathematical model of rock physics that is used on Earth to find oil deposits and groundwater reservoirs. The analyses confirmed that the porosity of the subsurface at this depth is around 17 percent and that the rock there is riddled with fine, water-filled cracks: “A middle crust of broken igneous rock saturated with liquid water best explains the existing data,” write Wright and his colleagues. If these measurements are representative not only of the location of the Mars probe, but also of the rest of the planet, then there could be a huge reservoir of liquid water in the middle crust of Mars. According to the researchers’ calculations, there would then be enough water down there to flood the entire surface of Mars with water to a height of one to two kilometers.
Refuge for Martian life?
According to Wright and his colleagues, these data suggest that the water in the ancient Martian ocean did not evaporate and escape into space. Instead, most of this water may have seeped underground and been preserved to this day. This deep water would not be usable for future Mars astronauts and Mars stations, as no drilling technology can reach that deep. Nevertheless, the existence of such an aquifer on Mars is potentially significant, as the team explains. “The realization that there is a large reservoir of liquid water there opens a window into the past climate of Mars,” says co-author Michael Manga of the University of California in Berkeley. “And since water is a prerequisite for life as we know it, I don’t see why the underground reservoir should not be a life-friendly environment.” After all, there is microbial life on Earth, even in the deepest mines or the bottom of the deep sea. “Although we have not yet discovered any evidence of life on Mars, we now at least have a place that would, in principle, support life,” said Manga.
Source: Vashan Wright (Scripps Institution of Oceanography, University of California San Diego, La Jolla) et al., Proceedings of the National Academy of Sciences, doi: 10.1073/pnas.2409983121