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InSight data may have revealed water deep below the surface of Mars

Tags: seismic imaging , seismology

photo of the InSight seismometer sitting on the ground a few feet from the lander body, dusty ground surrounds and the horizon is curved in the fisheye lens view
November 26, 2022 image from the InSight lander. (Credit: NASA/JPL-Caltech)

When NASA’s InSight mission went to Mars and put its seismic ear to the ground, it began to record marsquakes and reveal tectonic activity on the Red Planet. But seismic waves also tell us about the rock they pass through on the way to the seismometer. A new paper in PNAS led by Vashan Wright at Scripps Institution of Oceanography presents a noteworthy conclusion—that those seismic waves also passed through liquid water.

The InSight mission has produced an impressive amount of knowledge from a single seismometer running for just a few years. We’ve learned about the thickness and composition of Mars’ core, mantle, and crust. If there’s water—either liquid or ice—below the Martian surface, then that, too, is part of the picture drawn by these data. (InSight’s data can be accessed in the NSF SAGE archive operated by EarthScope.)

Seismic imaging involves working backwards from the ground motion detected at the seismometer to learn about the material the seismic waves passed through. Take that data, plug in some known numbers, and we can back out unknown physical properties of the Earth required to produce the ground motion as we observed it.

The goal in this paper was to characterize the materials between 11 and 20 kilometers beneath the surface where InSight sits. The team used the velocity of marsquake S-waves and P-waves traveling through that layer, as well as estimates of density based on a combination of those data and satellite gravity measurements.

The unknown physical properties they solved for included the amount of open pore space in the rock, whether those pore spaces are round or flattened in shape, the amount of liquid water in pores, and the density and compressibility of the volume of rock.

The values that best fit InSight’s observations point to “igneous rock with thin fractures filled with liquid water”, the researchers write. That is, they think a fair volume of liquid water (filled pores making up about 17% of the volume) is the best explanation for the way this region transmitted seismic waves.

The pressure below 20 kilometers depth is expected to be high enough to squeeze pores closed, and the temperature above 11 kilometers depth should be too cold for liquid water. But assuming the rock below InSight is representative of the planet, the volume of liquid water proposed by their model is actually larger than estimates of Mars’ surface oceans that have been lost to time.

One open question about Mars’ past is where all that water went. While there is ice at the poles, the rest of it could either have gone up (being lost to space) or down (disappearing into the interior of the planet). Confirming the presence of a significant amount of water deep below the surface would be relevant to that question. This illustrates the value of geophysical tools in revealing secrets hidden below the surface of other worlds, beyond the reach of orbital imagery and rovers. As ever, more data could reveal an even clearer picture—but that is certainly no easy task on Mars.