Persuasive new evidence supporting the possibility of liquid water deep underground on Mars has come to light in a new analysis of seismic data from NASA's InSight lander.
In 2024, researchers proposed that the deep subsurface of the Red Planet, particularly between 7.1 and 12.4 miles (11.5 and 20 kilometers) down, is soaked in liquid water, a conclusion they base on the velocities of seismic waves detected during marsquakes.
Now, researchers Ikuo Katayama of Hiroshima University and Yuya Akamatsu of the Research Institute for Marine Geodynamics in Japan have found supporting evidence for this claim of liquid water deep inside Mars. "Many studies suggest the presence of water on ancient Mars billions of years ago," said Katayama in a statement. "But our model indicates the presence of liquid water on present-day Mars."
Like the previous research, this new study is based on seismic data collected by the SEIS (Seismic Experiment for the Interior Structure) instrument that was part of the InSight mission, which operated on the surface of the red planet between 2018 and 2022.
SEIS was the first ever seismometer to operate on Mars, and it was sensitive to three different types of seismic wave emanating from marsquakes. These were: P-waves, which oscillate back and forth similar to how a sound wave propagates; S-waves which oscillate up and down, perpendicular to the direction of travel; and surface waves, which travel along the surface of Mars similar to ripples in a pond.
The new research was focused on the subterranean P-waves and S-waves. P-waves are the faster seismic waves, while S-waves are slower and cannot travel through water because liquid does not permit that kind of oscillation perpendicular to motion. Seismometers measuring these two different types of seismic wave can help reveal the density and composition of the underground medium (such as water, or rock) through which those waves travelled, based on how strong their signals are and how long it took them to reach the seismometer.
With that in mind, Katayama and Akamatsu honed in on two transitional regions in the seismic data, where there appear to be sudden changes in the properties of the interior of the Red Planet at depths of 6.2 to 12.4 miles (10 and 20 kilometers), very close to where previous studies claim to have found evidence for liquid water.
Previously, geophysicists had argued that these transitions represent the difference between volcanic materials above and ejecta from impacts buried below, and a change from porous rock (i.e. filled with cracks and hollows) to solid rock at 12 miles (20 kilometers) deep.
But there's more to it than this, say Katayama and Akamatsu. According to their analysis of the P-waves and S-waves detected by SEIS, there is water in the porous rock, filling the cracks and hollows between 6 and 12 miles (10 and 20 kilometers) deep.
To test their hypothesis based on the seismic data, the two researchers performed tests on diabase rocks (a type of igneous rock also known as dolerite) from Rydaholm in Sweden. These rocks are one of the best terrestrial analogs to Martian rocks, and Katayama and Akamatsu found that in wet conditions the diabase rocks returned similar seismic signatures as what SEIS was detecting.
Previous studies estimated that there could be enough water deep underground on Mars to cover the surface in a global ocean between 0.62 to 1.24 miles (1 and 2 kilometers) deep. The existence of so much liquid water, if confirmed, could suggest the possibility of "the presence of microbial activity," said Katayama.
Sadly, there's no way we could reach that water, or any life that might exist down there, with current technology. Until we can, the mysteries of Mars, along with its water, will remain buried for the time being.
Katayama and Akamatsu's research is published in the journal Geology.
