Image courtesy of NASA.
Today, Mars is a red, barren wasteland, devoid of any signs of water. However, over three billion years ago, Mars looked like an entirely different world. Geological patterns on the surface of Mars provide solid evidence that the planet once held rivers, lakes, oceans, and aquifers. Where did all this water go?
Over the years, scientists have come up with many hypotheses to explain the disappearance of liquid water on Mars. Perhaps the water was incorporated into minerals on Mars such as gypsum, which has water molecules encapsulated within its crystal lattices. Another possibility is the water was lost to space over time—if given enough thermal energy from the sun, the water could have slowly vaporized. However, new research from Vashan Wright, an assistant professor at the Scripps Institution of Oceanography, and his team at the University of California, San Diego, provides an alternative explanation: the water had seeped into the Martian crust and is now isolated as groundwater.
To test for the presence of water beneath Mars’ surface, the Wright Lab used years of data collected from NASA’s InSight lander. NASA’s InSight lander currently tracks seismic waves, which are vibrations that travel through Mars. By analyzing how these waves move, scientists hope to determine whether water lies beneath the surface without a direct view into Mars’ interior. Recently, researchers used data from the InSight lander to derive the velocities of compressional waves, which move parallel to the direction of travel through Mars’ interior. The presence of water molecules makes the crust less squishy, allowing compressional waves to travel faster. By interpreting the velocities of these waves, scientists can infer whether water lies beneath Mars’ surface.
With the wave velocity data from the lander, the lab used complex statistical methods to reconstruct the composition of the Martian crust. “We look at the data, then ask ourselves, ‘What rock properties do we need to match this data?’” Wright said. After running their models five million times, the team found that the wave velocities were consistent with the theory that there is a thin layer of rock containing pores full of liquid water located in Martian mid-crust, which is a layer of igneous rock stretching from 8.4 to twenty-five kilometers below the planet’s surface.
Now, Wright is trying to understand how his finding fits in with previous discoveries about Mars’ geological structure. If there was water underground, theoretically, there should be a thick layer of ice—known as a cryosphere—just above the water layer. Just as water can condense into ice crystals in clouds on Earth, water in a layer below Mars’ crust should freeze to form a cryosphere as it rises. However, the results showed no continuous cryosphere—only small pockets of ice within the rock. Although previous research suggests that there is a cryosphere, Wright’s team is confident that their models refute this belief. As updated data arrives from the InSight lander, the experiments could be repeated to arrive at a more assured conclusion as to the existence of a cryosphere.
Wright’s results challenge our current understanding of Mars’ history, its water cycle, and the potential for microbial life deep below the surface. Water is a key indicator of life forms on an extraterrestrial body. Additionally, this could be a huge step forward in the human exploration of Mars. While these water sources might be difficult to access for now, since they exist at a depth unexplored even on Earth, technological advancements may someday make accessing them possible.