Earth is a seismically active planet, and scientists have figured out how to use seismic waves from Earthquakes to probe its interior. We even use artificially created seismic waves to identify underground petroleum-bearing formations. When the InSIGHT (Interior Exploration using Seismic Investigations, Geodesy and Heat Transport) lander was sent to Mars, it sensed Marsquakes to learn more bout the planet’s interior.
Researchers think they can use Marsquakes to answer one of Mars’ most pressing questions: Does the planet hold water trapped in its subsurface?
Ground-penetrating radar can tell us what’s underground on Earth. However, it has limitations. It can reach about 30 meters underground in low-conductivity materials and as shallow as one meter in conductive materials. Scientists are developing other methods, including seismological interferometers, to use seismology to detect deeper aquifers, but those methods are not fully developed. There’s also so much water inside Earth that it creates noisy signals.
These methods are not applicable to Mars, where equipment is limited.
However, researchers from Penn State University think they can use a different type of seismology to detect Mars’ subsurface water. It’s called the seismoelectric method, and it combines seismology and electromagnetism. It senses the electromagnetic signals that come from the propagation of seismic waves in a planet’s interior.
Their new research, “Characterizing Liquid Water in Deep Martian Aquifers: A Seismo-Electric Approach,” has been published in JGR Planets. Nolan Roth, a doctoral candidate in the Department of Geosciences at Penn State, is the lead author.
“The scientific community has theories that Mars used to have oceans and that, over the course of its history, all that water went away,” Roth said. “But there is evidence that some water is trapped somewhere in the subsurface. We just haven’t been able to find it. The idea is, if we can find these electromagnetic signals, then we find water on Mars.”
This artist’s impression shows how Mars may have looked about four billion years ago. The young planet Mars would have had enough water to cover its entire surface in a liquid layer about 140 metres deep, but it is more likely that the liquid would have pooled to form an ocean occupying almost half of Mars’s northern hemisphere and in some regions reaching depths greater than 1.6 kilometres. Credit: ESO/M. KornmesserSeismology works by detecting elastic waves that propagate through the Earth. These waves are divided into subtypes, especially P-waves, or primary waves, and S-waves, or secondary waves. Each type of wave travels differently depending on the material it’s moving through. In broad terms, P-waves travel faster than S-waves, so they arrive at seismographic sensors at different times. The differences in those times and other factors reveal the characteristics and densities of the material the waves are travelling through.
The seismoelectric method detects the electromagnetic signals created by seismic waves rather than the waves themselves. As the waves travel through a planet, materials like rock or water move differently in response. Those differences create magnetic fields that surface sensors can detect.
“If we listen to the marsquakes that are moving through the subsurface, if they pass through water, they’ll create these wonderful, unique signals of electromagnetic fields,” Roth said. “These signals would be diagnostic of current, modern-day water on Mars.”
This method is especially suited to Mars. On Earth, water is mixed throughout the subsurface, not just in aquifers, making detection difficult. But Mars is extremely dry, other than potential subsurface aquifers. If we detect buried water on Mars with the seismoelectric method, it’s almost certainly a subsurface aquifer.
Artist’s impression of water under the Martian surface. Credit: ESA/Medialab “In contrast to how seismoelectric signals often appear on Earth, Mars’ surface naturally removes the noise and exposes useful data that allows us to characterize several aquifer properties,” said co-author Tieyuan Zhu, associate professor of geosciences at Penn State and Roth’s adviser.
The seismoelectric method involves two types of electromagnetic fields: co-seismic waves and interface responses (IR). There are two types of interface responses: radiating interface responses (RIRs) and evanescent interface responses (EIRs.)
“Interface responses (IRs) are generated when a seismic wave creates a charge imbalance across a saturated interface,” the authors explain. RIRs radiate from the interface independently at electromagnetic velocities, regardless of how much fluid is in the medium. EIRs are generated when a seismic wave impinges on a saturated interface at a particular angle. Both types of IRs are generated in the presence of mobile fluids, but they don’t require a saturated layer to propagate further. RIRs, in particular, can travel through kilometres of rock. The two types of interface responses can be separated and analyzed independently.
It all adds up to a new method of “seeing” inside Mars and finding saturated layers.
Roth and his co-researchers started by creating a model of subsurface Mars. Then, they added aquifers to simulate how the seismoelectric method could work. The results showed they could use the seismoelectric technique to uncover details about the aquifers, including their dimensions and chemical properties, like salinity.
“Aquifer depth, thickness, and quantity affect interface response arrival times and shape,” the authors write in their research. “Aquifer water saturation fraction, chemistry, and salinity strongly impact the interface response strength but have little to no affect on the waveform shape.”
“Seismo-electric signals can be used to constrain estimates of aquifer depth, volume, location, and bulk chemical composition,” they added.
This illustration from the research shows how the seismoelectric method could detect subsurface water on Mars. It shows three different cases: a dry Mars, a Mars with a deep aquifer, and an Earth-analog model. There’s a lot of complexity, but the main takeaway is that the different interface responses behave differently and arrive at sensors at different times. See the published research for more details. Image Credit: Roth et al. 2024. “SE measurements give us a way to detect and image Martian groundwater kilometres below the surface,” the authors write in their conclusion. “As SE exploration becomes more widespread on Earth, this study represents the first foray of the method to other worlds.”
“If we can understand the signals, we can go back and characterize the aquifers themselves,” Roth said in a press release. “And that would give us more constraints than we’ve ever had before for understanding water on Mars today and how it has changed over the last 4 billion years. And that would be a big step ahead.”
The most exciting part about using the seismoelectric method on Mars is that it doesn’t require a new mission. NASA’s InSIGHT lander acquired ample seismic data during its mission. It also had a magnetometer, and future work will combine the signals from both to open a new window into subsurface Mars.
If the method proves fruitful, seismometers and magnetometers could be included in future missions, not only to Mars but also to other worlds. Frozen ocean moons like Europa and Enceladus are prime exploration targets in the search for life, and the technique could work there.
“This shouldn’t be limited to Mars — the technique has potential, for example, to measure the thickness of icy oceans on a moon of Jupiter,” Zhu said. “The message we want to give the community is there is this promising physical phenomena — which received less attention in the past — that may have great potential for planetary geophysics.”
