Is the Underground Lake on Mars Just Volcanic Rock?

Is Mars home to an underwater lake? Different researchers are reaching different conclusions. Some say remote sensing from the Mars Express orbiter shows liquid water in an underground lake at Mars’ south polar region. Other researchers say clays or minerals explain the data better.

Who’s right? Maybe none of them.

A new study says that volcanic rock can explain the Mars Express data and that it’s a more plausible explanation.

The Martian lake hypothesis dates back to 2018 when a team of researchers published a paper presenting data from the MARSIS (Mars Advanced Radar for Subsurface and Ionosphere Sounding) instrument on the ESA’s Mars Express orbiter. The data showed a highly-reflective surface under the South Polar Layered Deposits (SPLD). In that paper, the researchers concluded that water was responsible for the signal and the Mars lake hypothesis gathered steam.

Then other researchers published other papers giving different explanations for the signal, showing how clays and minerals might be responsible. Then MARSIS data showed more reflective areas which scientists interpreted as more subsurface lakes. Recently the authors responsible for the 2018 paper that started it all published a research letter re-affirming their original interpretation of the data and refuting research that reached different conclusions.

Now a group of scientists published a paper saying the other researchers have it wrong. They conclude that volcanic rock is responsible for the MARSIS signal.

The title of their paper is “The Basal Detectability of an Ice-Covered Mars by MARSIS.” The journal Geophysical Research Letters published the paper, and the lead author is Cyril Grima, a planetary scientist at the University of Texas Institute for Geophysics (UTIG).

The hypothesis that there’s water under the SPLD relies on a couple of facts. The water must be briny to resist freezing, and the temperature can’t be too low. Obviously, there’s a lot more detail than that involved. But that’s the essence of it. The temperature is critical because different materials display different permittivity at different temperatures. And scientists don’t know exactly what the temperature is under the SPLD.

This is a map of the SPLD thickness, based on MARSIS measurements and MOLA surface topography. Image Credit: Plaut et al. 2007.

But this paper sets some of those concerns aside.

“For water to be sustained this close to the surface, you need both a very salty environment and a strong, locally generated heat source, but that doesn’t match what we know of this region,” lead author Cyril Grima said in a press release.

Rather than clays, minerals, or brines, this new research suggests that a type of volcanic rock that’s relatively common on Mars is responsible for the MARSIS data. If some of that volcanic rock were buried under the ice in the SPLD, then it would appear bright like water when MARSIS observed it.

In the study, Grima and his co-authors used computer models to add a global sheet of ice onto the Martian surface. This simulated one-mile-thick ice sheet allowed the researchers to compare features all across Mars’ surface with those under the real ice at the south pole. The SPLD is about 10% impure, and the team duplicated that in their simulated ice.

The result?

A radar map of Mars as seen through a mile of ice. UT Austin planetary scientist, Cyril Grima, built a computer model to cover the Red Planet in ice and observed how it changed the radar data. This caused volcanic plains (seen in red) to reflect radar in a manner that resembled liquid water. The finding challenges a 2018 study that appeared to find liquid water under Mars’ south polar cap. Credit: Cyril Grima

The team found bright reflections like those under the SPLD scattered across different latitudes. Many of them matched up with known locations of volcanic rock. Overall they found that between 0.3% and 2.0% of Mars’ surface could produce the same MARSIS signal detected under the SPLD. The bright terrains the team detected in their study are “… gathered within volcanic constructs of diverse geologic epoch,” the paper says.

Not all of Mars’ known volcanic terrains produce the same signal. But some pronounced volcanic features like shield volcanoes produced strong reflections. “A broad region of strong reflections is identified East of the Uranius Tholus interpreted as a shield volcano resulting from effusive eruptions of low viscosity lavas during the Hesperian-Amazonian transition,” the authors write.

The study showed a strong connection between known volcanic areas and reflectivity. This image shows the Uranius Tholus shield volcano in yellow. The team found strong reflections to the east of the volcano. Image Credit: Wikimedia

There’s a fascinating scientific debate playing out right now over the potential water under the SPLD. These results won’t end that debate, but they play a role. “It draws attention that the brightest terrains across the planet would produce basal echoes with a radiometric character in the range of the brightest ones observed at the SPLD by Orosei et al. (2018) and under similar assumptions for the composition of the overlying ice.” (Note: Orosei et al. 2018 is the original study presenting evidence for liquid water under the SPLD.)

“This radiometric similarity (or continuity) is indicative of the likelihood for a non-wet generic material currently available at Mars to be responsible for the bright basal SPLD reflection,” the paper’s conclusion says. The non-wet material is an iron-rich volcanic rock that’s common on Earth, too.

What do scientists on the other side of this issue think?

Dr. David Stillman is a geophysicist at the Southwest Research Institute (SwRI.) He’s a co-author of papers in support of the liquid water hypothesis.

He told Universe Today that Grima et al. is a robust study. “The Grima paper is very good,” Dr. Stillman said. But he identifies some potential discrepancies if we can call them that, and points them out.

“His paper makes the assumption that surface MARSIS amplitudes can be compared even though they were processed onboard Mars Express when Mars’ magnetosphere was varying. The reflectivity data used by the Italian group (Orosei et al. 2018) was not processed onboard so that amplitudes could be compared when Mars’ magnetosphere was varying (another issue with assumptions).” Dr. Stillman is referring to assumptions about Mars that all scientists have to make when studying the planet. In particular, scientists must work with assumptions about the subsurface temperature under the SPLD. The temperature affects the reflectivity of different compounds, altering the MARSIS signal.

Grima and his co-authors used a figure to present some of their findings in their paper. It highlights four areas on the surface of Mars that show high reflectivity under the simulated ice sheet and shows how volcanic rock can account for the signal.

This figure from the study shows the relative basal echo strength of Mars if the surface was entirely covered by a 1.4-km dirty ice sheet (10% volume impurity rate). Bottom inserts display only positive values for better identifications relative to the regional landforms. Image Credit: Grima et al. 2022.

According to Grima et al., the fact that these four regions are spread across longitudes is a significant strength in their results. “Four insets in Figure 3 highlight some of those regions where a positive Pss/Ps signature is consistent across longitudes instead of just being confined locally along an orbit (an indicator of possible data glitch),” the paper says.

But Dr. Stillman said there’s another possibility for those signals.

“Additionally, if you look at Fig 3 of Grima’s paper you will see very high surface reflection in the northern plains of Mars that likely does not have massive lava flows, but are due to artifacts due to the onboard processing,” he said.

“All those arrows point to high reflectivity that is likely just artifacts as the majority of these are in what we think are sediments and could not have high dielectric values or reflectivity,” Stillman pointed out. “Solis Planum also has pretty random high values, does this mean the whole thing has a high reflectivity or just like 10% of it?”

This won’t be the end of the debate, but it does reveal how intricate the problem is.

This issue is important to many in the planetary science community. If you scan the internet you can see it gets lots of attention and lots of commentary from other researchers even though Martian polar scientists form a fairly small, tightly-knitted community.

The coloured dots in this image represent sites where the ESA’s Mars Express Orbiter spotted bright radar reflections at Mars’ south polar cap. Some researchers interpret the reflections as subsurface liquid water, but other researchers have different explanations. Credits: ESA/NASA/JPL-Caltech

Isaac Smith is a Mars geophysicist at York University who’s not involved in any of these studies. In a press release, Smith explained that the highly reflective signal could be explained by a type of clay dissolved in water. This phenomenon is present on Earth and could be on Mars, too.

Smith also points out that if Grima is right about the reflective signal, it’s not all bad when it comes to the larger issue of Martian water.

“I think the beauty of Grima’s finding is that while it knocks down the idea there might be liquid water under the planet’s south pole today, it also gives us really precise places to go look for evidence of ancient lakes and riverbeds and test hypotheses about the wider drying out of Mars’ climate over billions of years,” Smith said.

We’re in a position between competing hypotheses. But we’re not stuck. This is how science works.

“Science isn’t foolproof on the first try,” said Smith. “That’s especially true in planetary science where we’re looking at places no one’s ever visited and relying on instruments that sense everything remotely.”

Dr. Stillman seems to agree and points out that everyone is forced to make some assumptions when it comes to Mars.

“Honestly, I do not know which assumptions are correct because we are studying a planet so far away with very limited data,” he told Universe Today.

None of the papers published so far proves there’s water, and none prove there isn’t. Instead, we’re inching our way toward knowing for sure.

We need better data, which means we need another mission to Mars.

That’s never a bad thing.