On both sides of the Atlantic, the debate between those who believe that a liquid water lake lies beneath the surface of the Red Planet and those who think there are less exciting explanations to account for the signals seen by Mars Express has gained a new chapter in Nature Astronomy. But everyone agrees on one point: it is still too early to retire Mars Express.

If you think that scientific controversies through written correspondence are a thing of the past, the debate over the existence of liquid water beneath the surface of Mars provides an opportunity to think again. The discussion has been ongoing since the discovery was reported in Science in 2018, with studies for and against it. The debate has recently moved to the pages of Nature Astronomy, particularly in the section "Matter Arising," which is a space for epistolary exchanges on outstanding scientific questions. Last September, three scientists from the Cornell Center for Astrophysics and Space Science in the United States – Daniel Lalich, Alexander Hayes, and the Italian Valerio Poggiali – deconstructed an article supporting the existence of liquid water, which was published two years earlier in Nature Astronomy. Yesterday, the "pro-water" team, led by Sebastian Emanuel Lauro and Elena Pettinelli from the University of Rome Tre, responded punctually with a reply.

Mars water controversy: Italian and American scientists debate liquid water on the Red Planet
Therefore, what did Lalich and colleagues say? Everything revolves around the interpretation of the radio reflections collected while flying over the south pole of Mars by the Italian radar Marsis on board the European Space Agency's Mars Express probe. Reflections in which the Italian scientists of the Marsis team - starting from Roberto Orosei of Inaf, first author of the 2018 Science article - see the signature of liquid water. However, according to the three Cornell experts, the signature is far from unmistakable: although the possibility that it is liquid water is exciting, they wrote in their letter last September, "here we demonstrate that similar reflections can be produced as a natural result of interference from thin layers, without invoking the presence of liquid water or other rare materials".

The Controversy Over Water on Mars Continues
Scientists are still debating whether or not liquid water exists beneath the surface of Mars, and a recent exchange of letters between two groups of scientists in Nature Astronomy has brought the controversy back into the spotlight. One team, from Cornell University, argued against the presence of liquid water based on a model of the Martian subsurface that suggested the bright radar reflections observed by the Marsis radar instrument could be explained by thin layers of frozen water sandwiched between layers of dry ice. The other team, led by Italian researchers, responded by arguing that the Cornell team's model was overly simplistic and did not account for all the available data. The debate is likely to continue as scientists seek to better understand the mysteries of Mars.

Mars' liquid water may actually be a mirage created by sandwiched layers of ice
In simple terms, the "thin layers" referred to by the Cornell scientists can be visualized as a sort of sandwich with alternating thin slices of "bread" - in their model, layers of CO2 (dry ice) - and one or two nice "burgers" (water ice). Depending on the thickness of the different layers and the "sandwich configuration" - two thin slices of CO2 enclosing a thick layer of water ice, or one slice of CO2 and one of ice only, or even three and two - specific constructive and destructive interferences can occur which, under the right conditions, can in turn give rise to radar reflections much brighter than one might expect. Reflections - Lalich and colleagues stress, also using data collected on Mars with another Italian radar, Sharad, aboard NASA's Mars Reconnaissance Orbiter - very similar to those observed by Marsis under the southern Martian polar deposits. Without the need, that is, to hypothesize the presence of liquid water, or brine. In short, if packaged in the right way, that "sandwich" of dry ice and water ice could be deceptive, and at the "bite" of the radar give the impression that it is liquid water.
The three frequencies mentioned by Lauro are 3 MHz, 4 MHz, and 5 MHz: to give misleading answers, each would require a different "sandwich". But the one "bitten" by Marsis is the same for all three, so the objection by the Cornell team falls away. Not to mention that the measurements were taken over a long period of time, with the alternating Martian seasons and from different orbits: a configuration of parallel layers of CO2 over a large and perfectly flat area, like the one that could produce misleading reflections, therefore does not seem very plausible.

Artistic representation of liquid water reservoirs in the Martian subsurface. Credits: Medialab, ESA 2001

Marsis Radar Controversy Continues as Cornell Team Challenges Findings

Will it be enough to convince Lalich's team? Probably not. "In general, we think that having three frequencies certainly brings a benefit, and whatever hypothesis is proposed must be consistent with the observations made at each frequency. Without going too much into the technical details of the paper, we do not believe that the differences between the various frequencies can be explained solely by attenuation," says Valerio Poggiali, one of the three Cornell scientists, in an interview with Media Inaf. "On a larger scale, the observations generally follow, as expected, a frequency-dependent behavior. But the variability between the observations is enormous, especially on a smaller or regional scale, and there are many areas where the expected relationship between the various frequencies does not occur or is even reversed. That kind of variability cannot certainly be explained by attenuation alone."

Mars Express mission extension sought by scientists for further study on water on Mars

At least on one point, the two teams are in agreement: Mars Express should not be retired. And this is a very current issue. In the coming weeks, in fact, the ESA will have to decide whether or not to extend the mission - launched almost twenty years ago, in June 2003 - to the 2023-2025 period. Since there is pessimism in the air about the outcome of the decision, last week an appeal was put online, already signed by almost eight hundred scientists, in favor of the extension. Among the reasons in support of the choice is also the contribution that Mars Express can still give to the solution of the water controversy. "It would allow us to continue to search if there are other highly reflective areas," explains Lauro, "other regions that we could associate with the presence of water beneath the ice. So yes, it would be very important, because it would allow us to collect more data."
And the Cornell team is no different. "Even if we might not have written it exactly that way, in general we agree and will add our names to support the letter. This is one of the most debated and important issues for research on Mars at the moment," Poggiali concludes referring to the presence or absence of liquid water in the subsurface, "and as of today Mars Express is the only [mission, ed.] that can make this type of direct observations. If we want to understand what is at the base of the South Polar Layer Deposit, we need Mars Express."