In the last month and a half, some articles have been published in Geophysical Research Letters that question the presence – suggested in 2018 in Science – of a lake of liquid water in the subsoil of Mars. How do the authors of that study counter? We asked two of them, the astrophysicist of INAF of Bologna Roberto Orosei and the geophysicist of the University of Roma Tre Elena Pettinelli
Was it really water? Or maybe ice? Or maybe clay? The all-Italian discovery of a lake of liquid water in the subsoil of Mars , published in the summer of 2018 in Science and bounced around in newspapers around the world, has been under attack for some time . How normal it is when the scientific method does its job well: flea at each result to see how far the hypothesis holds. A work that also has aspects of competition, of challenge between different research groups – it is useless to deny it. In the case of the discovery of Martian liquid water – now questioned by some articles published in Geophysical Research Lettersover the course of the summer – we are probably still in the initial phase of the meeting. We therefore went to hear from two of the protagonists of the discovery now in check – the astrophysicist of INAF of Bologna Roberto Orosei and the geophysicist of the University of Rome Tre Elena Pettinelli – what their point of view is on the ongoing confrontation.
Orosei, what happens? Your hypothesis about the presence of liquid water in the subsoil of Mars is becoming the punching bag of Geophysical Research Letters …
Orosei: “It is actually the third article published by Geophysical Research Letters in a few weeks in which an alternative explanation to liquid water is presented for the strong echoes detected by Marsis at the base of the Martian polar cap, and we know that there is a fourth arriving…”.
Friendly fire or enemy fire? I mean: are they researchers on the Marsis team or are they external?
Orosei: “All the articles were written by US authors who are part of the Marsis and Sharad teams (the Italian radar aboard NASA’s Mars Reconnaissance Orbiter), while so far no American or European researcher – apart from us – has published articles in favor of our interpretation. The debate is therefore limited to a group of radar experts on both sides of the Atlantic, while the vast majority of the community stands by. Those who can discuss this topic with competence are not many, given its extreme specificity, so in the end even the way in which the results of a scientific work are presented and disseminated influences the credibility that a theory can have in the community. Unfortunately,
Let’s go in order. Last June’s coup, the one launched by Aditya Khuller and Jeffrey Plaut , was based on an expansion – geographic and chronological – of the sample of data analyzed, right? Going to suggest that there might be ice down there, not liquid water. How does it counter?
Orosei: «The discovery of a first area with high radar reflectivity under the south polar cap of Mars was made by Plaut himself in 2007. A couple of years later an article with French and American authors, including Plaut, involved in his conclusions that the cause of these intense reflections was the presence of CO 2 on the ice surface , which is very transparent to radio waves: the strong echoes would therefore be due to the transparency of the overlying material, not to an anomalous composition at the base. When we started working on the water article we had these results in mind, and we took care to demonstrate that there was no CO 2 ice.on the surface. None of us ever claimed that strong radar echoes automatically imply the presence of water, and indeed we tried to consider all plausible alternatives, ultimately proving that these did not explain the properties of the radar signal. The recent article by Khuller and Plaut does not subvert our interpretation, because it does not say anything new from the point of view of the analysis of the radar signal, but it has the merit of identifying other areas where to concentrate our efforts to verify whether there may be or less water presence “.
Another criticism is that the salt present under Mars would not be enough to explain the liquid state… Pettinelli, don’t you convince you?
Pettinelli: «It is known that on Mars different types of salts, especially perchlorates, are ubiquitous, so it is plausible that they are also found below the south polar cap where we have detected particularly strong radar echoes. These perchlorates are particular salts that lower the crystallization temperature of water by many tens of degrees (even 80 ° C). Laboratory experiments say that some of these salts also have the particular characteristic of keeping water in a subcooled state down to -120 ° C. So the explanation of the salt water is not in contradiction with the low temperature ».
Now this second attack, much more direct, is launched by Isaac Smith and colleagues , based instead on laboratory tests and analysis, they write, not only of the real part – as you did – but also of the imaginary part of the detected dielectric constant. from the radar. And the bottom line, this time around, is that there is a particular type of clay down there, smectic clay. Is it plausible?
Pettinelli: «The dielectric properties of clays, especially smectites, have been extensively studied since the 1960s for the most diverse reasons: from the study of confined water, to the use of clays to confine radioactive waste. Therefore the dielectric behavior of clays as a function of temperature, frequency and the presence of water is known. In particular, the measurements published in the literature say that at the low temperatures typical of the base of the Martian polar cap the dielectric permittivity, both the real and the imaginary part, are very small and cannot explain the strong echoes detected by Marsis. In essence, the measurements reported by Smith and colleagues are not in agreement with the existing literature and, more importantly,
Here, however, it seems they have taken a liking to it. Do you expect a new attack? From where?
Orosei and Pettinelli: «There was a presentation at a conference in which it was argued that the origin of the echoes seen by Marsis was caused by the resonance of thin layers of CO 2 ice . An article on this subject has not officially been published yet, but there is a preprint on a public archive. We have also examined this possibility and we have remained rather skeptical ».
And you? Are you going to react? Are you preparing a defense of your hypothesis?
Orosei and Pettinelli: «We already have two articles submitted to journals that address the objections raised by the articles in Geophysical Research Letters and a third will be sent shortly. We are reasonably certain that we can rigorously refute the alternative hypotheses to the presence of water. As we said above, the debate is only just beginning ».
To know more:
- Read on Media Inaf the article ” Mars, there is an underground lake of liquid water “, dated 25/07/2018
- Read on Media Inaf the article ” Underground Martian lakes: new signals analyzed “, dated 29/06/2021
- Read on Media Inaf the article “ A lake with feet of clay ”, dated 06/08/2021
Provided by INAF