In recent decades we have become aware of much water on Earth that lies deep under ice. In some cases, we’ve been nervous about watching this water because it’s deep under the ice sheets, where it could smother the sliding of the sheets into the sea. But we’ve also discovered lakes that were trapped under ice near the poles, possibly for millions of years, raising the prospect that they could harbor ancient ecosystems.
Now researchers are applying some of the same techniques we used to find those lakes under the ice on data from Mars. And the results support an earlier claim that there are bodies of water trapped under the red planet’s polar ice.
Spotting liquids from orbit
Mars obviously has extensive water trapped in the form of ice, some of it circulating through the atmosphere as orbital cycles make one pole or the other a little warmer. But there won’t be pure liquid water on Mars — the temperatures just aren’t high enough long and the atmospheric pressure is far too low to keep liquid water from boiling in the atmosphere.
However, calculations suggest that liquid water is possibly on Mars – just not on the surface. With enough dissolved salts, a water-rich brine could remain liquid at Martian temperatures, even in the polar regions. And if it’s trapped beneath the Martian surface, there could be enough pressure to keep it liquid despite the thin atmosphere. That surface could be Martian soil, and people are thinking of that possibility. But the surface could also be one of the ice sheets we’ve seen on Mars.
That possibility helped motivate the design of the MARSIS (Mars Advanced Radar for Subsurface and Ionosphere Sounding) on the Mars Express orbiter. MARSIS is a radar device that uses wavelengths for which water ice is transparent. As a result, most of the photons returning to the instrument are reflected off the interface between ice and something else: the atmosphere, the underlying rock, and possibly any interface between the ice and a liquid brine below.
And that’s what the original results, published in 2018, seemed to indicate. In an area called Ultimi Scopuli near the south pole of Mars. The researchers saw a bright reflection at a number of specific locations under the ice, different from that caused by the underlying rock. And they interpreted this as indicating a boundary between ice and some liquid brine.
Now with more data
Since those previous results, two things have changed. One is that Mars Express has continued to pass through the polar regions of Mars, generating even more data for analysis. The second is that studies of ice-covered lakes on Earth are also advanced, with new ones identified from orbit using similar data. So part of the team behind the original work decided it was time to revisit the ice sheets at Ultimi Scopuli.
The analysis involves looking at details of the photons reflected back to the MARSIS instrument from an area of 250 x 300 square kilometers. One aspect of this is the basic reflectivity of the different layers that can be distinguished from the data. Other aspects of the signal can tell us how smooth the surface of the reflective boundaries is and whether the nature of the boundary changes suddenly.
For example, the transition from an glacial boundary to an icy brine boundary would cause a sudden shift from a relatively weak, uneven signal to a brighter, smoother signal.
The researchers generated separate maps of the signal’s intensity and smoothness and found that the maps largely overlap, giving them confidence that they were identifying real transitions in the surfaces. A separate measurement of the material (called dielectric constant) showed it to be high in the same location.
Overall, the researchers found that the largest area likely to have water under the ice is about 20 by 30 kilometers. And it is separated by rock features from a number of similar but smaller bodies. It is speculative to call these bodies “lakes” as we have no idea how deep they are. But the data is certainly consistent with some sort of under-glacial feature, even if we use the detection standards used for under-glacial lakes on Earth.
How did that get there?
The obvious question after assuming these bodies are filled with a watery brine is how that liquid got there. We know that these salty solutions can remain liquid at temperatures well below freezing. But conditions on Mars are such that most minimum temperatures for water to remain liquid are right on the edge of possible conditions at the polar ice cap site. So some people have suggested geological activity as a possible heat source to keep things liquid.
That’s not necessarily as unlikely as it sounds. Some groups have proposed that some features indicate that magma was on the surface of Mars as recently as 2 million years ago. But the researchers here argue that if it’s about to work under current climate conditions, there’s no need to use anything out of the ordinary.
Instead, they suggest that the types of salts we already know are present on Mars can absorb water vapor from Mars’ thin atmosphere. Once formed, these can remain liquid up to 150 Kelvin, when local temperatures at Ultimi Scopuli are likely near 160 Kelvin, increasing with depth.
And if that’s true, there could be liquid in many more locations on Mars’ poles. Not all of them are as good for orbital imaging as Ultimi Scopuli, but it’s a safe bet this team will try to find some more.
Natural Astronomy, 2020. DOI: 10.1038/s41550-020-1200-6 (About DOIs).