In the last few decades we have become aware of a lot of water on earth that lies deep under ice. In some cases, we have been nervously watching this water as it lies deep under ice sheets where it could smear the sheets' slide into the sea. But we've also discovered lakes that may have been trapped under ice near the poles for millions of years, increasing the chances that they could harbor ancient ecosystems.
Now researchers are applying some of the same techniques that we used to find these sub-ice lakes on data from Mars. And the results support an earlier claim that waters are trapped beneath the red planet's polar ice.
Detect fluids from orbit
Clearly, a lot of water is trapped in the form of ice on Mars, and some of it flows through the atmosphere as orbital cycles make one or the other pole a little warmer. But there will be no pure liquid water on Mars – temperatures just aren't high long enough and atmospheric pressure is far too low to prevent liquid water from boiling off into the atmosphere.
However, calculations suggest that liquid water is possible on Mars – just not on the surface. With enough dissolved salts, a water-rich salt solution could remain liquid at the temperatures prevailing on Mars – even in the polar regions. And when it's trapped beneath the Martian surface, despite the thin atmosphere, there can be enough pressure to keep it fluid. That surface could be Martian soil, and people are considering that possibility. But the surface could also be one of the ice sheets that we discovered on Mars.
This opportunity helped motivate the design of MARSIS (Mars Advanced Radar for Underground and Ionospheric Probing) on the Mars Express orbiter. MARSIS is a radar device that uses wavelengths to which water ice is transparent. As a result, most of the photons returning to the instrument will be reflected from the interface between ice and something else: the atmosphere, the bedrock below, and possibly any interface between the ice and an underlying liquid brine.
And that's exactly what the original results published in 2018 seemed to show. In an area called Ultimi Scopuli near the South Pole of Mars. In some places under the ice, the researchers saw a bright reflection that was different from that caused by the bedrock below. And they interpreted this as indicating a boundary between ice and some liquid salt solutions.
Now with more data
Two things have changed since these earlier results were achieved. For one, Mars Express continued to pass through the polar regions of Mars and generated even more data for analysis. The second is that studies of ice-covered lakes on Earth have also advanced, with new ones being identified from orbit based on similar data. Part of the team behind the original work decided it was time to revisit the ice sheet at Ultimi Scopuli.
During the analysis, details of the photons are considered, which are 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 various layers that can be distinguished from the data. Other aspects of the signal can reveal how smooth the surface of the reflective borders is and whether the nature of the border changes suddenly.
For example, the transition from an ice bedrock boundary to an ice brine would cause a sudden shift from a relatively weak, uneven signal to a brighter and smoother signal.
The researchers made separate maps of the intensity and smoothness of the signal and found that the maps largely overlapped, giving them confidence that they were identifying real transitions in the surfaces. A separate measure of the material (called permittivity) showed that it was high in the same place.
Overall, the researchers found that the largest area likely to have water under the ice is around 20 by 30 kilometers. And it is separated from a number of similar but smaller bodies by bedrock features. Calling these bodies "lakes" is speculative as we have no idea how deep they are. But the data certainly does agree with some kind of under-ice feature – even if we use the detection standards that were used for under-ice lakes on Earth.
How did that get there?
The obvious question of assuming these bodies are filled with an aqueous saline solution is how much fluid has ended up there. We know that these salty solutions can remain liquid at temperatures well below freezing. However, the conditions on Mars are such that most of the minimum temperatures below which water will remain liquid are right on the edge of the possible conditions at the location of the polar ice sheet. So some people have suggested geological activity as a possible source of heat to keep things fluid.
That's not necessarily as unlikely as it sounds. Some groups have suggested that some features suggest that magma was on the Martian surface as recently as 2 million years ago. However, the researchers here argue that there is no need to resort to anything out of the ordinary when things are on the verge of work in the current climatic conditions.
Instead, they suggest that the types of salts that we already know are present on Mars can absorb water vapor from the thin Martian atmosphere. Once formed, these can remain liquid up to 150 Kelvin if the local temperatures at Ultimi Scopuli are probably in the 160 Kelvin range and increase with depth.
And if that's true, there could be fluid in many other places at the poles of Mars. Not all of them are as suitable for orbital imaging as Ultimi Scopuli, but it is certain that this team will try to find more.
Nature Astronomy, 2020. DOI: 10.1038 / s41550-020-1200-6 (Via DOIs).