In recent decades, we have realized a lot of water on Earth that is deep under ice. In some cases, we have observed this water nervously, as it is deep under the ice sheets, where it can crush the sliding of sheets into the sea. But we also found lakes that are closed under ice near the poles, probably for millions of years, which increases the prospect that they can house ancient ecosystems.
Researchers are now applying the same techniques we used to find these icy lakes to data from Mars. And the results support an earlier claim that there are bodies of water that have fallen under the polar ice of the red planet.
Detection of fluids from orbit
Mars apparently has extensive water locked in the ice forum, and some of them cycle through the atmosphere as orbital cycles make one or the other pole a little warmer. But there will be no clean liquid water on Mars ̵
However, calculations suggest that liquid water is possible on Mars – just not on the surface. With enough dissolved salts, water-rich brine can remain liquid at temperatures prevailing on Mars – even in the polar regions. And if trapped below the Martian surface, there may be enough pressure to maintain fluid despite the thin atmosphere. This surface may be Martian soil, and people are considering it. But the surface may also be one of the ice sheets we’ve seen on Mars.
This capability helped motivate the design of the Mars Express (Mars Advanced Radar for Underurface and Ionosphere Sounding) orbiter Mars Express. MARSIS is a radar device that uses wavelengths for which water ice is transparent. As a result, most of the photons that return to the instrument are reflected by the interface between the ice and something else: the atmosphere, the underlying substrate, and potentially any connection between the ice and the liquid brine beneath it.
And this seems to show the initial results published in 2018. In an area called Ultimi Scopuli near the south pole of Mars. The researchers saw a bright reflection, different from that caused by the underlying rock base, in some specific places under the ice. And they interpreted this as indicating a boundary between ice and some liquid brines.
Now with more data
Two things have changed since those earlier results. One is that the Mars Express continues to pass over the polar regions of Mars, generating even more data for analysis. The second is that research on ice-covered lakes on Earth has also advanced, with new ones being identified from orbit using similar data. So some of the team behind the original work decided it was time to review the ice sheets at Ultimi Scopuli.
The analysis includes consideration of details of the photons reflected back in the MARSIS instrument of 250 x 300 square kilometers. One aspect of this is the basic reflectivity of the different layers that can be recognized by the data. Other aspects of the signal can tell us how smooth the surface of the reflecting boundaries is and whether the nature of the boundary changes suddenly.
For example, the transition from the boundary of the ice rock to the ice brine would cause a sudden transition from a relatively weak, uneven signal to a brighter and smoother one.
The researchers generated separate maps of signal intensity and smoothness and found that the maps largely overlapped, giving them confidence that they identified real transitions in the surfaces. A separate measure of the material (called dielectric constant) indicates that it is high in the same place.
In general, the researchers found that the largest area that probably has water under the ice is about 20 by 30 kilometers. And it is separated from the main features by a number of similar but smaller bodies. Calling these bodies “lakes” is speculative, given that we have no idea how deep they are. But the data certainly match some characteristic under ice – even if we use the detection standards that have been used for the Earth’s icy lakes.
How did this come about?
The obvious question following the assumption that these bodies are filled with aqueous brine is how so much liquid got there. We know that these saline solutions can remain liquid at temperatures well below freezing. But the conditions on Mars are such that most of the minimum temperatures for water to remain liquid are exactly on the verge of possible conditions in place of the polar ice sheets. So some people suggest geological activity as a possible source of heat to keep things liquid.
This is not necessarily as unlikely as it may sound. Some groups suggest that some characteristics indicate that there was magma on the surface of Mars 2 million years ago. But researchers here say that if things are on the verge of working in the current climate, we don’t need to resort to anything extraordinary.
Instead, they suggest that the varieties of salts we already know to be on Mars can absorb water vapor from the thin Martian atmosphere. Once formed, they can remain liquid up to 150 Kelvin, when local temperatures in Ultimate Scopula are likely to be in the region of 160 Kelvin and rise in depth.
And if that’s true, there could be fluid at many more places on the poles of Mars. Not all of them are as susceptible to orbital imaging as Ultimi Scopuli, but it is certain that this team will try to find additional ones.
Astronomy of Nature, 2020. DOI: 10.1038 / s41550-020-1200-6 (For DOI).