Now the researchers are applying some of the same techniques we used to find these lakes under the ice to data from Mars. And the results support an earlier claim that bodies of water are trapped under the polar ice of the Red Planet.
Identify liquids from orbit
Mars clearly has a large amount of water locked in the ice forum, and some of it passes 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 – temperatures just aren’t high enough for very long, and atmospheric pressures are far too low to keep liquid water from boiling in the atmosphere.
Calculations suggest, however, that liquid water East possible on Mars, but not on the surface. With enough dissolved salts, a water-rich brine could remain liquid at temperatures prevalent on Mars, even in polar regions. And if it’s trapped below the Martian surface, there could be enough pressure to keep it liquid despite the low atmosphere. This surface could be Martian soil, and people are pondering that possibility. But the surface could also be one of the ice caps we’ve spotted on Mars.
This possibility helped to motivate the design of the Mars Advanced Radar for Subsurface and Ionosphere Sounding (MARSIS) on the Mars Express Orbiter. MARSIS is a radar device that uses wavelengths at 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 bedrock, and potentially any interface between ice and a liquid brine below.
And that’s what the original results, released in 2018, seemed to indicate. In an area called Ultimi Scopuli near the south pole of Mars. The researchers saw a bright reflection, distinct from that caused by the underlying bedrock, in specific places under the ice. And they interpreted this to indicate a boundary between ice and certain liquid brines.
Now with more data
Two things have changed since these first results were obtained. The first is that Mars Express continued to pass over the polar regions of Mars, generating even more data for analysis. The second is that studies of ice-covered lakes on Earth have also advanced, with new lakes having been identified from orbit using similar data. So part of the team behind the original work decided it was time to revisit the Ultimi Scopuli ice caps.
The analysis involves examining the details of 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 various layers that can be discerned from the data. Other aspects of the signal can tell us how smooth the surface of the reflective borders is, and if the nature of the border suddenly changes.
For example, the transition from an ice-bedrock boundary to an ice-brine boundary would cause a sudden change from a relatively weak and irregular signal to a clearer, smoother signal.
The researchers generated separate maps of signal strength and smoothness and found that the maps overlapped widely, giving them the confidence to identify true transitions in surfaces. A separate measurement of the material (called permittivity) showed it to be high in the same location.
Overall, the researchers found that the largest area likely to have water under the ice was around 20 by 30 kilometers. And it is separated by basic characteristics of a number of bodies. similar but smaller. Calling these bodies “lakes” is speculative, since we have no idea how deep they are. But the data is certainly consistent with some sort of feature under the ice – even though we are using the detection standards that have been used for lakes under ice on Earth.
How did it get there?
The obvious question under the assumption that these bodies are filled with an aqueous brine is how that amount of liquid ended up there. We know that these salty solutions can remain liquid at temperatures well below the freezing point. But conditions on Mars are such that most of the minimum temperatures for water to stay liquid are right on the edge of possible conditions at the site of the polar ice caps. So, some people have suggested geological activity as a possible source of heat to keep things liquid.
This is not necessarily as unlikely as it sounds. Some groups have proposed that certain characteristics indicate that there was magma on the surface of Mars as recently as 2 million years ago. But the researchers here argue that if things are about to work under current climatic conditions, there is no need to resort to anything exceptional.
Instead, they suggest that the types of salts we know already present on Mars can absorb water vapor from the thin Martian atmosphere. Once formed, these can remain liquid up to 150 Kelvin, when local temperatures at Ultimi Scopuli are likely to be in the range of 160 Kelvin and increase with depth.
And if that’s true, there could be liquid in many other places at the poles of Mars. Not all are as suited to orbital imaging as Ultimi Scopuli, but it’s a safe bet that this team will try to find others.
Nature Astronomy, 2020. DOI: 10.1038 / s41550-020-1200-6 (About DOIs).