Over the past few decades, we have noticed a lot of water on Earth under the ice. In some cases, we watch this water nervously, as it is under ice sheets, where it can lubricate the sheet slide at sea. But we also discovered lakes trapped under ice near poles, probably for millions of years, fueling the hope that they might have ancient ecosystems.
Today, researchers are applying some of the same techniques we used to find underwater ice lakes in data from Mars. And the results support an earlier claim that there are bodies of water trapped under the red planet’s polar ice.
Detection of liquids from orbit
Clearly Mars has vast waters locked away on the ice forum, and some of them revolve around the atmosphere as the orbital cycles make one pole or the other a bit warmer. But it will not be pure liquid water on Mars ̵1; the temperature is not high enough for too long, and atmospheric pressures are too low to keep any liquid water from boiling in the atmosphere.
The calculations indicate, however, that liquid water ay possible on Mars – not just on the surface. With sufficiently soluble salts, a rich water brine can remain liquid at temperatures prevalent on Mars – even in polar areas. And if it is trapped beneath the Martian surface, there may be enough pressure to keep it liquid despite the thin environment. That surface could be Martian land, and people think of that possibility. But the surface can also be one of the ice sheets we see on Mars.
That possibility helped motivate the design of MARSIS (Mars Advanced Radar for Subsurface and Ionosphere Sounding) in the Mars Express orbiter. MARSIS is a radar device that uses wavelengths to make ice ice transparent. As a result, most of the photons that return to the instrument are represented by the interface between the ice and so on: the atmosphere, the underlying bedrock, and potentially any interface between the ice and a liquid brine beneath it.
And that is what the original results seem to indicate, published in 2018. In a place called Ultimi Scopuli near the southern pole of Mars. Researchers have found a clear reflection, different from the underlying cause of bedrock, in some specific locations under the ice. And they interpret it as indicating a boundary between ice and some liquid brine.
Now with more data
Two things have changed since the previous results were made. One is that Mars Express continues to traverse the polar regions of Mars, generating more data for analysis. The second is the studies of ice-covered lakes on Earth that have also advanced, with new ones identified from orbit using similar data. So some of the teams behind the original work decided it was time to revisit the ice sheets at Ultimi Scopuli.
The study involves looking at the details of the photons reflecting back to the MARSIS instrument from a 250 x 300 square area. One aspect of this is the basic reflection of the various layers that can be distinguished from the data. Other aspects of the signal can tell us about how smooth the surface of the reflecting boundaries is and whether the nature of the boundary has changed abruptly.
For example, moving 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 one.
The researchers created separate maps of the intensity and the smoothness of the signal and found that the maps were more overlapping, giving them confidence that they were recognizing real movements on the surfaces. A separate dimension of the material (called permittivity) showed that it was high in the same location.
In general, researchers have found that the largest area of water is likely to be about 20 to 30 miles[20 to 30 km]of ice. And bedrock features separate it from 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 corresponds to some kind of under-ice feature — even though we use the detection standards used for the underwater ice lakes on Earth.
How did you get there?
The obvious question following the assumption that these bodies are filled with a salty water is how much fluid ends up there. We know that this salty solution can remain liquid at temperatures far below the freezing point. But conditions on Mars like most minimum temperatures for water to stay liquid are right on the verge of possible conditions in the area of polar ice sheets. So some people have suggested geological activity as a possible source of heat to keep things liquid.
That is not necessarily unlikely to sound. Some groups have suggested that some features indicate that there was magma on the surface of Mars as recently as 2 million years ago. But researchers here argue that if things are on the verge of working under current climate conditions, there is no need to use anything extraordinary.
Instead, they suggested that the types of salts we already know exist on Mars could receive water vapor in the thin Martian atmosphere. When formed, they can remain liquid for up to 150 Kelvin, when local temperatures in Ultimi Scopuli are likely to be in the area of 160 Kelvin and increase in depth.
And if that is true, there may be liquid in many other locations on Mars’ poles. Not all of them are as conducive to orbital imaging as Ultimi Scopuli, but a safe bet that this group will try to find additional additions.
Nature Astronomy, 2020. DOI: 10.1038 / s41550-020-1200-6 (About DOI).