Mapping ice on Mars that could support future missions

Over the past two decades, plans to go to Mars or return to the Moon for longer stays have gradually shifted away from sci-fi “what if” scenarios and into something akin to planning. real. And these plans invariably include extracting water from local ice deposits. This water would help support the astronauts during their stay, thus reducing the weight we would have to move out of Earth orbit. But it could also be a source of hydrogen that helps fuel the astronaut’s return trip to Earth.

This obviously means that we want to land where the water is. On the Moon, this has meant focusing on the lunar poles, where deep craters create permanent shadows that can hold ice at stable temperatures. On Mars, the situation is considerably more complicated. In response to pilot funding from NASA, a team of scientists set up the SWIMM project, for Subsurface Water Ice Mapping on Mars, to analyze the data. The project has now released a progress report showing numerous ice deposits in areas we might want to land.

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Whether or not water ice is stable on the Moon is entirely determined by exposure to the sun. As long as the sun is never visible in one place, ice can survive. Mars is significantly more complicated, with an atmosphere that distributes heat and makes extreme temperatures much more moderate, as well as orbital oscillations that ensure seasonal changes in temperature.

Mars has polar ice, but the number of these deposits changes with the seasons (and much of it is frozen carbon dioxide). Further away from the poles, there is a region where temperatures would stabilize water ice, if it did form there. But this region is still far from the equator, which means more extreme cold and less solar power for any PV equipment we might bring with us. Ideally, it would be nice to find ice in temperate regions, and some reports have suggested places where it could reside.

The SWIM team decided to take a much more holistic approach, using data from several instruments to try to establish a degree of confidence in the presence of water. To do this, the team has developed its own on-ice scoring system.

This data comes from a number of instruments that we have put into orbit above Mars. These include a neutron counter (neutrons diffuse differently in ice than in rock) and two forms of radar that record the presence and depth of ice deposits. Additionally, water tends to transmit heat poorly, so heat flux measurements can be indicative of its presence. Finally, by comparing them to the glacial characteristics of the Earth, we can deduce the presence of ice caps from photographs of the terrain.

The authors created a scale for each of these five measurements ranging from -1 (ice extremely unlikely) to 1 (ice almost certainly present). They then averaged the five, creating an overall score for the possible presence of ice. This allowed some methods to compensate for the shortcomings of others. For example, neutron scattering is extremely sensitive but could be blocked by a dust layer less than half a meter thick. Radar is less sensitive but can pick up material much further below the surface.

Given the researchers’ averaging technique, having a decisive reading would create a score of 0.2 if all other methods were ambiguous. A score of 0.5 would mean that at least three of the methods strongly indicate the probable presence of water.

Go north, but not too far north

The first study, reported here, analyzed the northern hemisphere of Mars, from the equator to 60 ° latitude. There is a small region along the east-west axis that is not included, but otherwise the data includes most of the region where we could reasonably expect to land. Adding to the appeal, the area includes many open plains with suitable terrain for dropping something out of its orbit.

To some extent, the data is consistent with what we had previously suspected. Modeling the temperature profiles had identified northern areas of this region as likely to support ice, and the readings increase as you move north. An examination of some of the regions identified by the mapping project shows that impacts in the area tend to expose ice (the 13 ice exposure impacts that the researcher examined were within a pixel of an area considered likely to contain ice). Finally, some of the areas identified by the mapping correspond to regions where the geography had already been interpreted as indicating glacial history.

But the main conclusion is that some apparently ice-rich areas are further south than we would have predicted based on temperature modeling alone. There were areas that scored above 0.5 at about 35 ° north of the Martian equator, well in the relatively temperate areas of Mars (for comparison, this is roughly where you find Morocco on Earth). One of the strongest signals is in an area called Arcadia Planitia, a very flat area covered by recent volcanic flows.

The team will likely then travel to the southern hemisphere. And that’s going to be critical. While it’s great that we have a potential site well in the mid-latitudes of Mars, all of the landings there will be focused on the science case of exploring the region. Having multiple promising sites will give us the flexibility to choose based on something beyond the availability of water.

Nature astronomy, 2021. DOI: 10.1038 / s41550-020-01290-z (About DOIs).