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Mapping the ice on Mars that could support future missions

Image of two slopes with steep faces.
Enlarge / While we know of locations with ice on Mars, not all of them are located in places we’d want to land.

Over the last couple of decades, plans to return to the Moon for longer stays or to go to Mars have gradually moved away from sci-fi tinged “what if” scenarios, and shifted to something that resembles actual planning. And those plans invariably include extracting water from local ice deposits. This water would obviously help support any astronauts during their stay, cutting down on the weight we’d have to shift out of Earth orbit. But it can also be a source of hydrogen that helps power the astronaut’s return trip to Earth.

That, obviously, means we’re going to 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 temperatures where it’s stable. On Mars, things are considerably more complicated. So, in response to some NASA pilot funding, a team of scientists set up the SWIM projectM, for Subsurface Water Ice Mapping on Mars. The project has now published a progress report, and there is good news in the form of lots of ice deposits in areas we might want to land.

No poles, please

On the Moon, the temperature that dictates whether water ice is stable is set entirely by exposure to sunlight. As long as the Sun is never visible in a location, ice can survive. Mars is substantially more complicated, with an atmosphere that distributes heat and ensures that the temperature extremes are far more moderate, plus orbital wobbles that ensure seasonal changes in temperature.

Mars does have polar ice, but the extent of these deposits changes with the seasons (and a lot of it is frozen carbon dioxide). Further from the poles, there’s a region where temperatures would allow water ice would be stable should it form there. But further from the poles also means more extreme cold, and less solar energy for any photovoltaic equipment we might bring with us. Ideally it would be nice to find some ice in temperate regions, and some reports have suggested locations where it might reside.

The SWIM team decided to take a far more comprehensive approach, using data from multiple instruments to try to establish a degree of confidence in the presence of water. To do so, the team developed its own ice scoring system.

That data comes from a number of instruments we’ve put in orbit above Mars. These include a neutron counter (neutrons scatter differently in ice than in rock), and two forms of radar that register the presence and depth of ice deposits. In addition, water tends to transmit heat poorly, so measurements of thermal flux can be indicative of its presence. Finally, by comparison to glacial features on Earth, we can infer the presence of ice sheets from photographs of the terrain.

The authors created a scale for each of these five measurements, that ranged 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 allows some methods to compensate for the shortcomings of others. For example, neutron scattering is extremely sensitive, but could be blocked by a layer of dust less than a half-meter thick. Radar is less sensitive, but can pick up material much further below the surface.

Given their averaging technique, having one decisive reading would create a score of 0.2 if all the others methods were ambiguous. A score of 0.5 would mean that at least three of the methods strongly indicated the likely presence of water.

Go north, but not too far north

The first survey, reported here, has analyzed Mars’ northern hemisphere, from the equator up to 60º in latitude. There’s a small region along the east-west axis that’s not included, but otherwise, the data includes most of the area where we might reasonably be expected to want to land. Adding to the appeal, the area includes a lot of open plains with suitable terrain for dropping something out of orbit.

To an extent, the data is consistent with things we already had suspected. Modeling of temperature profiles had identified the northern areas within this region as likely to be able to support ice, and the readings do go up as you move north. And an examination of some of the regions that the mapping project identified showed that impacts in the area tended to expose ice (all 13 of the ice-exposing impacts they looked at were within one pixel of an area scored as likely to contain ice). Finally, a few of the areas identified by the mapping correspond to regions where the geography had already been interpreted as indicating a glacial history.

But the key finding is that some apparently ice-rich areas are quite a bit further south than we’d have predicted based on temperature modeling alone. There were areas that scored above 0.5 at about 35º north of the martian equator, well into Mars’ relatively temperate zones (for comparison, it’s roughly where you’d find Morocco on Earth). One of the strongest signals is in an are called Arcadia Planitia, a very flat area covered by recent volcanic flows.

The team will presumably move on to the southern hemisphere next. And that’s going to be critical. While it’s great that we have a potential site well into mid-latitudes of Mars, any landings there are going to be focused on the scientific case for exploring the area. Having multiple promising sites will give us the chance to pick and choose based on something beyond water availability.

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

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