The past few decades have had plans to go to Mars or return to the moon for longer stays, gradually moving away from the scientific “what if” scenarios and shifting to something resembling actual planning. And these plans always include extracting water from local ice deposits. This water can help support astronauts during their stay, and reduce the weight we have to move out of Earth’s orbit. But it can also be a source of hydrogen that helps drive the astronaut’s return to Earth.
This of course means that we want to land where the water is. On the moon, this meant we had to focus on the moon poles, where deep craters create permanent shadows that can hold ice at temperatures where it is stable. On Mars, the situation is considerably more complicated. In response to some NASA pilot funding, a team of scientists set up the SWIM project for Subsurface Water Ice Mapping on Mars to analyze the data. The project has now published a progress report showing many ice deposits in areas where we may want to land.
No poles, please
Whether the ice on the moon is stable is fully determined by exposure to sunlight. As long as the sun is never visible in one place, ice can survive. Mars is considerably more complex, with an atmosphere that dissipates heat and makes the extreme temperature much more moderate, plus orbital motions that ensure seasonal changes in temperature.
Mars does have ice, but the number of deposits changes with the seasons (and many of them are frozen carbon dioxide). Further from the poles there is an area where the temperature can make ice stable if it forms there. But the region is still far from the equator, which means more cold and less solar energy for any photovoltaic equipment we may bring. Ideally, it would be nice to find ice in temperate regions, and some reports have suggested places where it might be.
The SWIM team decided to take a much more comprehensive approach by using data from various instruments to try to achieve some confidence in the presence of water. To do this, the team developed its own icing system.
The data comes from a number of instruments we placed in orbit above Mars. These include a neutron counter (neutrons distributed differently in ice than in rock) and two types of radar that record the presence and depth of ice deposits. In addition, water tends to transfer heat poorly, and thus the measurement of thermal flux may be an indication of its presence. Finally, by comparing them with glaciers on Earth, we can deduce the presence of ice sheets from photographs of the site.
The authors made a scale for each of these five measurements ranging from -1 (ice is very unlikely) to 1 (ice is almost certain). They then score the average of the five and create an overall score for the possible ice presence. This has made some methods possible to make up for the shortcomings of others. Neutron distribution, for example, is extremely sensitive, but can be blocked by a layer of dust less than half a meter thick. Radar is less sensitive, but can pick up material much further below the surface.
Given the average technique of the researchers, a decisive reading would create a score of 0.2 if all the other methods were ambiguous. A score of 0.5 would mean that at least three of the methods strongly indicated the probable presence of water.
Go north, but not too far north
The first survey, reported here, analyzed the northern hemisphere of Mars, from the equator to 60º in latitude. There is a small region along the east-west axis that is not included, but otherwise the data covers most of the area where we could reasonably land. The area also contains many open plains with suitable terrain to drop something off the track.
The data is in line with what we already suspected. The modeling of temperature profiles has identified that the northern areas within this region are likely to support ice, and the readings increase as you move north. An investigation of some of the regions identified by the mapping project shows that the impact in the area tends to expose ice (all 13 of the ice exposure effects the researcher looked at were within one pixel of an area likely to be ice contains). Finally, some of the areas identified by the mapping correspond to regions where the geography has already been interpreted as an indication of an ice history.
But the most important finding is that some seemingly icy areas are further south than we would have predicted based on temperature modeling. There were areas that scored more than 0.5, about 35º north of the Mars equator, as far as the relatively temperate zones of Mars (by comparison, this is about where you would find Morocco on earth). One of the strongest signals is in an area called Arcadia Planitia, a very flat region covered by recent volcanic currents.
The team is expected to move on to the southern hemisphere next. And that’s going to be critical. While it’s amazing that we have a potential location up to the center of Mars, any landings there will be focused on the scientific matter of exploring the area. If we have several promising sites, we can pick and choose based on something beyond the availability of water.
Natural Astronomy, 2021. DOI: 10.1038 / s41550-020-01290-z (About DOIs).