Where would you land on Mars if the water had to be underneath you? It is not an idle question. Ice dug out of the Martian subsurface is not just drinking water, it’s oxygen, and it’s the feedstock for making propellant on the surface rather than hauling it from Earth. Manufacture your return fuel where you land and the mass of the whole mission drops enormously. This is what planetary scientists mean by in situ resource utilisation, and it is the difference between a plausible crewed mission and an implausible one.
Unfortunately, geography isn’t on our side since Mars keeps its ice at the poles, where the Sun barely climbs and solar panels are close to useless. The equator is where you want your astronauts, and there the ice is long gone. The atmosphere is so thin that surface ice does not melt so much as vanish, sublimating straight into the atmosphere. NASA’s Phoenix lander scooped up buried ice at high latitude and actually filmed it disappearing. Anywhere nearer the equator, you dig, and you dig deeper.
Ice in Phoenix’s “Dodo-Goldilocks” trench, photographed in June 2008. The lumps in the lower simply sublimated away. Salt couldn’t do that which is how we knew it was water (Credit : NASA/JPL-Caltech/University of Arizona/Texas A&M University)
So attention falls on the mid-latitudes and that is precisely where the maps have been least trustworthy.
Two papers in the Planetary Science Journal, led by Hanna Sizemore and Samuel Courville of the Planetary Science Institute, tackle the problem from opposite ends. Both draw on the SWIM project, which reads buried ice indirectly by watching how the ground warms and cools. Sand over ice holds heat differently from sand over dust, and thermal instruments in orbit, on Mars Global Surveyor and Mars Reconnaissance Orbiter, have been recording those rhythms across days and seasons for years.
Sizemore’s contribution is to stop trusting any single reading, from any instrument. Different instruments, different models and a dustier or clearer atmosphere all pull the answer around, so her team compared three independently produced maps and looked at where they agree. Broad agreement is reassuring. The disagreements are arguably more useful, because they mark exactly the ground we understand least, these are great places to send a robot, bad places to send a crew.
A wedge shaped pit in the Martian mid latitudes. Erosion has cut through the surface and opened a window onto a buried ice sheet (Credit : NASA/JPL-Caltech/UA/USGS)
Courville’s paper does something more subtle and, I think, more important. It converts confidence into probability. “Two techniques found ice here and one didn’t” is a statement about our methods. “There is a 64 per cent chance of ice at this location” is a statement about Mars, and it is the sort of number a mission planner can actually work with.
There is a catch and it is a wide one since all of this reads only the top metre of ground. Radar handles depths beyond five metres. Between one and five metres though, the range a crew would realistically excavate sits in a gap that no current instrument covers, and closing it needs a high-frequency radar nobody has yet flown. Until someone does, this is the best water map of Mars we have.
Source : Laying odds on the potential for Martian water
