Laying Odds On The Potential For Martian Water

[PSI] We know Mars has water ice in its polar regions, but astronauts (and their solar powered equipment!) would much rather experience life closer to Mars’ equator. As NASA looks toward landing humans on the Red Planet, the Planetary Science Institute (PSI) is mapping just where water should and shouldn’t be found, and using statistics to quantify their certainty.

Two new papers, led by PSI’s Hanna Sizemore (paper 1) and Samuel Courville (paper 2) highlight new global maps of subsurface water ice and identify what new missions and tools are needed to increase the certainty of their maps.

Today, water ice can’t exist on the majority of the Martian surface for long periods of time. The extremely low density air causes ice to sublimate – to change from a solid to a gas. Missions like NASA’s Phoenix lander have been able to use scoops to uncover buried ice and use cameras to watch it sublimate away. Phoenix landed in a high latitude region where ice within centimeters of the surface was found. Astronauts closer to mid-latitudes will likely have to dig deeper to find ice. Mapping just where water is located is a necessary first step in preparing for human exploration of Mars.

“Water ice excavated from the subsurface can be converted into hydrazine,” says Sizemore. “This can be used as rocket fuel for a return trip to Earth. Making fuel from Martian ice is one form of in situ resource utilization (ISRU) and it can significantly reduce the mass and cost of a human mission.”

Sizemore and Courville are members of the PSI led Mars Subsurface Water Ice Mapping or SWIM Team, which, in part, is leveraging data from NASA’s Mars Global Surveyor’s Thermal Emission Spectrometer (TES) and the Mars Reconnaissance Orbiter’s Mars Climate Sounder (MCS) to locate water ice within about a meter of the surface. Both of these instruments obtained thermal maps of the Red Planet that allow researchers to look at how Mars’ surface and atmosphere respond to changes in sunlight from day to night and across the seasons.

Looking at the cooling and warming rates at the surface can reveal what lies beneath. For instance, regions layering dry sand over ice, or layering crusty sand over dust, will change temperature differently than a region of just sand (or just dust, rock, or ice) over the course of a Martian day and through the year.

This isn’t the first time researchers developed these kinds of maps. In Sizemore’s new paper, published in the Planetary Science Journal (PSJ), the SWIM Team’s maps are compared to two other state-of-the-art maps that used different techniques to process the mission data.

“Deriving an ice-burial depth on the ground from temperature measurements taken from orbit is a complex process,” states Sizemore. “Differences in the thermal instruments (TES vs MCS), differences in numerical models, and changing conditions in the atmosphere (e.g., dustiness) can all affect results. So there is a lot of uncertainty if we only use one instrument and one model. If we make multiple maps with different data, different assumptions, and different models, we can combine the maps and be much more certain of where the ice is or isn’t – particularly at the boundaries of the icy regions.”

In general, good agreement is found between the compared maps, and the places where differences are found are the places where new data are needed to better understand what is going on. These are ideal places to send future robots, but maybe not the right places to initially send humans. Sizemore and Courville are reducing the uncertainty on where Mars ice is, but certainty is needed when sending humans into dangerous environments.

In a second paper in the PSJ, this time led by Courville, probability and statistics are leveraged to add certainty levels to the water ice maps. “Whereas previous SWIM approaches have evaluated whether current orbital data are consistent with ice, this new paper formally quantifies how certain we are that ice is actually there,” according to Courville.

“This approach is new because it adds a formal calculation of the probability of finding ice at a given location to the informal map of ice ‘confidence,’” Sizemore explains. “Ice confidence values tell us something like one mapping technique found ice here, but two didn’t. The statistical approach lets us say ‘We have a 64% chance of finding ice if we go to this location.’

The data used in this work only probes the first 1 meter of materials on Mars surface. Current radar technologies have been able to map ices at depths greater than 5m. To extend this work to the unknown middle depths and to increase certainty in the results, new instruments will be needed. Specifically, a high frequency radar, capable of detecting ice in the 1-5 m depth range, could greatly increase the certainty in these results. Until then, this could be the best water map we have, uncertainty and all.

These papers appear in The Planetary Science Journal Focus Issue, Human Exploration of Mars: Resources and Science Targets, which includes SWIM analysis for finding deeper ice. Our study of Mars is ongoing, and with each new mission, creative leap, and technological advance, we will understand more about this target for future human exploration.

This work was supported primarily by the NASA Mars Exploration Program through JPL subcontracts 1611855 and 1639821, and also partially by the NASA Mars Data Analysis Program through grant 80NSSC21K1