This value is lower than what is expected for pure water ice. Ideally, pure, solid water ice would have a dielectric of 3.0-3.2. In reality, we often have different materials in the mix. Rock, water ice, and porosity (i.e,. air) are in a three way tug-of-war over the dielectric constant result. Porosity pulls it down from 3, towards 1, and lithic (rock) content or dust brings it up towards 6–8. My work used a handy ternary diagram from Bramson et al. (2015), to come up with a possible combination of material that is consistent with our result. This amounted to a value of 50–80% water ice, 0–30% rocky content, and 15–50% porosity. So, think of a mostly icy but somewhat porous subsurface, with a bit of dust and lithic material mixed in.

This result implies a volume of water ice around 1.2 times the volume of Lake Superior. This is huge! And when you combine this result with work done by Bramson et al., 2015, which found a similar composition for the subsurface in Arcadia Planitia, it paints a picture of a northern plains rich in subsurface water ice, with widespread areas where these deposits are tens of metres thick.

Overall, it’s great to have geophysical support for what the geologists and geomorphologists have been saying for a while: that there are broad areas of subsurface ice throughout the northern plains thanks to deposition during high obliquity periods on Mars. What our evidence adds to that discussion are constraints on what kind of ice is there, in our case excess ice (i.e., more than 50% ice), and how thick it is throughout this area. The finding of excess ice contributes to the debate on how this water ice was emplaced, narrowing the range of plausible geologic histories for the area. And the constraints on thickness contribute to what we know about high-obliquity deposition of water ice in the mid-latitudes. We can see that the Utopia deposits are roughly twice the thickness of what’s measured in Arcadia Planitia, and much much thicker than what is expected for the latitude-dependent mantle generally (metres). This may speak to differential deposition of water ice during recent high-obliquity cycles, which could potentially act as a constraint for climate modellers down the road.

Finally, this is some of the lowest latitude water ice we’ve found on Mars, down to roughly 40°N (the lowest being Deuteronilus Mensae, where we see glacial features down to ~35°N). Considering that it’s no longer wacky to suggest that people are going to be visiting Mars someday, I don’t think it’s too radical to suggest that it’s important to characterize our low-latitude water ice deposits as potential resources. If our future missions are going to have astronauts, those astronauts are going to need something to drink. With these results, we see that Utopia Planitia is a great place for that…provided you find a way to deal with all the radiation and dust storms.