339 — Radar evidence of subglacial liquid water on Mars

I don’t usually manage to read papers this hot off the press! (That’s what I get for procrastinating today, I guess.)

Today’s paper, which was also published today, details some pretty conclusive proof that there is liquid water on Mars.

The idea of liquid water on Mars isn’t new: We’ve long imagined that the huge abundance of water-positive signs (such as ancient canal-like structures, as well as the presence of polar ice) probably meant that there was liquid water somewhere on the planet. Planets are pretty big, right?

But the catch is that it’s hard to prove that a feature is a permanent body of water. As far as we know, liquid water is a pretty important requirement for life to begin. That’s not to say that things can’t survive in ice — it just means that our understanding of how life starts — that is, amino acid formation — requires long-standing liquid water.

But how do you prove that things that look like water is actually liquid water? After all, the spectrometry of ice is pretty darn similar to that of liquid water. How can you tell the difference?

Near the thickest part of the south pole of Mars — an icy surface that appears to have deep ice underneath it, much like the south pole of Earth — there’s a highly reflective region, with a correspondingly high relative dielectric permittivity (in other words, the permittivity to electric charge of a material versus that of a vacuum). The permittivity of a vacuum, definitionally, is 1; the permittivity of ice is around 88, and the permittivity of liquid water is much lower, around the mid 50s (although salty or brackish water is even lower, in the mid teens).

The permittivity of this subsurface region of Mars read in the vicinity of 15. On Earth, values greater than 15 almost assuredly indicate the presence of liquid of some type or another. On Mars, does that hold as well?

The only way to find out is to better understand the chemical makeup of martian soil: After all, the region of Mars under the polar cap is well below 0ºC. So in order for this region to contain liquid water, it would need to contain enough in-solution material to suppress freezing.

Indeed, magnesium, calcium, and sodium are found in high concentration on Mars. And so it is very reasonable to assume that these permittivity measurements, which suggest liquid water, are indeed signs of liquid water with high dissolved ion concentrations.