I posted previously about the announcement that buried glaciers had been discovered on Mars, but now I’ve had time to actually take a look at the article in Science. The important point of the article is not that these formations were discovered (we’ve actually known about them for some time) but that, using radar on the Mars Reconaissance Orbiter, scientists were able to penetrate the surface of these mysterious glacier-looking things and prove that they are mostly ice!

You may be wondering how you could have a big, lobate, flowing tongue of stuff that isn’t mostly ice. Well, all you need is something viscous to flow. A pile of rocks, with just a tiny bit of ice holding them together and lubricating the flow can mimic the shape of a glacier of pure ice. Conveniently enough, I got interested in these last year when I was taking a geomorphology class, and found some pictures of terrestrial examples in the Andes with Google Earth:

You can see the lobate flows oozing their way down from the peaks, but in this case, you’re looking at something that is almost entirely rock. Many people thought that the glaciers that people were seeing on Mars were similar, but there was no way to test this until MRO got there with its radar sounder, SHARAD. Radar waves bounce off of rock, but are quite good at penetrating ice. So, when MRO flew over the glaciers, the scientists bounced radar off the glaciers and saw this:

This shows along the top what the surface would look like if it was just made of rock, and then it shows the actual data below. You can see that the lobes of “stuff” have a radar reflection at their surface, but some of the radar penetrates and bounces off of another boundary beneath the surface. This is showing that some of the radar reflects off of the surface but some travels down through the ice until it hits solid ground underneath. Rock glaciers wouldn’t do this, but glaciers of almost pure ice would!

The paper has a very nicely written concluding paragraph that I will post here, with slight annotations. It explains why the glaciers are there and why they are significant:

Why would such large quantities of snow and
ice accumulate in the eastern Hellas region in
particular? Over time scales of millions of years,
Mars undergoes large changes in spin-axis obliquity (tilt), forcing changes in insolation (sunlight energy that reaches the surface), and hence
in climate and the subsequent distribution of
ice. Climate simulations performed with
a model that includes the current water cycle but
assumes an obliquity of 45° predict snow accumulation
in the eastern Hellas region from a
south polar water source that operates efficiently
at the southern summer solstice, when the southern
polar cap releases large amounts of water
vapor. This vapor moves northward and is deflected
by cold air moving southward from the
Hellas basin; the subsequent cooling causes strong
condensation and precipitation in the area of the
LDAs (Lobate Debris Aprons), which we have shown to contain
primarily water ice. We therefore conclude that
these deposits harbor large quantities of water ice
derived from high-obliquity epochs, now concealed
beneath a thin protective layer. This ice
survives from climatic conditions markedly different
from today’s and is potentially accessible
to future landed missions, not only for scientific
study but as a resource to support exploration.