Mars is made of Swiss cheese

  

If the Moon is made of Green Cheese, then what cultured dairy confection makes up Mars? Why Swiss Cheese, of course! This week, Alex takes us on a tour of the pitted south polar terrain of Mars whose interplay of sunlight, water and carbon dioxide ices result in something that looks visibly similar to Swiss Cheese. Naming planetary terrains after food is not new, nor is it limited to the inner solar system. If you were putting together a platter of hors d'oeuvres, Cantaloupe makes an excellent accompaniment to Swiss Cheese. Perhaps we will have to take a closer look at Neptune's moon Triton in the future...

By Alex Innanen

Long-time PVL blog enthusiasts may recall that my planetary journey began at the Martian north pole looking at many, many HiRISE images. Over the past year I’ve returned to the Martian poles – the south pole this time.

Both poles have layered deposits of mostly water ice and dust, and residual water ice caps left behind when the winter layer of CO
2 ice sublimates in the summer. The south polar residual cap (or SPRC for the acronym fans) is mostly made up of carbon dioxide ice as well, overlying water-ice. The terrain of the SPRC is as varied as the North pole, but has some features that are unique to it. One of these are circular or circular-ish pits with steep sides and flat bottoms. The terrain they carve out is similar to a piece of Swiss cheese, giving the features their nickname. 

The distinctive pits of Swiss cheese terrain, from the HiRISE instrument.
[NASA/JPL/University of Arizona]

In Swiss cheese – the kind you can eat – the distinctive holes are formed by carbon dioxide bubbles that are released by the cheese-making bacteria. The Swiss cheese features of the SPRC are much larger than the ‘eyes’ in a piece of cheese – on the order of tens to a few hundreds of metres in diameter. No bacteria are forming these holes, instead they’re likely formed from fractures in the residual cap, which are widened into pits through sublimation from their walls. In the southern spring and summer, the steep, dark sides of the pits get more sunlight than the flat floors, causing the walls to sublimate and grow outwards by a few metres per year.

If the pits grow large enough, they can even grow into each other, creating intricate, branching features that can cover large swaths of the residual cap, like you can see in the HiRISE image here. It’s been suggested that based on this rate of growth, every century or so the entire SPRC could be entirely carved out by Swiss cheese features, causing a total resurfacing. 

[NASA/JPL/University of Arizona]

The Swiss cheese features occasionally show more ephemeral features such as bright, surrounding halos or dark fans emanating from higher standing areas. There’s a fairly clear halo around the feature shown at the top of this post – sometimes nicknamed the ‘Happy Face’. It looks almost like the feature is glowing, but what we’re really seeing is a localized region of higher albedo (i.e. more white) surrounding the Swiss cheese feature. These halos have only been observed during the Southern summer of Mars year 28 (2007, for Earthlings), and their appearance happened to follow a global dust storm. It’s likely, though, that these halos aren’t actually a ring of material getting lighter, but rather the SPRC as a whole getting darker from settling dust, except in the areas close to the pit walls. The mechanism that was proposed to explain this in a 2014 paper, is that the sublimation from the pit walls that I discussed above raises the amount of CO2 in the atmosphere and pushes the settling dust from the storm away from the edges of the pits. Lower rates of sublimation on flat areas allow the dust to settle normally.

The dark fans are much smaller and harder to pick out of even HiRISE images – on the scale of 1-10 m². They tend to appear at the edges of high-standing areas, ‘fanning’ into the lower areas. They appear in the southern spring, and unlike the halos they have been seen over multiple Mars years. Moving into the summer, as CO
2 ice sublimates, the terrain around the fans darkens until the fans disappear. Their formation is also much more exciting – they’re formed when jets of gas rupture through the CO
2 ice layer, lifting dust and depositing it outward in the fan shape. Dust can then get trapped in layers of ice, making it darker, absorbing more sunlight, and leading to more sublimation, creating more trapped gas to explode out and create more fans.

Until now I’ve been talking about CO
2 ice which makes up the majority of the SPRC. But what about water ice? The polar layered deposits are composed mostly of water ice and dust, and in the Southern summer the SPRC shrinks and exposes some of the water ice of the south polar layered deposits. It is possible that the flat floors of Swiss cheese pits also expose water ice in the summer. There have been detections of water vapour associated with the pits, but this could also be from their walls, which could be layers of CO
2 and water ice. In either event, the work I’ve been doing looks as if it is possible for the water ice in the Swiss cheese pits to have any appreciable contribution to atmospheric water vapour. The polar caps are the major source of surface water ice, and the yearly formation and retreat of overlying CO
2 ice, exposing water ice, drives Mars’ water cycle. I’m interested in finding out how much, if any, water vapour could be released from the Swiss cheese pits, and in the event of most or all of the SPRC being removed by Swiss cheese pits, whether this could have a significant impact on the amount of atmospheric water vapour.