11 March 2008

Layers and Swiss Cheese

Posted by Ryan Anderson

No, this isn’t a post about sandwiches. There just happen to be layers and swiss cheese (terrain) in the ice caps on Mars.

The morning session that I attended today was all about the north and south polar ice caps, and what people are seeing there, especially with new high-resolution data. The poles of Mars are really interesting because every winter the atmosphere condenses out to form layers of carbon dioxide and every summer those layers sublimate away.

One of the first talks I saw this morning was by Kathryn Fishbaugh, about tracing layers in the north polar cap with very high resolution HiRISE images, and the digital elevation models (DEMs) that can be made from those images. DEMs work on the same priciple as depth perception: using two slightly different points of view (one from each eye) your brain can construct a 3D view of the world around you. Similarly, using images taken from slightly different locations in orbit, scientists can make really high resolution topographic maps. Fishbaugh used these maps, and the images from HiRISE, to identify important “marker” layers that are easy to follow over long distances in the north polar cap. She found that the spacing of these thick layers might correspond to changes in the way that Mars tilts. The tilt of a planet can drastically change its climate, and Mars can change from spinning nearly vertically to nearly tipping over on its side (tilting up to nearly ~60 degrees sometimes).

Another talk by Wendy Calvin showed that even where the north polar caps look dark, there is evidence for ice. Everywhere they look with CRISM, light and dark layers both have ice in them. The dark layers might just have a little more dust. Dust can acti like a pigment, and a little bit can make a big difference in the color of the surface.

Moving on to the south pole, Shane Byrne gave a cool talk about explaining the “swiss cheese” terrain that is seen in the carbon-dioxide ice at the south pole. To give you an idea of why it is called “swiss cheese” terrain, take a look at this:


In the picture, the really smooth area is higher up. It is part of a slab of dry ice that is gradually turning back into gas. As it sublimates, the terrain develops really weird textures: the lower-elevation pits in the CO2 layer are like holes in a slice of swiss cheese. Byrne came up with a computer model that shows that, weird though it may look, the swiss cheese terrain is just the result of normal cycles of deposition and sublimation of carbon dioxide. The cycle works like this: start with a flat area and start depositing dry ice on it from the atmosphere. Inevitably, there will be some places that get a little more than other places, and the surface will become more rough. Once roughness gets started, it will become exaggerated as more CO2 is added (maybe a rough spot that gets less sunlight will accumulate a litte more CO2). Eventually, pits form in the CO2 layer where more CO2 sublimates than condenses. Once the pit reaches through the CO2 layer, it grows and grows, until it meets up with other pits and eventually the CO2 layer is eaten away. Pretty cool stuff. A key factor in his models is that dust storms can change the rate at which CO2 is deposited, so they can have a big influence on how the swiss cheese terrains form and evolve.

There was a talk about Martian spiders too, but I’ll let Melissa fill you in on that one.