Ice filled fractures on Mars.

 Now to reveal the answer to our first guess the planet post...

Here is another view of the features I posted on Monday, this time with the proper context information. North is to the top left, and the image is over a kilometre across. The sun symbol indicates that illumination in this image is coming from the bottom left corner, although in this case that doesn't have too great an effect on how the features look.

 

This image comes from Mars and its location is indicated by the star in the inset context map. It is a section of HiRISE image PSP_007571_2490 and the credit goes to NASA/JPL/ University of Arizona. Their High Resolution Imaging Science Experiment, or HiRISE instrument, on the Mars Reconnaissance Orbiter (MRO) produces the most detailed images of Mars. The Pixels on HiRISE images can be as small as 25 centimetres across. So they have comparable resolution to the air photos you see in Google Earth. This means that metre scale objects can be “resolved”, in other words those are the size of feature which you can reliably tell are present. This means that HiRISE images give us a great view of the landscape. 

This image shows heavily fractured ground in the high northern latitudes. Cracks have opened up in the ground, forming a massive network of cracks which covers many kilometres of the planets surface. These cracks have become filled with bright ice, which makes them stand out from the ground around them. This landscape has resulted from the seasonal temperature changes on Mars. Although it has a cold, dry climate there is actually a lot of seasonal variation in temperature on Mars. This has some interesting effects on ice rich soil.

Changes in temperature result in very small amounts of expansion and contraction of solid materials. As the temperature drops the material that makes up the ground will contract slightly and this will induce tensile stress. If the tensile stress due to contraction exceeds the tensile strength of the material then cracking will occur, releasing the tension and deforming the ground surface. Once small cracks form they start to fill with ice, and these ice wedges will force the fractures open even further, until wide fractures like these are formed. The pattern covers several kilometres of the surface, and some of the fractures are several metres across.

You can see the full image at this link, although full resolution HiRISE images are huge file sizes. http://www.uahirise.org/PSP_007571_2490