The features seem to be twice smaller than previously calculated

May 27, 2008 13:41 GMT  ·  By
Image of one of the polygonal patterns on the northern polar surface of Mars
   Image of one of the polygonal patterns on the northern polar surface of Mars

Amongst some of the first images sent back by the Phoenix Mars Lander in the outcome of its landing on the surface of Mars on the evening of last Sunday lies this particular one, showing the now famous polygon patterns characteristic to the north polar plains of the Red Planet. In a previous article I wrote that NASA first observed these weird shapes with the help of the Mars Reconnaissance Orbiter, which circles the planet at an altitude of about 760 kilometers above the surface.

Now, take a good look at the picture displayed here. What do you see? First of all, the polygons are too small. According to principal investigator Peter Smith from the University of Arizona, the image shows a feature measuring between 1.5 to 2.5 meters across, which is too small to be observed by the Mars Reconnaissance Orbiter.

"I think it means that there are polygons within polygons within polygons. At different climate times there may have been big polygons and at other times there may be small polygons, and it just may go back and forth over time", said Smith.

The patterns, similar to some in Earth's Arctic regions, may have been produced by a successive refreezing process that created fractures in the soil as ice beneath the surface expanded and contracted repeatedly. The contraction of the ice creates small cracks several millimeters across, which are then filled with Martian dust. When the ice expands again, it has no more room and starts buckling upwards to create these polygonal shapes.

The sizes of these features are the most important factor in understanding when exactly they might have formed. "We knew they would be there, but we didn't know what they would look like", says Michael Mellon, a Phoenix mission co-investigator from the University of Colorado. The sizes of the polygons are directly related to the thickness of the soil layers sitting on top, since ice located closer to the surface would be subjected to greater temperature influences, creating smaller polygons.

In the case of larger polygons however, the ice below would need to be covered by a layer of dust at least 30 centimeters thick, smaller shapes no longer being obvious. Climate patterns also influence the size of the shapes. The polygon shape seen in this image may have been created very recently, otherwise wind would have long covered it with dust, says Smith.

The process that created it might still be active and, if true, then the process would manifest itself through cryoturbation, churning up soil and rock.

On Earth polygonal shapes reach sizes of up to 15 or 20 meters, while calculations show that on Mars they should be at least 5 meters in size, which leaves the investigating team dealing with some kind of a mystery. "It has certainly got our brains turning", says Mellon.

One explanation would be that Phoenix might have captured images of small polygons because it was looking in the wrong direction. In other places, the features could easily be of 5 meters in size. In the meantime, scientists have to think their way out of the situation, just in case larger polygons are not found in the vicinity of the lander.