Wednesday, November 8, 2017

The ice-sands of Mars

This week Alex gives an update on some of her work with Giang investigating small-scale topography of the Northern Polar Cap of Mars. As always with Alex's posts, expect her flowing prose to be punctuated with fascinating images culled from the many she has examined.

By Alexandra Innanen

With nearly 600 frames from all over the Martian north pole, my efforts have recently been turning to categorization of the nearly 600 variations on ‘polar cap’ which we’ve seen. This is the kind of repetitive work that I can do with the help of a good podcast and some tea, taking a break from my studies to flip through the catalogue of black and white features. Some of the various myriad features which I have been looking at are dunes. The Martial polar cap is lousy with dunes. Some of them are quite obvious, bringing to mind those classic sandy desert landscapes, while others are more hidden; zoom in on a seemingly uniform HiRISE image and suddenly stripy linear dunes start to emerge. There are basically four types of dunes that we’ve seen on the pole: longitudinal, transverse, star and barchan.

Longitudinal dunes, like the ones at the top of this article, are large dunes which from parallel to the direction of the prevailing winds. They tend to form in areas where the winds are strong and sustained. While the dunes above are hundreds of meters long, longitudinal dunes can be hundreds of kilometers in length. You can also see some of the dunes above branching off each other and merging together, which is a common feature in longitudinal dunes. The small black arrow in the upper left corner shows the direction of the prevailing winds, which lines up pretty perfectly with these dunes. We were able to determine the direction of the prevailing winds by looking at the directions of nearby troughs, which have been shaped over the ages by these winds.
Transverse dunes, on the other hand, lie perpendicular to the direction of the prevailing winds. Transverse dunes are actually formed of smaller dunes that align and form a ridge. Looking from above as HiRISE does, it can be difficult to determine whether the linear dunes we see are longitudinal or transverse, but taking into account the wind direction, we are able to differentiate between the two. In addition, while longitudinal dunes tend to be fairly symmetrical, transverse dunes tend to be steeper on the leeward side. So, while the longitudinal dunes tend to resemble corduroy, transverse dunes look more like waves or ripples in water.  

Star dunes are a lot less ordered. They tend to be formed from mounds which develop radial ‘arms’, or slip faces from the centre. They tend to form in areas where the winds are multidirectional, hence the various directions of the ‘arms’. Despite being much less uniform than the longitudinal and transverse dunes, a lot of the star dunes we’ve seen tend to line up in a certain direction. In the above image, there’s a definite directionality, almost perpendicular to the wind direction like in the case of the transverse dunes. This is not all that unusual. Oftentimes two different regimes can be observed in a single region where different winds and even geological process are working together to shape the landscape.

Barchan dunes are perhaps my favourite of the dunes I’ve seen on the north pole. They tend to be crescent shaped, reminding me a bit of the Starfleet delta insignia. From such a height as this image is taken, however, you can just see that seeming randomness of the terrain, and the large, rounded dunes. Like transverse dunes, the steeper slipface is on the leeward side of the dune and are formed in regions where the wind is fairly consistent in direction. In the dunes above you can also see smaller linear dunes on top of the larger barchan dunes, in the direction of the prevailing winds. Again, as with the star dunes above, we can see the two different regimes both existing here, the longitudinal dunes on the surface of the barchans.
Of course, it’s impossible to capture the sheer variety of features that we’ve seen on the pole, not only variations upon these four types of dunes above but also in rocky terrain, pits, aeolian ridges and features that defy categorization.  However, grouping features into categories can help shed some light on the processes that formed them.

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