The sand dunes on the surface of Mars closely resemble their terrestrial cousins, only bigger. Formed from wind-driven sand and dust, the Martian versions can grow ten-times bigger than any dunes we have on Earth. The High Resolution Imaging Science Experiment (HiRISE) on board NASA’s Mars Reconnaissance Orbiter (MRO) has been taking shots of these distinctive shapes, and although there are examples all over the planet, they seem to have the same characteristics no matter where they form or how they swarm. Now researchers are investigating how these Mars dunes grow and why they are so large…
On the 11th April, I reported on some new HiRISE images of barchan sand dunes that appeared to form on the Red Planet. In this particular case, a flat-topped mountain (or mesa) was being ravaged by the Martian wind, sweeping fine grains of sand and dust downstream. What formed in the mesa’s wake turned out to be quite impressive. Long strands of sand banks, stretching hundreds of miles beyond the mesa mixed with swarms of similarly-shaped dunes. What struck me were the impressive similarities with Earth-based sand dunes we find on our beaches and deserts.
Since this initial report, I have found HiRISE to be quite a prolific dune-seeker. On April 16th, HiRISE released more images of sand dunes in the northern polar regions of Mars, some heavily eroded (pictured below), and others tightly packed and clumsily shaped. I was intrigued. As you may have noticed from some of the Mars stories I cover, I like to see terrestrial processes happening on the surface of Mars. Seeing an impressive Mars avalanche, or a simple rock rolling down a hill, I love it. I think it gives us a special connection with an alien world when we can see processes we commonly associate with Earth happening on an eroding Mars.
So what processes are behind these giant Mars dunes? It might seem obvious (wind blowing sand ain’t that hard to understand after all), but researchers at the Federal University of Ceará in Fortaleza, Brazil have been modelling the effects that the tenuous Mars atmosphere and weak gravity have on sand dune construction. Murilo Almeida and his team have found that when blown around on the surface of Mars, grains of sand “bounce” much higher than their terrestrial counterparts. In fact, they bounce 100 times higher and further. They are also blown 10 times faster. This has the effect of producing a series of sand dunes with very long wavelengths.
This also has a knock-on effect as a possible mechanism that drives the savage dust storms in the thin atmosphere. As there is more bouncing action in Martian grains of sand than here on Earth, more dust particles are thrown aloft and suspended in the air.
Almeida’s work is published in the Proceedings of the National Academy of Sciences (DOI: 10.1073/pnas.0800202105)
Source: New Scientist