Friday, 26 April 2013

Blowing in the Martian Wind

Dune field near Northern Polar Cap
This image of a vast dune field near the northern pole of Mars was taken in August 2010, by the Thermal Emission Imaging System instrument on NASA's Mars Odyssey orbiter.
Image Credit: NASA/JPL-Caltech/ASUption
Over the past two weeks, I have gotten involved in the Hawai'i Space Exploration Analog and Simulation (HI-SEAS) mission as a First Tier Support person. What this means is that every Tuesday night, from 10 pm to 2 am my time (4-8 pm Hawai'i time), I am on call to help the crew in whatever they may need. The crew is simulating a mission to Mars, so they are on a 20 minute time delay, each way, and cannot access information quickly. This is where I come in.

Last Tuesday, I was asked to provide information on martian winds.  There was a strong wind advisory at the HI-SEAS mission site and crew member Kate Green wanted to compare what she was experiencing with potential conditions on Mars.  What I found and passed on was so interesting, I thought I should write about it.

The atmosphere of Mars is very thin, about 100 times less dense than the Earth's. In principal, one would expect that such a thin atmosphere could NOT lift sand and dust particles and move them around. And in fact, this is exactly what models that estimate the global circulation of the martian atmosphere predict, that winds there are too weak to lift sand and dust grains. The model predictions are confirmed by various martian landers; wind speeds that are strong enough to pick up sand grains have been measured only very rarely.
Dunes in Herschel Crater
Dunes on the floor of Herschel crater on Mars. The wind here blows from the top of the image to the bottom, forming these crescent-shaped dunes, with long tails that merge into each other. The tiny ripples on the dune surfaces are also formed by the wind.
Image Credit: NASA/JPL-Caltech/University of Arizona

But, as far back as the 1970's we have known about the existence of vast dune fields on Mars from images sent back by the Viking missions. Dunes are formed by winds moving large amounts of sand and dust around. So, if current conditions aren't right for getting sand grains moving, these dunes must be fossils from Mars' ancient past, and reflect a time when the atmosphere was much thicker.

However, several recent studies have found evidence of current activity in Mars' sand dunes. In March of 2010, NASA released an image set from the High Resolution Imaging Science Experiment camera on NASA's Mars Reconnaissance Orbiter ,which shows the appearance of new streaks on a dune slope in Nili Patera. These streaks indicate that winds have moved sand up the gentle side of the dune, over the dune crest, and down the steep side of the dune in a span of about 15 weeks.
Nathan Bridges of the Applied Physics Laboratory at Johns Hopkins University, working with a team of researchers from various other institutions, has measured the active dune motion in Nili Patera. He and his team looked both at the full dunes and also the smaller scale sand-ripples that can form on dune surfaces. They found that ripples moved a distance of up to 4.5 meters over a period equal to about 100 Earth days. Dune crests moved about 0.3 meters over a period of about 3 Earth years. These results show that dunes on Mars are currently active, and not just at the surface, the entire sand volume of the dune must be mobile.
Changes on Dune Slip Face, Nili Patera, Mars
Two images of the same dune face in Nili Patera, Mars (June 30, 2007 on the left and Oct. 13, 2007 on the right) show creation of new streaks.  Winds here blow from right to left.
Image Credit: NASA/JPL-Caltech/U of Arizona/International Research School of Planetary Sciences

Granted, these movement rates are slow, about 10 to 100 times slower that what we see for similar-sized dunes on Earth. However, this is only slightly slower than sand migration in the Victoria Valley dune fields of Antarctica on Earth, where the dunes have sand volumes 1000 times smaller than the martian Nili Patera dunes.  Such comparisons allow us to conclude that martian dunes form about 1000 times slower than dunes on Earth, which means the Nili Patera dune field probably formed in about 10,000 years. Now, this may seem like a long time, but the climate changes that Mars experiences due to fluctuations in its orbit take much longer than this to occur. Thus, these dunes probably formed in the current climate and are not just left over relics from an earlier time being moved around a little bit by the modern weak atmosphere.

So, how can the current thin atmosphere move sand and dust around when models and surface measurement say this should be very difficult to do? Well, the answer has two parts.

Part One: Models of the entire martian climate are, out of necessity, very course. The computing power and time needed to perform such calculations for the whole globe are rather large, so modellers look only at the big features. This means that variations on small scales aren't considered. But, very strong and localized winds can be created by abrupt changes in topography, where sudden differences in elevation produce changes in temperature and initiate convection.  These kinds of localized winds, which are believed to be responsible for various dune fields all over Mars, would not show up in global circulation models and would not be measured by landers, unless they happened to land in just the right place.

Part Two: The wind speed required to pick up sand grains and get them moving is not the same as that required to keep them going once they are started. Wind moves sand by picking up and throwing individual sand grains, and doing this continuously with millions of grains at a time. But, these thrown grains eventually drop back down to the ground and when they do, they can dislodge other grains, causing them to move as well. This added energy from the falling grains means that slower winds can keep already moving sand going. On Earth, only 80% of the wind strength needed to pick up a grain is enough to keep grains moving. On Mars, the lower gravity means that once a sand grain is thrown, it travels higher, gathering more momentum, and the thinner atmosphere means that air resistance is lower, so a sand grain on Mars has more energy when it comes back down. As a result, only 10% of the wind energy needed to pick up sand grains on Mars will be enough to keep the whole process going once it gets started.

So, despite a relatively thin atmosphere, winds on Mars can and do move sand around. Local strong winds get things started and then even weak winds can keep things going. The results are spectacular dunes forms on the surface, which are active even today.

Kok, 2012, Martian sand blowing in the wind, Nature, V 485, May 17, 2012, 312-313.

Bridges et al., 2012, Earth-like sand fluxes on Mars, Nature, V 485, May 17, 2012, 339-342.

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