On November 17, 2011, NASA published new images showing that winds are driving the pristine sand dunes of the planet Mars – that they are in motion – as you might think is only logical on a windy desert world.
Or is it so logical? Dust is easily blown around the Red Planet. There are sometimes planet-wide dust storms, which earthly observers have seen through telescopes since early in the 20th century. Sand is heavier, though, and Mars’ thin atmosphere means that strong winds are required to move grains of sand. And while winds are known to contribute to dune movement on Earth, computer simulations of Mars’ atmosphere had suggested that strong-enough winds would be rare there.
Yet the grains of sand do move, due to wind, across the surface of many parts of Mars. The image above shows a rippled dune front in Herschel Crater – located just south of the Martian equator in the cratered highlands of Mars. NASA’s Mars Reconnaissance Orbiter saw the dune front move an average of two meters (about two yards) between March 3, 2007 and December 1, 2010. See the pattern of ripples on the dune surface? The pattern has changed completely between the two images.
Images from NASA’s Mars Reconnaissance Orbiter – launched from Earth in 2005 – show sand dunes and ripples moving across the surface of Mars at dozens of locations. The Orbiter’s sophisticated HiRISE camera captured these images of shifting sands. Clearly, the planet’s sandy surface is more dynamic than planetary scientists once thought. Nathan Bridges is a planetary scientist at the Johns Hopkins University’s Applied Physics Laboratory and lead author of a paper on planet-wide sand motion on Mars published November 14, 2011 in the journal Geology. He said:
Mars either has more gusts of wind than we knew about before, or the winds are capable of transporting more sand. We used to think of the sand on Mars as relatively immobile, so these new observations are changing our whole perspective.
Sand dunes are a common feature on the surface of Mars, and they can appear exotic, as in the image above. This image was taken by Mars Global Surveyor (MGS). It was the Astronomy Picture of the Day (APOD) for February 26, 2001, which compared the dark dunes above to “shark’s teeth.” These dunes are located in Proctor Crater, a 170-kilometer-wide crater first seen to house sand dunes by Mariner 9 more than 35 years ago. The image illustrates the complex relationship between Martian sand dunes and the wind, but no one was sure, when this image was taken 10 years ago, whether the winds still blew strongly enough on Mars to move the dunes in real time.
The Mars Global Surveyor, which operated from 1997 to 2006, gave us the first hints that Martian dunes do move. But the spacecraft’s cameras lacked the resolution to definitively detect the changes. NASA’s Mars Exploration Rovers also detected hints of shifting sand when they touched down on the Red Planet’s surface in 2004. The mission team was surprised to see grains of sand dotting the rovers’ solar panels. They also witnessed the rovers’ track marks filling in with sand. Still, the extent of sand movement across Mars remained elusive.
The image above is from 2009. It illustrates another way in which dunes were seen to move on Mars in the past – not moved by wind, but by another process. Some dunes near Mars’ polar areas were observed to move when the carbon dioxide ice (dry ice) on Mars changes directly from a solid to a gas – with no liquid state in between, as with water ice.
Several studies by Mary C. Bourke that year characterized Martian sand dunes, in particular by comparing them to cold climate dunes in Antarctica. The north pole of Mars is surrounded by a vast circumpolar region of sand dunes. These dunes exist in mean annual temperatures of -80°C, are covered by frosts for 70% of the year and are subject to annual snowfalls. On Earth, cold region sand dunes often contain inter-bedded sand, snow and ice. The HiRISE camera captures images of these Martian dunes, too. Using data from Antarctica and other cold region deserts, Bourke was able to suggest that dunes in the Martian north polar region may also contain snow and ice deposits.
I think I love the image above most of all. It’s another HiRISE camera image, taken from the Mars Reconnaissance Orbiter in 2009 when planetary scientists were beginning to glimpse the truly dynamic quality of the dunes – but still weren’t positive how widespread dune movement is on Mars.
What is this image? It looks like flowing liquid. On Mars, actual liquids freeze and evaporate quickly into the thin Martian atmosphere. But persistent winds clearly make large sand dunes appear to flow and even drip like a liquid, as shown above.
So – until now – active sand dunes in many parts of Mars had not been confirmed. That is this week’s big announcement, and it’s intriguing indeed.
However, it seems not all of the sand on Mars moves, as the wind blows. The November 14, 2011 study also identifies several areas where sand on Mars did not move. The scientists say the unmoving sand dunes could have larger grains, or their surface layers might be cemented together. Or they might move on longer time scales, triggered by climate cycles on Mars that are believed to last tens of thousands of years. These scientists said:
The tilt of Mars’ axis relative to its orbital plane can vary dramatically. This, combined with the oval shape of Mars’ orbit, can cause extreme changes in the Martian climate, much greater than those experienced on Earth. Mars may once have been warm enough that the carbon dioxide now frozen in the polar ice caps could have been free to form a thicker atmosphere, leading to stronger winds capable of transporting sand.
What will we learn next?
Bottom line: Astronomers have been watching dust storms rage on Mars through telescopes for many decades, but spacecraft also revealed the presence of sand dunes on this cold dry desert world. Until now, it was not known how widely the thin atmosphere of Mars could pick up the courser grains of sand – in contrast to the fine dust – and move it. But images from the Mars Reconnaissance Orbiter HiRISE camera shows that wind does move sand on Mars in real time, in dozens of locations on Mars. The study was published in the journal Geology on November 14, 2011.