美宇航局观测到火星上沙丘的移动

Last year, images from NASA's Mars Reconnaissance Orbiter captured sand dunes（沙丘） and ripples moving across the surface of Mars -- observations that challenged previously held beliefs that there was not a lot of movement on the red planet's surface. Now, technology developed by a team at the California Institute of Technology (Caltech) has allowed scientists to measure these activities for the very first time. The new method for data processing is outlined in an advance online publication of the journal Nature.

"For many years, researchers have debated whether or not the sand dunes we see on Mars are fossil features related to past climate, since it was believed that the current atmosphere is too thin to produce winds that could move sand," says Jean-Philippe Avouac, the Earle C. Anthony professor of geology at Caltech, who initiated the study. "Our new data shows that wind activity is indeed a major agent of evolution of the landscape on Mars. This is important because it tells us something about the current state of Mars and how the planet is working today, geologically."

Using the COSI-corr software (for Co-registration of Optically Sensed Images and Correlation), which was invented at Caltech, a team of researchers gathered high-resolution imagery from Mars to look at a specific field of sand dunes called Nili Patera. The images came from the High Resolution Imaging Science Experiment (HiRISE) on the Mars Reconnaissance Orbiter.

The team focused on precise, subpixel measurements of movement between pairs of images. On the dunes at Nili Patera, the software automatically measured changes in the position of sand ripples from one image to another over a 105-day period, resulting in the surprising findings that the ripples are moving fast -- some upwards of 4.5 meters during that time -- which contributes to the total motion of the sand dunes.

"This is the first time that we have full, quantitative measurement of an entire dune field on a planetary surface, as opposed to the localized manual measurements that were done before," explains Francois Ayoub, a coauthor of the paper and a scientist engineer in Avouac's lab. "Using this technique, you could monitor other dune fields, or you could also follow a particular area over a longer time frame to see the seasonal or annual evolution of the sand dunes. This is a huge step in terms of the data that you can obtain from the surface of Mars."

The team also found that the dunes at Nili Patera appear to move similarly to those found on Earth in Victoria Valley, Antarctica. This implies that the rates of landscape modification due to wind are similar on the two planets. Interestingly enough, getting these measurements was much easier on Mars -- the researchers could not quantify dune ripple migration rates on Earth using the same technique because that would require satellite imagery of our planet at a resolution that makes it classified information.