Sand avalanches stir up trouble on Mars
Recent avalanche leaves its mark
February 1, 2000
Web posted at: 3:09 p.m. EST (2009 GMT)
From staff reports
PASADENA, California (CNN) -- Driven by dust devils and sand storms, avalanches have pushed around dunes on Mars as recently as the past few months, as dramatically illustrated in a series of graphic images released by NASA this week.
Besides sand avalanches, the pictures, snapped by a camera onboard a satellite orbiting Mars, depict a variety of haunting sand landscapes, from stormy scenes with dark streaks on steep dunes, to peaceful picture replete with small wind ripples on gentle slopes.
Mars has been known for centuries to experience large dust storms, but it was not until the Mariner 9 mission in 1971-1972 that pictures from the planet showed sand dunes.
Active and inactive dunes
The Groovy Dunes of Herschel
Good evidence that the martian atmosphere still transports loose sediment and deposits it elsewhere, dunes are deposits of sand that have accumulated usually over thousands of years.
The Mars Global Surveyor have snapped pictures that since 1997 have provided clues about martian dune activity. The onboard camera has found that some dunes currently seem active.
The camera has also spotted sand dunes that appear inactive because they are either covered by dust and landslide debris, cemented and eroded into ridged and grooved terrain or pockmarked by old impact craters.
The streaks in the avalanche images indicate that sand has moved down the slopes in the past few years. Sand dunes move forward by the combined action of wind that drives sand up the shallow slope on the windward side of the dune, and the avalanching of this sand down the steeper, lee-side slope.
The groovy dunes of Herschel
The dark streaks indicated by arrows are evidence for sand avalanches that occurred within a few months or years of the time when the picture was taken in March 1999. Other streaks that are seen criss-crossing the dunes may be the result of passing dust devils.
Normal, active sand dunes have smooth slopes, except for small wind ripples on their surfaces. However, some dunes found in the Herschel Basin of Terra Cimmeria have rough, grooved surfaces instead. These grooves indicate that the
dune surfaces are immobile. Wind has had to scour the sand to remove it and transport it away from the dunes.
What has caused the sand to become cemented remains unknown; dunes like these are extremely rare on Mars and have only been seen in Herschel Basin. This image was acquired in May 1999.
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