Earth Observations taken by the Expedition 22 Crew

ISS022-E-015154 (24 Dec. 2009) --- Tsauchab River and Sossus Vlei Lakebed, Namibia are featured in this image photographed by an Expedition 22 crew member on the International Space Station. Taken on Christmas Eve of 2009, this image shows the lower 45 kilometers of the Tsauchab River, a famous landmark for Namibians, tourists, and for orbiting astronauts. The Tsauchab River bed is seen jutting into the sea of red dunes near Namibia?s hyper arid coast, where it ends in a series of light-toned silty muds of the dry lake floor, known locally as Sossus Vlei (small lake). Because of the present arid climate, few people have ever seen the Tsauchab River with flowing water or a lake in Sossus Vlei. In times past, however, the Tsauchab appears to have reached the Atlantic coast, another 55 kilometers further west. Like several other rivers of the coastal Namib Desert, the Tsauchab brings sediment down from the hinterland to the coastal lowland. According the scientists, this sediment is then blown from the river beds, and over probably tens of millions of years, has accumulated as the red dunes of the impressive Namib Sand Sea. In this view the sand is heaped up in the form of numerous ?star dunes,? each characterized by long arms extending in several directions. These compare with the better-known barchan dunes (not present in image) that display two horns pointing downwind, which form in areas where winds generally blow from one direction only. By contrast, star dunes are apparently generated in a variable wind regime. In this part of the Sand Sea, winds are mainly from the south, but easterly winds, channeled along the Tsauchab valley, provide another component. And warm dry winter winds?similar to the Santa Ana winds of California, which can cause the hottest annual temperatures to occur briefly on winter days?blow from the northeast. These northeasterly winds are likely responsible for the regular dune arms that point into the valley from both sides. These large dunes facing the river valley are promoted as the highest dunes in the world. Although continuous dune slopes allow hikers to gain more than 300 meters of altitude from the river bottom, this could be misleading since the main base of the dunes lies on a terrace 180 meters above the river.