Editor's Note: Today’s caption is the answer to Earth Observatory’s monthly satellite puzzler.
Perhaps no word better describes Australia’s Simpson Desert than extreme. The 140,000-square-kilometer (54,000-square-mile) desert, located in a large drainage basin in central Australia, is one of the driest regions on the continent. But it is also experiences torrential rainstorms and flash floods that occasionally spread huge sheets of water across the landscape.
The Advanced Land Imager (ALI) on NASA’s Earth-Observing 1 (EO-1) satellite captured this view of the far eastern edge of the Simpson Desert on May 20, 2004. The image shows a part of the desert at a point about 40 kilometers (25 miles) southwest of Bedourie, a small settlement in southwestern Queensland.
One of the defining features of the Simpson Desert is its long, red, linear dunes. In some areas, chains of unbroken sand ridges stretch for hundreds of kilometers. Linear dunes generally form parallel to the direction of prevailing winds. In this case, the “tuning-fork” junctures that open to the southeast indicate that the prevailing winds that sculpted the dunes blew from that direction.
Unlike the dune fields in the heart of the Simpson Desert, the dunes in this image are not densely packed and there are only a few per kilometer. In areas to the west and south, where dried-out lakebeds provide abundant sand, there are upwards of 15 dunes per kilometer. On the margin of the desert (above), the main source of sand for dune building is the Diamantina River, a tributary of Lake Eyre.
Sands in the Simpson Desert range from brilliant white to dark red, with a range or pinks and oranges in between. Two main factors affect the color—the distance the sand has moved from its source, and the proximity to river channels that flood regularly. The lightest-colored sands are usually closest to the source. As it ages, sand develops a red sheen because a layer of hematite forms on the surface of the grains. The presence of water in floodplains mixes light-colored sands into the environment, reducing the redness of dunes near drainage basins.
Though arid, the landscape above falls in the midst of a part of Queensland known for its ephemeral rivers and flash floods called Channel Country. The area shown above receives about 250 millimeters (10 inches) of rain per year. But in the summer, monsoonal rains falling hundreds of kilometers away send floodwaters coursing through the network of braided river channels that cut through the Simpson Desert. All of the rivers lead toward Lake Eyre, the large inland lake at the lowest point of the basin.
The rivers lack established beds, so water instead arrives in sheets that wind their way through low-elevation areas between dunes. The flow creates waterholes and other ephemeral waterways that can rapidly transform barren desert soil into lush oases of green. (Click on the larger image to see Murraturley Creek, a dried stream bed just to the southeast of the area shown, and the Rooka Bougacea waterhole.)
NASA Earth Observatory image by Robert Simmon, using EO-1 ALI data from the NASA EO-1 team. Caption by Adam Voiland, with information from Kathryn Berg of the Royal Geographical Society of Queensland. Congratulations to Carl Schardt, Conan Witzel, and David Haycock for being some of the first readers to solve the puzzler.