QUICK FACTS
Where is it? Grand Erg Oriental, Sahara [30.13306820, 9.306920931]
What’s in the photo? The stark contrast where a field of “star dunes” meets rocky hills
Which satellite took the photo? EarthKAM camera, on board the International Space Station
When was it taken? Oct. 31, 2017
This striking satellite image shows the stark difference between a giant field of sandy “stars” and a barren rocky wilderness in the Sahara Desert. The contrasting biomes also overlap with the border where two African countries meet.
The sea of orange sand visible in this image is part of the Grand Erg Oriental — a giant field of dunes, or erg, that covers around 55,000 square miles (140,000 square kilometers) in the northern Sahara.
This part of the erg has hundreds of prominent “star dunes,” which have at least three ridges coming from a central peak, giving them a star-like appearance when viewed from above. Star dunes can grow to be over 300 feet (90 meters) tall and only form in locations where wind directions change constantly, which allows their different slopes to form, according to the National Park Service.
The dunes overlap with a windswept rocky terrain, which is among “the driest parts of the Sahara Desert” and almost completely devoid of vegetation, according to NASA’s Earth Observatory. This terrain is covered with overlapping geological features, giving it an otherworldly quality.
One of the most striking features is a large white patch in the heart of the rocky terrain, which is the remnant of an ancient lake. You may also be able to see a series of parallel grooves in the rock (in the top half of the rocky terrain), which were carved by eons of unidirectional winds that blew long before the multi-faced dunes were formed.
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The dunes in the image are all located within Algeria, while most of the rocky terrain is in Libya. The invisible boundary between the two countries is largely located down the very center of this image. However, at one point, just to the left of a Libyan town called Ghadames (visible as a dark spot near the top of the image), the border between the countries almost exactly overlaps with the line where the sand and rock meet.
While the contrasting ecosystems appear to have nothing to do with each other, they are more linked than you might imagine. Geologists believe that a majority of the erg’s sand was deposited by ancient rainwater that took loose sediments from the rocky terrain and dumped them within a natural depression, or sink, in the landscape.
This process likely took hundreds of thousands, if not millions, of years — and if you look closely, you can see that some of the erg’s sand has spilled back out into the ancient “winding water courses” that helped to deposit the sand within the erg, according to the Earth Observatory.
The erg is also partly formed above the bed of an ancient river. When rain infrequently falls on the erg’s sands, the moisture is funneled away into underground aquifers, which can support pockets of vegetation on the surface. If this did not happen, this valuable moisture would quickly evaporate back into the atmosphere.