This is a tiny fragment of the Empty Quarter, a sand sea larger than France. Ar Rub' al Khali is an erg that stretches across four countries, covers most of the Arabian Peninsula, holds half as much sand as the larger Sahara Desert despite being 1/15th the size, and is absolutely gorgeous.
A small fraction of the Ar Rub'al Khali erg measuring 54.8 kilometers by 61.9 kilometers on December 2, 2005. Image credit: NASA/GSFC/METI/ERSDAC/JAROS, and U.S./Japan ASTER Science Team
Ar Rub' al Khali (الربع الخالي) translates as "Quarter of Emptiness," an entirely suitable name for a vast sand sea spread across Saudi Arabia, Oman, Yemen, and the United Arab Emirates. Extending over 660,000 square kilometers (255,000 square miles), this is the world's largest erg. The Sahara Desert is more geographically impressive at 15 times the raw size, but where that desert has gravel plains and rocky outcrops, Ar Rub' al Khali is sand, sand, and more sand. The result is that Ar Rub' al Khali is the largest erg on the planet, a concentration of sand to test the most ambitious of camels.
The southern margin of the erg, north of the Saudi Arabia-Yemen border, contains a small rocky numb sheltering the border town and airport of Sharurah (Sharorah) in this image from February 1, 2003. Image credit: USGS/NASA
The erg lays within a structural basin, a geological recess surrounded by wind-blocking obstructions that trap sand no matter how long the wind blows. The basin is tilted, with the thinnest sheet of fine, soft sand in the west where the elevation is highest, and a thick mantle of sand that supports dunes and salt flats in the lower-elevation east.
A highland ridge splits the winds, exposing enough rock for the town of Sharurah (Sharorah) while specks of clouds cast shadows. Image credit: USGS/NASA
The original source of the sand is up for debate. Over the past few million years, fluctuating water levels in the Persian Gulf could have periodically exposed a sandy sea floor ripe for plucking by prevailing winds onto the peninsula. The pink-red tint of sands in the south section of the erg have a more tantalizing source: the iron oxide rich grains may have been carried by wadis, ephemeral streams flowing only during the rainy season.
Longitudinal dunes alternate with sabkahs until changes in wind patterns and sand supply break them into scattered star dunes in the northeast and eat. Image credit: NASA
Ar Rub' al Khali is dominated by long, linear dunes that form parallel to the prevailing winds. These longitudinal dunes are so massive that they can sprout secondary surficial dunes: splatters of crescent-shaped barchan dunes, and even a few picturesque star dunes modify the long, skinny dunes with a decorative touch. Between the dunes are sabkahs, interdune salt flats that can be hard enough to support a vehicle or soft and squishy as they fade into sand.
A puny, fragile human highlights the vast scale of dunes within Ar Rub' al Khali. Image credit: lintmachine
The erg is a hyper-arid desert, fitting its stereotypical image. It averages less than 3 centimeters (1.2 inches) of rain a year with an occasional year where not a single drop falls within the desert. Hitting record high temperatures of s 51°C (124 °F), it's also one of the hottest places on the planet. Yet, the desert hasn't been this impassible long. The region has been hit hard by desertification in the past two millennia: as recently as 300 AD it was a regular route for trade caravans, while fossilized bits of hippopotamus, water buffalo, and long-horned cattle suggest a damper history.
A road to Kharkhir sneaks through longitudinal dunes at the top edge of this image taken on January 8, 2014. Image credit: ESA
The exotic terrain may not have much vegetation, but it's fertile grounds for fiction. From inspiring H.P. Lovecraft's City of Pillars and the Nameless City to being the location of Machine City, Zero-One in the Matrix, the harsh terrain of Ar Rub' al Khali spawns dystopian dreams.
Longitudinal dunes trace out the path of prevailing winds. Image credit: USGS/NASA