Aorounga Impact Crater is located in the Sahara Desert, in
north-central Chad, and is one of the best preserved impact structures
in the world. The crater is thought to be middle or upper Devonian to
lower Mississippian (approximately 345–370 million years old) based on
the age of the sedimentary rocks deformed by the impact. Spaceborne
Imaging Radar (SIR) data collected in 1994 suggests that Aorounga is
one of a set of three craters formed by the same impact event. The other two suggested impact structures are buried by sand deposits.
The concentric ring structure of the Aorounga crater—renamed
Aorounga South in the multiple-crater interpretation of SIR data—is
clearly visible in this detailed astronaut photograph. The central
highland, or peak, of the crater is surrounded by a small sand-filled
trough; this in turn is surrounded by a larger circular trough. Linear
rock ridges alternating with light orange sand deposits cross the image
from upper left to lower right; these are called yardangs
by geomorphologists. Yardangs form by wind erosion of exposed rock
layers in a unidirectional wind field. The wind blows from the
northeast at Aorounga, and sand dunes formed between the yardangs are
actively migrating to the southwest.
Astronaut photograph ISS020-E-26195
was acquired on July 25, 2009, with a Nikon D3 digital camera fitted
with an 800 mm lens, and is provided by the ISS Crew Earth Observations
experiment and Image Science & Analysis Laboratory, Johnson Space
Center. The image was taken by the Expedition 20 crew. The image in this article has been cropped and enhanced to improve contrast. Lens artifacts have been removed. The International Space Station Program
supports the laboratory to help astronauts take pictures of Earth that
will be of the greatest value to scientists and the public, and to make
those images freely available on the Internet. Additional images taken
by astronauts and cosmonauts can be viewed at the NASA/JSC Gateway to Astronaut Photography of Earth. Caption by William L. Stefanov, NASA-JSC.