This astronaut photograph features one of the largest of a series of
ten mostly fresh water lakes in the Ounianga Basin in the heart of the
Sahara Desert of northeastern Chad. The lakes are remnants of a single
large lake, probably tens of kilometers long, that once occupied this
remote area approximately 14,800 to 5,500 years ago. As the climate
dried out during the subsequent millennia, the lake shrank, and large,
wind-driven sand dunes invaded the original depression, dividing it
into several smaller basins.
The area shown in this image is approximately 11 by 9 kilometers.
The lakes’ dark surfaces are almost completely segregated by linear,
orange sand dunes that stream into the depression from the northeast.
The almost-year-round northeast winds and cloudless skies make for very
high evaporation rates; an evaporation rate of more than 6 meters per
year has been measured in one of the nearby lakes. Despite this, only
one of the ten lakes is saline.
The reason for the apparent paradox—fresh water lakes in the heart
of the desert—is that fresh water from a very large aquifer reaches the
surface in the Ounianga Depression. The aquifer is large enough to keep
supplying the small lakes with water despite the high evaporation rate.
Mats of floating reeds also reduce the evaporation in places. The lakes
form a hydrological system that is unique in the Sahara Desert.
The aquifer was charged with fresh water and the original lake
evolved during the African Humid Period (about 14,800 to 5,500 years
ago), when the West African summer monsoon was stronger than it is
today. Associated southerly winds brought Atlantic moisture well north
of modern limits, producing sufficient rainfall in the central Sahara
to foster an almost complete savanna vegetation cover.
Pollen data from lake sediments of the original 50-meter-deep
Ounianga Lake suggest to scientists that a mild tropical climate and a
wooded grassland/savanna ecosystem existed in the region. Ferns grew in
the stream floodplains. The same vegetation groups are now only
encountered 300 kilometers farther south. Even shrubs that now occur
only on the very high, cool summits (above 2,900 meters) of the Tibesti
Mountains have been found in the Ounianga Lake sediments.
Astronaut photograph ISS021-E-26475
was acquired on November 14, 2009, with a Nikon D2Xs 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 21 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 M. Justin Wilkinson, NASA-JSC.