The Sahara Desert
The Sahara desert covers about 9 million square miles — an area roughly the size of Australia. It can be very hot during the day, with temperatures reaching 120°F in the shade, but the lack of humidity means there are no clouds so the nights can be near freezing. When we think of the desert, we often imagine a scene like this one:
Desert sand grains. Source: National Geographic.
But what is the desert really made of? Sand, of course. These images show pictures of sand from dunes in Egypt. Notice that the sand grains are all about the same size and are pretty well rounded. Those features result from the action of the wind. Wind is a very fussy actor, and won't carry a wide range of sediment sizes like you would find in a river. As a result, a single sand dune will often have a nearly uniform grain size and shape.
Desert sand grains. Source: From the personal collection of Dr. Tanya Furman.
The wind shapes sand into a wide range of shapes. Here notice that there are two different sizes of very similar shapes: the huge dunes and the tiny ripples on their surface. Sometimes the ripples stay in the low spots between the dunes, and sometimes they climb the dune faces.
Dunes with ripples (left); dunes and basal ripples (right). Source: From the personal collection of Dr. Tanya Furman.
Not surprisingly, it takes a lot of wind to make those beautiful dunes and ripples. The most powerful of these are the Harmattan, warm dry winds that carry dust throughout the Sahel region and across the Gulf of Guinea to the Cape Verde Islands. The Harmattan blow across the Sahara from December to March, and are likely responsible for triggering episodic epidemics of meningitis in western Africa. The winds are not all bad, however, as the dust they bring across the Atlantic Ocean carries important nutrients that essentially support lush plant growth in the Amazon rainforest.
Linear dunes west of Rosso. Source: USGS.
Thus far we've focused on the wind as an agent of sand transport (dust storms) and deposition (dunes). It is also an agent of erosion and in this capacity carves the desert floor. For every grain of sand piled up in a dune, there is a small area of rock left bare. Thus the desert is an area of juxtaposed rocks and sand, the combination of which create patterns that are remarkable both close up and from space. Most of these images were taken from the Space Shuttle, and the view you see covers hundreds of miles.
Desert pavement. Source: Copyright [2000] by Andrew Alden, www.geology.about.com, reproduced under educational fair use (http://geology.about.com/library/bl/images/bldesertpave.htm)
The rocks that are left behind (not carried by the wind) often end up polished by the abrasion of thousands of tiny sand grains. This picture is from the southwestern US, but the phenomenon occurs in the Sahara and in all other desert areas as well. Notice that the rocks are pretty large — the wind has removed all of the smaller stones.
Martian surface - evidence for wind erosion. Source: National Geographic
We see a similar image as we look at the surface of Mars. In this picture we see a flat desert-like surface, dotted with larger rocks. Like in the Sahara, these large rocks are simply the ones that the Martian wind was not strong enough to remove. Martian researchers use photos like this one to estimate the wind speed on Mars.
