Extraterrestrial Geology The Hills of Mars: An Earth-Based Perspective by Erik P. Kvale Updated March 31, 2004

Gusev Crater on Mars. Click for a larger view with caption.

In order to know the anatomy of each mountain range, you have to know details of sedimentary history.

—David Love as quoted in "Rising From the Plains" by John McPhee

What does the Namibian desert of South Africa have in common with Mars? Both apparently have many of the same landforms including dry river valleys, sand dunes, alluvial fans, and horizontally stratified or layered rocks. In fact, the surface of Mars has many features that we associate with seasonally dry desert areas here on Earth.

This comparison has led many geologists to believe that Mars once had surface water and possibly supported life.

How we interpret information gleaned from our recent and current explorations of Mars is based largely on our understanding of the Earth. As far back as the ancient Greek and Roman philosophers, people have been trying to understand floodplains, the origin of springs, erosion, the development of rivers, and many other surface features and the processes that formed them. It wasn’t until the early 1800s, however, and the realization that "the present is the key to the past" that the systematic evolution of the landscape was appreciated. John Playfair (professor of mathematics and philosophy at Edinburgh) noted in 1802:

"Every river appears to consist of a main trunk, fed from a variety of branches, each running in a valley proportioned to its size, and all of them together forming a system of valleys, communicating with one another, and having such a nice adjustment of their declivities, that none of them join the principal valley, either on too high or too low a level, a circumstance which would be infinitely improbable if each of these valleys were not the work of the stream which flows in it."

By understanding the concept that large streams and their tributaries cut valleys, it isn’t too much of a stretch to view a dry desert valley with smaller valleys leading into it and assume that water once flowed through those valleys.

The study of the processes that form and shape the surface of the Earth and the products of those processes is called "geomorphology." Our understanding of these earthly processes is now being applied to unraveling the mysteries of the Martian landscape. Below are several images that compare geomorphic features on the Earth to similar features on Mars.

The far left image (click images for larger views) shows a satellite view of wind blown sand dunes present in the Namib Desert along the Atlantic coast of southwest Africa. The near left image shows similar dunes on Mars. Both sand dune fields appear as "wrinkles" in the images.
—Far left image from Universities Space Research Association's Lunar and Planetary Institute Web site (STS-41D, August-September 1984; Picture #14-45-03). Near left image from The Geology of Mars Web site.

The sand dunes in the left image above were formed by dry westerly winds blowing eastward (from the left) off the Atlantic Ocean. Some of the dunes in this image exceed 300 meters (1,000 feet). The dune-free area is a dry wash that is periodically swept by flash floods draining the rocky hills (on the right side of the image).

The image of Martian dunes (above, right) is about 60 kilometers across. Because Mars has a lower atmospheric pressure, wind speeds must be very fast to mobilize sand and form dunes. Typical wind speeds in the Martian atmosphere exceed 200 km/hr (125 miles/hr). Gusts can often reach 500 to 600 km/hr (300 to 375 miles/hr).

The image at right shows sand dunes in the Mojave National Preserve.
—Image from U.S. Geological Survey Web site.

This image from Mars (at left, click image for a larger view) shows a very distinctive layering (arrow) in the scarp above channel-like features that emanate from the base of the hill. Such layers resemble sedimentary units that on Earth are typically deposited from water or wind. For a visual image of what this feature might resemble from the ground think of Monument Valley in Arizona.
—Image from The Geology of Mars Web site.

Even the driest places on the Earth typically show evidence of the presence of flowing water. The image to the right (click image for a larger view) shows a dendritic drainage system present in the Republic of South Yemen, which lies on the margin of a vast sand sea (the Rubh-al-Khali). The term "dendritic" comes from the Greek word "dendrites," meaning "treelike."
—Image from Universities Space Research Association's Lunar and Planetary Institute Web site (STS-41G, October 1984; Picture #17-36-036).

Dendritic channel patterns similar to those shown in the image from South Yemen are also present on the Martian surface. In the image to the right (click image for a larger view), individual segments of the channels are no more than 50 kilometers long and 1 kilometer wide.
—Image from The Geology of Mars Web site.

The image to the left (click image for a larger view) is of the south wall of Valley Marineris. It shows what appears to be a debris flow on the valley floor and below the scarp face (arrow) from which it originated. Debris flows typically involve significant amounts of water. The scarp face from which the debris flow originated clearly cuts the edge of the large impact crater on the right side of the photo and must therefore be younger. The image is approximately 60 kilometers (40 miles) across.
—Image from Universities Space Research Association's Lunar and Planetary Institute Web site.

The image to the right (click to go to a NASA Web site for a larger view with an explanation of the features) shows several geologic features that geologists working with NASA have interpreted to reflect significant movement of surface water on Mars.
—Image from NASA's Center for Mars Exploration Web site.

The image below left shows the Colorado River and the Grand Canyon in Arizona. The Colorado River has deeply entrenched itself into the mesa but has still preserved its tight meandering bends (arrow) typical of many rivers. Note how short the Colorado River tributaries are where they drain the upland mesas.
—Image from NASA's Gateway to Astronaut Photography of Earth Web site (STS066-090-086, Colorado River and Lake Powell, Arizona and Utah, U.S.A., November 1994).

Compare the image above right of a very large Martian channel network to that of the Colorado River. Note the tight meander bends of this Martian channel (arrow) and how relatively short the tributaries are on this image as well. Many Martian channels are interpreted as having originated from springs emanating from near surface ground water.
—Image fromThe Geology of Mars Web site.

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