Extraterrestrial Geology

The Hills of Mars: An Earth-Based Perspective

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The image of Mars to the left (click for a larger view) is a southward-facing view that shows the channel referred to as "Ma'adim Vallis" (between the two arrows). This channel appears to have been cut by water that flowed into Gusev Crater (blue area in foreground). Gusev Crater is currently being explored by Spirit, the NASA robotic field geologist.
—Image by R. P. Irwin III and G. A. Franz, National Air and Space Museum, Smithsonian Institution.

The Opportunity Rover has provided some of the most intriguing evidence of water at its landing site in the Meridiani Planum region. Images of the Opportunity Ridge outcrop (image at right, click for a larger view) show a series of broad intersecting lineations in the rock.

To the right is a close-up image of a segment of the Opportunity Ridge outcrop showing some of the lineations in detail. The rock lineations are actually similar to the "lineations" one would see if they looked at the end of a stack of papers. In this Martian example the "pages" are dipping away from the observer.
—Image from NASA Mars Exploration Rover Mission Web site.

The figure at left (click for a larger view) illustrates how these lineations might have formed. As currents generated by wind or flowing water move over loose sediments, they can create "ripples." Perhaps you have seen small ripples in gutters after a rain storm. One can tell the direction the current moved by looking at the shape of the ripple. The steep side is always down flow. The steep side is called the avalanche face (A in the figure) because grains pushed by the currents tend to cascade or roll down the steep side of the ripple as the ripple migrates or moves downstream. Sandstones commonly preserve the avalanche faces (dashed lines in B and C). The recognition of avalanche surfaces in rocks allows geologists to determine the flow direction of ancient currents.
—Figure from Harms, J.C., et al., 1975, Depositional environments as interpreted from primary structures and stratification sequences, Short Course Notes No. 2, Dallas: SEPM.

The photo at right (click for a larger view) shows sandstone that preserves ancient avalanche faces formed by flowing water. Flow was away from the camera. Note the similarity between the lineations to the right of the person and those in the close-up image of Opportunity Ridge.

Scientists are now pretty certain that these Martian features were made by currents generated by water. This interpretation also supports other evidence at this site that indicates that water once existed there (for more information, read these NASA articles: Opportunity Rover Finds Strong Evidence Meridiani Planum Was Wet and More Clues Emerge to Mars' Watery Past).

The evidence for surface water having once existed on Mars is compelling. With the presence of water also comes the possibility of life. Who knows what Spirit and its twin explorer Opportunity (image at right, click for a larger view) may find over the next few months? You can rest assured, however, that whatever data the two rovers generate will be completely scrutinized by Earth-based field geologists, because the key to understanding Mars is going to be our understanding of Earth.
—Image from NASA fact sheet (313K pdf file; requires Adobe® Reader®)

For more information about the geology of Mars, visit these Web sites:

• water
• NASA's Center for Mars Exploration
• NASA's Mars Exploration Rover Mission
• NASA's Jet Propulsion Laboratory, California Institute of Technology
• The Geology of Mars (by Albert T. Hsui, University of Illinois)
• Cornell site about the Mars mission
• Rover images

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