Crater Clues

    Left: a flyover image of the Mars Crater; Right: a slope map of the northeast section of the crater. The formation of debris flows occurs for slopes between 15 and 20 degrees, represented as the yellow region in the map. At the center of the crater are flat sediments from an ancient lake. Credit: Marisa Palucis

    Standing on the dusty rim of Meteor Crater in Northern Arizona, UC Berkeley Professor William Dietrich thought, “It looks just like Mars!” The crater (called the Barringer Crater by scientists but officially named after the nearest post office in Meteor, AZ) was created approximately 50,000 years ago, when a 45-meter-wide asteroid impacted earth. The crater is nearly one mile wide, but Professor Dietrich was particularly impressed by the erosion gullies that had formed at the crater’s rim. These gullies, he thought, bear some striking similarities to those observed on Martian craters.

    Debris flows—liquified landslides of mud, rock, and water—are responsible for the gullies cascading down Meteor Crater’s rim. Marisa Palucis, a graduate student in Professor Dietrich’s lab in the Department of Earth and Planetary Sciences, believes debris flows also formed the gullies observed on Mars. The automobile-sized boulders found at the bottom of Martian craters are a big tip-off. Debris flows on Earth are able to carry boulders—as well as logs, cars, and even buildings—for very long distances. The Martian gullies also culminate in lobe-like fans of eroded material, which are characteristic of debris flow gullies on Earth.

    According to Palucis, flood-like processes cannot be responsible for these features. She believes floods require more liquid water than would have been available on Mars at the time of gully formation. While there is ample evidence that oceans and river deltas were present for at least some of Martian history, scientists believe that Mars’s water is now frozen in ice caps or locked away in below-ground reservoirs.

    Tantalizingly, many of the Martian debris flows seem to have originated after Mars froze. If Palucis is able to prove that the Martian meteor gullies were caused by debris flows, she would also prove that a small amount of liquid water was present in more recent times. Transient events, such as a shift in Mars’s orbit closer to the sun, may have unlocked enough liquid water to form a debris flow. However, Palucis says the harder questions are, “What size of rainstorm is that? Do you need a torrential downpour, or could a moderate size storm form this feature?” Palucis needs to prove that the amount of water required for debris flows is reasonable based on both Martian hydrology and climate.

    To answer these questions, Palucis’ laboratory experiments estimate how much water it would take to form a given debris flow feature. Instead of running after a debris flow with clipboard and lab coat flying, Palucis uses giant rotating drums—located at UC Berkeley’s Richmond Field Station—to create “essentially a debris flow that’s forced to stay in place.” By varying the proportions of clay, water, sand, and soil loaded into the drum, Palucis is able to observe when the mixture transitions from something less like UC Berkeley’s Strawberry Creek to something more like a raging slurry of mud and gravel.

    Palucis is also carrying out an extensive mapping survey at Arizona’s Meteor Crater to determine the age and frequency of debris flow formation. Using cosmogenic dating—a method to estimate the ages of rocks from chemical changes that occur during sun exposure—Palucis determined that debris flows on Meteor Crater are not modern. They date from around 20,000 years ago, during the Pleistocene, when the American southwest was a warmer and wetter place than today. She notes that, “if one [debris flow] happened a year, then a pretty average rainstorm or snowmelt event during the Pleistocene would cause it. If they happened every 100 years, then that means it’s a more unlikely, bigger event that happened.”

    Palucis’s ultimate goal is to use her laboratory experiments and fieldwork at Meteor Crater to integrate both geology and hydrology into a model of debris flow formation. This “joint hydrologic sediment transfer model” would allow her to back-calculate how much water it would take to form a given debris flow feature, both on Earth and on Mars. Contemplating this prospect, Palucis muses, “If we find water, can we find life [on Mars]?”

    However, she cautions that before taking her model to Mars, she has to prove it can accurately model phenomena on Earth. This in itself would be an enormous step toward understanding debris flow processes. For citizens of Switzerland, Taiwan, Japan, and other areas with steep terrain prone to landslides, Palucis’s models could become part of an integrated storm hazard warning system. And who knows? After NASA terraforms Mars her models might be used in storm warning systems for Martian citizens living in Crater Rock City.

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