The Gullies of Mars: Wet or Dry?

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Over a century ago, some astronomers thought that numerous straight lines seen all over the planet Mars were giant artificial canals, carrying water from the poles to large cities. Percival Lowell, the recognized leader of this theory, was certain that an ancient and wise civilization of Martians had built these vast waterways to hold off their eventual extinction as the planet slowly dried up.

The first robot probes from Earth began arriving on Mars in the 1960s, sending back close up and detailed images of the Red Planet's surface. These early explorers proved that the "canals" seen by astronomers decades before were just optical illusions caused by viewing Mars from millions of miles away through Earth's thick and blurring atmosphere.

The returned data also showed that the Red Planet could not have been home to anything much higher on the biological ladder than very hardy microbes, if at all. Mars, being half the size of Earth and twice as far as our world from the Sun, was discovered to be a very cold and very dry place, with an atmosphere no thicker than what is found 20 miles above the surface of our planet.

Other findings from those space probes left open the possibility that Mars might have been much warmer and wetter a long time ago. Huge canyons and long flowing channels covered the planet. Some areas even looked like the remnant shores of ancient seas or oceans of water.

The Mars Exploration Rovers (MER), using geological technology designed by Cornell University, recently proved that, at least in the two landing areas, bodies of water existed there in the distant past.

Some of the data from the three unmanned satellites currently orbiting Mars have delivered evidence that the Red Planet might not be quite as dry as originally perceived. Vast reserves of water ice may dwell not far below the planet's otherwise dusty surface in this present day.

In 2000, NASA released images of Mars which showed what appeared to be gullies streaming down the sides of canyons and craters. Were these features on Earth, most geologists would automatically assume they were created by flowing water.

But this is Mars, where the temperature averages 50 degrees below zero Celsius and the air pressure at the surface hovers between seven and 10 millibars (by comparison, Earth's surface air pressure averages 1,013 millibars). By all accounts, water could not last in liquid form under such conditions.

So what did create these intriguing slices into the Martian skin?

The Dry Look

This was the topic of a seminar held at Cornell's Upson Hall on Oct. 26 titled "Are Martian Gullies Generated by Granular Flows?" by Dr. Troy Shinbrot of Rutgers University.

An Associate Professor of Biomedical Engineering at the New Jersey institution, Shinbrot's regular area of study is how various pharmaceutical products flow and interact with each other.

At a conference he co-organized with Cornell Professor Michel Louge in 2002, Shinbrot was discussing the "beautiful pictures" he saw of the Martian gullies. To Shinbrot, these surface features looked like they were made by the sandy particles themselves moving down the sides of banks, canyons, and craters, and not by liquid water.

In addition to the bitterly cold temperatures and thin atmosphere already discussed, Shinbrot also pointed out that most of the Martian gullies "faced poleward," away from the feeble though not insignificant warmth of the Sun. They also seem very young in geological terms, created perhaps only 10,000 years ago or less. There are no visible craters and no detectable boulders or rocks sitting in deposits at the bottom of these gullies, the usual clues to determining advanced age on planetary surfaces. The slopes they flow from are also quite steep.

From his previous granular flow research, Shinbrot knew that very light particles in a lower gravity environment act like a liquid before they eventually settle together to behave as regular solids.

What This May Mean

The idea of the Red Planet's gullies being formed by water through various means or even other processes such as underground heat or even certain gases like carbon dioxide has certainly not been ruled out. However, Shinbrot's granular flow research has given planetary scientists something new to consider when dealing with alien worlds.