Mars rover to probe ancient clay sediments

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At a location on Mars called Yellowknife Bay, NASA’s Curiosity rover last March found clay rocks with hydrogen, oxygen, nitrogen, carbon and sulfur.

Clay rocks with these elements normally form in a wet, low-salt environment — the type that could support life. But today Mars has become a dry, acidic desert, which is not a good environment for life.

On July 4, Curiosity took the first steps — well, actually drove 59 feet — toward Mount Sharp, a 3.4-mile-high peak just over five miles away. Scientists hope the lower layers of Mount Sharp will reveal the secrets of how the Martian atmosphere changed over time.

With a top speed of 0.09 mph, Curiosity’s journey will take at least nine months; with stops to explore, it may take a year.

Based on images taken from Mars orbiters, mission planners have selected a route that is mostly straight but tries to avoid boulders, steep slopes and other major obstacles. The biggest challenge is to cross a bank of sand dunes near the base of Mount Sharp.

The first planned stop will occur in about a month, when Curiosity stops to drill and sample the dirt for more analysis.

So why is Curiosity making the journey to Mount Sharp?

The mountain sits in a 96-mile-wide crater formed by a meteor impact about 3.6 billion years ago. It is believed that over time the crater filled with sediments, and swirling winds in the crater carved the sediments away to form the mountain in the center.

This peak is higher than Washington’s Mount Rainier, making the exposed sediments more than three times higher than the Grand Canyon is deep. This may be the greatest exposed section of layered sedimentary rock in the solar system.

The sequence of rock changes in Mount Sharp is the result of changing environments as the layers were deposited, with the oldest layers at the bottom of the mountain and the youngest at the top.

Data from Mars orbiters indicate the bottom layers are clay minerals that form under very wet conditions. There may have been a lake in Gale crater at the time these sediments were laid down, or wind-driven sediments may have soaked up groundwater.

Above these layers are layers with sulfate minerals intermixed with the clays. Sulfates often form when water in which they are dissolved evaporates. These would have been laid down during a time when there was less water on Mars.

The next layer has only sulfate minerals, with no detectable clays. This layer would have been laid down during a time of even less water.

On the top of the mountain are thick, regularly spaced layers that bear no detectable water-related minerals. These were laid down when there was little or no water present.

Rock exposures with compositions like those of Mount Sharp have been mapped elsewhere on Mars, and it has been proposed that the planetary chronology of clay-producing conditions, followed by sulfate-producing conditions, followed by dry conditions, is the environmental history of Mars.

But Mount Sharp is the first location where a single stack of layers has been found to contain this history in a clearly defined sequence from older to younger rocks.

As Curiosity climbs Mount Sharp, it will use its suite of instruments to analyze these layers, giving us a more complete understanding of the planetary history of Mars. At the same time, the high-resolution camera on the mast should give us pictures and movies of spectacular valleys and canyons. We will see the canyon walls on each side of the rover climb to over 100 feet.

This extraterrestrial sightseeing tour alone will be worth the trip to Mount Sharp.

Marty Scott is the astronomy instructor at Walla Walla University, and also builds telescopes and works with computer simulations. He can be reached at marty.scott@wallawalla.edu.

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