For much of its nearly two-year mission, NASA's Mars Exploration Rover Spirit has been the forgotten child. Its twin, Opportunity, was the first to find geological layering and evidence for past liquid water, and its landing site is covered by diverse rocks and soils. But for many months, whenever Spirit sampled a rock, it always sniffed the same thing — volcanic basalt. The floor of Gusev Crater, Spirit's home, was uniform in composition and, put bluntly, somewhat boring.
Spirit's fortune changed when it reached the Columbia Hills in June 2004. Since it began its trek up Husband Hill, the highest of the peaks, the intrepid rover has found no fewer than five different classes of rocks. As reported by rover team leader Steve Squyres (Cornell University) at this week's American Astronomical Society Division for Planetary Sciences meeting in Cambridge, England, these rocks not only provide evidence for past water activity at Gusev Crater, they also give clues to the hills' geologic history.
As Spirit approached the hills, the first new rock type it stumbled upon was called Clovis class (each class of rock is named for the first rock the rover saw of the given type). Clovis rocks are finely layered and soft enough that the rover can dig into them easily with its rock abrasion tool. They contain grains of various sizes, and they look like basaltic glass. Therefore rover scientists think Clovis rocks are most likely impact ejecta that were later altered by water.
While climbing up the northwest flank of Husband Hill, Spirit found very little bedrock as it rolled across the loose, steep terrain. Then, around December 1, 2004 (the rover's 325th Martian day, or sol), the intrepid rover began to encounter Wishstone-class rocks. These stones contain coarse, angular grains and sport knobby textures. When drilled with Spirit's rock abrasion tool, the interiors of Wishstone-class rocks revealed the minerals pyroxene and olivine. The samples look very much like volcanic tuff — ash ejected from Martian volcanoes billions of years ago that later congealed into rock.
Spirit found Peace-class rocks at higher elevations. These finely layered specimens contain abundant magnetite and up to 20 percent sulfate salts. These samples probably formed by sand that was deposited by wind or water. At some point in time the sandy layers became wet and the sulfate salts that formed in the water became the glue that cemented the sand grains together.
Higher still, along the Cumberland Ridge, Spirit encountered bedrock. At a point called Larry's Lookout (around Sol 400, in mid-February), the rover uncovered a fourth class of rock dubbed Watchtower. The vehicle's instruments detected clear evidence for water interactions. The cameras and spectrometers revealed millimeter-scale layering, minerals such as magnetite, and large amounts of titanium and phosphorus. It seems, says Squyres, that Watchtower rocks are a mixture of Wishstone and Peace rocks, and they might be pieces of impact ejecta.
As the rover neared the summit (around Sol 541, July 11th), it found a fifth set of rocks — Backstay class. These rocks are similar to the ones on the plains of Gusev. They might be evidence of dikes — fingers of igneous rock that run through the Columbia Hills.
So what are all these rocks telling us? Rover scientists are still analyzing the situation, but as Squyres reports, it's possible that the Columbia Hills were uplifted from the floor of Gusev Crater, probably after a large meteorite impact. "Nothing about these rocks says they were deposited in liquid water. They were soaked later," says Squyres. We might be seeing "an island in a lake."