Spirit was rovering in an area called Troy, within a larger area called Columbia Hills.
It was driving through a dark, crusty surface, along the edge of a small crater, which suddenly broke away, caused by a patch of loose soil.
When it tried to move backward its left-front wheel tossed out some light, fluffy soil originally beneath the crust.
Columbia Hills on Mars is a range of low altitude hills inside of the Gusev crater. The Mars Exploration Rover (MER) Spirit landed about two miles (three kilometers) from the hills in 2004.
The range was named, on February 2, 2004, in memory of the seven-member crew of the NASA space shuttle Columbia, who died on February 1, 2003 while returning from their mission in space.
The peaks of Columbia Hills were each named after the seven astronauts that perished in the disaster over Texas that destroyed the Columbia space shuttle
The 12-2-2009 NASA media brief “Sandtrapped Rover Makes a Big Discovery” quotes Dr. Raymond "Ray" E. Arvidson (of the Department of Earth and Planetary Sciences, Washington University, St. Louis, Missouri).
Page two discusses the discovery of sulfates on Mars from the perspective of Dr. Arvidson, and why it is important to eventually learning the watery past of the planet.
Dr. Arvidson, who is directly involved with the Mars Exploration Rover (MER) mission, says, “The rover's spinning wheels have broken through a crust, and we've found something supremely interesting in the disturbed soil.”
Arvidson states the different type of missions for both rovers, "It's been easy for Opportunity to find such minerals. Opportunity landed in an ancient lake bed. Spirit has had to work much harder. Spirit landed in basaltic plains formed by lava flows chewed up by repeated meteoroid impacts. There's been little evidence of anything that was ever very wet."
However, this difficult mission of Spirit changed when it moved into the Columbia Hills region on Mars.
Arvidson states, “Spirit came across iron hydroxide, a mineral that forms in the presence of water. That alerted us to the change. We started coming across more and more rocks formed in the presence of water."
When Spirit got stuck and tried to remove itself from its captivity, its left-front wheel churned up some loose soil beneath the crust. And, because of that activity, it made an exciting discovery.
It found sulfates. So, why are sulfates important for the search of water on Mars?
Page three explains.
First a quick explanation of sulfates.
Dr. Arvidson explains, "Sulfates are minerals just beneath the surface that shout to us that they were formed in steam vents, since steam has sulfur in it. Steam is associated with hydrothermal activity – evidence of water-charged explosive volcanism. Such areas could have once supported life."
He adds, "And most amazingly, the boundary between the sulfate-rich soil and the soil with just the generic concentration of sulfates runs right down the middle of the stranded rover. Spirit is lodged on the edge of a crater -- sitting astride the boundary!"
Arvidson continues, "Also, the robot found that the top of the sulfate material is crusty. Ancient sulfates probably formed this crust as they were processed by variations in climate associated with changes in Mars' orbit over millions of years."
The NASA article summarizes what might have happened on Mars millions of years ago. It states, “Here's what the scientists think: When a Martian pole faces the sun in Martian summer, it gets warmer at that pole and the water ice shifts to the equator. It even snows there! Warm dark soil under the snow causes the bottom layer of snow to melt. The water trickles into the sulfates, dissolving the water-soluble iron sulfates and forming a crust with the calcium sulfates remaining.”
Spirit is giving scientists valuable information on the ancient water cycle on Mars, and the cycle that is continuing even today.