"The rover's spinning wheels have broken through a crust, and we've found something supremely interesting in the disturbed soil."From the beginning, the rovers' motto has been "follow the water." Both rovers have been searching Mars for minerals formed in the presence of H2O. Mars appears dry today, but minerals can provide clues that water was once there.
But when Spirit reached an area of Mars called the "Columbia Hills," the whole complexion of the mission changed. "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 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.