Martian surface. Dry, rocky surface of the Gusev Crater on Mars, taken by the panoramic camera of the Mars Exploration Rover Spirit. Ripples in the sandy soil are thought to have been formed by gentle winds. Spirit landed successfully in the Gusev crater on the planet on 4th January 2004. NASA scientists chose this location because the 145-kilometre-wide crater may once have contained a lake. Spirit has been examining soil and rocks in the area with its array of scientific instruments. An identical rover, called Opportunity, is also carrying out explorations on the opposite side of the planet.