This image taken at JPL shows the Moessbauer spectrometer, an instrument on the Mars Exploration Rover Spirit that detects iron-bearing minerals in martian rocks and soil. Located on the rover's instrument deployment device, or "arm," this machine uses two pieces of radioactive cobalt-57, each about the size of pencil erasers, to determine with a high degree of accuracy the composition and abundance of iron-bearing minerals too difficult to detect by other means.