This image taken at NASA's Jet Propulsion Laboratory shows the Moessbauer spectrometer, an instrument on NASA's Mars Exploration Rover Spirit that detects iron-bearing minerals in martian rocks and soil.

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.

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