NASA / SCIENCE PHOTO LIBRARY NASA / SCIENCE PHOTO LIBRARY
Sojourner. Mars Pathfinder mosaic image of the robotic Sojourner vehicle on the surface of Mars analysing the rock known as "Barnacle Bill" (at upper left). Bill" was the first rock to be studied by Sojourner with its Alpha Proton X- Ray Spectrometer (APXS). The APXS determines the chemical composition of rocks & soil by bombarding them with alpha particle radiation and monitoring the spray of particles released in response. All elements except hydrogen can be detected during the 10 hours the APXS takes to complete an analy- sis. Image taken by the Imager for Mars Pathfinder (IMP) on the third Martian day (Sol 3) spent on Mars since it landed on 4 July 1997.
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