Which instrument on Sojourner analyzed the composition of Martian rocks?

The instrument on Sojourner that analyzed the composition of Martian rocks was the Alpha Proton X-ray Spectrometer.

Usually abbreviated APXS, the instrument identified chemical elements by exposing a rock or soil surface to radiation from a curium-244 source. It measured returned alpha particles, protons, and X-rays, with each signal revealing information about different elements. This let scientists estimate the composition of targets without bringing samples back to Earth.

Sojourner carried the APXS on a small deployment mechanism attached to its robotic arm. The instrument made close-contact measurements of targets including Barnacle Bill and Yogi, two rocks examined during the Pathfinder mission. A full measurement could take as long as ten hours, reflecting the instrument’s careful but relatively slow detection process.

The APXS was different from Mini-TES, which was a thermal infrared spectrometer used on later Mars Exploration Rovers such as Spirit and Opportunity. RIMFAX belongs to Perseverance, and Mastcam-Z is Perseverance’s stereo imaging system. The APXS name is also sometimes shortened to “alpha proton X-ray spectrometer” without capitalization changes.