Located on the arm of the Mars Exploration Rover Spirit, the alpha particle X-ray spectrometer uses alpha particles and X-rays to determine the chemical make up of martian rocks and soils. This type of information helps scientists understand how the planet's crust was weathered and formed. Mars Exploration Rover team members used this palm-sized instrument on a small patch of martian soil just after Spirit rolled off the Columbia Memorial Station. They found that although the soil was very similar to what they had seen previously on Mars, the instrument's improved sensitivity allowed them to see new elements and subtle differences not detected before.