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Alpha Particle X-Ray Spectrometer (APXS)
The APXS is designed to study the alpha particles and x-rays emitted by rocks and soils in order to determine their elemental chemistry. Alpha particles are emitted during radioactive decay and X-rays are a type of electromagnetic radiation, like light and microwaves. The elemental composition of a rock describes the amounts of different elements that have come together to form all of the minerals within the rock. Knowing the elemental composition of Martian rocks provides scientists with information about the formation of the planet's crust, as well as any weathering that has taken place.
As with the other instruments on the arm of the rover, the APXS sensor head is small enough to hold in your hand. Its electronics are housed in the warm electronics box (WEB) located in the body of the rover. Most APXS measurements will be taken at night and will require at least 10 hours of accumulation time, although just x-ray alone will only require a few hours.