NASA's twin Mars Exploration Rovers, Spirit and Opportunity, launched toward Mars on June 10 and July 7, 2003, in search of answers about the history of water on Mars and the geology of two distinct landing sites.
Three scientists in ARES - Dick Morris, Doug Ming, and Dave Mittlefehldt - play important roles in the operation of Opportunity, particularly in the operation of the Alpha Particle X-ray Spectrometer (APXS) instrument. One also serves as Long Term Planning lead for Science Operations Working Group meetings where science activities are planned. The APXS is located on the end of the Instrument Deployment Device (IDD), a.k.a. the arm. The IDD deploys the APXS onto the sample surface. The APXS bombards the surface with alpha particles from a radioactive 244-curium source. One decay product of 244-curium is radioactive 240-plutonium which emits X-rays that also bath the sample surface. This dual irradiation of the surface by energetic particles and X-rays cause elements in the rocks and soils to in turn fluoresce X-rays. Each element emits X-rays with a characteristic energy. Detectors in the APXS count the emitted X-rays, which are then converted into element concentrations in the surface rocks and soils through comparison with calibrations done on reference rocks on Earth. The IDD also hold a Mössbauer spectrometer that was used to determine the distribution of iron among the minerals in rocks and soils. This instrument used a 57-cobalt source to irradiate the sample surface with gamma-rays, which probed differences in energy levels of 57-iron nuclei in response to their environment, such as the crystal structure of a mineral. The half-life of 57-cobalt is only 271.8 days, and thus, thirteen years into the mission, the cobalt source is far too weak for measurements to be made.
In addition to working on tactical rover operations, the ARES MER team helps interpret data that have been returned by Spirit and Opportunity. A major focus of the ARES MER team is to characterize the compositions and mineralogies of rocks and soils on Mars, and use this information to understand ancient aqueous processes and environments. An important component of our work has been on using the datasets from the APXS and Mössbauer instrument to unravel the chemical composition and iron mineralogy of rocks and soils. However, as part of a mission team, we coordinate our investigations with other team members and bring results from the full suite of instruments into geological interpretations of the origins of the rocks.