NASA's Curiosity Mars rover examined both the "Greenhorn" and "Big Sky" targets with the rover's Alpha Particle X-ray Spectrometer (APXS) instrument. Greenhorn is located within an altered fracture zone and has an elevated concentration of silica (about 60 percent by weight). Big Sky is the unaltered counterpart for comparison.
The bar plot on the left shows scaled concentrations as analyzed by Curiosity's APXS. The bar plot on the right shows what the Big Sky composition would look like if silica (SiO2) and calcium-sulfate (both abumdant in Greenhorn) were added. The similarity in the resulting composition suggests that much of the chemistry of Greenhorn could be explained by the addition of silica. Ongoing research aims to distinguish between that possible explanation for silicon enrichment and an alternative of silicon being left behind when some other elements were removed by acid weathering (see PIA20274).
NASA's Jet Propulsion Laboratory, a division of the California Institute of Technology, Pasadena, manages the Mars Science Laboratory Project for NASA's Science Mission Directorate, Washington. JPL designed and built the project's Curiosity rover.
More information about Curiosity is online at http://www.nasa.gov/msl and http://mars.jpl.nasa.gov/msl/.