Retrieval of Compositional End‐Members From Mars Exploration Rover Opportunity Observations in a Soil‐Filled Fracture in Marathon Valley, Endeavour Crater Rim

The Opportunity rover investigated a gentle swale on the rim of Endeavour crater called Marathon Valley where a series of bright planar outcrops are cut into polygons by fractures. A wheel scuff performed on one of the soil‐filled fracture zones revealed the presence of three end‐members identified on the basis of Pancam multispectral imaging observations covering ~0.4 to 1 μm: red and dark pebbles, and a bright soil clod. Multiple overlapping Alpha Particle X‐ray Spectrometer (APXS) measurements were collected on three targets within the scuff zone. The field of view of each APXS measurement contained various proportions of the Pancam‐based end‐members. Application of a log maximum likelihood method for retrieving the composition of the end‐members using the 10 APXS measurements shows that the dark pebble end‐member is compositionally similar to average Mars soil, with slightly elevated S and Fe. In contrast, the red pebble end‐member exhibits enrichments in Al and Si and is depleted in Fe and Mg relative to average Mars soil. The soil clod end‐member is enriched in Mg, S, and Ni. Thermodynamic modeling of the soil clod end‐member composition indicates a dominance of sulfate minerals. We hypothesize that acidic fluids in fractures leached and oxidized the basaltic host rock, forming the red pebbles, and then evaporated to leave behind sulfate‐cemented soil. Plain Language Summary The Opportunity rover investigated a region on the rim of Endeavour crater on Mars called Marathon Valley where a series of bright outcrops are cut by fractures. A scuff performed by one of the rover wheels on the fractures revealed the presence of three different compositional end‐members. A novel technique was applied to retrieve the composition of the end‐members using measurements by the rover's Alpha Particle X‐ray Spectrometer and Pancam instruments. The presence of a magnesium‐sulfate‐rich soil end‐member and hematite‐rich pebbles in the scuffed fracture imply alteration in an acid‐sulfate environment. Results add to growing evidence of alteration along the rim of Endeavour crater that was concentrated along fractures, which likely provided a conduit for subsurface fluid flow. The timing of formation of these altered deposits is unclear; they could have formed during transient postimpact hydrothermal activity or perhaps significantly later utilizing groundwater from the southern highlands. Key Points A log maximum likelihood method was used to determine end‐member composit