OUTCROP-SCALE PROPERTIES OF SEDIMENTARY ROCKS ON MARS

The outcrop-scale physical properties of the sedimentary rocks at Meridiani Planum were determined from Opportunity rover data and are in the range of analogous terrestrial rock masses, such as sandstone and shale. These properties, obtained by using the rock mass rating system (RMR), include the contributions of lithology (the intact rock material), fractures, and pore fluid (groundwater conditions). Physical properties were calculated for both present-day dry and past conditions. We find that Burns Formation rocks are relatively weak, with an outcrop-scale tensile strength of –0.24 MPa and unconfined compressive strength of ~50 MPa, and of small stiffness, with a deformation modulus of 21.1 GPa. The intact rock material itself is porous, with Φ = 4.5%, and fine-grained, giving a critical pressure for the onset of grain crushing of as much as 19.5 GPa. All values were reduced by less than ~50% during previous water-saturated conditions. The approach we used is general and therefore allows standardization of rock mass strength analysis so that one outcrop can be compared to another, regardless of age, location, or composition. Application of this method to other outcrops of the Burns Formation would permit determination of the spatial variation of physical properties, such as porosity, unconfined compressive strength, and degree of jointing, of a geologic unit on Mars. Understanding the physical properties of Martian materials on the outcrop scale would promote a greater understanding of the range of types and distribution of materials present on Mars' surface. It may also facilitate understanding of the changes in geological processes, both on Mars' surface and in the interior, through time.