LARGE RIPPLE-LIKE BEDFORMS: EXAMPLES FROM THE MARS ORBITER CAMERA

Mars Orbiter Camera (MOC) images are being surveyed to document the occurrence and dimensions of large aeolian ripple-like features on Mars, as part of an on-going NASA MDAP investigation. All released MOC Narrow Angle (NA) images, through the Primary mission, have been studied for Mars Charts (MC) 13, 24 and 27, as well as selected NA images from MC 2, 8, and 16. Ripple-like features are ubiquitous on Mars; they are not confined to particular physiographic or geologic settings but tend to accumulate within local topographic lows. MOC images show isolated ripple-like features on terrain surrounding concentrations of rippled deposits; isolated individual transverse features are difficult to reconcile with a ripple origin but are consistent with the possibility of individual dunes. There is little evidence in MOC images of possible slip faces on the ripple-like features; this observation is difficult to reconcile with a dune origin for these features unless they have been inactive for a sufficient length of time to obscure the slip face avalanche deposits. The majority of ripple-like features from the equator to both northern and southern mid-latitudes have average wavelengths of 35 m and range between 10 and 60 m. Poleward of 25 latitude, both hemispheres exhibit ripple wavelengths between 60 to 90 m, with some examples exceeding 100 m. Terrestrial studies of large aeolian ripples reveal a correlation between ripple wavelength and the largest particles on the features. Nighttime THEMIS data will provide relative thermal properties of sufficiently large ripple patches, which we predict should correlate with ripple wavelength. Continued data collection will help to provide a semi-quantitative assessment to determine whether Martian ripple-like features are more likely to be formed by dune or ripple processes.