LARGE RIPPLE-LIKE BEDFORMS: EXAMPLES FROM EARTH

Aeolian bedforms transitional between conventional aerodynamic ripples and dunes occur in rare, but diverse, locations. Their wavelengths are longer than those of aerodynamic ripples by an order of magnitude or more, so an origin mechanism directly related to the saltation/reptation process, like that responsible for aerodynamic ripples, is unlikely. They lack slipfaces, so they are unlike dunes also. Large ripple-like bedforms are observed in numerous locations on Mars, making them a topic tailor-made for study from a comparative planetology perspective. Recent field studies have been conducted at Rogers Dry Lake, Coyote Dry Lake, the Kelso Dunes, Great Sand Dunes National Monument (GSDNM), and in the Coachella Valley, all locations of previous studies of ripple-like bedforms in the literature. The Rogers and Coyote sites had ripple-like bedforms with particles of pebble size atop a deflating playa base. Their cross-sections were similar to, but subtly different from, ripples of similar dimensions found in sandy environments (Kelso, GSDNM, and Coachella). The following generalizations can be drawn from observations at all sites: 1. The ripple index for all cases was ~15, although the data are sparse and concentrated toward small wavelengths. 2. A bimodal distribution of particle sizes seems to be required for large ripple-like bedforms to develop. Bedforms of Kelso, GSDNM, and Coachella (granules atop sand) occurred only where granules were present, although not every area covered with granules had developed larger bedforms than aerodynamic ripples. 3. Additional observations are needed to determine whether the granules merely veneer the crests of the bedforms, which was observed at GSDNM and Coachella, or comprise the main body of the bedform, as observed at Kelso. 4. There is a direct correlation between the wavelengths of the large ripple-like bedforms and the size of the particles comprising them. The last observation is particularly useful in the Mars context. By terrestrial analogy, the particle size (as determined from THEMIS observations) of the larger of the observed martian ripple-like bedforms should be demonstrably greater than that of the smaller ripple-like bedforms.