Large Aeolian Ripples: Extrapolations from Earth to Mars

Introduction: The shapes produced on terrestrial surfaces that are continually subject to aeolian processes range in size from tiny ripples to giant draa [1]. The physical mechanism of the formation of ripples, the smallest of aeolian bedforms, is intimately related to saltation and reptation processes. Sand and granule ripples are typically tens of reptation path lengths wide and do not alter the flow of wind that passes over them. Dunes are greater than tens of saltation path lengths wide, large enough to affect the local wind regime, and typically exhibit avalanchegenerated slip faces on their lee side. On Earth, the differences between aeolian ripples and dunes are quite apparent and distinct [e.g., 1]. On Mars, however, the thin atmosphere leads to greatly increased saltation path lengths for sand-sized particles [2], which may cause the physical dimensions of large ripples and small dunes to overlap despite their unique formation mechanism [3, 4, 5]. Since bedforms intermediate between ripples and dunes are rare in terrestrial aeolian environments, the largest terrestrial granule-covered ripples are used as analogues for ripple-like bedforms on Mars as imaged by the Mars Orbiter Camera (MOC). Comparing the physical characteristics of granule ripples on Earth to ripple-like bedforms on Mars is useful to further the understanding of large ripple formation on both planets. Large Ripples on Earth: Large terrestrial granular ripples vary with grain size and have wavelengths that range from ~1 m up to 20 m. We have documented several granule or pebble-covered ripples with wavelengths between 1 m and 10 m in the southwestern United States and at Great Sand Dunes National Monument (GSDNM) in Colorado. The ripples at all scales have a fairly consistent index (height/wavelength) that agrees with the 1/15 value given by Sharp [6], whereas the dunes at Parker, AZ, have a significantly different index (1/51), as compared to the ripples. Laser profiling of ripples (Figure 1) was conducted perpendicular to the crest axis to measure the wavelength, height and shape of granule ripples at GSDNM. Figure 2 depicts a field site where granule ripples were measured, and Figure 3 shows the resulting profile of distance versus height. Ripple-like Bedforms on Mars: With its increased surface resolution, MOC has provided the detail necessary to document and examine ripple-like bedforms on Mars. These common bedforms are found in topographic depressions such as troughs and Figure 1. Laser profiling of granule ripples at Great Sand Dunes National Monument.