Evidence for episodic alluvial fan formation in far western Terra Tyrrhena, Mars

• Published in: Icarus (New York, N.Y. 1962) N.Y. 1962), 2011, Vol.211 (1), p.222-237
• Main Authors: Williams, Rebecca M.E., Deanne Rogers, A., Chojnacki, Matthew, Boyce, Joseph, Seelos, Kimberly D., Hardgrove, Craig, Chuang, Frank
• Format: Article
• Language: English
• Subjects: Astronomy; Catchment basins; Earth, ocean, space; Exact sciences and technology; Geological processes; Mars, Surface; Solar system; Spectroscopy
• ISSN: 0019-1035; 1090-2643
• DOI: 10.1016/j.icarus.2010.10.001

A Late Noachian-aged alluvial fan complex within Harris Crater in far western Terra Tyrrhena, Mars, is comprised of two well-defined source regions and associated discrete depositional lobes. Three fan units were recognized based on common morphological characteristics, thermal properties and spectral signatures. Although the entire fan complex has been subjected to extensive erosional degradation, the preserved morphologies record episodic fan formation and indicate the type of flow processes that occurred; the bulk of the fan surface has morphology consistent with fluvial emplacement while one fan unit exhibits a rugged surface texture with boulders consistent with a debris flow. This transition from fluvial to late-stage debris flow(s) suggests a decline in available water and/or change in sediment supply. The thermal inertia values obtained for all three fan surface units (mean values ranged from 318 to 344 J m −2 K −1 s −1/2) are typical for coarse-grained and/or well-indurated materials on Mars, but subtle variations point to important distinctions. Variations in aeolian bedform coverage as well as the density of ridges (inferred inverted channels) and boulders contribute to these subtle fan thermophysical differences and likely reflect changes in the fan depositional mechanisms and variations in post-depositional modification histories. The majority of the alluvial fan surface has a spectral signature that is broadly similar to TES “Surface Type 2” (ST2), with some important exceptions at long wavelengths. However, a unique spectral component was identified in one of the fan units (unit 3), that likely reflects lithological differences from other fan materials. This spectral attribute of unit 3 matched locations within the western catchment providing confirmation of provenance and supporting the contention that sediment supply changed over time as the fan developed. Finally, we applied simple modeling to a well preserved subsection of the fan complex to quantify the developmental history. Using the computed eastern fan volume (32 km 3), significant water, likely from precipitation, was involved in fan construction (>50 km 3) and an extensive period of fan formation occurred over millennia or longer.