EVIDENCE FOR FLUVIAL EMPLACEMENT AND SECONDARY ALTERATION IN ALLUVIAL FAN DEPOSITS DERIVED FROM THERMAL EMISSION AND TEMPERATURE DATA

Alluvial fans are important for understanding the paleohydrology of Mars as they record widespread fluvial activity from the late Hesperian into the sub-optimal climate of the Amazonian. The amount of water needed to form fans can be constrained by characterizing fan surface deposits to interpret transport processes. In terrestrial studies, this is accomplished through stratigraphic analysis, including grain size and sorting determination. On Mars, however, the limited resolution of orbital image data and the low number of in-situ observations constrain alluvial fan investigations. Fortunately, the relationship between the thermophysical properties (i.e., thermal inertia) and surface physical characteristics (i.e., particle size, clast abundance, and degree of weathering) is well established. Here we use apparent THEMIS thermal inertia and emissivity spectral slopes along with HiRISE visible image data to examine sub-meter surface characteristics on alluvial fans within two unnamed craters (centered at: -30.445 N, 313.359 E & -30.411 N, 315.379 E). These data sets can help determine the spatial distribution of particle sizes as well as the presence of surface induration. Ultimately, this will assist in distinguishing between primary transport processes and secondary aqueous alteration.

Average thermal inertia values (200 – 600 TIU) indicate that fan surfaces comprise sand to pebble-size clasts. THEMIS daytime emission data are negatively sloped and "bluing" in decorrelation stretch analyses, suggesting a high probability of sub-pixel temperature mixing on fan surfaces. In HiRISE image data, fan surfaces along the eastern rim appear fluted near their source and channelized near the toe suggesting fluvial emplacement. In contrast, fans along the crater's southwest rim exhibit layered, lobe-shaped deposits, inverted channels, and well-preserved small impact craters (< 0.25 km diameter). Combined with the moderate thermal inertia values (400-800 TIU), these features may suggest induration of the southwestern fan surfaces. Additional thermophysical and spectral slope modeling is needed to test these hypotheses.