CANDIDATE BOULDER-RICH MARTIAN FANS BASED ON THERMOPHYSICAL AND MORPHOLOGICAL CRITERIA

A global survey of fan deposits identified in CTX images on Mars shows they are abundant landforms (n=1500) across the cratered highlands [1]. However, their depositional environment is debated. Furthermore, sediment caliber, a key parameter for modelling fan depositional process, is poorly constrained. In this study, the thermophysical properties of 16 fans from two morphological classes is examined. We report a general correlation between fan type and the time-varying thermal response, founded on thermal slopes obtained from a terrestrial analog site. These findings suggest an approach for identifying candidate boulder-rich fans on Mars, and has implications for constraining threshold flow conditions in fan formation.

The catchment to fan volume ratio is used to distinguish depositional environment [2]. Type 1 fans have a volume ratio significantly less than 1 indicating sediment loss. In addition, these fans typically have a larger and higher order feeder, channel system. Type 1 fans are interpreted as deltaic deposits [1]. Type 2 fans have a volume ratio of ~1. Type 2 fans have a single, short feeder valley, attributes that are consistent with mass wasting emplacement.

The time-dependent thermal response of the surface can reveal detailed information about the nature of the surficial materials. The greatest surface thermal response occurs during the early morning warming and the late afternoon cooling periods. Capitalizing on the expansive THEMIS record, the afternoon (15:00-20:00) temperature gradient is extracted for fans from each morphological class. Temperature data is seasonally-constrained to within a 60 degree Ls range.

The afternoon thermal gradient differs between Type 1 and Type 2 fans (~16.5 versus ~11.5 º/hour). A shallower thermal slope may be due to smaller grain size, cementation, or diagenetic overprinting. A steeper thermal slope is consistent with coarse-grained deposits. Included in this sample is the Jezero delta (Type 1), which has groundtruth image data confirming the presence of boulders [3]. Utilizing morphological and thermophysical criteria together can aid in identifying candidate boulder-rich fans. These findings raise the possibility of high-energy floods at more locations than previously recognized.

[1] Morgan et al., 2022. [2] Vaz et al., 2020. [3] Mangold et al., 2020.