CONSTRAINING EFFECTS OF SPATIAL RESOLUTION ON FAN GRAIN SIZE ESTIMATES FROM THERMAL INERTIA: INSIGHTS FROM THE MILNER CREEK FAN, CALIFORNIA

Grain size is a crucial parameter needed to interpret the hydrologic and climatic history of martian alluvial fans. Constraining fan grain sizes on Mars is difficult as these are often at or below the spatial resolution of orbital visible images. Previous studies have suggested that thermal inertia may provide a useful proxy for estimating fan grain sizes on Earth and Mars, as coarser sediments (>10 cm) generally have high thermal inertia, whereas finer-grained sediments have low thermal inertia. However, fan deposits consist of a range of grain sizes that vary spatially, and we lack constraints on what size fraction within that distribution is reflected in thermal inertia data at different spatial scales. This analog study focuses on the Milner Creek fan in California and uses orbital- and drone-based thermal images to systematically constrain how spatial resolution impacts the interpretation of grain size from apparent thermal inertia (ATI).

Orbital-based ATI images were calculated for the Milner Creek fan using Landsat 8 images (~90 m/pxl), whereas the drone-based ATI were from a DJI Mavic 3T (~3-10 cm/pxl). The fans’ grain size properties were determined in the field by conducting pebble counts at three drone flight locations in an incised channel on the fan. Preliminary results indicate that overall orbital ATI values and grain size generally decrease down fan with increasing distance away from the apex. ATI values show a relative decrease, reflecting small changes in the grain size distribution from ~1,000 m (closer to the apex) to 4,271 m, with sand-sized particles further down fan. Generally, ATI values reflect no notable correlations of size fractions per location. The Landsat 8 results will be compared with the drone-based ATI to test if changing spatial resolutions affects the grain size that dominates the ATI values. Understanding the relationship between grain size and thermal inertia at varying scales is important for interpreting past aqueous processes on Earth and Mars.