Discordance analysis on a high-resolution valley network map of Mars: assessing the effects of scale on the conformity of valley orientation and surface slope direction

Fluvial valleys incised into planetary surfaces display morphologies, geometries and orientations that can be used to understand their controlling processes. Comparisons between valley orientations and topographic surface slope direction have been used to understand how Martian surfaces have evolved (Luo and Stepinski, 2012; Black et al., 2017); however, new insights from terrestrial studies have revealed that the slope needs to be defined on a scale commensurate with the length of valley (Lipp and Roberts, 2020). We propose and test a new map-based framework for interpreting the difference between Martian valley orientation and surface slope direction (discordance) at varying spatial scales. Valleys and inverted channels were manually mapped in a latitudinal strip from pole to pole between 20°E–20°W using High/Super Resolution Stereo Colour Imager images (15 to 25 m per pixel). The orientation of incisional valleys, associated with fluvial activity, were compared with topographic slope direction at varying scales (463 m, 1 km, 10 km and 50 km). High-resolution mapping resulted in ~1.5 times the density (0.02 km/km2 on average) of the previous valley map (Hynek et al., 2010), which raises the possibility that water volumes necessary to carve valleys were greater than previously thought. The distribution of valleys is similar to previous valley maps, however there are areas where valleys were identified that have previously remained undetected in low resolution images. The scale over which one compares valley orientation to surface slope direction has a clear effect on the apparent conformity between Martian valley orientation and topographic slope direction, with smaller valleys conforming more closely with smaller scale topography. To account for scale we perform discordance analysis with the spatial scale of surface slope direction varying based on valley length. When scale is accounted for, there is a greater conformity between valley orientation and surface slope direction, however only ~38% of valleys display conformity, indicating other sources of high discordance. We find that the source of the high values of discordance are likely a result of a combination of valley immaturity, attesting to the arid nature of the Late Noachian – Early Hesperian climate, but also the palaeolake outburst and, possible, subglacial origin of some valley networks. Considering a large proportion of valleys do not display conformity with topography, we suggest that a discordance analysis of the kind reported here is an important preliminary to any hydrodynamic analysis that relies on topographic information.