The Evolution of Martian Valley Network Formation Timescales

Abstract

<p><strong>Introduction:</strong>  Mars’ surface is carved by an array of dendritic valley networks, which are evidence for ancient water cycles and surface run-off on Mars. The majority of these networks appear to have formed during the Late Noachian – Early Hesperian (3.8 to 3.6 Ga) and resemble terrestrial precipitation-fed systems. After this period, other than localised valleys present on the flanks of several volcanoes, valley network formation appears to have rapidly decreased, indicating that Mars’ climate experienced a sudden change from a warm and wet state to hyper aridity. However, a recent analysis of Amazonian – Hesperian aged flat crater-bottom deposits and alluvial fans indicates that localised areas of wettest persisted after the Early Hesperian. By analysing the morphological, morphometric, and paleohydraulic characteristics, and formation timescales of valley networks of different ages, one can gain a better understanding of the evolution of Mars’ aridity.</p><p>In this study, we aim to perform a detailed analysis of valley networks of differing ages to determine their formation origin and the duration of aqueous activity required to incise their troughs. At present we have performed formation timescale analysis on an Amazonian-Hesperian aged valley network – the results are presented below.</p><p><strong>Data and Methods:</strong> A combination of GIS software packages were used to perform the analysis: SAGA GIS was used to determine full water depth estimates and flow width via the multiply flow direction method; GRASS GIS was used to determine flow accumulation, flow direction, and upstream slope; ArcGIS Pro was used to perform spatially variable drainage area calculations for Hack’s Law and Flint’s Law calculations. Valley networks were initially identified using the Hynek et al. (2010) valley map, and narrowed to different surface ages based on the Tanaka et al. (2014) surface age map. Detailed mapping and morphological analyses of these valleys was performed using Context Camera images (5 m per pixel). High-Resolution Stereo Camera (HRSC) DEMs were used for paleohydraulic and formation timescale analysis. For the formation timescale calculation, the estimated volume (km<sup>3</sup>) of each pixel was divided by the volumetric transport rate (km<sup>3</sup>/yr).</p><p><strong>Initial Results:</strong> At present, we have applied the technique to a valley network located north-east of Lowell Crater (49.82 °S 77.16 °W). The source is within a Middle Noachian highland unit, with the majority of the network incising an Amazonian-Hesperian aged impact unit. The valley network has a main valley length of ~123 km, an almost linear profile, and an average slope (dz/dl) of ~ 0.012. Based on a calculated average water velocity of 6.8 m/s and an average 12.25 m water depth, the average formation time for the whole study area is 23235.3 yr (1 sigma standard deviation = 39401.2 yr).</p><p><strong>Discussion: </strong>It is apparent the examined young valley is immature compared to previously examined Late Noachian – Early Hesperian Martian valley networks, which have minimum formation timescales ranging from 10<sup>5</sup> to 10<sup>7</sup> years. Applying formation timescale and paleohydraulic calculations to valley networks from a range of ages, we will be able to better understand the evolution of fluvial activity that formed them.</p>