Interrogating the lava hypothesis for the origin of Kasei Valles with analyses of streamlined island morphometry

 Introduction:  Outflow channels are the largest erosive structures on Mars [1,2,3,4]. Their formation postdates that of most valley networks, at the Hesperian-Amazonian transition circa 3 Ga. This period is marked by the end of the milder, perhaps habitable conditions that had characterized Mars' climate to this point [1,2]. Understanding the formation of the outflow channels is key to investigate the nature and duration of this climate collapse.Previous interpretations of Kasei Valles formation (Figure 1) part from observations of its scale, morphology, and location [2,4,5]. They infer that it was incised by cataclysmic outburst floods sourced from the collapse of vast subsurface aquifers, based on terrestrial analogues [5,2]. However, this hypothesis suffers from unrealistic discharges and the impossibility to replenish aquifers in the Amazonian [6,7]. Moreover, landforms such as potential eskers [8,9], lava terraces, or basal grooves suggest that other processes such as glacier sliding [10,3] or lava flows may have played a major role in the formation of Kasei Valles.Here we test the hypothesis that Kasei Valles was eroded by lava [6,7], leaving the test for ice or megafloods for further work. To make progress, we compared the Length-to-Width ratios (L/W) of streamlined islands within Kasei Valles to a lava channel, Olympica 1b, sourced from Olympica Fossae (Figure 1C) [11]. Our findings yield a gradient of L/W ratios in Kasei Valles that indicate a changing of behavior of the erosive agent inconsistent with lava flow.Methods: Previous studies characterized island streamlining by measuring their L/W ratios using island contour lines [11]. Here we followed the same approach using MOLA-HRSC DEM (200 m/px) and CTX images (6 m/px) to define contours, calculated in ArcPro ('Contours' tool), employing an equidistant cylindrical projection centered on 35.0°N, 70.0°W. We computed L/W ratios using the Minimum Bounding Rectangle tool in ArcPro to obtain 407 streamlined islands within Kasei. To ground truth our methodology, and to provide a quantitative test for the lava hypothesis, we compared the ratio statistics and distribution with those in Olympica 1b [11]. Specifically, we looked at the L/W histograms and evaluated the gradient in L/W values moving downflow within both canyons. FIGURE 1 – (A) MOLA-HRSC DEM showing Kasei Valles and Olympica 1b. (B) Map of Kasei Valles displaying the elevation and Length-to-Width ratios. The flow direction is indicated by the blue arrow. (C) Map of Olympica lava channel with elevation and Length-to-Width ratios. The flow direction is indicated by the blue arrow. Results and DiscussionThe analysis at Kasei Valles focused on evaluating the L/W ratios of streamlined islands as a distribution and in relationship with other parameters, including length along valley, type of obstacle (crater vs. outcrop), and island area. The L/W ratios mapped in Kasei Valles (Figure 1B) range from 1.0 to 7.0 and display a gradient that increases downflow. Within Olympica 1b (Figure 1C) we found an average L/W ratio of 3.10 over 386 islands which is consistent with [11], who reported an average L/W ratio of 3.09 over 468 streamlined forms.    We observed that the Olympica 1b islands display a gradient in L/W ratios that decreases downflow. This decrease in L/W ratios appears coherent with lava intrinsic properties: the lava viscosity increases as the fluid temperature drops downflow, resulting in a decrease in Reynolds number and a shortening of the streamlined islands [5].Our results on streamlined island L/W ratios for Kasei Valles indicate that the opposite trend was true with a statistical tendency to streamlined island lengthening towards the terminus of Kasei Valles, hinting at downflow acceleration. Moreover, the presence of very elongated streamlined island at the beginning of Kasei and poorly elongated ones at the end suggests that multiple events occurred.Conclusions:The origin of Kasei Valles is key to understand the shift in climatic conditions that took place at the end of Mars' Hesperian period. In this study we implement a novel approach based on the principle that drag minimization sets the equilibrium shape of streamlined forms to interrogate the morphometry and evolution of streamlined islands within Kasei Valles, focusing on the test of the lava flow origin hypothesis by considering the evolution of the inner islands L/W ratios along the valley.Our results yield a L/W ratios gradient in contradiction with the lava hypothesis. Our observations indicate that the gradient is not due to the islands properties but rather a consequence of changes in fluid dynamics which derived from a flow acceleration downstream. This quantitative study paves the way for the use of computational fluid dynamics applied to the observed geomorphology to better constrain the origin of Kasei Valles.Acknowledgments:This project has received funding from the European Research Council (ERC) under the European Union’s Horizon Europe research and innovation programme (Grant agreement No. 101165197 ICEFLOODS).References:[1] Turbet, M., Forget, F., Head, J. W., and Wordsworth, R. (2017). Icarus, 288, pp. 10–36. [2] Baker, V. R. (2001). Nature, 412(6843), pp. 228–236. [3] Lucchitta, B. K. (1982). Journal of Geophysical Research: Solid Earth, 87(B12), pp. 9951–9973. [4] Baker, V. R., and Milton, D. J. (1974).  Icarus, 23(1), pp. 27–41. [5] Komar, P. (1983). Geology, 11, pp. 1–10. [6] Leverington, D. W. (2011). Geomorphology, 132(3), pp. 51–75. [7] Leverington, D. W. (2004). Journal of Geophysical Research: Planets, 109(E10). [8] Chapman, M. 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