Reconstructing a Martian channel-fan formation iteratively with mapping and mechanical modeling
Authors: Molina Antonio , Herreros Isabel , Martínez-Pérez Teresa , Mata Susana , Gómez Belén 
Published: July 2021

INTRODUCTION
Mars&#8217; surface shows evidence of a decreasing geological activity over the years, leaving some geologic structures almost intact during the Amazonian era. This geologic age is characterized by the dry and inert conditions that can be found today. However, water-related features like channels and deltas prove that Mars was wetter and more active in previous eras, at least for brief periods. The study of surficial images and topographic data compared with analog structures on Earth allows us to infer the nature of these events. However, some processes are difficult to constrain and observations are not enough to answer some fundamental questions. Mechanical modeling applied to geological processes emulation is becoming increasingly powerful with the development of technology and new numerical solutions. Here we propose the iterative combination of both techniques, topographic reconstruction by mapping and mechanical modeling (detailed in [3]), to provide a better constraint for past surficial processes on Mars.METHODOLOGY
Before running numerical modeling over real surficial data, it is necessary to reconstruct the previous shape of the terrain. Even with a profound knowledge of the geologic history of the study area, some assumptions have to be made. Although the general shape of ancient features on Mars is still preserved, ulterior modifications may hinder proper modeling. Impact, tectonic, and gravitational processes alter original stream beds; aeolian deposits hide these channels&#8217; original depth and shape.&#160;We start reshaping the contours to reverse the terrain mobilized by the process to be studied, helped by what can be identified in the imagery. In this case, we remove the fan and fill the channel until a knickpoint [4]. To support the assumption that both the fan and knickpoint are coetaneous, we compare the volume of material of both reshapings. Running the model [3, 5] on the reshaped surface (with the channel filling volume as the starting condition), new information is obtained to enhance the reconstruction, starting an iterative process. The alternation between reshaping the topography and running the model until the result of the latter fits the shape and characteristics found in the initial unaltered topography allows tuning the topography. More than one hypothesis can be tested, providing insights during the iteration process itself.Our testing case is a fan system found in Coprates Catena (14&#186; 59&#8217; S; 60&#186; 15&#8217; W) [1, 2, 4]. This structure shows more than one differentiated formation stage, according to slope variations, which was supported by previous studies (e.g., [2]). We hypothesize that initially, the material was deposited over a water body, producing a flat and extensive delta. The much steeper and concealed remaining fan was produced during a later episode(s), flowing this time into a dry surface on top of the previous delta. The channel and the outflow&#8217;s surface displayed a few modifications. The tectonics related to the Coprates horst/graben system acted after the channel/fan creation, producing several shrinks and faults that alter their shape. The reconstruction of those features has to be done without overly altering the original shape of the surface in order to provide a valid input for the mechanical modeling.&#160;RESULTS
The first step was to remove the upper part of the fan and fill the channel for the starting condition for the model (Figure 1B and 1C). When it was first run, the material flowed into a shrink instead of accumulating where the fan was. Those collapses likely postdate the fan deposition, as were identified in the images (Figure 1A). In later runs, the fluid found other obstacles identified in the imagery, as landslides along the channel. When all those flow impediments were removed, a fan with a resemblance to the observed one was obtained (Figure 1D, [3]). We tested different materials, water contents, and slopes; finding that this upper part of the fan is compatible with a kaolinite mudflow, with a water content of about 53% [3].&#160;Figure 1. (A) HRSC derived topography (20m contours) of the fan (14&#186; 59&#8217; S; 60&#186; 15&#8217; W), (B) reconstructed topography previous fan formation, (C) reconstructed initial condition, and (D) result of the numerical model simulation [3].&#160;CONCLUSIONS
This workflow, through reconstruction of real data and application of a mechanical modeling, proved effective to simulate and constrain the formation of a channel-fan system on Mars. Our next step is to simulate the delta phase beneath this structure, and test if it is compatible with the deposition over a water body. This will allow us to refine this iterative process and test its capabilities for later application to other examples.ACKNOWLEDGEMENTS
A.M. is funded by the Project &#8216;MarsFirstWater&#8217;, European Research Council Consolidator Grant no. 818602. T.M. was founded by the grant JAEIntro-2020-CAB-03. The authors thank the Agencia Estatal de Investigaci&#243;n (AEI) project no. MDM-2017-0737 Unidad de Excelencia &#8216;Mar&#237;a de Maeztu&#8217;.&#160;&#160;REFERENCES
[1] Di Achille, G., Ori, G. G., Reiss, D., Hauber, E., Gwinner, K., Michael, G., &amp; Neukum, G. (2006). A steep fan at Coprates Catena, Valles Marineris, Mars, as seen by HRSC data. Geophysical research letters, 33(7).[2] Grindrod, P. M., Warner, N. H., Hobley, D. E. J., Schwartz, C., &amp; Gupta, S. (2018). Stepped fans and facies-equivalent phyllosilicates in Coprates Catena, Mars. Icarus, 307, 260-280.[3] Herreros, M.I., Molina, A., Mart&#237;nez-P&#233;rez, T.,&#160; Bacallado, A., Mata, S.,&#160; G&#243;mez, B. (2021). Alluvial fan at Coprates Catena in Valles Marineris, Mars: new modeling insights. EPSC2021-116.[4] Weitz, C. M., Irwin III, R. P., Chuang, F. C., Bourke, M. C., &amp; Crown, D. A. (2006). Formation of a terraced fan deposit in Coprates Catena, Mars. Icarus, 184(2), 436-451.[5] Pastor, M.,&#160; Herreros, I., Fern&#225;ndez Merodo, J.A.,&#160; Mira, P.,&#160; Haddad, B.,&#160; Quecedo, M., Gonz&#225;lez, E.,&#160; Alvarez-Cedr&#243;n, C.,&#160; Drempetic, V. (2009). Modelling of fast catastrophic landslides and impulse waves induced by them in fjords, lakes and reservoirs. Engineering Geology, 109(1&#8211;2): 124-134.