Emanuele Baratti

We suggest the landing of ExoMars in Oxia Planum between 16° and 19° of latitude north and-23° to-28° of eastern longitude, and below-2 km of MOLA elevation. This region exhibits one of the widest Mg/Fe phyllosilicates exposures as mapped globally with OMEGA and with CRISM multispectral data. The outcrop of Mg/Fe phyllosilicates is so wide that several potential landing ellipses (19 x 110 km) fitting the engineering constraints is possible. The exposed terrains are 4 Ga old (Hartmann's age system) and have undergone intense erosional processes until 3.6 Ga. The region also reveals fluvial related morphologies such as valleys and a delta fan attesting the water-related history of this region. Moreover, the region is current under erosion so that the exposition age of the fresher phyllosilicate rich surfaces is younger than 100 My attesting the potential preservation of putative biosignature. This proposed site fulfills ExoMars objectives.

One of the most interesting morphological feature on the surface of Mars are paleolakes: time capsules and low-energy depositional settings that preserve hints on the ancient environment and, possibly, traces of life. On Earth, lakes host a huge diversity of habitats where life is flourishing; hence, on Mars, they may have once supported life too. In order to understand the paleohydrology of these basins, the estimation of the water discharge flowing into their tributary and/or outlets can give important clues on the water cycle on the planet. On Mars, the most common modeling used to reconstruct the hydraulic properties are based on the adaptation of terrestrial empirical relationship, less focusing on more sophisticated hydraulic models. The main objective of this study is to separately analyze the hydraulic characteristics of the tributary and the outlet of a paleolake located in the Menmonia quadrangle by using an hydraulic model based on the energy and momentum equations of the body of water enclosed in the channels. The lengths of the analyzed river reaches are ∼ 10 km and ∼ 19 km, the outlet and the tributary respectively. A Monte Carlo procedure coupled to geomorphological evidences of the paleowater surface (e.g. terraces levels) were used to constrain the hydraulic parameters of the system. The Mars Express High-resolution stereo camera digital elevation model, H31850000DA4 with a spatial resolution on 75 m, is hence used to characterize the geometry of the study area. In this contribution we provide estimates on the magnitude of the bankfull discharge and roughness coefficient of the tributary and the outlet of the paleolake; moreover, on the basis of these results, the probable water depth of the lake is inferred. Finally, a discussion on the possibility that the system inlet-lake-outlet was acting simultaneously is also presented. Consequently, a sensitivity analysis is performed to examine the accuracy of the results to the main sources of uncertainty concerning the methodology, as well as, the uncertainty in the geomorphological evidences used to constrain the hydraulic modelling. Given the comparable discharge estimates of the tributary and the outlet, as well as the coherent elevation and width of their mouths into the lake, the results suggest that both channels were most probably acting simultaneously during their last evolutionary phase, with a discharge equal to ∼ 6000 m 3 s −1 and a water surface elevation in the lake equal to ∼-1400 m. This contribution supports the hypothesis of an ancient and articulated hydrologic system acting in the study area.

This contribution presents the results of a detailed hydraulic study of the tributary and the outlet of a Martian paleolake located in the Memnonia quadrangle between 167 • 0'0 " W and 167 • 20'0 " W longitude and between 9 • 25'0 " S and 9 • 45'0 " S latitude [1] (Fig. 1). We used an hydraulic model capable of performing one-dimensional water surface profile calculations for steady gradually varied flow in natural channels adapted to the Martian conditions (e.g. martian gravity equal to 0.38 Earth's) [2]. Geomorphic evidences, i.e. fluvial terraces, were used to identify the probable bankfull level of the tributary and the outlet [2,3,4]. The identified terracing levels were used to constrain the past water discharge flowing on the surface, the Manning's roughness coefficient of the channels and the water level of the paleolake. The Mars Express high-resolution stereo camera digital elevation model, HRSC DEM, H31850000DA4, presents a spatial resolution of 75 m and it was used to characterize the geometry of the channels, i.e. their cross sections and the fluvial terraces elevation. The outlet and the tributary reaches are 10.0 km-long and 22.2 km-long, respectively. Fifty-one cross-sections along the outlet were extracted from the DEM and were used to characterize the geometry of this river-reach in the hydraulic model, whereas the hydraulic model of the tributary is composed of 111 cross sections. The mean distance between cross sections is ∆x = 200 m. Fig 1 shows the upper and the lower cross sections for both the outlet and the tributary studied through the above mentioned hydraulic model. Note that the water surface profiles are computed from one cross section to the following one by solving the Energy equation through an iterative procedure called the standard step method [5]; where the energy equation is not considered applicable, we computed the wa-Figure 1: Context map of the case study area showing HRSC H3185000DA4 DEM elevation values in meters , the latitude and longitude grid and a scale bar reference. The upper and lower cross section of the outlet (red) and the tributary (white) are marked.

Liquid water was flowing on the surface of Mars in the past, leaving behind a wide range of geomorphic features. The ancient major Martian water fluxes vanished about 3.5 Ga. Meteoritic impacts, wind-erosion, gravity-related phenomena, tectonic deformations and volcanic activities deeply altered the landforms during the ages. Hence, the reconstruction of water-shaped landscapes is often complicated. Fluvial and lacustrine terraces analysis and correlation is a useful approach to understand and reconstruct the past changes in Martian landscape evolution. These features are commonly used as reference for the top of water bodies on Earth, since they are void of the uncertainties or errors deriving from erosional or slumping processes that could have acted on the valley flanks or in the plateau, where the hydrological network was carved in. The study area is located in the western hemisphere of Mars, in the Memnonia quadrangle, between latitude 9˚10'-9˚50'South and longitude 167˚0'-167˚30' West and it constitutes a transition region between the southern highlands of Terra Sirenum and the northern lowlands of Lucus Planum. Many water-shaped features have already been described near the study area, the most prominent of them being the Ma'adim Vallis and the Mangala Valles system. Our results derive from the observations and the analysis of HRSC images (12.5 m spatial resolution) and Digital Elevation Models (DEMs) derived from the MEX-HRSC (75 m resolution), that allow the identification of elevation differences up to the tens of meter scale. We were able to reconstruct six main evolutionary stages of a complex hydrologic systems consisting of two main palaeorivers (up to 5 km wide) connected one another by a palaeolake that formed within a meteor crater (∼20 km diameter). On the basis of Earth analogs, these stages/terraces should have evolved during a long period of time, at least thousands years long. Furthermore, crater counting date back the deactivation of the system to ca 3.5±0.1 Ga ago, suggesting the presence of a stable environment with subaerial water fluxes during the Late Hesperian, very close to the liquid-water disappearance. Apart from the above mentioned reasons, the increasing interest and ongoing programs of on-site Martian exploration are additional reasons to study fluviolacustrine depositional environments. Together with the technology improvements that lead to more flexible safety constraints for landing/exploring, the possibility to focus on specific and more detailed scientific aspects is enhanced.