Lakes in Valles Marineris, Mars

Introduction: The question whether the Valles Marineris harbored lakes has been debated ever since McCauley [1] proclaimed that the horizontality and lateral continuity of the interior layered deposits (ILD) suggested that they were deposited in lakes. Recent evidence, summarized here, sheds new light on this old problem. Morphology and composition: The increasingly better resolution of images and refinements in topographic data from recent missions made it apparent that the layers within the Valles Marineris are not nearly as uniform as previously thought. Some appear massive but at high resolution turn out to be finely layered [2], others are cross bedded [3] and have numerous disconformities [4] or major angular uncon-formities [5]. Some beds are competent, some weak, and some cyclical [6]. Their thickness and lateral continuity vary widely. Most, but not all beds are light-toned. Many ILDs occur in free-standing mesas or mounds separated from the walls by " moats " [7]. In many places the mounds have flat-lying beds in the center but beds that conform to the local slope on the outside [2, 6,8,9,10]. All of these characteristics have to be accounted for if the ILD were deposited in lakes. In addition, most ILD mounds contain hydrated sulfates [11,12,13] and ferric oxides in the form of crystalline grey hematite [4,14] and fine-grained iron oxides [15] mostly at the base of ILD scarps [13,16]. CRISM data show that different types of hydrated sulfates are specific to individual layers within the ILD, suggesting different circumstances during deposition or later alteration along selected beds [17]. The prevalence of hydrated sulfates and ferric oxides indicates the involvement of water either during or after em-placement of the ILD. Are the ILD exhumed from the walls? The idea that the ILD are ancient Noachian lake deposits later exhumed from the Valles Marineris walls [18] was revived after the MGS mission showed that layered terrain was ubiquitous on Mars and mostly formed in ancient craters [19]. Light-toned patches are indeed seen in several places in the walls, and therefore a number of authors picked up on Malin and Ed-gett's [19] contention [20,21,22,23]. However, the recent discovery of light-toned layered deposits on top of the plateaus near the Valles Marineris [24,25,26], and the recognition that these deposits are different in morphology as well as mineralogy from the ILD inside the troughs (they contain hydrated silica) [27,22] makes it possible that light patches in the walls belong …