Juvenile Chemical Sediments and the Duration of Aqueous Activity on Ancient

Introduction: Recent orbital and landed exploration has painted a complex geochemical picture of the earliest martian surface. It is clear that that water was at times present on early Mars, but it is the chemistry and persistence of that water that influences habitabil-ity [1]. Chemically benign conditions appear to surround carbonate and clay-bearing assemblages, while saline minerals reflect a harsher chemical environment before liquid surface water was permanently lost [2,3]. Aside from chemistry, even more basic is the requirement that liquid water persist long enough to mediate any first chemical steps toward life. But despite the seemingly disparate chemical environments indicated by aqueous minerals, almost all of the aqueous mineralogy identified on Mars reflects the consequences of an ancient surface that saw liquid water in episodes that lasted for only a geologic instant. Characterizing the habitability of the early martian waters is now as much a question of timing and duration as it is of chemistry. Here we discuss mineralogical evidence for geologically brief episodes of liquid water on early Mars and a general lack of diagenetic maturation after initial deposition. This evidence is consistent with recent studies of valley networks, drainage basins and impact crater degradation, all of which point to transient rather than persistent liquid water on early Mars [4-7]. Such a martian surface may present an additional challenge for biology, but helps reconcile a number of difficulties associated with a protracted climate where liquid water persisted for significant geologic time. Opaline silica and SiO 2 diagenesis: Silica is a common product when water interacts with mafic mineralogy [8], and there are numerous examples of its occurrence [8-11]. With time and available water, sil-ica diagenesis progresses through opal-A, opal-CT, and finally, microcrystalline quartz [12-14]. Despite numerous examples of hydrated silica on Mars, there are no detections of quartz (the TES instrument should detect quartz at a level ≥ 5% [15]). One exception is the orbital detection of quartzo-feldspathic materials thought to be a result of igneous differentiation [16]. At the molecular level, liquid water drives the transformation of initially precipitated silica to less soluble polymorphs by dissolution-reprecipitation. This mechanism is supported by textural relationships, 18 O/ 16 O ratios, and thermodynamic and kinetic evidence [12-14]. For example, Williams et al. [12] have shown that the thermodynamic driving force behind the maturation of amorphous silica is the effect of sur