Silica deposits in the Nili Patera caldera on the Syrtis Major volcanic complex on Mars

The martian surface features abundant volcanoes and evidence for past liquid water. Extant or relict martian volcanic hydrothermal systems have therefore been sought in the pursuit of evidence for habitable environments1. The Mars Exploration Rover, Spirit, detected deposits highly enriched in silica with accessory minerals, suggesting formation by hydrothermal leaching of basaltic rocks by low-pH solutions2. However, extensive erosion has obscured the context of the formation environment of these deposits. Silica deposits have also been identified remotely, but also with limited contextual clues to their formation; aqueous alteration products of basalt and volcanic ash are the most likely sources3,4. Here we report the detection from orbit of hydrated silica deposits on the flanks of a volcanic cone in the martian Syrtis Major caldera complex. Near-infrared observations show dozens of localized hydrated silica deposits. As a result of the morphology of these deposits and their location in and around the cone summit, we suggest that the deposits were produced by a volcanically driven hydrothermal system. The cone and associated lava flows post-date Early Hesperian volcano formation. We conclude that, if a relict hydrothermal system was associated with the silica deposits, it may preserve one of the most recent habitable microenvironments on Mars. The Syrtis Major volcanic complex is a type example of Hesperian-aged plains volcanism onMars. Crater statistics indicate that the complex was formed in the Early Hesperian period with major resurfacing completed by ∼3.7Gyr ago5. Orbital observations at thermal infrared and visible and near-infrared wavelengths constrain the modal mineralogy and show the rocks to be consistent with an olivine-bearing basaltic lava enriched in high-calcium pyroxene6,7. Near the centre of Syrtis Major are two named calderas, Nili Patera and Meroe Patera, at the northern and southern ends of an elongated depression that may result from multiple episodes of caldera formation5,8. The Nili Patera caldera (Fig. 1) hosts a 100-m-high degraded cone on the northeastern side of its floor, and a related lighter-toned lava flow to the west of the cone. The Nili Patera volcanic cone (Fig. 2) is unique in its region and is the only local late-stage volcanic feature with significant topography, possibly indicating a relatively viscous lava composition. Thermal infrared spectral properties of the flow have been interpreted as due either to a glassy dacitic composition or possibly a high-Si glass alteration product9. The west flank of the cone exhibits amoderate slopewith surface streaks probably formed by downslope movement of unconsolidated material. The east flank seems to have been removed, possibly by explosive eruptive processes or extensive erosion. Recent observations of the volcanic cone and associated lava flow by the Compact Reconnaissance Imaging Spectrometer