Ancient volcanism and aqueous erosion on Mars

Volcanism plays an important role in the formation and thermal evolution of crusts of all the terrestrial planets. Martian volcanoes have been extensively studied, and it has been suggested that the volcanism on Mars that created the visible volcanic features was initiated in the Noachian (>3.8 Ga), and continued to the Late Amazonian (0.1 Ga). However, ancient volcanism styles, their links with the earliest volcanic constructions and thermal evolution of the planet are little known. Here we show that numerous Early Noachian (>4.0Ga) small shield volcanoes are preserved in the heavily cratered southern highlands. All of these are central volcanoes with diameters ranging from 50 to 100 km, and elevations of 2-3 km high. These edifices are heavily cut by radial channels, suggesting that an early phase of aqueous erosion occurred and ended prior to the emplacement of the Hesperian lavas. These shield volcanoes and their morphology provide strong evidence that volcano construction and aqueous erosion were occurring during the earliest recorded period on Mars, and support an early, relatively wet phase in Martian history. The presence of numerous channels on the small shields identified in this study, together with the lack of channeling on the younger Hesperian lavas that surround them, suggests that an early phase of aqueous erosion ended prior to the emplacement of the Hesperian lavas. In addition, several of these volcanoes have numerous moderate-sized craters that post-dated the channeling, suggesting that the channels formed before the end of the heavy bombardment period of Martian history. These observations are consistent with an early, relatively wet phase in Martian history. This phase has been suggested to be related to impact-generated melting of ground ice and the generation of a short-lived, relatively thick atmosphere that may have produced precipitation and channeling. The small shield volcanoes identified in this study provide strong evidence that volcanic construction formation and aqueous erosion were occurring during the earliest recorded period on Mars.