Another Look at the Summit Caldera of Apollinaris Patera

Introduction: Highland paterae [1-2] represent one of the most distinctive landforms on the Martian surface. The term patera (paterae if plural) was originally applied to any large, flat-floored depression with irregular margins identified in Mariner 9 and Viking imagery and included volcanoes as well as impact craters [3]. However, highland paterae represent an individual class of patera that is volcanic in origin and concentrated mostly around the Hellas basin region [1-3]. This class of patera includes Tyrrhena, Hadriaca, Amphitrites, and Peneus Paterae and is characterized by very low slopes of <1° to ~1.5° that are deeply in-cised by channel complexes which are radial to a prominent central caldera [3-5]. The similar morphologies and characteristics of these four paterae suggest a common origin that, based on the presence of abundant channels that cut into an apparently easily erodible surface, may have included explosive activity with effusive eruptions likely occurring later in their eruptive history [4-5]. They are among the oldest central vent volcanoes on Mars [6-7] and may represent a transition from flood lava eruptions, which dominated the early volcanic history of Mars, to more localized eruptions [4]. Here, I describe a recently started project that addresses the processes involved in the volcanic evolution of Martian highland paterae. These processes include the style(s) of volcanic eruptions, processes involved in caldera formation, and the construction and subsequent erosion of the edifice. Specifically here, I focus on Apollinaris Patera. Although Apolli-naris Patera, which is located along the northern low-land-southern highland transition, was not originally classified as a highland patera [i.e., 3; see also, 8], it contains many similar morphologic aspects including a large central caldera and extensively channeled and dissected flanks [8]. Thus, it may have an origin similar to Tyrrhena, Hadriaca, Amphitrites, and Peneus Paterae. Motivation: The broad, low-sloped, heavily chan-neled flanks of highland paterae make them an intriguing class of central vent volcanoes on Mars [i.e., 3]. Based on crater modeled ages of ~3.5 Ga [9-10], they also represent some of the oldest recognized central vent volcanoes on the planet. Therefore, studying highland paterae is critical because they further our understanding of Martian volcanic systems by providing a glimpse into volcanic processes on early Mars. In addition, because the constructs have existed for a majority of the planet's history, they undoubtedly must