Overview of Possible Ice-related Morphologies in the Transition Zone between Elysium and Utopia

Introduction: The transition zone between Elysium and Utopia is a highly complex area, characterized by Galaxias Chaos along the northern boundary and huge outflow channels dominating the northwestern part. Several very enigmatic morphologies are observed including some very distinctive crater morphologies [1], having bipartite crater ejecta consisting of an inner pattern of breached folds close to the crater rim, while the ejecta further away seems smoother, bulging and slightly elevated. Moreover, ring mold-like craters (RMC) are observed in many areas [2] and thermally distinct craters have also been reported [3] and both have been related to existence of ice. Therefore we characterize the different types of craters distributed within the area, with special emphasis on the craters showing bipartite and excess ejecta [21, 22]. Moreover we evaluate their distribution and compile an overview in connection with other landforms, which have been ascribed to the presence of ice. Background and Geologic Setting: Observations of landforms in the research area span from volcanic (lava flows, sills and dike emplacement [4-6]), outflow channels [5,7], glacial features such as eskers and thumbprint terrain [8], and ice-volcano interactions such as volcaniclastic flows, mega-lahars and subglacial volcanic edifices [3,5,9-16]. Several authors have found evidence for existence of H 2 O in the region based on crater ejecta studies [17,21,22] and Mars Odyssey Neutron Spectrometer results [18]. Furthermore, the Vasti-tas Borealis Formation, which has been considered to be a residue from an H 2 O-rich material deposited by huge floods, is situated in the study area near the proposed shoreline, contact 2 [19, 20]. Thus, using HiRISE, CTX, MOC and THEMIS data we here evaluate enigmatic morphologies investigating whether they can be explained by ice-related processes. Observed Morphologies: Within the research area several different crater morphologies are observed. The biggest and most prominent craters are the craters having bipartite crater ejecta, which have a diameter up to 25 km. The unusual crater ejecta is divided into an inner ejecta facies consisting of breached folds and an outer ejecta facies, which is smooth, has a tongue-like outline and is slightly elevated with respect to the inner ejecta; different examples can be seen in Fig. 1. B-F. Especially Fig. 1E & F, shows the abrupt change in diameter, which seems to be closely related to the distance from the crater rim. Within the outer ejecta, circular knobby features are observed having similar characteristics to