THE NATURE AND DISTRIBUTION OF POLLYWOG CRATERS ON MARS

The working paradigm of the hydrologic evolution of Mars over the past several decades has envisioned a sudden decline in fluvial activity at the Noachian-Hesperian boundary, and the Hesperian and Amazonian epochs were generally considered less favorable for fluvial activity other than occasional hydrothermal runoff and outburst floods. Analyses of Mars Reconnaissance Orbiter (MRO)-era data, largely due to the increased resolution and (or) global coverage by the High Resolution Imaging Science Experiment (HiRISE, ~0.25 m/pixel) and Context Camera (CTX, nearly global coverage at ~6 m per pixel), have contributed to a growing inventory of well-preserved, post-Noachian fluvial landforms that are challenging the paradigm that the climate in the Hesperian and Amazonian were sub-optimal for widespread aqueous activity. These landforms include 1) mid-latitude and equatorial fluvial valleys that are smaller, and presumably younger, than the classic late Noachian valley networks, 2) alluvial fans and deltas, and 3) small craters with one or more exit breach and an exterior rim channel (“pollywogs”).

The informally named “pollywog” craters have breached rims and 1 or more exit channels(s). These valleys are consistent with outlets, rather than inlets, because they do not incise to the level of the interior crater floor. Furthermore, there are no associated material(s) (e.g., fan or lobe-shaped deposits) on the interior crater floor that would be consistent with flow into the crater. This newly identified landform tends to occur in association with fresh, shallow valleys, yet they occasionally occur as isolated features. The morphology of pollywog craters implies overflow of a deep, possibly ice-covered lake within the crater. Although the source of water that caused the host crater to fill, overflow, and form the valley is unknown, it is either related to surface runoff (e.g., water and (or) ice fills crater until it overflows and forms channel), groundwater upwelling, or some combination of both. We expand the inventory of pollywog craters beyond the study in northern Arabia Terra by Wilson et al. (2016), and look at variables such as elevation, slope, and crater size, to help constrain the possible source(s) of water and formation mechanisms.