An assessment of evidence for pingos on Mars using HiRISE

Pingos are small hills with cores of ice, formed by injection and freezing of pressurized water. The possibility of pingos on Mars is of particular interest because of the associated implications for liquid water. We have systematically searched for candidate pingos using images from the High Resolution Imaging Science Experiment (HiRISE) camera. Since pingos are expected to develop surface fractures due to extension of the frozen ground over the ice core, we have searched for fractured features and identified a variety of mounds. These features are confined to the martian mid-latitudes, in the bands where gullies are also most common. The observed fractured mounds have a variety of morphologies and are likely of multiple origins. Isolated fractured mounds found on the floors of gullied craters in the southern hemisphere match the general morphologic characteristics of terrestrial pingos and are the best candidates for martian pingos, but there is currently no direct evidence for presence of ice cores and it is difficult to produce the necessary water volumes, so these features should still be interpreted with caution. Other fractured mounds appear more likely to be erosional remnants of an unusual mantling layer or possibly thermokarst structures. Flat-topped mounds in Utopia have some characteristics (fracture pattern and latitudinal distribution) consistent with pingos but differ in other aspects such as shape and setting. While we do not rule out a pingo origin, we prefer an erosional model for these enigmatic features.