A billion or more years of possible periglacial/glacial cycling in Protonilus Mensae, Mars

The long-term cyclicity and temporal succession of glacial-periglacial (or deglacial) periods or epochs are keynotes of Quaternary geology on Earth. Relatively recent work has begun to explore the histories of the mid- to higher-latitudinal terrain of Mars, especially in the northern hemisphere, for evidence of similar cyclicity and succession in the Mid to Late Amazonian Epoch.

Here, we carry on with this work by focusing on Protonilus Mensae [PM] (43–490 N, 37–590 E). More specifically, we discuss, describe and evaluate an area within PM that straddles a geological contact between two ancient units: [HNt], a Noachian-Hesperian Epoch transition unit; and [eHT] an early Hesperian Epoch transition unit. Dark-toned terrain within the eHt unit (HiRISE image ESP_028457_2255) shows continuous coverage by structures akin to clastically-sorted circles [CSCs]. The latter are observed in permafrost regions on Earth where the freeze-thaw cycling of surface and/or near-surface water is commonplace and cryoturbation is not exceptional.

The crater-size frequency distribution of the dark-toned terrain suggests a minimum age of ~100 Ma and a maximum age of ~1 Ga. The age estimates of the candidate CSCs fall within this dispersion. Geochronologically, this places the candidate CSCs among the oldest periglacial landforms identified on Mars so far, by at least one and possibly two orders of magnitude.

Unit HNt is adjacent to unit eHt and shows surface material that is relatively light in tone. The coverage is topographically irregular and, at some locations, discontinuous. Amidst the light-toned surface, structures are observed that are akin to clastically non-sorted polygons [NSPs] and polygonised thermokarst-depressions on Earth. Terrestrial polygon/thermokarst assemblages occur in permafrost regions where the freeze thaw cycling of surface and/or near-surface water is commonplace and the permafrost is ice-rich. The crater-size frequency distribution of the light-toned terrain suggests a minimum age of ~10 Ma and a maximum age of ~100 Ma. The age estimates of the candidate ice-rich assemblages fall within this dispersion. Geochronologically, this places them well beyond the million-year ages associated with most of the other candidate ice-rich assemblages reported in the literature.

Stratigraphically intertwined with the two possible periglacial terrains are landforms and landscape features (observed or unobserved but modelled) that are indicative of relatively recent glaciation (~10 Ma - 100 Ma) and glaciation long past (≥ ~ 1 Ga) to decametres of depth: glacier-(cirque) like features; viscous-flow features, lobate-debris aprons; moraine-like ridges at the fore, sides and midst of the aprons; and, patches of irregularly shaped (and possibly volatile-depleted) small-sized ridge/trough assemblages. Collectively, this deeply-seated intertwining of glacial and periglacial cycles suggests that the Mid to Late Amazonian Epochs might be more Earth-like in their cold-climate geology than has been thought hitherto.