Candidate subglacial palaeolake deposits associated with Amazonian-aged debris-covered glaciers and eskers in the mid-latitudes of Mars

Introduction

Mars’ Amazonian period (3 Gyr to present) is thought to have been cold and hyperarid, with a limited role of liquid water in modifying the surface. Mars’ mid-latitude regions host thousands of buried glaciers (termed ‘viscous flow features’), which formed within the last 10s-100s Myr, during the mid-to-late Amazonian period [e.g., 1]. These glaciers are thought to have been deposited by snowfall during Martian ‘ice ages’ driven by cyclical variations in Mars’ orbital obliquity. It has long been assumed that the glaciers have remained predominantly cold-based under cold Amazonian climate conditions. A small number of eskers (ridges of glaciofluvial sediment deposited in ice-confined meltwater tunnels) extending from glaciers in the Phlegra Montes and Tempe Terra regions [e.g., 2-4] have been attributed to rare, spatially restricted subglacial melting of their parent glaciers, driven by transient, localized geothermal heating events. This is consistent with their locations within glaciated tectonic grabens in major volcano-tectonic provinces.

Observations and interpretations

Here, we present two key advances based on observations from orbital image and elevation datasets [5]. First, we identify three additional eskers associated with glaciers in Tempe Terra, which, when combined with three identified by previous studies [3,4], indicate regionally widespread occurrences of Amazonian subglacial melting distributed along a 650 km long mountain chain in Mars’ northern mid-latitudes.

Second, two of the newly identified eskers in Tempe Terra are associated with a landsystem consistent with meltwater drainage into a subglacial palaeolake (37.44°N, 86.42°W). The landsystem comprises two major viscous flow features (debris-covered glaciers), two landforms interpreted as eskers, an esker-terminal fan, subglacial channels, a medial moraine, and a quasi-flat-topped deposit interpreted as glaciofluvial sediments deposited in the subglacial lake cavity [5]. It also contains landforms consistent with postglacial erosion and slumping. Impact crater size-frequency distributions on the surfaces of the two major debris-covered glaciers return a minimum age of 130 Myr (late Amazonian). Considering uncertainties, and that the glaciers have retreated to expose the eskers and subglacial palaeolake deposits, we cautiously estimate that esker formation and meltwater drainage into the palaeolake occurred ~130 Myr to 1 Ga (mid-to-late Amazonian) [5].

Implications

The implications of our observations are twofold. First, evidence for at least six occurrences of subglacial melting evidenced by eskers distributed along a 650 km-long mountain chain in Tempe Terra [3-5] likely necessitates a much more spatially extensive heat source than previously expected (e.g., a geothermal hotspot and/or regional climate change), within the last 100s Myr to 1 Gyr.

Second, the landsystem of which two of these eskers are a part implies that formerly thicker and more extensive Amazonian mid-latitude ice masses on Mars were capable of supporting subglacial lakes. Glaciers in Mars’ mid latitudes are prime targets for next-generation missions aiming to search for life [e.g. 6]. Subglacial palaeolake deposits proximal to these glaciers, which have been exposed by ice retreat, would represent high-priority science targets for such missions. Subglacial lakes on Earth support microbial ecosystems under extreme conditions [7], and palaeolake deposits have high potential for preserving biosignatures in the geologic record [8]. Hence, our discovery should motivate an extensive search for glacial palaeolake deposits across Mars’ mid latitudes as an important new class of exploration targets.

References

[1] Levy et al. 2014. Geophys. Res. Lett

[2] Gallagher and Balme 2015 Earth Planet Sci. Lett.

[3] Butcher et al. 2017 JGR Planets.

[4] Woodley et al. 2022 Icarus.

[5] Butcher et al. 2026 In Review.

[6] National Academies of Science Engineering and Medicine 2023 Origins, Worlds, and Life: Planetary Science and Astrobiology in the Next Decade

[7] Christner et al. 2014 Nature

[8] Summons et al. 2017 Astrobiology