Cold as ice, red as Mars: Polythermal glaciation and implications for surface composition as a record of past climate

Introduction: The surface of Mars exhibits clear geomorphic evidence for recent and ongoing glacial and periglacial processes, but the ancient history of ice on Mars is not well understood [1,2]. Most glaciers and ice sheets on Mars are predicted to have experienced little or transient melting at their base, and because melt is a major driver of erosion and transport in a glacier, these cold-based and/or polythermal glaciers may not have left behind clear physical signs in the geologic record. Thus, while abundant evidence exists for at least transient warmer and wetter conditions on ancient Mars, we lack a clear record of the duration and extent of ice-dominated climates. Here we suggest that an alternative signature of past glaciation on Mars may be the geochemical record, due to alteration by interactions with cold-based glaciers. However, the mineralogy of cold-based glacial alteration products is poorly constrained.

Recent work on temperate glacial alteration products of Mars-relevant bedrock shows silica cycling is the dominant alteration process throughout the glacial system, resulting in significant amounts of silica dissolution and precipitation [3], but that warm-based ice may not be the best analog to hypothesized cold-based Mars glaciers. Thus, polythermal glaciers, which are combinations of temperate and cold ice, are key sites to help constrain glacially driven alteration products in a Mars-related setting.

In this study, we analyze glacial sediment, bedrock, and water samples from a Mars-relevant polythermal glacier system overlying mafic bedrock to better constrain alteration and resulting alteration products. We hypothesize that interactions with these glaciers will result in distinct alteration signatures (i.e. mineral assemblages) which can be used to identify evidence of past polythermal or cold-based glacial activity under past climates on Mars.

Field Site:  Storglaciӓren is an Arctic, polythermal glacier located in the Kebnekaise Mountains of Sweden. Storglaciӓren was chosen as it overlies Mars-like, iron-rich, mafic bedrock [4-6]. The underlying bedrock is primarily metamorphosed doleritic and mafic dykes, hornblende-rich amphibolites, and mylonitic gneiss [4,5]. The field site also exhibits relatively low water-rock ratios approaching hypothesized conditions on past Mars.

Methods: Rocks, sediment and water samples were collected from the glacial margin and outwash plain of Storglaciӓren. In situ and laboratory visible/near-infrared spectra (VNIR; 0.3-2.5 μm) were collected to determine the mineralogy of glacially altered sediments. Spectrally dominant minerals were identified based on comparison to spectral libraries [7]. A UAV was also used to collect multispectral aerial imagery of the study site.

Results: VNIR spectra of the glacial sediments (Fig. 2) are dominated by a strong triplet band near 2.32 μm, the strength of which is correlated with a broad band at ~1.2 μm. Both are consistent with chlorite, a phyllosilicate that is a significant component of the mafic metamorphic rocks in the region. Additionally, fine glacial sediments also show an additional strong shoulder on this band near 2.21 μm that is consistent with hydrated silica. This signature is strongest (and the chlorite signatures are weakest) in sediments collected from moraine seeps where cold-based ice is melting in situ. Fresh proglacial sediments from the warm-based portion of the glacier also show moderate chlorite and silica signatures, along with a strong red slope that we hypothesize may be related to subglacial oxidation.

Aqueous geochemistry results indicate that chemical weathering of the bedrock is likely driven by a combination of both carbonic acid (H₂CO₃) and sulfuric acid (H₂SO₄), resulting in major cations (e.g., calcium, magnesium), silica, and iron being released from subglacial sediment into the meltwater.

Discussion: We hypothesize that amorphous silicates and opaline silica are precipitating from dissolved SiO₂ during freeze/thaw cycles [3] and that acidic weathering is driving heterogeneous oxidation in the subglacial environment [3]. Initial results indicate that the former appears to be more effective in the more stable, cold-based margins of the glacier where residence times of sediment are longer, while the latter is more prevalent in the warm-based portions dominated by subglacial meltwater, glacial sliding, and seasonal flushing of sediment and water. These results suggest that there may be differences in weathering processes between cold- and warm-based glaciers and that these differences may be detectable in relict glacial terrains and deposits on Mars [8].

References: [1] Souness C. et al. (2011) Icarus, 217, 243-55. [2] Fastook, J.L., and J.W. Head. (2015) Planetary and Space Science, 106, 82-98. [3] Rutledge, A.M. et al. (2018) Geophysical Research Letters, 45, 7371–7381. [4] Baird, G.B. (2010) [5] Andreasson P. and D.G. Gee (1989) Geografiska Annaler. Series A, Physical Geography, 71, 235-239. [6] Holmlund P. and P. Jansson (2002). [7] Kokaly, R.F. et al. (2017) USGS Data Series 1035, 61p. [8] Havig, J.R. and T.L. Hamilton (2019) Geochimica et Cosmochimica Acta, 247, 220-242.

Acknowledgement: This work was supported by NASA SSW# 80NSSC20K1236.