Establishing Criteria for Identifying Ancient Perennially Ice-covered Lakes in the Sedimentary Record of Mars

With the discovery and analysis of lacustrine sedimentary deposits on Mars, the apparent duration of an early “warm” Mars is challenging climate models. However, there are different implications for early Mars climate if... [ view full abstract ]

With the discovery and analysis of lacustrine sedimentary deposits on Mars, the apparent duration of an early “warm” Mars is challenging climate models. However, there are different implications for early Mars climate if the martian lakes were perennially ice-covered. Unfortunately, diagnostic criteria do not yet exist to distinguish the deposits of perennially ice-covered lakes (PICLs) from those of other lakes in the rock record, and thus the presence of an ice cover cannot be reliably identified. Unlike open water lakes that are dominated by the accumulation of mud-sized sediment and dropstones or tills if icebergs are present, PICL deposits can include a distinctive distribution of coarser grains. These large grains are transported onto the ice by wind or water, and then pass through the ice into otherwise mud-sized sediments. In addition, sand can form characteristic deposits such as sand mounds and ridges.
As a first step toward developing more complete criteria for identifying PICLs in the rock record, we have characterized sedimentary deposits from PICLs in the McMurdo Dry Valleys, Antarctica. Sediment cores acquired from lakes Joyce and Vanda were coupled with field observations and images taken by SCUBA divers for context to interpret the samples. In addition, sediment on the ice cover of Lake Joyce was sampled and compared to the sediments in the cores.
Sediments in PICLs reflect processes occurring in and on the ice cover. Once on the ice, sediment absorbs solar radiation, and melts surrounding ice. Our observations suggest that both sediment size and albedo (i.e., composition) affect grain/ice interactions. In addition, ice cover thickness may determine the grain size and spatial distribution of the sediment deposited in PICLs. Coupled, these processes may produce sedimentary deposits diagnostic for perennial ice cover. For example, spatially restricted pockets of coarse sediments appear to be deposited in PICLs with rough ice covers > ~ 3 m thick (e.g., lakes Joyce), whereas lakes with thinner ice covers have disseminated grains (e.g., Lake Vanda).
At Gale Crater, Mars, Murray formation sandy mudstones are interpreted as lacustrine but the depositional process for the sand is unknown. From our analysis of modern PICLs, ice cover can promote sand deposition in lacustrine muds. If this ancient lake on Mars had a perennial ice-cover, even if intermittently, it could explain these deposits. The Murray formation is an ideal target to start analyzing for evidence of ancient PICL deposits on Mars.