Investigating the formation conditions of glacier-like forms using Bayesian inversion.

Glacier-like forms (GLFs) are one subtype of glacial features found on the Martian surface. They are located within the mid-latitudes of Mars (30-60 degrees) in both hemispheres. These features having formed within the Amazonian period during a period of higher obliquity than Mars' is at today which allowed for the preferential accumulation of icy material in the mid-latitudes. While previous studies have investigated the geographic controls on GLF formation, their former extent, and their former dynamics (Souness, et. al., 2012; Brough, et. al. 2016, 2019), the boundary conditions under which GLFs formed remain poorly constrained, particularly on a local-scale.

Our primary aim is to improve our understanding of how Martian GLFs formed and evolved with respect to their climactic and geomorphological setting using terrestrial rock glaciers as analogues. As there is still ongoing debate as to the formation dynamics of rock glaciers on Earth, be they permafrost-derived or derived from debris-covered glaciers, with the issue being that both start points can adequately describe the end-state of palaeo rock glaciers, we need to take an approach which acknowledges this issue of equifinality. Bayesian inversion is one such method that can do this. We start with the assumption that these GLFs represent permafrost-derived ice bodies where ground-temperature is a key boundary-condition for their formation. With this method, we use observed glacier geomorphology to reconstruct the former extent, volume, and thickness of the GLF to compute a posterior probability distribution for ground temperatures that are physically consistent with the reconstructed geometry of the palaeo glacier. We also consider near-surface air temperature as a secondary factor in accumulation feasibility. 

Here we present our ongoing work in this effort. We manually demarcated the geomorphological constraints of multiple GLFs on Mars within GIS software based on identifiable geomorphology within the orthorectified imagery that mark the former maximum extent of the glacier, and extract morphometric data using the georeferenced HiRISE DEM. We then used the perfect-plasticity approximation to reconstruct palaeo ice-thicknesses and volume of the palaeo glacier. These morphometrics are then compared with modelled outputs for glacier deformation, employing Bayesian logic to constrain a boundary range of long-term mean ground temperature that would be compatible to produce the reconstructed glacier morphology. We also investigate several terrestrial rock glaciers in order to assess the accuracy and validity of our approach against measurable analogue examples, which further enables us to compare the dynamics of terrestrial and Martian glaciers.

References:

Brough, Stephen, Bryn Hubbard, and Alun Hubbard. 2016. “Former Extent of Glacier-Like Forms on Mars.”, Icarus 274 (August): 37–49. https://doi.org/10.1016/j.icarus.2016.03.006.

Brough, S., Hubbard, B., & Hubbard, A. (2019, 02). Area and volume of mid latitude glacier-like forms on mars. Earth and Planetary Science Letters, 507 , 10–20. Retrieved from https://linkinghub.elsevier.com/retrieve/pii/S0012821X18306903 doi: 10.1016/j.epsl.2018.11.031

Souness, Colin, Bryn Hubbard, Ralph E. Milliken, and Duncan Quincey. 2012. “An Inventory and Population-Scale Analysis of Martian Glacier-Like Forms.” Icarus 217 (1): 243–55. https://doi.org/10.1016/j.icarus.2011.10.020.