Frost Heave on Mars: Rate-limiting boundary conditions and the role of salts

Science Motivation: Observations of excess ice (ice that exceeds the pore volume of its host soil) in the shallow subsurface of Mars are a persistent challenge to understanding the physical processes that delivered ground ice to its current location. Three primary hypotheses have been proposed to explain the origin of excess ice. The first hypothesis contends that excess ice was deposited as extended snow packs during recent high obliquity periods and persists today under a shallow sublimation lag. The second hypothesis contends that thermal contraction cracking produces enhanced diffusion and ice deposition that gradually inflates the regolith on 10^6 year timescales. The third hypothesis contends that excess ice forms via in situ segregation, the process that produces ice lenses and frost heave on Earth. In reality, all three processes may have contributed to the observed excess ice in a way that is geographically and temporally complex. Further, all three hypotheses face challenges in that they must be reconciled with a complex and growing set of observations of excess ice.

Objectives: The purpose of our proposed work effort is to test the hypothesis that in situ segregation and resultant frost heave have contributed substantially to observed excess ice in the upper meter of the Martian regolith. We will focus on numerical simulations of the development of ice lenses, with the goal of estimating the volume of excess ice that can be produced in this form during the past 1-4 Ma of Martian history in two specific regions where the origin of excess ice is debated: the Phoenix landing site and mid-latitude fresh crater sites in Utopia Planitia. Observations of excess ice in these locations are distinct from one another. Assessing the contribution of frost heave at each location will shed light on the relative importance of all three hypotheses for excess ice production in different geographic regions of Mars.

Uniqueness: For the first time, a self-consistent treatment of premelting in a salty regolith will be included in a numerical model of Martian frost heave, and the model will be validated against new theoretical and empirical data. Additionally, self-consistent simulations of frost heave and vapor diffusion will be used to place new constraints on Martian frost heave rates.

Approach: (1) Modify an existing model of Martian frost heave to include physics that was neglected in previous work. (2) Validate model inputs and results against new Monte Carlo simulations of soil freezing and new empirical data. (3) Carry out self-consistent simulations of frost heave and water vapor diffusion to determine which process is rate limiting at specific times and locations. (4) Assess whether frost heave alone can produced observed excess ice and/or protect deep ice in two regions.

Relevance: Our proposal is directly relevant to the SSW Program, as it will develop a theoretical basis for understanding the evolution and modification of the Martian surface and seeks to quantify fluid inventories that interact with the surface.