Preservation of Midlatitude Ice Sheets on Mars

Abstract

Excess ice with a minimum age of tens of millions of years is widespread in Arcadia Planitia on Mars, and a similar deposit has been found in Utopia Planitia. The conditions that led to the formation and preservation of these midlatitude ice sheets hold clues to past climate and subsurface structure on Mars. We simulate the thermal stability and retreat of buried excess ice sheets over 21 Myr of Martian orbital solutions and find that the ice sheets can be orders of magnitude older than the obliquity cycles that are typically thought to drive midlatitude ice deposition and sublimation. Retreat of this ice in the last 4 Myr could have contributed ~6% of the volume of the north polar layered deposits (NPLD) and more than 10% if the NPLD are older than 4 Myr. Matching the measured dielectric constants of the Arcadia and Utopia Planitia deposits requires ice porosities of ~25–35%. We model geothermally driven vapor migration through porous ice under Martian temperatures and find that Martian firn may be able to maintain porosity for timescales longer than we predict for retreat of the ice.

A Migration Model for the Polar Spiral Troughs of Mars

“…Within this lag deposit, we allow for the resupply of ice into the pore spaces. We assume that the pore‐filling ice is always in equilibrium with the atmosphere and that pore spaces at and below the equilibrium depth are always completely filled with ice (Bramson et al, ; Schorghofer, ). Experiments on the diffusivity of Mars regolith simulants support these assumptions (Hudson & Aharonson, ; Hudson et al, ).…”

“…d is the volumetric fraction of dust in the ice (which we assume to be 3%, consistent with Grima et al, ), and ρ ice is the density of pure ice (taken to be 920 kg/m 3 ). Additional details about the thermal model and equations for diffusive ice loss calculations can be found in Bramson et al (). …”

“…We have also made updates to the thermal model presented in Bramson et al () to include the following additional three effects: Extinction due to attenuation from the atmosphere (which matters more at poles because of the higher incidence angles). We multiply our top‐of‐atmosphere solar flux by a term described in equation 2 in Schorghofer and Edgett () and equation 6 in Aharonson and Schorghofer (), which parameterizes attenuation through the path length of the atmosphere as a function of the elevation of the Sun above a horizontal horizon.…”

“…We use a thermal inertia for the dry (porous and ice‐free) regolith of 196 J·m −2 ·K −1 ·s −0.5 , consistent with a value intermediate of Martian dust‐ and sand‐sized particles (Edgett & Christensen, ; Paige et al, ) and lag deposits previously modeled in the northern plains (Bramson et al, ). We assume the lag deposit has a porosity of 40% and that NPLD ice has a dust content of 3%, consistent with that estimated for the bulk composition of the NPLD (Grima et al, ).…”

Widespread Shallow Water Ice on Mars at High Latitudesand Midlatitudes

“…In addition, the seasonal equatorial cloud belt (Benson et al, ; Heavens et al, ; Wilson & Guzewich, ) mutes the subtle cyclical temperature signal we seek to leverage, preventing us to derive ice depths with this proposed approach and modeling tools. Similarly, in the southern midlatitudes, where spring corresponds to the dusty season (Montabone et al, ), that is, a period of buffered surface temperatures (and therefore warmer nighttime temperatures), the trend we seek to identify is swamped. We explicitly ignore low thermal inertia terrains (i.e., TI < 100 J m −2 K −1 s −1/2 , masked out in Figure ; Putzig et al, ) because they are often frosted at night (Piqueux et al, ) and excluded from consideration for crewed exploration, even though their very high insulating properties could make them conducive to persistent shallow ice (Bramson et al, ; Watters et al, ).…”