Formation and stability of martian mid-latitude water ice deposits at high obliquity

Introduction: Remnants of glacial and periglacial geomorphological features are visible up to mid-latitudes on Mars. Notably, an out-of-equilibrium “latitude-dependant mantle” extends to 30° latitude in both hemispheres [1]. These patches were likely deposited as snowfall in the recent past (less than ~2Myr) in response to climate change driven by shift in obliquity, similar to Earth glacial/interglacial periods [2]. However, martian climate models usually struggle to reproduce environmental conditions required to form LDM under recent paleoclimatic orbital forcing. We present a new set of simulations with refined description of surface and sub-surface water processes and their relevance regarding the deposition and above all stability of large ice patches up to mid-latitudes in both martian hemispheres.

Water-ice clouds in recent paleoclimates: As present-day martian atmosphere is extremely dry, water ice clouds have second-order effects in global climate models [6]. Modeling higher obliquity episodes, ie shifts from 25° up to 35°, atmospheric humidity is enhanced by polar warming and water-ice cloud become a key element of martian climate [7,8]. Their radiative effect strongly warms the atmosphere, amplifies meridional circulation and water transport toward tropical latitudes. Previous work showed that radiatively active water-ice clouds allow for the transportation and deposition of water ice patches up to mid-latitudes [7]. However, these deposits can only be perennial under extreme atmospheric dust scenarios, in order to limit summertime sublimation.

Frost and ice albedo: Surface water ice has a typical albedo of 0.35 in our present-day Mars model [9]. Mid-latitude ice patches resulting from an intensified water cycle at higher obliquity would rather have a 0.7 albedo, due to less dust content and fresh snowfall. By limiting solar heating, this albedo parametrization favors the persistence of mid-latitude ice throughout summer.

Latent heat of sublimation: The latent heat of water sublimation has been neglected in present-day climate models, because of the low amount of water and thus energy flux involved. The sublimation of water was hitherto simply computed using surface temperature and water vapor equilibrium. Similarly to the frost albedo effect, taking into account the latent heat of water becomes relevant when the water cycle, and thus the energy flux involved, is intensified at higher obliquity. It also favors the year-long persistence of mid-latitude ice by adding an energy cost to sublimation, which decreases surface heating.

Nudging subsurface thermal inertia: Martian soil’s thermal inertia is driven by the presence of subsurface perennial water ice, regulated by long-term equilibrium with water vapor [10]. This equilibrium is modified with the water cycle at higher obliquity. Instead of waiting for natural equilibrium to occur, we artificially accelerate the relocation of subsurface water ice, and accordingly increase subsurface thermal inertia. Subsurface ice inventory is computed each year using annual mean water vapor, as proposed in [10].

Conclusions with idealized orbital forcing: The recent excursions to 35° obliquity are thought to be the main drivers of martian glaciations. We use our new parametrizations along with idealized orbital forcing, that is a 35° obliquity and a null-excentricity, to show that the effect of water-ice clouds, frost albedo and latent heat of sublimation allow for the preservation of mid-latitude ice deposits when equilibrium is reached. In the last ~2 Myr on Mars, obliquity has reached 35° a dozen times, for approximately 1000 years each time [11]. Under our idealized hypothesis, the accumulation rate is compatible with hundreds of meters thick latitude-dependent mantle of ice-rich deposits.

References:

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