3D modelling of the early martian climate under a denser CO2 atmosphere: Temperatures and CO2 ice clouds

Abstract

On the basis of geological evidence, it is often stated that the early martian climate was warm enough for liquid water to flow on the surface thanks to the greenhouse effect of a thick atmosphere. We present 3D global climate simulations of the early martian climate performed assuming a faint young sun and a CO 2 atmosphere with surface pressure between 0.1 and 7 bars. The model includes a detailed representation of radiative transfer using revised CO 2 gas collision induced absorption properties, and a parameterisation of CO2 ice cloud microphysical and radiative properties. A wide range of possible climates is explored using various values of obliquities, orbital parameters, cloud microphysic parameters, atmospheric dust loading, and surface properties.Unlike on present-day Mars, for pressures higher than a fraction of a bar surface temperatures vary with altitude because of adiabatic cooling / warming of the atmosphere. In most simulations, CO 2 ice clouds cover a major part of the planet. Previous studies suggested that they could have warmed the planet thanks to their scattering greenhouse effect. However, even assuming parameters that maximize this effect, it does not exceed +15 K. Combined with the revised CO 2 spectroscopy and the impact of surface CO 2 ice on the planetary albedo, we find that a CO 2 atmosphere could not have raised the annual mean temperature above 0 • C anywhere on the planet. The collapse of the atmosphere into permanent CO 2 ice caps is predicted for pressures higher than 3 bar, or conversely at pressure lower than one bar if the obliquity is low enough. Summertime diurnal mean surface temperatures above 0 • C (a condition which could have allowed rivers and lakes to form) are predicted for obliquity larger than 40 • at high latitudes but not in locations where most valley networks or layered sedimentary units are observed. In the absence of other warming mechanisms, our climate model results are thus consistent with a cold early Mars scenario in which non climatic mechanisms must occur to explain the evidence for liquid water. In a companion paper by Wordsworth et al., we simulate the hydrological cycle on such a planet and discuss how this could have happened in more detail.

Habitability of planets on eccentric orbits: Limits of the mean flux approximation

“…However, Sotin et al (2007) found that ocean planets are likely to have a high pressure ice layer between the liquid water ocean and the silicate mantle. Therefore, Lammer et al (2009) classified the ocean planets as class IV habitats, the lowest class of potentially habitable worlds (see also Forget et al 2013). However, Léger et al (2004) and Forget et al (2013) both highlighted the possibility of enriching the ocean with minerals from meteoritic impacts, showing that these planetary objects should not be discarded as potential life habitats.…”

“…Therefore, Lammer et al (2009) classified the ocean planets as class IV habitats, the lowest class of potentially habitable worlds (see also Forget et al 2013). However, Léger et al (2004) and Forget et al (2013) both highlighted the possibility of enriching the ocean with minerals from meteoritic impacts, showing that these planetary objects should not be discarded as potential life habitats. In addition, some other mechanisms were put forward to bring minerals to the liquid water layer, such as solid convection (e.g., the presence of 36 Ar at the surface of Titan could be explained by solid convection within the subsurface high pressure ice layer; Niemann et al 2010;Tobie et al 2006).…”

Constraints on Climate and Habitability for Earth-like Exoplanets Determined from a General Circulation Model

“…To date no 3-D simulations have calculated the maximum CO 2 greenhouse limit. However, several 3D studies have explored paleo-Earth, paleo-Mars, and high-CO 2 exoplanets scenarios, which may serve as useful steps towards determining the outer edge of the habitable zone around different stars (Wordsworth et al 2011;Urata & Toon, 2013;Forget et al 2013;Wordsworth et al 2013;Shields et al 2016). Several of these data points are included on Fig.…”

A Numerical Study of Convection in a Condensing CO2 Atmosphere under Early Mars-Like Conditions

“…Investigation of possible structures of circulation fields and properties of clouds in such an atmosphere, especially a dense CO 2 atmosphere, is not only interesting as a problem of fluid mechanics, but also as an important target of research on the early Mars, where the scattering greenhouse effect of CO 2 ice cloud could have played an indispensable role in maintaining possible warm climate (Forget and Pierrehumbert 1997;Mischna et al 2000;Colaprete and Toon 2003;Mitsuda 2007;Forget et al 2013). Previous studies have shown that the intensity of scattering greenhouse effect depends on a number of cloud properties, such as the size distribution of cloud particles, optical depth, cloud cover, and cloud height (Forget and Pierrehumbert 1997;Mischna et al 2000;Kitzmann et al 2013). For the advancement of our understanding on the early Martian climate, it is, therefore, valuable to investigate possible properties of CO 2 clouds consistent with circulation fields of convection associated with condensation of the major atmospheric constituent.…”

“…In the previous studies employing one-dimensional models, distributions of cloud particles are specified irrelevant to flow fields. Forget and Pierrehumbert (1997) and Mischna et al (2000), for instance, assume that clouds are localized at specified vertical levels in the condensation layer. Mitsuda (2007) determines the equilibrium distribution of cloud condensates by the local radiative balance, neglecting possible contributions of atmospheric motion and latent heating.…”