3D modelling of the early Martian Climate under a denser CO2 atmosphere: Temperatures and CO2 ice clouds (1210.4216v1)

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 CO2 atmosphere with pressure between 0.1 and 7 bars. The model includes a detailed radiative transfer model using revised CO2 gas collision induced absorption properties, and a parameterisation of the CO2 ice cloud microphysical and radiative properties. A wide range of possible climates is explored by 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 the adiabatic cooling and warming of the atmosphere when it moves vertically. In most simulations, CO2 ice clouds cover a major part of the planet but greenhouse effect does not exceed +15 K. We find that a CO2 atmosphere could not have raised the annual mean temperature above 0{\deg}C anywhere on the planet. The collapse of the atmosphere into permanent CO2 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{\deg}C (a condition which could have allowed rivers to form) are predicted for obliquity larger than 40{\deg} at high latitudes but not in locations where most valley networks 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.

• The paper demonstrates that high surface pressures lead to subfreezing temperatures due to enhanced Rayleigh scattering.
• The paper reveals that extensive CO2 ice clouds provide a maximum warming of about 15 K, insufficient for stable liquid water.
• The paper employs sensitivity analyses of obliquity, dust loading, and surface properties to highlight the complex climate dynamics on early Mars.

Analyzing Early Martian Climate: Insights from 3D Modeling in a Denser CO$_2$ Atmosphere

The paper detailed in the paper titled "3D Modelling of the Early Martian Climate under a Denser CO$_2$ Atmosphere: Temperatures and CO$_2$ Ice Clouds" presents a comprehensive exploration of the early Martian climate through advanced climate simulations. Employing a 3D global climate model (GCM), the researchers address the prevailing hypothesis that the early Mars atmosphere, comprising primarily CO$_2$, may have sustained liquid water, influenced by the greenhouse effect, amidst a fainter young sun. This paper provides a nuanced understanding of the climate dynamics, temperature variations, and cloud formation processes, proposing that a cold climate could have been prevalent.

Key Findings and Methodology

The authors utilized a robust GCM with advanced radiative transfer calibrations to simulate various early Martian climatic conditions. The model incorporated updated CO$_2$ gas collision-induced absorption properties and detailed parameterization of CO$_2$ ice cloud microphysics. Exploring scenarios of surface pressures ranging from 0.1 to 7 bars, the paper adjusts variables such as obliquity, orbital parameters, atmospheric dust, and surface properties to elucidate their impact on climate outcomes.

Several pivotal conclusions arise from the simulations:

1. Surface Pressure and Temperature Dynamics: Surface temperatures do not exceed freezing across any parameter set, with annual mean temperatures consistently below 0°C. High surface pressures induce significant Rayleigh scattering, counteracting the greenhouse effect and limiting warming. Additionally, surface temperature variations correlate with topography, highlighting adiabatic atmospheric processes.
2. CO$_2$ Ice Clouds: CO$_2$ ice clouds primarily drive the greenhouse effect in these models but provide insufficient warming to permit stable liquid water given the modeled atmospheric compositions. Regardless of assumptions to maximize cloud opacity, the warming effect caps at approximately 15 K. The simulations depict clouds as extensive but variable across the planet, influenced strongly by simulated atmospheric circulation patterns and orographic features.
3. Atmospheric Collapse: Atmospheric collapse into stable CO$_2$ ice caps occurs at pressures below ~1 bar for low obliquities, or above 3 bars when obliquity is higher, suggesting a self-limiting mechanism on atmospheric pressure in scenarios lacking additional heat sources.
4. Obliquity Influence: Higher obliquities enhance seasonal temperature variability, potentially permitting episodic and localized melting under optimal orbital configurations. However, such conditions do not align with known sedimentary and fluvial features at mid to low latitudes, which remain unexplained by baseline conditions in the model.
5. Sensitivity Analysis: Variations in cloud microphysics, atmospheric dust loading, and surface properties reveal modest impacts on the overall climate picture. Increased dust opacity moderately warms the surface, primarily by elevating atmospheric infrared opacity and reducing cloud cover.

Implications and Future Considerations

The paper presents evidence supporting a cold and predominantly frozen early Martian climate, challenging the notion of a long-standing "warm and wet" environment. This concurs with geological findings indicating episodic and transient liquid water presence, possibly driven by non-climatic mechanisms such as geothermal or impact-induced heating.

However, the exploration of other greenhouse gases like CH$_4$ and NH$_3$, and alternative warming mechanisms, remains an open terrain for further paper. Additionally, the assumption of a purely CO$_2$ atmosphere may benefit from considering complex atmospheric chemistry or transient atmospheric compositions.

In summary, this research contributes to the broader understanding of planetary climate systems under conditions markedly different from Earth's current environment. It underscores the necessity for interdisciplinary methods combining climatology, atmospheric science, and geology to unravel the geophysical histories of terrestrial planets.