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Warming Early Mars with Carbon Dioxide Clouds That Scatter Infrared Radiation

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Science  14 Nov 1997:
Vol. 278, Issue 5341, pp. 1273-1276
DOI: 10.1126/science.278.5341.1273

Abstract

Geomorphic evidence that Mars was warm enough to support flowing water about 3.8 billion years ago presents a continuing enigma that cannot be explained by conventional greenhouse warming mechanisms. Model calculations show that the surface of early Mars could have been warmed through a scattering variant of the greenhouse effect, resulting from the ability of the carbon dioxide ice clouds to reflect the outgoing thermal radiation back to the surface. This process could also explain how Earth avoided an early irreversible glaciation and could extend the size of the habitable zone on extrasolar planets around stars.

It is most likely that the martian atmosphere 3.8 billion years ago was composed primarily of CO2, with a surface pressure ranging from a few hundred to several thousand millibars, and some H2O (1). At that time, the solar luminosity was about 25% lower than it is at present. Under such conditions, calculations performed with a one-dimensional (1D) climate model by Kasting (2) showed that the atmospheric CO2 should condense in the atmosphere for surface pressures larger than a few tens of millibars. Kasting found that the condensation of CO2 decreases the atmospheric temperature lapse rate and reduces the magnitude of the greenhouse effect, making it impossible to warm the surface of Mars enough to allow the presence of fluid water together with a CO2-H2O gaseous atmosphere. Several alternative mechanisms such as geothermal heating (3), an early more massive sun (4), or the greenhouse effect of methane (5) and ammonia (6) have been considered but none has provided a likely solution to the early Mars climate enigma (5).

Another consequence of the condensation of CO2 is the formation of CO2 ice clouds. Because they are perfect scatterers at solar radiation wavelengths, the CO2 ice particles should raise the planetary albedo. In the thermal infrared (IR), CO2 ice is at least 500 times more transparent than water ice, except near 15 μm where the ν2 absorption band is located and above 90 μm where two broad lattice vibration bands were measured (7). Thus, CO2 ice clouds should not be able to contribute to an absorption-emission greenhouse effect as cirrus clouds on Earth do. On this basis, Kasting (2) estimated that CO2 ice clouds should cool the planet through reflection of sunlight uncompensated by IR trapping.

We have studied the IR properties of the CO2 ice clouds using a two-stream, hemispheric mean, source function code that allows for multiple scattering, absorption, and emission by atmospheric particles (8). The CO2 ice particle single-scattering properties were obtained from the refractive index measured by Hansen (7), using Mie theory with a modified gamma size distribution of effective variance 0.1 (9). As expected by Kasting, a cloud composed of CO2 ice particles smaller than a few micrometers should be almost transparent in the IR, except near 15 μm. However, larger particles can be expected in CO2 ice clouds. Crystal size is determined by the time required for crystal growth versus the time it takes for the particles to fall out of a supersaturated layer (sedimentation). On Earth, despite the fact that the growth of water ice particles is limited by the diffusion of water vapor through air, particles 80 μm or larger are often observed in cirrus ice clouds, and the observed radiative properties of Earth's cirrus clouds can be fit by assuming equivalent spheres with a radius of 16 μm (10) with optical depth up to 30. On early Mars, because it is the primary atmospheric constituent that is condensing, the CO2 cloud particles should grow faster for a comparable sedimentation rate (11). Although not much is known about the exact microphysical processes involved, particle radii from 10 to about 100 μm can be expected (12). Such particles can more readily scatter the IR radiation (Fig. 1). A cloud composed of such particles would be able to scatter the IR radiation back to the ground and thus contribute to surface warming. IR scattering by clouds or aerosols is not important on Earth, but its warming effect has been considered for other planets (13). It has been studied as a means of accounting for the observed IR spectrum of Mars (14), Venus (15), and even Titan (16). In particular, CO2 ice clouds that scatter IR radiation are thought to have an impact on the radiative budget of the polar regions of Mars at the present time (17).

Figure 1

The reflectivity α (solid curve), transmissivity β (dashed curve), and emissivity ɛ (dotted curve) of a pure CO2 ice cloud (r = 10 μm) of τ = 10, corresponding to a mass of CO2 ice about 100 g m–2. Except in the far IR where the cloud particles are too small to scatter the radiation, and near the 15-μm CO2 absorption band, the main effect of the cloud is a reflection of the IR radiation.

We included the effect of the CO2 ice clouds in the 1D radiative-convective model designed for early Mars by Kasting (2). In the IR wavelength, this model is based on a traditional radiative transfer band model. To account for scattering by CO2 ice clouds, we used the multiple scattering code and assumed that the effect of a given cloud could be mimicked in the band model by a single-layer cloud with the same transmissivity, reflectivity, and emissivity (18). At solar wavelengths, we used a δ-Eddington code to compute multiple scattering by the CO2 ice particles consistently with Rayleigh scattering by the CO2 gas molecules, and the system was integrated to radiative-convective equilibrium with the use of a straightforward time-stepping method (19). Overall, this climate model gives results similar to the model of Kasting (2) when the cloud optical depth is set to zero. When a cloud composed of particles with radii larger than 6 to 8 μm is included, the model predicts a warming of the troposphere and the surface (Figs. 2 and 3). Such a warming may seem surprising because one would expect the IR scattering–induced warming to be balanced by a strong cooling due to scattering of sunlight. In fact, the two effects are not compensating. At solar radiation wavelengths, the importance of the clouds is limited by the fact that the planetary albedo without clouds is already quite high because of Rayleigh scattering in the clear atmosphere (the Rayleigh scattering coefficient for CO2 is 2.5 times that for air on Earth), whereas in the IR the CO2 clouds tend to block the outgoing thermal radiation at wavelengths where it would otherwise freely escape. For instance, the planetary albedo (A) of Mars with a 2-bar cloud-free CO2 atmosphere would be A clear = 0.38. The cloud of Fig. 1increases the planetary albedo to A cloudy = 0.65, reducing the absorbed solar energy by about 40% while trapping more than 60% of the outgoing thermal radiation (Fig. 3B). The cloud reduces the outgoing thermal radiation by reflecting the IR flux from below on the one hand, and by absorbing it and reemitting at a lower temperature on the other hand. This last effect corresponds to the “conventional” cloud greenhouse effect that is observed for terrestrial clouds. However, because the cloud can only emit where the atmosphere is opaque, the absorption-emission radiative forcing of CO2 clouds is almost negligible (Fig. 3B), except for a few watts per square meter in the 15-μm band wings. Consequently, the cloud-induced warming does not depend on the cloud temperature. Thus, it is almost insensitive to the cloud altitude, except when the cloud is artificially put near the surface. In the lower troposphere, the vertical heat transport due to convection and turbulence is larger than the radiative heat flux, and the model predicts a reduced cloud-induced warming. In reality, however, CO2 clouds should be higher, at the level where condensation temperatures are reached (Fig.2).

Figure 2

Calculated mean temperature profiles for a 2-bar CO2 atmosphere, assuming a 25% reduced solar luminosity corresponding to the early Mars conditions. The effect of the cloud from Fig. 1 (τ = 10, r = 10 μm) is shown in the cases of a wet (fully saturated troposphere; dashed curves) and a dry (solid curves) atmosphere. The dotted curve shows the CO2 condensation temperature profile.

Figure 3

(A) Calculated surface temperature as a function of the cloud τ for r = 50 μm (curves with dots) and r = 10 μm (curves without dots) in the cases of a wet (dashed curves) or dry (solid curves) 2-bar CO2 atmosphere on Mars. A 25% reduced luminosity is assumed. (B) The corresponding cloud radiative forcings with r = 10 μm. The surface warming results from the excess of radiative forcing in the IR as compared with the negative solar forcing. The radiative forcings were defined as ΔF Sol =S(A clearA cloudy) and ΔF IR =F clearF cloudy, with S the mean incoming solar flux and F the outgoing thermal fluxes computed in the clear-sky and cloudy-atmosphere cases. The scattering component of ΔF IR was computed assuming that it was proportional to αI subcloud at every wavelength, with I subcloud the upward radiance from the subcloud atmosphere. Similarly, the absorption-emission component was assumed to be proportional to ɛ[I subcloudB(T cloud)], withB(T cloud) the Planck function at the cloud temperature.

If the early martian atmosphere was in contact with water on the surface, the atmosphere probably contained enough water to increase the IR opacity of the atmosphere (20). Assuming a fully saturated troposphere, the greenhouse effect is strongly increased, especially for high surface temperatures (Fig. 2 and Fig.3A). Water clouds, or possibly CO2-H2O clathrate hydrate clouds, probably formed below and within the CO2 ice cloud layer. With CO2 ice clouds above, their impact on the planetary albedo was probably small. However, they may have contributed to the greenhouse effect by their IR absorption. Calculations performed with water ice particles mixed with the CO2 ice particles in the modeled cloud reveal a possible warming due to an increase of the absorption-emission cloud radiative forcing, uncompensated by the decrease of the IR scattering forcing (21).

Overall, even in the absence of other greenhouse gases except CO2 and H2O, the magnitude of the total greenhouse effect of the early Mars atmosphere should have been relatively strong, depending on the assumed humidity, CO2and H2O cloud properties, and fractional cloud cover. This last parameter was probably below 100% because cloud formation may have been inhibited in regions of atmospheric subsidence. Surface temperatures calculated assuming a 75% fractional cloud cover with visible optical depth τ = 10 were found to be 20 to 30 K colder than those for a 100% cloud cover. It might be expected that clear sky regions were harder to maintain on early Mars than on modern Earth, as the clouds in the former case arise from condensation of the atmosphere's primary constituent, whence subsidence must maintain anomalously warm conditions in a deep layer to inhibit condensation. The issue of fractional cloud cover is an important one that is inextricably tied up with dynamics and can only be treated in the context of a full 3D climate model.

Eventually, without taking into account the additional effect of water clouds, we found that a surface pressure lower than 1 bar may have been sufficient to raise the global mean temperature of early Mars to the melting point of water (Fig. 4). In fact, it has been suggested that the geomorphic observations on Mars can be explained with mean temperatures a few tens of degrees below freezing (3, 22). According to Fig. 4, this would allow a surface pressure as low as 0.3 bar or a mean cloud optical depth of only 1. In any case, conditions suitable for life could easily have been reached. The requirement for life is the presence of liquid water, regardless of mean temperature and pressure. As seen in Earth's polar regions, major liquid water habitats supporting life can be maintained by the insulating properties of an ice cover or by geothermal activity, even when temperatures are well below freezing (22).

Figure 4

Mean surface temperature as a function of surface pressure for several values of the mean cloud τ in the cases of a wet (dashed curves) or dry (solid curves) CO2 atmosphere on Mars. A 25% reduced luminosity is assumed.

The high surface temperatures (Fig. 4) indicate that the greenhouse effect of a thick, wet, condensing CO2 atmosphere can be extremely powerful. This general mechanism should be taken into account in the estimation of the “habitable zone” (suitable for life on extrasolar planets) around stars. For instance, a wet 10-bar CO2 atmosphere filled with τ = 10 CO2 ice clouds would allow a mean surface temperature above the freezing point of water at more than 2.4 astronomical units (AU) from a sunlike star, beyond the outer edge of the habitable zone found by Kasting et al. (23) at 1.37 AU. Similarly, CO2 ice clouds may have played a role in warming Earth when the sun was fainter than today (24), assuming that enough CO2 was available on early Earth (5). As for Mars, given our current knowledge of the environmental conditions 3.8 billion years ago, our conclusions must remain speculative. Nevertheless, it must be emphasized that our CO2 ice cloud scenario is simple. It does not require any ad hoc combination of physical processes. Assuming that the atmosphere was composed of more than a few 0.1 bar of CO2, it is likely that CO2 clouds formed and that they contributed to warming the planet enough for liquid water to flow on the surface.

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