Here we introduce a new term for a general habitable zone (HZ, Kasting et al., 1993), the "life supporting zone" (LSZ). In our model the LSZ consists of different habitable zones for different solvents (e.g. a water-HZ, or a formamide-HZ, etc.). Our first studies (Leitner et al., 2010) have shown that the resulting formamide-HZ is larger than the classical water-HZ, for the situation of an architecture of a planetary system similar to our solar system.

In order to extend our model for a LSZ, it is necessary to include climate buffers, different atmospheric compositions, cloud albedo, etc. to calculate the inner and outer edge of a HZ for one alternative solvent or water-solvent composites more precisely. The primary goal of this sub-project is to define possible habitable zones for life based on different solvents in order to calculate the life supporting zones for different spectral-type central stars.

Droplet Formation with/without pre-existing nuclei from alternative solvents

Clouds dominate the radiative transfer through a planet's atmosphere. Depending on the clouds' optical depth, the optical properties of the cloud droplets, the clouds' altitude and cloud coverage, clouds may cool or warm a planet's surface. On Earth clouds consist of water because of the large water oceans and because water may exist as vapor, liquid or solid in the temperature range on Earth. Other gases like methane or carbon dioxide exist as vapors in Earth's atmosphere. On other planets, however, clouds consisting of other substances are formed. On Mars there are carbon dioxide-ice clouds (Montmessin et al., 2006), clouds consisting of sulfuric acid and water exist on Venus (Porshnev et al., 1987) and methane and ethane clouds on Titan, a satellite of Saturn (Tokano et al., 2006; Griffith et al., 2006).

A cloud model is used to compute the properties of clouds which may consist of water or alternative solvents. This model was developed for terrestrial water clouds at the research group Aerosol Physics and Environmental Physics (AEP) of the Faculty of Physics of the University of Vienna (Neubauer, 2009). The physico-chemical properties of the alternative solvents necessary for the calculations are taken from the literature. For the formation of clouds a condensable gas and condensation nuclei onto which the gas may condense are necessary. As nothing is known about possible cloud condensation nuclei (CCN) in the atmosphere of exoplanets, we assume that aerosol particle formation follows the same physical principles of nucleation from the gas phase and erosion of bulk material as on Earth. The presence of soluble and wettable CCN, respectively, is assumed.

The cloud model is a cloud parcel model which describes an ascending air parcel containing the droplets (following Kornfeld, 1970; Houze, 1993; Pruppacher and Klett, 1997). The model includes the microphysical processes of nucleation, condensation and coagulation and radiative effects (Chen and Lamb, 1994). Turbulent diffusion is also considered (Mason, 1960). The cloud model provides cloud droplet size distributions during the ascent of the air parcel which are stored as a dataset.

Optical properties of clouds formed from alternative solvents

Light is multiply scattered when passing through clouds. The size distribution and the optical properties of the cloud droplets must be known to compute the radiative transfer through clouds.
The cloud droplet size distribution is taken from a dataset calculated by the cloud model for the alternative solvents. The optical single scattering properties of the cloud droplets are modeled with classical Mie theory (Bohren and Huffmann, 1983). The refractive index of the solvents is taken from the literature. Mie calculations for homogeneous droplets as well as for droplets consisting of a liquid shell and an insoluble core ("coated sphere" - model) will be performed to obtain optical depth, asymmetry parameter and single scattering albedo of the cloud droplets.

Radiative effects of clouds formed from alternative solvents and computation of life supporting zones

Radiative transfer through the atmosphere of exoplanets and radiative effects of clouds formed from alternative solvents are investigated using a modified version of the radiative transfer model Streamer (Key and Schweiger, 1998; other parts are described by Toon et al., 1989; Stamnes et al., 1988, 1994 and 2000; Streamer-Homepage). Streamer is already in use at the research group Aerosol Physics and Environmental Physics (AEP) of the Faculty of Physics of the University of Vienna.

Energy balance calculations show that the widths and the positions of life supporting zones for alternative solvents are quite different from those for the classical habitable zone for water based life because of the different temperature ranges for liquid alternative solvents.

The temperatures in the atmosphere and of the surface of exoplanets and thereby the LSZs are calculated with a radiative convective model (RCM) based on the model of Manabe and Strickler (1964) and Manabe and Wetherald (1967). The RCM computes temperatures in radiative convective equilibrium. The atmosphere is divided into vertical, plane parallel layers. Space and the planetary surface represent the upper and the lower boundaries. In every layer radiation may be scattered, absorbed or emitted. In the case of radiative equilibrium every layer receives as much radiation as is leaving the layer. For this reason as much radiation is leaving a planet's atmosphere as is received from space when the reflection of a part of the radiation at the planet's surface is taken into account. Radiative equilibrium may result in a strong decrease of temperature with increasing altitude. In the case of radiative convective equilibrium also convection in the atmosphere is accounted for. Air of warmer, lower alti tude layers may rise and, in doing so, heat up higher altitude layers. In the model a convective adjustment is used. The atmospheric lapse rate calculated for radiative equilibrium is adjusted not to exceed a given lapse rate (e.g. the applicable dry adiabatic lapse rate).

Besides the spectral class of the star and the distance between the star and the (exo)planet, the surface temperature strongly depends on the amount of atmospheric gases (in particular greenhouse gases), aerosol particles, clouds and surface albedo. Different scenarios (e.g. varying cloud coverage, surface albedo, amount of atmospheric gases, etc.) are investigated for each solvent to calculate the width and the location of the respective LSZ.

The chemical composition of a planet's atmosphere is important for radiative transfer. Most atmospheres of planets and satellites in the solar system consist of a main constituent and trace amounts of other gases. Some of these trace gases absorb shortwave and long wave radiation and thereby influence radiative transfer (greenhouse gases). Small amounts of a greenhouse gas may change the surface temperature of a planet if the gas absorbs in a spectral range where the atmosphere is not opaque for radiation. As nothing is known about the chemical composition of atmospheres of terrestrial exoplanets, we assume different scenarios where the atmosphere consists of a main constituent and varying amounts of different greenhouse gases.

The surface albedo of planets and satellites in the solar system strongly varies as their respective surface consists of different materials. A pure water surface for example absorbs more stellar radiation than a rock surface. Scenarios for a large range of possible surface albedos will be investigated. Besides for a G2V star like the Sun, the computations will be performed also for other spectral classes.


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