From climate models to planetary habitability: temperature constraints for complex life
The liquid water criterion for planetary habitability implies a temperature range wider than the thermal limits of organisms able to affect the atmospheric composition. On the other hand, the search for life on exoplanets relies on the detection of atmospheric biosignatures. A recent paper introduces a new temperature-based habitability index suited for the potential emergence of complex life and generation of atmospheric biosignatures, and explores with a climate model its consequences for the habitable zone.
The quest for life in planets outside the Solar System (exoplanets) must necessarily rely on the search of atmospheric biosignatures rather than in situ measurements. Life on Earth heavily affects the atmospheric composition particularly through the continuous production of oxygen via photosynthetic activity. If oxygen were not continually replenished it would react with other compounds, and the atmospheric composition would reach a final state of thermodynamic equilibrium. On these notions, since long it was suggested that the signature of biological activity on exoplanets could be provided by spectral lines revealing “the presence of compounds in the planet’s atmosphere that are incompatible on a long-term basis” (Lovelock 1965). Spectroscopy of the atmospheres of gas giant planets is within reach of current instruments. It will be probably accomplished for rocky planets with the next generation instruments. It is therefore important to estimate the potential habitability of rocky exoplanets, in order to select the best candidates for the observational search of biosignatures.
Due to the essential role of water on terrestrial life, the habitability has been usually defined as the possibility to host liquid water on the planet’s surface. But the liquid water habitability is not tailored for the most promising biosignature-bearing targets, and the thermal limits for terrestrial surface life able to heavily affect the atmospheric composition are tighter than those of the liquid water.
In a paper to appear in the International Journal of Astrobiology (Laura Silva, Giovanni Vladilo, Giuseppe Murante from INAF/OATs; Antonello Provenzale from CNR/IGG; Patricia M. Schulte from the Dep. of Zoology, Univ. British Columbia), a new temperature-based habitability index suited for the potential emergence of complex life and generation of atmospheric signatures has been introduced, and its consequences have been explored with the climate model described in Vladilo et al. (2015).
In line with the studies of the effects of climate change on the survival of different species through the investigation of their thermal responses, this work explores the thermal limits of poikilotherms, i.e. those plants, invertebrates and ectothermic vertebrates whose internal temperature varies with the ambient temperature. The range identified for these organisms to complete their life cycle is 0°C-50°C, that also brackets most photosynthetic cyanobacteria species. From a careful analysis of the mechanisms underlying the thermal responses from the molecular to the complex life level, it can be inferred that this temperature range may be appropriate for any form of water-based chemical life with aerobic metabolism.
The climate model used to compute the planetary habitability adopts a surface energy balance calculation, coupled with radiative-convective atmospheric column vertical transport, and a physical parametrization of the horizontal transport. It computes the latitude and orbital-dependent surface temperature, as a function of a large range of orbital and planetary parameters. The new habitability index for complex life h050, is computed as the orbital-averaged planetary surface fraction satisfying the 0°C-50°C temperature constraints. In Fig.1 the habitability maps on the plane atmospheric columnar mass Natm vs flux received by the planet S for the liquid water (left) and complex life criteria (center) are shown, both for a terrestrial CO2 abundance. The right panel is h050 but with 100 fold increase of CO2.
Fig.1 – Liquid water hlw (left) and complex life h050 (center) planetary habitability maps as a function of the stellar flux (normalized to the Earth value S0=1360 W/m2) and of the atmospheric columnar mass Natm (=1033 g/cm2 for Earth) for a terrestrial CO2 abundance. On the right h050 for a 100 fold increase of CO2. The red line marks the onset of the runaway instability caused by the accumulation of water vapor in the atmosphere, that may lead to the total loss of water. The green lines are the 0, 25 and 50°C isotherms (mean global planetary T). The shaded region highlight the Natm values for which the radiation dose from galactic cosmic rays becomes >100 mSv/yr for a planet without a magnetic field (yellow) or with a magnetic field as the Earth one (orange).
The new complex life habitable zone is much tighter than the classic liquid water one and is not subject to the uncertainties inherent to the calculations of runaway instability caused by the increasing amount of water vapor (a strong greenhouse gas) in the atmosphere at increasing temperature. The high values of maximum insolation often adopted to estimate the habitability are found to be incompatible with the thermal limits for surface complex life. It is found that the characteristics of the atmosphere strongly affect the surface T gradient, its seasonal variability, the habitability time scale and the possibility to develop complex life, and also the surface radiation dose from galactic cosmic rays. Planets with low atmospheric mass have shorter habitability time scales, and undergo large temperature excursions and high radiation doses, that may induce a fast rate of Darwinian evolution. Therefore in order estimate extrasolar habitability it is necessary to consider the effects of the planetary, orbital, and atmospheric characteristics, together with a careful evaluation of the appropriate thermal limits.