HABEBEE: Habitability of Eyeball-Exo-Earths

2.1. Potential limits of a stable Eyeball state

A stable Eyeball Earth consists of an ice shell over a rocky interior and a stable liquid water ocean at the substellar point. If the planet has an atmosphere or surface/ocean that effectively transports heat from the substellar point, the formation of an Eyeball Earth state is less likely to occur than would a completely frozen Snowball Earth state (Lucarini et al., 2010). We will construct various models of Eyeball Earth temperature distributions based on the planetary conditions, such as mass, distance from star, and incoming stellar radiative flux, to determine the range of conditions where Eyeball Earth states are stable. These calculations will determine the mass, orbital distance, and stellar types where Eyeball Earths can exist in either a stable or transient state and will produce surface temperature distributions for these states that will be used in later analyses. We will determine potential Eyeball Earth candidates in the current catalogue of extrasolar planets and in future survey results. Our work will provide general and extreme cases for potential stable Eyeball Earths that we can use to predict observable limits for their detection and characterization.

2.3. Astrochemical implications due to the action of ionizing agents in the surface of an eyeball planet

To understand the radiation environment on an M dwarf planet, the flux of energetic electrons, protons, ions, stellar photons, and meteoroids must be considered (Vasyliunas and Siscoe, 1976). The surface of the planet could be modified as a consequence of an environment that would cause sputtering (physical changes caused by erosion of the surface from incoming energetic particles) and chemical changes, caused by the impact of ionizing radiation such as stellar wind, cosmic rays, and so on (e.g., Plainaki et al., 2010). Dissociation and chemical reactions produced by ionizing radiation (radiolysis and photolysis) could also release energy that can cause the ejection of chemically unstable atoms or molecules. The existence of such an ionizing environment would increase the chemical complexity of the system over time.

We propose to determine the formation and destruction rates of molecules in ice by ionizing agents by utilizing the Brazilian National Synchrotron Light Source (LNLS, www.lnls.cnpem.br) to simulate the action of ionizing agents in the surface of our modeled Eyeball Earth planet. Synchrotron radiation experiments will simulate the chemical transformations that occur in the planet's surface and atmosphere. Our analyses will enable us to describe the surface composition variability with respect to various parameters such as ice composition, density, temperature, atmospheric temperature, presence or absence of magnetic field on the planet, radiation flux at the planet, the distance between the planet and the star it orbits, as well as characteristics of the planetary system in which the planet resides, such as type of star (mass) and age of star.

3.1. Remote sensing of biosignatures

In general terms, a biosignature is a feature that is interpreted to have formed through a biological mechanism and cannot be explained nor generated through abiotic processes. For a biosignature to be detectable, it must (1) have a biological generation rate greater than any abiotic sources and greater than decomposition mechanisms allowing atmospheric accumulation: (2) be detectable through spectral observations (spectral signature in a wavelength region accessible to instrumentation, concentration such that there is a significantly large signal-to-noise ratio, and a unique signature distinguishable from other atmospheric components; after Domagal-Goldman et al., 2011).

The metabolic activity of microorganisms on Earth produces several disequilibrium gasses that would constitute spectrally observed biosignatures (e.g., Seager et al., 2012; see Berkner and Marshall, 1965, for a discussion of the rise of O₂ on Earth as a consequence of biological activity). Observations of Earth's spectrum reflected off the dark side of the Moon (Earthshine) allow Earth to be viewed as an extrasolar planet. Turnbull et al. (2006) concluded that the “simultaneous presence of oxygen and methane [is a] strong indicator of biological activity.” As such, the microbial biomass on Earth is sufficient to produce metabolic by-products that can be detected spectrally and interpreted as biosignatures when Earth is viewed as an extrasolar planet through “earthshine” observations (Turnbull et al., 2006; Seager et al., 2012, and references therein). Important Earth-based biosignature gasses include O₂ (and O₃), N₂O, and CH₄ (Seager et al., 2012). There are several remotely observable biosignatures, including spectral observations of disequilibrium gas phases produced by photosynthesis such as CO₂, in addition to O₂. The detection of atmospheric oxygen is an important biosignature (Léger et al., 2011) particularly if the astronomical study can exclude hydrogen escape as a possible abiotic source of oxygen. Presently, only CO₂, O₂, O₃, and H₂O would be detectable on an Earth-like exoplanet (Selsis, 2004; Selsis et al., 2008).

Despite the ability to recognize Earth-based biosignature gasses and predict the presence of out-of-equilibrium gasses that may be interpreted as biosignatures on extrasolar planets, the detection, spectroscopic observation, and subsequent characterization of such biosignatures from exoplanets is currently out of technological reach (Seager et al., 2012, and references therein). In this proposal, we aim to develop a set of criteria to identify potentially habitable Eyeball Earth exoplanets for further spectroscopic observation. Questions of specific biosignature gasses and the active geological processes required to allow for the buildup and persistence of atmospheric biosignatures on putatively habitable Eyeball Earths are problems to be tackled as technology advances. The data sets generated by HABEBEE will provide the fundamental framework for directed observational studies of potentially habitable tidally locked exoplanets around M dwarf stars.

3.2. Assessing the habitability potential of photosynthetic organisms

A habitable Eyeball Earth must thermodynamically support active microbial metabolism. Photosynthetic organisms are limited by the thermostability of chlorophyll, and survivability rapidly declines at temperatures over 75°C (Pikuta et al., 2007). To assess the plausibility of photosynthesis as a viable metabolism, the putative limits of photosynthetically active radiation (PAR) available to a photosynthetic community on an Eyeball Earth must be determined. We propose to study PAR through the hypothetical ice cap on a habitable Eyeball Earth. Using the models and observations from our astronomical teams, we will establish depth limits of a putative photosynthetic community living in the ice shelves of an Eyeball Earth. Part of our analysis will involve testing the capacity of ice as a protective shield against UV radiation. This will lead to further microbial survivability studies that will constrain the UV shielding effects of ice as well as the depth to which PAR is available (Fig. 1). Since tidally locked planets do not have day/night cycles, our experiment will also test continuous exposure to incoming radiation.

The absence of sufficient PAR, however, does not preclude the existence of life on Eyeball Earths. Assuming a geologically active planet, active submarine hydrothermal systems such as those on Earth may theoretically support chemolithoautotrophic microbial communities. Additionally, given the distribution of elements in the Solar System, where basalt is the dominant rock type, and the potential that this can be extrapolated to other planetary systems, it is hypothesized that chemoautotrophic metabolisms based on basaltic minerals (e.g., olivine and serpentine) may exist within the lithosphere or at the lithosphere-atmosphere boundary of an Eyeball Earth exoplanet. The potential of alternative metabolic systems will also be tested within the simulation chamber constrained by the modeled physical parameters derived by the astrochemistry team. Using known terrestrial chemolithotrophic metabolisms, we can model the thermodynamic possibility of such metabolisms existing on a hypothetical Eyeball Earth.

3.3. Sampling and characterization of Earth analog extremophiles

By studying the conditions under which extremophilic organisms thrive on Earth, theoretical limits to life can be imposed, and hypothetical habitable environments can be extrapolated to occur on other planets such as Eyeball Earths. A sampling expedition to Antarctica (in cooperation with the Brazilian Antarctic Program, PROANTAR, www.mar.mil.br/secirm/proantar.htm) will be completed to study a microbial community from an ice-water interface analogous to the potential substellar ocean on a habitable Eyeball Earth exoplanet. Concurrent with the astronomical observational analyses, metagenomic and functional gene analysis of the analog microbial community from the Antarctic Peninsula samples will be conducted.

Characterizing the microbial community at the Antarctic ice-water boundary will involve available metagenomic analyses (e.g., high-throughput shotgun 16S metagenomic analysis to determine the dominant species in the community followed by functional gene assays). Such genomic and functional assays will provide information on putative microbial metabolisms active in this transition zone on an Eyeball Earth and can be conducted within the biology laboratories of the National Center of Research in Energy and Materials (CNPEM, www.cnpem.br).

3.4. Experimental setup of a simulation chamber

Based on the models and observations of the astronomy teams, we will construct the environmental parameters for possible habitable Eyeball Exo-Earths. Samples from the above-mentioned analog environments will be subjected to modeled conditions in the planetary/space simulation chamber built at the Brazilian Astrobiology Laboratory (AstroLab, www.astrobiobrazil.org). We will then access microbial survivability and functional gene expression, and compare these data with that from the samples not subjected to the simulation.