Exobiology and Planetary Habitability. Giovanni Vladilo INAF-OATs

Transcription

1 Exobiology and Planetary Habitability Giovanni Vladilo INAF-OATs

2 Definitions Exobiology Search for life out of the Earth Used in Solar System missions (ESA) Bioastronomy Mostly used by the astronomical community (IAU) Search for extrasolar planets, intersterstellar molecules of biological interest, possible signatures of biological activity, Astrobiology Study of the living Universe Study of the origin, distribution and evolution of life in the Universe Used in Solar System missions (NASA) Used by biologists and chemists interested in studies of the origin of life

3 Motivations Introducing the Copernican revolution in biology Four centuries after our planet was displaced from the centre of the universe, our perception of biology is still Earth-centric Fostering multidisciplinary studies Life sciences, chemistry, sciences of the universe, earth sciences, Spinoffs Technology for space exploration, space medicine, links with biotecnologies, climate research, space colonization

4 Research fields Origin of life Appearance of life in the primitive Earth Prebiotic chemistry (in laboratory, in space) } Interstellar chemistry Delivery of organic material from space (comets and meteorites) Search for life in the Solar System Space missions in the Solar System }Solar System research Terrestrial life in extreme conditions Studies of extremophiles Search for the last universal common ancestor Comparison of genomic sequences

5 Exobiology & astronomical research Exoplanets Search for habitable exoplanets Search for biomarkers in exoplanet atmospheres Protoplanetary disks Formation history of habitable planets Delivery of water and organic material on terrestrial-type planets Organic chemistry in space Interstellar distribution of organic molecules

6 Life definition There is no commonly accepted definition of life Life is usually defined using a set of properties shared by terrestrial organisms There is no common agreement about which set of properties should be adopted Example of set of properties: Metabolism Reproduction Information coding and transmission Self organization Evolution Each property is the result of interactions that take place at the molecular level

7 Problems with the definition of life Some abiotic chemical reactions may mimic metabolic reactions Not all living organisms are able to reproduce Viruses encode and express information but do not have their own metabolism There is no single property that is intrinsic and unique to life Life escapes definition since it is an evolving phenomenon Definition of life in astrobiology In the context of astrobiology, life-defining criteria should be useful to detect life in remote astronomical bodies Some properties that are important to define life (e.g., evolution) may have little use to detect remote life

8 Thermodynamical properties of life Life requires incoming energy to sustain the metabolism Life needs to keep a high internal order (low entropy) In order not to violate the 2 nd principle of thermodynamics, life must increase the entropy of its environment ΔStot = ΔSlife + ΔSenv > 0 Life with active metabolism yields a biological imprint (a biosignature ) on its environment This conclusion, as well as other conclusions purely based on thermodynamical principles, apply to any form of life, not just the terrestrial one

9 Exobiology and astronomical observations (1) Search for biosignatures (2) Search for habitable environments Focus of the present talk: exoplanets rather than Solar System bodies Not possible to collect and analyse samples Search for atmospheric biosignatures generated by surface life Surface habitability

10 Habitability and biosignatures Both are inherent to the existence of life The imprint of life generates a feedback that affects the habitability Habitable environment jjjjjj jjjj Feedback Encoding and expression of instructions jjjjjj Biosignatures

11 Habitability Physico-chemical limits of life To define criteria of habitability we need to cast light on the physicochemical limits of life Limits of terrestrial life Extremophiles Terrestrial life shows examples of organisms that live in environments with extreme physico-chemical conditions (from our point of view) These organisms, called extremophiles, cast light on the physicochemical limits of life, or at least of terrestrial life The existence of extremophiles proves that life might exist in remote astronomical bodies with harsh ambient conditions To some extent, extremophilic life might survive transportation in space ( panspermia )

12 Extremophiles Mostly micro-organisms Temperature Thermophiles & hyperthermophiles (high temperature) Psycrophiles (low temperature) ph Acidophiles, alcalophiles Pressure Barophiles (high pressure) Salinity Halophiles (high salinity) Humidity Xerophiles (low humidity) Ionizing radiations Radioresistant

13 Poly-extremophiles Extremophiles are often adapted to more than one extreme Examples Some hyperthermophiles thrive at extreme values of pressure Radioresistant microrganisms my survive dehydration conditions Multicellular extremophiles In rare cases, extremophiles can be multicellular Example: Tardigrades ( water bears ) Survive extreme temperature, pressure, dehydration, and ionizing radiation They can suspend their metabolism and go into a state of cryptobiosis, from which they can resuscitate after a long time (years) Also studied on board of the International Space Station (ISS) and in other space experiments ~0.5 mm

14 Are there any limits for life? What can we conclude from the remarkable properties of extremophiles? At first sight one may (wrongly) conclude that there are virtually no limits for life This would make impossible to define quantitative criteria of habitability We can infer physicochemical constraints by narrowing the type of life that we wish to detect Let us focus on the temperature limits

15 Temperature limits of terrestrial life Clarke et al. (2014)

16 Defining thermal limits for terrestrial life Life with active metabolism and capability of reproduction Clarke et al. (2014) 0 o C T 50 o C Multicellular poikilotherms Organisms whose functioning of all vital processes is directly affected by ambient temperature (e.g., plants, invertebrates, and ectothermic vertebrates)

17 The thermal limits of multicellular poikilotherms are relevant for all multicellular life and also for the generation of biosignatures Reasons: (1) Poikilotherms are the most diffuse and ancient form of multicellular life on Earth (2) The emergence of homeotherms requires the previous existence of poikilotherms (3) The same thermal limits are relevant for the photosynthetic production of oxygen - Plants share the same thermal limits - Most cyanobacteria share the same limits (4) Oxygen is an essential ingredient for the emergence of muticellular life and a key atmospheric biomarker (Catling et al. 2005)

18 How universal are the thermal limits of life? Terrestrial organisms evolved from independent evolutionary pathways share a similar thermal response This suggests the existence of universal mechanisms of thermal response Some of the processes underlying thermal response are universal: (1) Arrhenius law temperature dependence of the rate of chemical reactions reaction rate = A exp( Ea/kT) (2) Denaturation of biomolecules kt >> Echemical bonds Thermal performance curve

19 Chemical bonds and thermal limits of life Every property used to define life has a counterpart at the molecular level The operation of life molecular processes requires different types of chemical bonds High-energy covalent bonds (few ev/bond) are required to build the backbone of functional molecules Low-energy hydrogen bonds (~10 1 ev/bond) are required for intermolecular interactions that do not affect the covalent-bond structure

20 Hydrogen bonds are required for directional intermolecular interactions: intermolecular recognition, replication, self-organization (e.g. folding) Hydrogen bonds are probably essential in any possible form of biochemistry, not just the terrestrial one

21 Implications of hydrogen bond requirements: thermal limits of life The strengths of hydrogen bonds, typically 0.1 ev/bond, constrain the temperature range of life processes Below T ~ 2 x 10 2 K there is not enough thermal energy for their activation Above T ~ 4-5 x 10 2 K biomolecules sustained by hydrogen bonds are denaturated These limits lend support to the existence of universal thermal limits of life centered around T ~ 3 x 10 2 K

22 Implications of hydrogen bond requirements: the chemical elements of life Only a small number of chemical elements are able to form hydrogen bonds This limitation provides severe constrains to possible types of biochemistry O, N and C have the highest capability of forming hydrogen bonds In terrestrial life, most hydrogen bonds in nucleic acids and proteins are N H O bonds

23 Implications of hydrogen bond requirements: the medium of life Among cosmically abundant molecules (CH4, NH3, and H2O) water has the highest capability of forming a network of hydrogen bonds Water is an optimal medium for life processes

24 The liquid water criterion Given the importance of hydrogen bonding, and given the unique hydrogen-bonding capabilities of water, the liquid water criterion might be a universal criterion for the existence of any possible form of biochemistry, not just the terrestrial one

25 The liquid water habitable zone The concept of liquid water habitable zone was established before the detection of the first exoplanets The quantitative estimates of the inner and outer edge of the habitable zone require climate calculations The inner edge is defined according the runaway greenhouse instability Kasting et al. (1993)

26 Climate and surface temperature

27 Earth-like Surface Temperature Model

28 Physical parametrization of the term D 2 π R 2 Φ cos φ Net rate of energy transport across a circle of constant latitude φ p : Surface pressure g : Surface gravity R : Planet radius v : Meridional velocity m : Moist Static Energy cpt : Sensible Heat Lv rv : Latent Heat y : Meridional coordinate lmix : Mixing length q : Relative humidity p* : Saturation vapor pressure

29 Validation of the model with 3D aquaplanet simulations Kaspi & Showman (2015)

30 Calibration of the model with Earth data Surface temperature versus latitude and time (phase of the orbital period) Simplified geography Constant fraction land/ocean in each latitude strip No orography

31 Impact of climate factors on the surface temperature Insolation Surface atmospheric pressure

32 Impact of climate factors on the surface temperature Rotation period Axis obliquity Planet radius

33 Atmospheric mass, surface temperature and habitability The atmospheric columnar mass, Natm=p/g, strongly affects the planetary climate and habitability 1) The surface pressure enters in the definition of the liquid water temperature range (the ambient pressure provides powerful constraints on the existence of life) 2) In conjunction with the atmospheric composition, Natm governs the vertical transport (e.g. the strength of the greenhouse effect) 3) The atmospheric columnar mass also plays a key role in the horizontal transport (i.e. the energy distribution along the planet surface) 4) The atmospheric mass acts as a shield to protect life from ionizing radiations (cosmic rays and high energy photons)

34 From surface temperature to surface habitability Complex Life habitability criterion 0 o C 50 o C Habitability index h050

35 The atmospheric-mass habitable zone at constant atmospheric composition pco2= 380 ppmv The h050 index bypasses the need to calculate the runaway greenhouse limit Natm=p/g Runaway greenhouse limit Outer edge h050 Insolation Silva et al. (2016), International Journal of Astrobiology, in press (arxiv: )

36 The atmospheric-mass habitable zone at constant atmospheric composition pco2= 10 ppmv Natm=p/g h050 The location and shape of the habitable zone changes with the atmospheric composition Insolation Silva et al. (2016), International Journal of Astrobiology, in press (arxiv: )

37 The atmospheric-mass habitable zone at constant atmospheric composition pco2= 380 ppmv Natm=p/g h050 The location and shape of the habitable zone changes with the atmospheric composition Insolation Silva et al. (2016), International Journal of Astrobiology, in press (arxiv: )

38 The atmospheric-mass habitable zone at constant atmospheric composition pco2= ppmv Natm=p/g h050 The location and shape of the habitable zone changes with the atmospheric composition Insolation Silva et al. (2016), International Journal of Astrobiology, in press (arxiv: )

39 The habitable zone at low values of Natm Decreasing Natm below ~300 g/cm 2 does not help to decrease the greenhouse effect pco2= 380 ppmv h050 Surface dose of 100 msv/yr of Galactic cosmic rays (Atri et al. 2013) Below ~300 g/cm 2 the surface is exposed to high doses of ionizing radiation Insolation Silva et al. (2016), International Journal of Astrobiology, in press (arxiv: )

40 Modelization of the surface temperature and habitability of a specific exoplanet: Kepler 452 b Best candidate rocky planet in the habitable zone of a solar-type star R=1.63 R S=1.1 S (Jenkins et a. 2015)

41 Quantitative estimates of the habitability of Kepler 452b Impact of surface atmospheric pressure and atmospheric composition for different models of internal structure

42 Quantitative estimates of the habitability of Kepler 452b Evolution of surface habitability: the impact of the luminosity evolution of the central star PARSEC stellar evolution tracks (Bressan et al. 2012)

43 Applications to terrestrial-type planets Exoplanet surveys suggest that terrestrial-type planets are common and will be discovered in large numbers Occurrence rate Foreman-Mackey et al. (2014)

44 ARTECS Archive of Terrestrial-type Climate Simulations This archive compiles results obtained from a large number of climate simulations of terrestrial-type planets The purpose of these simulations is to understand how variations of planetary quantities not measurable with present observational techniques may affect the surface temperature and habitability of extrasolar planets The simulations have been performed using the ESTM

45 Conclusions Exobiology is becoming an important driver of many fields of astronomical research (Solar-System exploration, characterization of extrasolar planets, observations of protoplanetary disks) The search for universal physicochemical limits of life (not just the liquid water criterion) is required to predict the potential presence of biosignatures in remote astronomical bodies The large number of terrestrial-type exoplanets that are expected to be discovered in the near future are promising targets to the search for life outside the Solar System A detailed modelization of their properties, with the aid of specially designed climate models, is required to select the best targets for follow up observations aimed at detecting atmospheric biosignatures

46 Searching for biosignatures in transmission spectra of exoplanet atmospheres The problem of searching for atmospheric biosignatures is two-fold: Improving the detection limit to obtain atmospheric spectra of terrestrial-type exoplanets Identifying molecular species that can be used as reliable biosignatures

47 Atmospheric biosignatures: redox disequilibrium The canonical concept for the search for atmospheric biosignatures is to find an atmosphere severely out of thermochemical redox equilibrium Redox chemistry adds or removes electrons from an atom or molecule (reduction or oxidation, respectively) Redox chemistry is used by all life on Earth and is thought to enable more flexibility than non-redox chemistry The idea is that gas by-products from metabolic redox reactions can accumulate in the atmosphere and would be recognized as biosignatures because abiotic processes are unlikely to create a redox disequilibrium For instance, earth s atmosphere has oxygen (a highly oxidized species) and methane (a very reduced species) several orders of magnitude out of thermochemical redox equilibrium

48 Atmospheric biosignatures: redox disequilibrium In practice it could be difficult to detect both molecular features of a redox disequilibrium pair For example, the present-day Earth, has a relatively prominent O2 absorption at 0.76μm, whereas CH4 absorptions are extremely weak Reflection spectra in the visible/near IR of Earth, Venus and Mars Mid IR thermal emission spectra, with the black body emission of a planet of the same radius (dashed lines)

49 Habitability versus Biogenic conditions Habitability conditions Set of physicochemical conditions required for the maintainance of life Biogenic conditions Set of physicochemical conditions required for the emergence of life from the abiotic world In general, habitability and biogenic conditions will be different Biogenic conditions are probably more restrictive than habitability conditions Important to understand biogenic conditions (still very uncertain) in order to select the best targets for remote detection of life