1. Dr. David Crisp (Jet Propulsion Laboratory/California Institute of Technology Dr. Victoria Meadows (California Institute of Technology) Understanding the Remote-Sensing Signatures of Life in Disk-averaged Planetary Spectra: 3

2. Exploring Terrestrial Planet Environments Modern Earth –Observational and ground-measurement data Planets in our Solar System –Astronomical and robotic in situ data The Evolution of Earth –Geological record, models Extrasolar Terrestrial Planets –Models, validation against Solar System planets including Earth.

3. The Earth Through Time

4. The Earth’s Primordial Atmosphere Our primordial atmosphere was created by “outgassing” from molten rock prior to 200 Myr 40 Ar/ 36 Ar in our present atmosphere indicates that core formation, and release of gases, took no more than a few 10s of millions of years. This released most of the water vapor and gases (CO 2, N 2, and H 2 S or SO 2 ). –Earth would have initially had a steam atmosphere –As the planet cooled, this condensed to produce our oceans and atmosphere. Impacts may also have delivered volatiles directly to the surface after the Earth formed. –However, the D/H ratio in comets is too high to have supplied most of the earth’s oceans.

5. The Faint Young Sun Paradox And yet, the Earth’s surface temperature has been maintained within the tolerance limits of living organisms for more than 3 billion years, despite substantial changes in solar luminosity Kasting et al., Scientific American (1988) The Sun today is considered to be 30% brighter than it was 4.6Ga.

6. Evolution of the Earth’s Atmospheric Composition Prebiotic Atmosphere > 3.5Gya Archean Atmosphere 4.0-2.3Gya Modern Atmosphere <2.3Gya Surface Pressure N 2 O 2 CO 2 CH 4 H 2 CO 1-10 bars 10-80% ~0 30-90% 10-100ppm 100-1000ppm 1-2 bars 50-80% ~0 10-20% 1000-10000ppm 1 bar 78% 21% 0.036% 1.6ppm 0.5ppm 0.1-0.2ppm The Earth

7. Planetary Evolution TPF/Darwin and LifeFinder will be able to observe planetary systems at different stages of evolution N 2 CO 2 CH 4 N 2 O 2 CO 2 CH 4 ?

8. Earth’s Prebiotic Atmosphere Dominantly N 2 and CO 2 –< 10bars CO 2 prior to continents (<~300Myrs) –0.1-0.3 bars CO 2 required to offset the faint young Sun –H 2 and CO from impactors, volcanism –H 2 concentration determined by balancing volcanic outgassing with escape to space Abiotic net source of O 2 – Photolysis of H 2 O and CO 2, and escape of H to space –But O 2 would have reacted with reduced volcanic gases to form CO 2 and H 2 O High-altitude O 2 source: Photolysis of CO 2 followed by O + O + M  O 2 + M

9. Weakly Reducing Early Atmosphere J. F. Kasting, Science (1993) The Earth’s Prebiotic Atmosphere was a “weakly reducing atmosphere”. It contained small concentrations of reduced gases and almost no free O 2 NB: This would not have supported prebiotic synthesis via CH 4 and NH 3

10. The Archean Atmosphere Life arose by at least 3.5Gya –Evidence from microfossils and stromatolites. –Possible evidence for life at 3.8Gya from 13 C depletion The Earth was inhabited - but the atmosphere was anoxic (no O 2 ) prior to ~2.3 Gya Photosynthesis may have been invented, but originally used H 2 S (or H 2 ) to reduce CO 2 –Not H 2 O, as used today, so no O 2 production! Even oxygenic photosynthesis would not have immediately produced an O 2 -rich atmosphere. –O 2 would have been consumed by reduced atmospheric gases or reduced surface materials.

11. Life and Archean Methane Methane may have become abundant soon after life arose –Abiotic methane is produced by outgassing from mid- ocean ridge hydrothermal vents –The potential biotic source of CH 4 is much larger RNA sequencing indicated that some methanogens are very ancient. Methanogenic bacteria can use CO 2 + 4H 2  CH 4 + 2H 2 O Could have produced 1000ppm of CH 4, globally –Longer lifetime because no O 2 ! Many ramifications for Archean climate –Helps solve the faint young Sun problem (provided 15C of warming). –But warming would drive down CO 2 –Rapid loss of CH 4 via an oxygenated atmosphere may have triggered an ice-age

12. The Rise of O 2 Somewhere around 2.3Ga, O 2 levels in the atmosphere rose dramatically Geological evidence includes –Banded iron formations, formed in an anoxic ocean, mostly found more than 1.8 Gya (but 0.6-0.8Gya also…during widespread glaciation) Detrital Uraninite and Pyrite –Found prior to 2.3Ga and could only have been weathered in an O 2 poor atmosphere Paleosols and Redbeds –Most paleosols prior to 2.2Gya have lost iron Fe released during weathering in an O 2 poor atmosphere would have been leached away. –Redbeds indicate oxidizing atmospheric conditions at the time of their formation. Earliest found 2.2Gya. Sulfur Isotope Data

13. Earth’s Evolution as a Terrestrial Planet The formation of an atmosphere containing N 2 and CO 2 and an H 2 O ocean appears to be a natural consequence of planetary accretion. Numerous sources of geological evidence point to atmospheric O 2 levels being low prior to ~2.3 Ga O 2 levels rose naturally, but not immediately as the result of photosynthesis and organic carbon burial –Explaining why O 2 first rose at 2.3 Ga while cyanobacteria arose prior to 2.7 Ga is still an ongoing task An effective ozone screen against solar UV radiation was established by the time pO 2 reached ~0.01 PAL Many of these general conclusions imply that Earth may not be unique.

14. Modern Earth 355ppm CO 2 V. Meadows

15. Proterozoic 0.1PAL O 2 100ppm CH 4 15% decrease in ozone column depth Meadows, Kasting,Crisp,Cohen Atmosphere from Climate Models by Pavlov et al., 2004 V. Meadows

16. Archean N 2 99.8% 2000ppm CO 2 1000ppm CH 4 100ppm H 2 Karecha, Kasting, Segura, Meadows, Crisp, Cohen Atmosphere from Ecosystem Models by Karecha et al., 2005

17. O3O3 Earth’s Reflectivity Through Time CH 4 H2OH2O H2OH2O CO 2 O2O2 Rayleigh Scattering CH 4 ARCHEAN PROTEROZOIC MODERN O2O2 CO 2 H2OH2O H2OH2O V. Meadows

18. In the MIR, Mid- Proterozoic Earth-like atmospheres show strong signatures from both CH 4 and O 3 In the visible, the O 2 absorption is reduced, but potentially detectable, CH 4 is probably less detectable for the mid-Proterozoic case. Earth Through Time - Biosignatures CH 4 O2O2 O3O3 IAUC200: Kaltenegger et al., Tuesday, Session V V. Meadows

19. Super Earths

20. The Evolutionary Trajectories of Super Earths (some speculations!) A more massive planet would have a longer geothermal lifetime If it rotates, it could maintain a dynamo and a significant magnetosphere longer than a much smaller planet Outgassing (and tectonic processes) would continue for a longer time A larger planet would maintain more of its lighter volatiles longer (for better or worse) giving an ecosystem longer to evolve Planetary differentiation processes may contribute to the environment in unpredictable ways as a function of planetary mass (ie different materials may be sequestered in the core) Differentiation affects formation of continents –continental weathering provides a source of phosphorus, –continents increase mixing within an ocean basin

21. How much O 2 was present prior to the origin of life? What did the Earth look like at that time? When did oxygenic photosynthesis evolve? When did atmospheric O 2 first become abundant? What exactly caused the rise of O 2 ? When did ozone become abundant enough to provide an effective solar UV screen? Lessons from the Earth Through Time

22. Exploring Terrestrial Planet Environments Modern Earth –Observational and ground-measurement data Planets in our Solar System –Astronomical and robotic in situ data The Evolution of Earth –Geological record, models Extrasolar Terrestrial Planets –Models, validation against Solar System planets including Earth.

23. Terrestrial Planets Around Other Stars

24. Modeling Planetary Environments: The Virtual Planetary Laboratory The Virtual Planetary Laboratory (VPL) is a numerical model developed to Simulate a broad range of planetary environments. –Planets other than Earth, around stars other than our Sun. Include abiotic and inhabited planets –Oxygen/non-oxygen producing life Generate realistic full-disk spectra that cover a broad range of wavelengths ultimately provide a comprehensive, flexible tool which can be used by a broader community. Climate Model Synthetic Spectra Observer Task 4: The Abiotic Planet Model Atmospheric and surface optical properties Task 3: The Coupled Climate-Chemistry Model Task 5: The Inhabited Planet Model Task 2: The Climate Model (SMARTMOD) Task 1: Spectra Atmospheric Composition Atmospheric Chemistry Model Radiative Transfer Model UV Flux and Atmospheric Temperature Exogenic Model Biology Model Atmospheric Thermal Structure and Composition Stellar Spectra Atmospheric Escape, Meteorites, Volcanism, Weathering products Atmospheric Thermal Structure and Composition Radiative Fluxes and Heating Rates Geological Model Biological Effluents Virtual Planetary Laboratory Atmospheric Thermal Structure and Composition

25. Summary of Processes Included in the VPL

26. VPL Architecture Initialize Exogenic Process Translator Geology Geo Input Geo Output Biology Bio Input Bio Output Translator Chemistry Chem Input EP Input Chem Output Climate Clim Input Clim Output Convergence ? Geo DB EP DB Chem DB Clim DB Bio DB Translator Major Time Step Loop Common Database Conv Input Final Spectrum EP Output The VPL simulates equilibrium planetary environments as an initial value problem by marching forward in time from an assumed initial state. It includes a series of modules that share environmental data through a common database as they progress through each time step

27. Stellar Radiation Thermal Radiation Atmospheric Composition Convection Cloud Why a climate model? Climate affects a planet’s reflected and emitted spectrum Climate will change with stellar type orbital distance A Simple Climate Model One Dimensional (vertical) Three heat transport Processes Radiative Transfer Solar heating Thermal cooling Vertical Convection Latent heat Cloud condensation, evaporation, precipitation Goal: Investigate spectra of planets in thermodynamic equilibrium Initial Guess Final Profile T Altitude Thermodynamically Balanced Planets

28. Stellar Radiation Thermal Radiation Atmospheric Composition Convection Cloud VPL Climate Model A 1-dimensional climate model is being developed to simulate the environments of plausible extrasolar terrestrial planets. –provides only a globally-averaged description of the planet’s surface temperature and atmospheric thermal structure –Includes all physical processes that contribute to the vertical transport of heat and volatiles throughout the atmospheric column Radiative heating and cooling rates: Comprehensive, spectrum-resolving model of the solar and thermal fluxes and radiative heating rates in realistic scattering, absorbing, emitting planetary atmospheres Vertical convective heat and volatile transport: Mixing length formulation based on a state-of-the-art planetary boundary layer model (U. Helsinki) Diffusive heat transport: Diffusive heat transport within the surface and near-surface atmosphere, and within the exosphere is simulated by a multi- layer vertical heat diffusion model Latent heat transport: A versatile cloud/aerosol model that simulates airborne particle nucleation, condensation, evaporation, coagulation, and precipitation of any species identified as an active volatile or passive aerosol (dust) in the climate system –Equilibrium climate derived by solving the vertical heat/volatile transport equation as an initial value problem, starting from an assumed state

29. Segura et al., 2005

30. Model Atmospheres 1-Bar “Earth-like” atmospheres –vary O 2 from present atmospheric levels (20.99%) to 1x10 -5 of its present-day values. (Krelove and Kasting) Atmospheric T and composition were allowed to evolve to a near-equilibrium state at 1 AU from a solar-like (G2) star. –Abundance of O 3,H 2 O, CH 4 and N 2 O decrease with O 2 abundance Particularly in the stratosphere –Stratospheric temperatures cooled substantially with loss of ozone T O3O3 CH 4 N2ON2O H2OH2O V. Meadows

31. F, G, K Planet Spectra Very little change in the visible (except O 3 ) MIR shows changes in CH 4, O 3, and CO 2 –F2V planet has 2x O3 column depth –K2V planet at 1PAL same surface flux, more atm CH 4 (Results published in Segura et al., Astrobiology, 2003, 3, 689-708.). O3O3 CO 2 O2O2 F2V G2V K2V O3O3 CH 4

32. Segura et al., 2005, F2V planet at 1PAL - 2X O 3 column depth K2V planet at 1PAL - same surface flux, more atm CH 4 Relative Detectability of CO 2, O 3 and CH 4

33. Ozone and Temperature at Different O 2 Levels Radiative-convective climate model Photochemical model Calculations by Kara KreloveGraphs by Darrell Sommerlatt Absorption of UV radiation by O 3 heats the stratosphere, and temperature affects ozone chemistry, so the most accurate calculations consider both photochemistry and temperature

34. Equilibrium environments with reduced O 2 have –Less stratospheric ozone –Lower stratospheric temperatures Less ozone heating –Strong 9.6  m O 3 band Less stratospheric emission 0.01xPAL O 2 case almost indistinguishable from 1xPAL case Masking and Exaggerating Biosignatures O3O3 T O 3 variations vs O 2 Concentration O 2 Absorption at visible wavelengths O 3 Absorption at Thermal wavelengths Temperature vs. O 2 Concentration Thermal IR observations of O 3 alone will not provide quantitative constraints on O 2

35. Ozone column depth vs.pO 2 Kasting et al. (1985) Why the nonlinearity? O 2 + h  O + O O + O 2 + M  O 3 + M As O 2 decreases, O 2 photolysis occurs lower down in the atmosphere where number density (M) is higher

36. F Star K Star G Star Earth-like planetary spectra at different O 2 abundances around different stars - look similar in the visible – O 2 most detectable down to 10 -2 PAL - are similar in the MIR for G and K stars - O 3 most detectable down to 10 -3 PAL of O 2 - quite different for F stars, which are most sensitive to 10 -1 – 10 -2 PAL of O 2 O 2 and O 3 detectability vs O 2 abundance

37. Clouds, Thermal Structure and The Detectability of Biosignatures PAL 0.01 O 2 (Krelove, Kasting, Crisp, Cohen, Meadows) High-altitude clouds –Mask surface albedos and temperatures –Dramatically reduce the spectral strength of the ozone and CO 2 bands Cloudy planets hide their secrets

38. Planets Around M Stars Segura, Kasting, Meadows, Cohen Crisp, Tinetti, Scalo O 2 photolysis N2ON2O O3O3 If a planet had O 2, would O 3 form?

39. CO 2 CH 3 Cl CH 4 O 3 + N2ON2O H2OH2O Earth AD Leo planet Active M Star Planets Segura et al., Astrobiology, 2005.

40. Active M Star Planets Earth AD Leo planet O3O3 CH 4 O2O2 O2O2 CO 2 H2OH2O H2OH2O H2OH2O H2OH2O Segura et al., Astrobiology, 2005.

41. Surface Biosignatures on M- Star Planets: The Infrared Edge

42. Conclusions planets in our Solar System are a good starting point, but –terrestrial planets may be larger in the sample that TPF finds. –terrestrial planets may exist in planetary systems very unlike our own Modeling will be required to interpret the data returned from TPF-C, TPF-I and Darwin –To explore a wider diversity of planets than those in our Solar System – To help interpret and constrain first order characterization data

43. The Virtual Planetary Laboratory Team The NAI’s Virtual Planetary Laboratory (VPL) Team is an example of the highly interdisciplinary team needed to Assess detectability of biosignatures on extrasolar planets Support the development of TPF and future missions to search for life in the universe.