Transcript Slide 1
Astronomical Biosignatures: Detecting Life From Space V. Meadows Remote Detection of Life • We will not be able to “resolve” the extrasolar planet • Everything we learn about the planet will be obtained from disk-averaged data. • The signs of life must be a global phenomenon Characterizing jj Extrasolar Terrestrial Planets Mass and Orbital Parameters Solar System Environmental Characteristics parent star, placement in solar system, preliminary orbit, other planets Photometric Characteristics brightness, color Spectra composition, physical properties. A Habitable Planet • A habitable planet is one that has conditions that can support life (in all its extremes). • A planet that can maintain liquid water on its surface • A Habitable Planet may not be inhabited. (…but how likely is this?) So signs of habitability do not (yet!) automatically equate to signs of life Recognizing Habitable Planets • Within the star’s “habitable zone” – distance from the parent star • Terrestrial, rocky worlds – Mass, brightness, color • ….with an atmosphere – Photometric variability (clouds, possibly surface) – Spectra that show CO2 and H2O vapor – Spectra that show signs of a UV shield (e.g. O3). • Surface conditions that support liquid water – Observations of MIR brightness, spectral determination of atmospheric composition, esp. greenhouse gases The Distant Signs of Life • Astronomical Biosignatures are photometric, spectral or temporal features indicative of life. • These biosignatures must be global-scale to enable detection in a disk-averaged spectrum. • Life can provide global-scale modification of: – A planet’s atmosphere – A planet’s surface – A planet’s appearance over time • Biosignatures always be identified in the context of the planetary environment – e.g. Earth methane and Titan methane Surface Flux (1012 moles yr-1) Atmospheric Biosignatures 10000 Hydrogen Methane Isoprene Dimethyl Sulphide Ammonia Nitrogen Nitrous Oxide Carbon Monoxide Carbon Dioxide Oxygen 1000 100 10 1 0.1 0.01 Earth Without Life Tim Lenton, Centre for Ecology and Hydrology • Oxygen • A reduced gas in the presence of oxygen (e.g. O2 and CH4) • Any species that can be determined to be out of chemical equilibrium Surface Biosignatures Crisp, Meadows Temporal Biosignatures • Cyclical or seasonal behavior that is not due to photochemistry or other abiological source. • On the Earth, although CH4 and CO2 both “breathe” with the seasons, the amplitudes are extremely small. Biosignatures in the Earth’s Visible Spectrum Reflectivity O3 H2O H2O O2 O2 (VPL) Data: Woolf, Traub and Jucks 2001 Model: Tinetti et al., 2004 Wavelength (m) • O2 (life) & water (habitability) are relatively easy to detect. • Surface biosignatures such as chlorophyll may also be detectable. Biosignatures in the Earth’s MIR Spectra MGS-TES: Christensen & Pearl, 1997 VPL Earth Model: Tinetti et al, 2004 60% cloud cover O3 CH4 H2O CO2 H2O The MIR is sensitive to atmospheric trace gases which could indicate habitability or life. Terrestrial Planet Spectra Vary Widely in Solar System ? CO2 CO2 VENUS X 0.60 EARTH-CIRRUS O2 O3 H2O H2O H2O O2 H2O MARS Iron oxides H2O ice EARTH-OCEAN Terrestrial planets in our Solar System offer diverse spectra which aid in their characterization. Terrestrial Planets in the MIR N2O CH4 H2O O3 OCS H2O SO2 CO2 CO2 CO2 ice Earth Through Time: Biosignatures CH4 O3 O2 CH4 • Life may have been easier to detect earlier in the Earth’s history. • In the MIR, MidProterozoic Earth-like atmospheres show strong signatures from both CH4 and O3 • In the visible, the O2 absorption is reduced, but potentially detectable, but CH4 is less detectable for the midProterozoic case. Earth’s Reflectivity Through Time ARCHEAN PROTEROZOIC MODERN Rayleigh Scattering O2 CH4 H2O O3 H2O H2O CH4 CO2 CH4 H2O O2 CO2 Understanding Earth-like Planets Around Other Stars F2V G2V K2V O3 CH4 O3 O2 CO2 • An Earth-like planet around another star may have different spectral characteristics due to different photochemistry and atmospheric temperature structure. – Synthetic spectra derived via a coupled climate-photochemical model for Earthlike planets around stars of different spectral type (Segura et al., Astrobiology, 2003, 3, 689-708.). Earth-like Planets Around M Stars Vis/NIR Reflectivity MIR Spectra • Molecular biosignatures may have longer atmospheric lifetimes for Earth-like planets around M stars, and the simultaneous presence of O2/O3 and CH4 may be easier to detect (Segura et al., 2005, in press). H2O O2 H2O H2O Earth in the MIR – spectral resolution Tinetti, Meadows, Crisp, Fong, Velusamy, Snively