Astronomical Biosignatures Detecting Life From Space V Meadows

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 diskaveraged 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 CO 2 and H 2 O vapor – Spectra that show signs of a UV shield (e. g. O 3). • 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 diskaveraged 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

Atmospheric Biosignatures Tim Lenton, Centre for Ecology and Hydrology • Oxygen • A reduced gas in the presence of oxygen (e. g. O 2 and CH 4) • 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 CH 4 and CO 2 both “breathe” with the seasons, the amplitudes are extremely small.

Biosignatures in the Earth’s Visible Spectrum Reflectivity O 3 H 2 O O 2 (VPL) Data: Woolf, Traub and Jucks 2001 Model: Tinetti et al. , 2004 Wavelength ( m) • O 2 (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 O 3 CH 4 H 2 O CO 2 H 2 O The MIR is sensitive to atmospheric trace gases which could indicate habitability or life.

Terrestrial Planet Spectra Vary Widely in Solar System ? CO 2 EARTH-CIRRUS O 3 O 2 H 2 O CO 2 H 2 O VENUS X 0. 60 O 2 H 2 O MARS Iron oxides H 2 O ice EARTH-OCEAN Terrestrial planets in our Solar System offer diverse spectra which aid in their characterization.

Terrestrial Planets in the MIR N 2 O CH 4 H 2 O O 3 OCS H 2 O SO 2 CO 2 ice

Earth Through Time: Biosignatures CH 4 O 3 O 2 CH 4 • Life may have been easier to detect earlier in the Earth’s history. • 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, but CH 4 is less detectable for the mid-Proterozoic case.

Earth’s Reflectivity Through Time ARCHEAN PROTEROZOIC MODERN Rayleigh Scattering O 2 CH 4 H 2 O O 3 H 2 O CH 4 CO 2 CH 4 H 2 O O 2 CO 2

Understanding Earth-like Planets Around Other Stars F 2 V G 2 V K 2 V O 3 CH 4 O 3 O 2 CO 2 • 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 O 2/O 3 and CH 4 may be easier to detect (Segura et al. , 2005, in press).

H 2 O O 2 H 2 O

Earth in the MIR – spectral resolution Tinetti, Meadows, Crisp, Fong, Velusamy, Snively