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

jjCharacterizing Extrasolar Terrestrial Planets

Mass and Orbital Parameters

Solar System Environmental Characteristicsparent star, placement in solar system, preliminary orbit, other planets

Photometric Characteristicsbrightness, 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

Atmospheric Biosignatures

• 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

Tim Lenton, Centre forEcology and Hydrology

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

Data: Woolf, Traub and Jucks 2001Model: Tinetti et al., 2004

• O2 (life) & water (habitability) are relatively easy to detect. • Surface biosignatures such as chlorophyll may also be detectable.

Ref

lect

ivity

Ref

lect

ivity

Wavelength (Wavelength (m)m)

(VPL)(VPL)OO22

HH22OOHH22OO

OO22

OO33

Biosignatures in the Earth’s MIR Spectra

The MIR is sensitive to atmospheric trace gases which could indicate habitability or life.

COCO22

HH22OO

OO33

HH22OOCHCH44

MGS-TES: Christensen & Pearl, 1997MGS-TES: Christensen & Pearl, 1997

VPL Earth Model: Tinetti et al, 2004VPL Earth Model: Tinetti et al, 200460% cloud cover60% cloud cover

Terrestrial Planet Spectra Vary Widely in Solar System

O2

Iron oxides

CO2

H2O H2O

CO2

EARTH-CIRRUS

VENUSX 0.60

MARS

EARTH-OCEAN

H2O H2O

H2O ice

?

O3O2

Terrestrial planets in our Solar System offer diverse spectra which aid in their characterization.

Terrestrial Planets in the MIR

SO2

CO2

O3

CH4

H2O

N2O

CO2

OCS

H2O

CO2 ice

• 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 CH4 and O3

• In the visible, the O2 absorption is reduced, but potentially detectable, but CH4 is less detectable for the mid-Proterozoic case.

Earth Through Time: Biosignatures

O3

CH4

CH4O2

O3

Earth’s Reflectivity Through Time

CH4

H2O

H2O

CH4

CO2

O2

Rayleigh Scattering

CH4

ARCHEANPROTEROZOICMODERN

O2

CO2

H2OH2O

Understanding Earth-like Planets Around Other Stars

• 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 Earth-like planets around stars of different spectral type (Segura et al., Astrobiology, 2003, 3, 689-708.).

O2

F2VG2VK2V

O3

O3

CO2

CH4

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).

O2

H2O H2OH2O

Earth in the MIR – spectral resolution

Tinetti, Meadows, Crisp, Fong, Velusamy, Snively