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: 2

Rationale Understanding the origin and evolution of terrestrial planets, and their plausible diversity, will help inform the search and characterization of extrasolar terrestrial planets. –The emphasis is not only on understanding the likely planetary environments, but Understanding their appearance to astronomical instrumentation Understanding whether they are able to support life –As we search for habitable worlds, superEarths Are likely to be the first extrasolar terrestrial planets that are characterized represent a class of terrestrial planet that may also support life –And this could all happen in our lifetimes!!

? Planetary Environmental Characteristics Is it a terrestrial planet? (Mass, brightness, color) Is it in the Habitable Zone? (global energy balance?) –Stellar Type - luminosity, spectrum –Orbit radius, eccentricity, obliquity, rotation rate In general, moderate rotation rate, low obliquity and a near circular orbit stabilizes climate. –Bolometric albedo – fraction of stellar flux absorbed Does it have an atmosphere? –Photometric variability (clouds, possibly surface) –Greenhouse gases: CO 2,H 2 O vapor present? –UV shield (e.g. O3)? –Surface pressure –Clouds/aerosols What are its surface properties? –Presence of liquid water on the surface Surface pressure > 10 mbar, T> 273 K –Land surface cover Interior: What is the geothermal energy budget?

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.

The Planet We Know and Love G. Chin GSFC

Habitability Markers and Biosignatures in the MIR CO 2 – atmosphere, greenhouse gas, vertical T structure, secondary indicator of possible UV shield. H 2 O SO 2, OCS, H 2 S – volcanism, lack of surface water Selsis et al., 2002; Tinetti, et al., Potential Biosignatures: O 3,CH 4, N 2 O,SO 2, DMS, CH 3 Cl, NH 3, H 2 S

The Earth at TPF Resolution

Biomarkers at Visible Wavelengths O2O2 H2OH2O O4O4 O3O3 Changes in disk-averaged reflectivities with phase are due to clouds Data: Woolf, Traub and Jucks 2001 Models: Tinetti et al., 2005

The Photosynthetic Red Edge Life Changes a Planet’s Surface Harry Lehto

~40% Vegetation in the diurnal cycle Earth, clear sky case Earth with clouds NDVI Tinetti et al., 2005c

NDVI at Dichotomy Tinetti et al., submitted, 2005 The red-edge could be potentially observed even on a cloudy planet using filters. - but the “red” edge may shift for different plants and star types!

Would need to be at significantly higher concentration than modern Earth Biosignatures for Ocean Life Tinetti et al., 2005b

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.

Solar System Planets

Origin of the Terrestrial Atmospheres Terrestrial planets did not capture their own atmospheres –Too small and warm –Our atmospheres are considered “secondary” Instead, terrestrials were enriched with impact delivered volatiles. –Water, methane, carbon dioxide and other gases were trapped in the Earth’s interior rock Venus and Earth, forming relatively close together in the solar nebula, probably started with a similar inventory of volatiles.

Terrestrial Planet Atmospheres Nitrogen, N 2 Oxygen, O 2 Argon, Ar Water Vapor, H 2 O Carbon Dioxide, CO Carbon Dioxide, CO 2 Nitrogen, N 2 Argon, Ar Water Vapor, H 2 O and 7x and 0.01 Earth – 1bar % Composition Mars and Venus ~ 0.01 and 100 bars

Thermal IR Spectra of Terrestrial Planets crisp

Venus’ Climate History Although Venus and Earth are believed to have started with the same amount of volatiles, they followed very different evolutionary paths. The early Venus may have been habitable with water oceans –Evidence of loss of water seen in the present day D/H ratio This water was most probably lost to space via a “runaway greenhouse effect” –Venus’ closer proximity to the Sun increased the amount of water vapor in its atmosphere, which enhanced the greenhouse effect in a positive feedback loop –The water vapor was photolyzed, and the H lost to space –Over billions of years, Venus may have lost an ocean of water this way (lower limit is a global ocean several meters deep).

Mars’ Climate History Mars may have had a much warmer climate in its past –Geological evidence from erosion patterns suggest that liquid water was stable on the surface. (picture) Warming was probably due to an enhanced greenhouse effect. –A CO 2 atmosphere at 400 times present density would work for the present Sun Volcanism may have been a source of CO 2 –However, the faint young Sun would require that Mars had an extra means of warming the surface. CH 4 has been postulated as the missing greenhouse gas Source of CH 4 for early Mars?

Modeling Solar System Planets Solar System planets offer diverse spectra for characterization.

Solar System Planets at R~70 Earth Venus Titan Neptune H2OH2O H2OH2O CO 2 H2OH2O H2OH2O H2OH2O CH 4 IAUC200: Fortney and Marley, Tuesday, Session V

Temporal Variability- Seasonal Changes Seasonal changes are visible in the disk-averaged spectra - As either changes in intensity or spectral shape The ice cap is most detectable for :  m, due to wavelength dependent emissivity of CO 2 ice. Tinetti, Meadows, Crisp, Fong, Velusamy, Snively, Astrobiology, 2005 Modern Mars Frozen Mars

Haze is thought to form from photolysis (and charged particle irradiation) of CH 4 (Picture from Voyager 2) Titan’s Organic Haze Layer

Titan Anti-greenhouse Effect Pavlov et al., JGR (2001)

Conclusions Our Solar System planets 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