Dr. David Crisp Dr. David Crisp (Jet Propulsion Laboratory/California Institute of (Jet Propulsion Laboratory/California Institute of Technology Dr. Victoria Meadows Technology Dr. Victoria Meadows (California Institute of Technology) (California Institute of Technology)

Understanding the Remote-Sensing Signatures of Life in Disk-averaged

Planetary Spectra: 3

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 Earth Through Time

The Earth’s Primordial Atmosphere

• Our primordial atmosphere was created by “outgassing” from molten rock prior to 200 Myr

• 40Ar/36Ar 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 (CO2, N2, and H2S or SO2). – 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.

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.

Evolution of the Earth’s Atmospheric Composition

Prebiotic Atmosphere

> 3.5Gya

Archean Atmosphere

4.0-2.3Gya

Modern Atmosphere

<2.3Gya

Surface Pressure

N2

O2

CO2

CH4

H2

CO

1-10 bars

10-80%

~0

30-90%

10-100ppm

100-1000ppm

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

Planetary Evolution

TPF/Darwin and LifeFinder will be able to observe planetary systems at different stages of evolution

N2 CO2 CH4

N2 O2 CO2 CH4

?

Earth’s Prebiotic Atmosphere

• Dominantly N2 and CO2

– < 10bars CO2 prior to continents (<~300Myrs)– 0.1-0.3 bars CO2 required to offset the faint young

Sun– H2 and CO from impactors, volcanism– H2 concentration determined by balancing volcanic

outgassing with escape to space

• Abiotic net source of O2

– Photolysis of H2O and CO2 , and escape of H to space

– But O2 would have reacted with reduced volcanic gases to form CO2 and H2O

• High-altitude O2 source: Photolysis of CO2 followed by O + O + M O2 + M

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 O2

NB: This would not have supported prebiotic synthesis via CH4 and NH3

The Archean Atmosphere

• Life arose by at least 3.5Gya – Evidence from microfossils and stromatolites.– Possible evidence for life at 3.8Gya from 13C depletion

• The Earth was inhabited - but the atmosphere was anoxic (no O2) prior to ~2.3 Gya

• Photosynthesis may have been invented, but originally used H2S (or H2) to reduce CO2

– Not H2O, as used today, so no O2 production!

• Even oxygenic photosynthesis would not have immediately produced an O2-rich atmosphere.

– O2 would have been consumed by reduced atmospheric gases or reduced surface materials.

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 CH4 is much larger

• RNA sequencing indicated that some methanogens are very ancient.

• Methanogenic bacteria can use CO2 + 4H2 CH4 + 2H2O• Could have produced 1000ppm of CH4, globally

– Longer lifetime because no O2!

• Many ramifications for Archean climate– Helps solve the faint young Sun problem (provided 15C of

warming). – But warming would drive down CO2

– Rapid loss of CH4 via an oxygenated atmosphere may have triggered an ice-age

The Rise of O2

•Somewhere around 2.3Ga, O2 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 O2 poor atmosphere

•Paleosols and Redbeds– Most paleosols prior to 2.2Gya have lost iron

• Fe released during weathering in an O2 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

Earth’s Evolution as a Terrestrial Planet

• The formation of an atmosphere containing N2 and CO2 and an H2O ocean appears to be a natural consequence of planetary accretion.

• Numerous sources of geological evidence point to atmospheric O2 levels being low prior to ~2.3 Ga

• O2 levels rose naturally, but not immediately as the result of photosynthesis and organic carbon burial– Explaining why O2 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 pO2 reached ~0.01 PAL

• Many of these general conclusions imply that Earth may not be unique.

Modern Earth

355ppm CO2

V. Meadows

Proterozoic

0.1PAL O2

100ppm CH4

15% decrease in ozone

column depth

Mead

ow

s, K

astin

g,C

risp

,Coh

en

Atmosphere from Climate Models by Pavlov et al., 2004

V. Meadows

ArcheanN2 99.8%2000ppm CO2

1000ppm CH4

100ppm H2

Karecha, Kasting, Segura, Meadows, Crisp, Cohen

Atmosphere from Ecosystem Models by Karecha et al., 2005

O3

Earth’s Reflectivity Through Time

CH4

H2O

H2O

CH4

CO2

O2

Rayleigh Scattering

CH4

ARCHEANPROTEROZOICMODERN

O2

CO2

H2OH2O

V. Meadows

• 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, CH4 is probably less detectable for the mid-Proterozoic case.

Earth Through Time - Biosignatures

CH4O2

O3

CH4

IAUC200: Kaltenegger et al., Tuesday, Session VV. Meadows

Super Earths

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

• How much O2 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 O2 first become abundant?

• What exactly caused the rise of O2?

• When did ozone become abundant enough to provide an effective solar UV screen?

Lessons from the Earth Through Time

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.

Terrestrial Planets Around

Other Stars

Modeling Planetary Environments:

The Virtual Planetary LaboratoryThe 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

SyntheticSpectra

Observer

Tas

k 4:

The

Abi

otic

Pla

net M

odel

Atmospheric and surface optical properties

Ta

sk 3

: T

he

Co

up

led

Clim

ate

-Ch

em

istr

y M

od

el

Tas

k 5:

The

Inh

abite

d P

lane

t M

odel

Tas

k 2:

The

Clim

ate

Mod

el

(SM

AR

TM

OD

)T

ask

1: S

pect

ra

AtmosphericComposition

AtmosphericChemistry

Model

RadiativeTransferModel

UV Flux andAtmosphericTemperature

Exogenic Model

Biology Model

AtmosphericThermal

Structure and Composition

Stellar Spectra

Atmospheric Escape, Meteorites, Volcanism,

Weathering products

AtmosphericThermal

Structure and Composition

Radiative Fluxes

and Heating Rates

GeologicalModel

BiologicalEffluents

Vir

tua

l Pla

ne

tary

La

bor

ato

ry

AtmosphericThermal Structureand Composition

Summary of Processes Included in the VPL

VPL Architecture

Initialize

ExogenicProcess Translator Geology

GeoInput

GeoOutput

Biology

BioInput

BioOutput

Translator Chemistry

ChemInput

EPInput

ChemOutput

Climate

ClimInput

ClimOutput

Convergence?

GeoDB

EPDB

ChemDB

ClimDB

BioDB

Translator

TranslatorTranslator

Translator

Major Time Step Loop

Common Database

ConvInput

FinalSpectrum

EPOutput

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

StellarRadiation

ThermalRadiation

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

Alti

tude

Thermodynamically Balanced Planets

StellarRadiation

ThermalRadiation

Atmospheric Composition

Convection

Cloud

VPL Climate ModelA 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

Segura et al., 2005

Model Atmospheres•1-Bar “Earth-like” atmospheres

– vary O2 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 O3,H2O, CH4 and N2O decrease with O2 abundance

• Particularly in the stratosphere

– Stratospheric temperatures cooled substantially with loss of ozone

T

O3

CH4N2OH2O

V. Meadows

F, G, K Planet Spectra

•Very little change in the visible (except O3)•MIR shows changes in CH4, O3, and CO2

– F2V planet has 2x O3 column depth– K2V planet at 1PAL same surface flux, more atm CH4

(Results published in Segura et al., Astrobiology, 2003, 3, 689-708.).

O3

CO2O2

F2VG2VK2V

O3

CH4

Segura et al., 2005,

F2V planet at 1PAL - 2X O3 column depth

K2V planet at 1PAL - same surface flux, more atm CH4

Relative Detectability of CO2, O3 and CH4

Ozone and Temperature at Different O2 Levels

Radiative-convectiveclimate model

Photochemicalmodel

Calculations by Kara Krelove Graphs by Darrell Sommerlatt

Absorption of UV radiation by O3 heats the stratosphere, and temperature affects ozone chemistry, so the most accurate calculations consider both photochemistry and temperature

• Equilibrium environments with reduced O2 have– Less stratospheric ozone– Lower stratospheric

temperatures• Less ozone heating

– Strong 9.6 m O3 band• Less stratospheric emission• 0.01xPAL O2 case almost

indistinguishable from 1xPAL case

Masking and Exaggerating Biosignatures

O3

T

O3 variations vsO2 Concentration

O2 Absorption at visible wavelengths

O3 Absorption at Thermal wavelengths

Temperature vs.O2 Concentration

Thermal IR observations of O3 alone will not provide quantitative constraints on O2

Ozone column depth vs.pO2

Kasting et al. (1985)

• Why the nonlinearity?

O2 + h O + OO + O2 + M O3 + M

• As O2 decreases, O2 photolysis occurs lower down in the atmosphere where number density (M) is higher

F Star K StarG Star

Earth-like planetary spectra at different O2 abundances around different stars - look similar in the visible – O2 most detectable down to 10-2 PAL - are similar in the MIR for G and K stars - O3 most detectable down to 10-3 PAL of O2

- quite different for F stars, which are most sensitive to 10-1 – 10-2 PAL of O2

O2 and O3 detectability vs O2 abundance

Clouds, Thermal Structure and The Detectability of Biosignatures

PAL

0.01 O2

(K

relo

ve, K

astin

g, C

risp,

Coh

en, M

eado

ws)

• High-altitude clouds

– Mask surface albedos and temperatures

– Dramatically reduce the spectral strength of the ozone and CO2 bands

Cloudy planets hide their secrets

Planets Around M Stars

Segura, Kasting, Meadows, Cohen Crisp, Tinetti, Scalo

O2 photolysis

N2O

O3

If a planet had O2, would O3 form?

CO2

CH3Cl

CH4

O3

+

N2O

H2O

EarthAD Leo planet

Active M Star Planets

Segura et al., Astrobiology, 2005.

Active M Star Planets

EarthAD Leo planet

O3

CH4 CH4CH4

O2

O2

CO2

H2OH2O

H2O

H2O

Segura et al., Astrobiology, 2005.

Surface Biosignatures on M-Star Planets: The

Infrared Edge

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

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.