dr. david crisp
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Understanding the Remote-Sensing Signatures of Life in Disk-averaged Planetary Spectra: 3. Dr. David Crisp (Jet Propulsion Laboratory/California Institute of Technology Dr. Victoria Meadows (California Institute of Technology). Exploring Terrestrial Planet Environments. Modern Earth - PowerPoint PPT PresentationTRANSCRIPT
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.