Lecture 13: Venus – Plate Tectonics, Runaway

Greenhouses, and the Inner Edge of the Habitable Zone

Abiol 574

Venus

• 93-bar, CO2-rich atmosphere• Practically no water (10-5

times Earth)• D/H ratio = 150 times that on Earth

What went wrong with it?

The Medea and Rare Earth hypotheses

Peter Ward

Medea hypothesis: Life is harmful to the Earth!Rare Earth hypothesis: Complex life (animals, including humans) is rare in the universe

2009 2000

List of Rare Earth arguments

1. Plate tectonics is rare2. Exoplanets may lack magnetic fields3. The animal habitable zone is narrower than the

habitable zone4. The Sun is anomalously metal-rich5. Evolutionary events like the origin of eukaryotes and

the Cambrian explosion are unlikely6. Nitrogen may not be abundant in a planet’s

atmosphere if life is not present (from Lovelock)7. Large impacts may be more frequent in planetary

systems that lack Jupiters8. A planet’s obliquity may be chaotic if it lacks a large

moon

Venus and plate tectonics

• One of Ward and Brownlee’s key “Rare Earth” arguments is that plate tectonics is rare– Argument: There are

~20 rocky planets and large moons in the Solar System. Of these, only Earth has plate tectonics

• Does Venus have plate tectonics? Image made using synthetic

aperture radar (SAR)

http://www.kidsgeo.com/geography-for-kids/0012-is-the-earth-round.php

Earth topography

• Earth’s topography shows tectonic features such as midocean ridges

http://sos.noaa.gov/download/dataset_table.html

Earth topography

• Linear mountain chains are also observed

Venus asseen byMagellan

Image made using synthetic aperture radar (SAR)

http://www.crystalinks.com/venus703.jpg

• Venus does not show such

features, suggesting that plate tectonics does not operate• But, the lack of liquid

water on Venus is probably responsible, so this should not be taken as evidence that plate tectonics is rare

Interesting observation(s):1. Craters are located randomly on

Venus’ surface (see next slide)2. There are no craters less than 3

km in diameter

• What do these observations imply?

Equal-areaprojection showing842 impact craters

Simplecylindricalprojection

G.G. Schaber et al., JGR 97, 13257 (1992)

Possible answers:1)Venus never had any water to begin withor2) Venus’ climate got out of control because of positive feedback loops in the climate system

Question:What went wrong with Venus?

Positive feedback loops(destabilizing)

Water vapor feedback

Surfacetemperature

AtmosphericH2O

Greenhouseeffect

(+)

• This feedback becomes more and more important as the atmosphere becomes warmer

Negative feedback loops(stabilizing)

IR flux feedback

Surfacetemperature

(-)

OutgoingIR flux

• This feedback can break down when the atmosphere heats up and becomes H2O-rich

Classical “runaway greenhouse”

Goody and Walker, Atmospheres (1972)After Rasool and deBergh, Nature (1970)

Assumptions:• Start from an airless planet• Outgas pure H2O or a mixture of H2O and CO2

• Solar luminosity remains fixed at present value• Calculate greenhouse effect with a gray atmosphere model

1 bar

Problems with the classical runaway greenhouse model

• Gray atmosphere approximation• No convection• No variation in solar luminosity• Planets acquire atmospheres

during accretion by impact degassing of incoming planetesimals

Alternative runaway greenhouse calculation

• Imagine a thought experiment in which you push the present Earth closer to the Sun

J. F. Kasting, Icarus, 1988

• Do this by gradually increasing the surface temperature in one’s climate model

H2O surface pressure vs. Ts

J. F. Kasting, Icarus (1988)

• Surface pressure approaches the saturation vapor pressure of water at high Ts

• Pressure exerted by a fully vapor- ized ocean is ~270 bars

100oC

Liquid watervanishes here

Vertical temperature structure

• Lower atmosphere temperature structure should be approximately adiabatic• Get moist or dry adiabat near the surface, depending on whether liquid water is present

Ocean present No ocean

J. F. Kasting, Icarus (1988)

Calculated T and H2O profiles

Temperature Water vapor

• The troposphere expands as the surface temperature rises• Water vapor becomes a major constituent of the stratosphere at surface temperatures above ~340 K (Ingersoll, JAS, 1969)• Hydrogen can then escape rapidly to space because the diffusion limit is overcome

J. F. Kasting, Icarus (1988)

Tropopause cold trap

• Temperature decreases rapidly with height in the troposphere, then levels out (or increases) in the stratosphere

• The H2O vapor pressure decreases with height in the troposphere, then remains constant (or increases) in the stratosphere

• H2O saturation mixing ratio, fsat = Psat/P, must therefore go through a minimum at some height. We call that height the tropopause cold trap

Cold trap

(= Psat/P)

Alternative runaway greenhouse calculation

• Now, calculate radiative fluxes. Define

FIR = net outgoing IR fluxFS = net absorbed solar flux for the

present solar luminosity• Then

SEFF = FIR/Fs = solar flux (relative to today) needed to sustain that temperature

Runaway greenhouse: FIR and FS

J. F. Kasting, Icarus (1988)

• Outgoing IR flux levels out above ~360 K (90oC) because the atmosphere is now opaque at those wavelengths

Present Earth

Simpson-Nakajimalimit

Planetary albedo vs. surface temperature

• The albedo decreases with increasing Ts initially because of increased absorption of solar near-IR radiation by H2O• At higher Ts, the albedo increases because of increased Rayleigh scattering by H2O

Back to the infrared…

• The key to understanding the runaway greenhouse is to think about the behavior of the outgoing IR flux, FIR

Negative feedback loops(stabilizing)

IR flux feedback

Surfacetemperature

(-)

OutgoingIR flux

• Above 360 K, the negative feedback loop is broken, so the surface temperature is free to run away

J. F. Kasting, Icarus (1988)

(Seff)

• Recall that Seff = FIR/FS

• The stratosphere becomes wet (and the oceans are thus lost) at Seff = 1.1. The corresponding orbital distance is 0.95 AU• But, stay tuned: these results have just changed!

The (liquid water) habitable zone

http://www.dlr.de/en/desktopdefault.aspx/tabid-5170/8702_read-15322/8702_page-2/

• By using climate models, we can estimate the boundaries of the habitable zone, where liquid water can exist on a planet’s surface• The habitable zone is relative wide because of the negative feedback provided by the carbonate-silicate cycle

New albedo calculations using the HITEMP database

Goldblatt model Kasting (1988) model

• As first pointed out to us by Colin Goldblatt (U. Victoria), our old climate model may have seriously underestimated absorption of visible/near-IR radiation by H2O. New data are available from the HITEMP database

Runaway greenhouse thresholds: old and new

New model(Kopparapu et al., Ap.J., 2013)

Old model(Kasting et al., 1988)

• Our own calculations using updated absorption coefficients for both H2O and CO2 suggest that the runaway greenhouse threshold is much closer than previously believed (runaway: 0.97 AU, moist greenhouse: 0.99 AU)

Revised conventional HZ limits

• The runaway and moist greenhouse limits on the inner edge of the HZ have recently been revised. They now lie much closer to Earth’s orbit

Kasting et al., PNAS, submitted (Figure by Sonny Harman)

Revised conventional HZ limits

• But, these calculations assume fully saturated atmospheres, and they neglect cloud feedback. The real inner edge could be anywhere within the red zone. This calculation needs to be done with 3-D climate models..

Revised ZAMS habitable zone in distance coordinates

Kasting et al., PNAS, submitted (Figure by Sonny Harman)

• There could be ways to broaden the habitable zone on both the inner and outer edges

• Let’s think about the inner edge first. (We’ll get back to the outer edge later.)

“Dune” planets• Abe et al., Astrobiology

(2011) suggested that dry planets with water oases at their poles might remain habitable well inside the inner edge of the conventional HZ– Seff = 1.7, or 0.77 AU

• Do such planets really exist, though?– In the science fiction novel,

much of the planet’s water has reacted with the crust, and they are working hard to recover it

• Let’s go back to an older diagram and consider some other factors that might affect planetary habitability

Kasting et al., Icarus (1993)

ZAMShabitable

zone

• The habitable zone is considered to be reasonably wide as a consequence of stabilizing feedbacks between atmospheric CO2 and climate• Bad things happen, though, to planets around stars much different from the Sun --F and A stars: high stellar UV fluxes, short main sequence lifetimes --Late K and M stars: tidal locking, stellar flares, initial volatile inventories?

• Gliese 581 is an M3V star, 0.31 Msun, 0.0135 LSun, so its habitable zone is at roughly 1/10th the distance of the Sun’s HZ

3-D climate model calculations for M- and K-

star planets• Clouds dominate the

sunny side of tidally locked planets orbiting M and late-K stars, raising their albedos

• The inner edge of the HZ is therefore pushed way in– Seff 2 for a

synchronously rotating planet around a K star (dark blue curves)

Yang et al., ApJ Lett (2013)

• Negative cloud feedback may well have pushed early Venus into the liquid water regime

• Venus lost its water anyway because the stratosphere became wet, leading to rapid photolysis and escape of H– The loss of water may have happened very early.

Hamano et al. (Nature, 2013) argue that a steam atmosphere formed during accretion and never collapsed after that

• Once the water was gone, volcanic CO2 (and SO2) built up in Venus’ atmosphere, leading to its present, hellish state

Evolution of Venus’ atmosphere

Habitable zone inner edge• The inner edge of the habitable zone is

determined by water loss and/or the runaway greenhouse

• The actual inner edge for a Sun-like star probably lies somewhere between the orbits of Earth and Venus– Simple 1-D climate models put it close to Earth’s

orbit (1 AU)– The ‘recent Venus’ limit at 0.76 AU is a reasonable

optimistic estimate for the inner edge– The ‘Dune-planet’ estimate of 0.77 AU agrees well

with the recent Venus limit, so both arguments point to this distance representing an optimistic inner edge

– Tidally locked planets could be habitable even closer in (Seff = 2 (equivalent to ~0.7 AU for the Sun) because of widespread cloudiness on their sunlit side