kevin zahnle nasa ames yutaka abe ayoko abe-ouchi university of tokyo norman h sleep stanford...

Kevin Zahnle

NASA Ames

Yutaka Abe

Ayoko Abe-Ouchi

University of Tokyo

Norman H Sleep

Stanford

Atmospheric evolution of Venus as a habitable planet

Earth is an ocean planet

The inner edge of the HabitableZone is determined by oceansevaporating at the critical flux

Mars, Titan*, and Arrakis are Land Planets

Land planets can be habitable closer to the Sun

*Methane plays the role of water on Titan

QuickTime™ and aTIFF (LZW) decompressor

are needed to see this picture.

Earth: tropics are stabilizedby heat transport to the poles

Ocean planet: runaway greenhouse when poles cannot radiate all the heat that comes from the tropics

Land planets can radiatemore efficiently fromdry tropics - poles stayhabitable

energy transport

energy transport

energy transport

IdealizedGCM expts confirm thata generic land planetis more stable at high insolationthan an ocean planet

relative humidityP

reci

pita

ble

wat

er [m

/m2 ]

Planetary Longwave (thermal) Radiation [W/m2]

Temperature (C)

0

0

30

30

60

60

runa

way

gre

enho

use

The Land Planet remains Habitable up to the runaway

(relative to flux @Earth)

stratosphericwater vapor

The stratosphereof a land planetis extremely dry

relativehumidity

Planetary Longwave (thermal) Radiation [W/m2]

Land planetsare stable againstH escape

The dry stratosphereinhibits HydrogenEscape

0.0001

0.01

1

100

104

106

200 250 300 350 400 450

Emitted Thermal IR Radiation [W/m-2]

land planet

Energy-limited escape

10-2 Ocean

10-3 Ocean

ocean planet Kasting 1988diffusion-limited escape

10X Energy-limited

ocean planet Abe 1988diffusion-limited escape

Planetary Longwave (thermal) Radiation [W/m2]

Land planetsare stable againstH escape

The dry stratosphereinhibits HydrogenEscape

0.0001

0.01

1

100

104

106

200 250 300 350 400 450

Emitted Thermal IR Radiation [W/m-2]

land planet

Energy-limited escape

10-2 Ocean

10-3 Ocean

ocean planet Kasting 1988diffusion-limited escape

10X Energy-limited

ocean planet Abe 1988diffusion-limited escape

Planetary Longwave (thermal) Radiation [W/m2]

0.4

0.5

0.6

0.70.80.9

1

2

2 4 6 8 10Time [Myrs]

SolarLuminosity

Venus Ocean

Venus Land

Earth Ocean

Earth Land

0.05

NOW

As the Sun gets brighter, first Venus then Earthencounter the Runaway Greenhouse Effect

Time [Gyrs]

Caladan - a (fictional) ocean planet

In this image, Caladan appears to be deep in a moist greenhouse

The Moist Greenhouse

Before the runaway, there is a brief time when the planet is habitable and the stratosphere is wet.

H escape is fast.

This sets Kasting’s inner bound to the Habitable Zone

Stratosphere

The MoistGreenhouse:

H escape is fast

Can the ocean escape before the planet becomes uninhabitable?

0.0001

0.01

1

100

104

106

200 250 300 350 400 450

Emitted Thermal IR Radiation [W/m-2]

land planet

Energy-limited escape

10-2 Ocean

10-3 Ocean

ocean planet Kasting 1988diffusion-limited escape

10X Energy-limited

ocean planet Abe 1988diffusion-limited escape

Planetary Longwave (thermal) Radiation [W/m2]

Earth: Kasting’s (1988) moist greenhouse model with H escape

0

0.2

0.4

0.6

0.8

1

1.2

200

250

300

350

400

450

500

4 5 6 7 8 9

Time (Gyr)

Kasting 1988

Ocean VolumeSurfaceTemperature

albedo

NOW

0

0.2

0.4

0.6

0.8

1

1.2

300

350

400

450

500

0.5 1 1.5 2Time (Gyr)

Ocean Volume

Ts

constantalbedo=0.35

"Venus" can lose an Ocean

0

0.2

0.4

0.6

0.8

1

1.2

300

350

400

450

500

0.5 1 1.5 2Time (Gyr)

Ocean Volume

Ts

constantalbedo=0.35

"Venus" other albedoes

Ts

Ts

albedo=0.32

albedo=0.38

0.4

0.5

0.6

0.70.80.9

1

2

2 4 6 8 10Time [Myrs]

SolarLuminosity

Venus Ocean

Venus Land

Earth Ocean

Earth Land

0.05

NOW

As the Sun gets brighter, first Venus then Earthencounter the Runaway Greenhouse Effect

Time [Gyrs]

Sur

face

Tem

pera

ture

[K]

Time

350

750

1300

<10Myr

Carbonate decrepitates“Dune”

“Venus”

The End is Quick

A habitable ocean planet can evolve into a habitable land planet without passing through a runaway greenhouse state • The oceans are lost during moist greenhouse state • The land planet is a kind of afterlife

It is possible that this will happen to Earth

It is possible that this has already happened to Venus

There may be observable consequences