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


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


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


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

Kevin Zahnle NASA Ames Yutaka Abe Ayoko Abe-Ouchi University of Tokyo Norman H Sleep Stanford...

Documents

Transcript of Kevin Zahnle NASA Ames Yutaka Abe Ayoko Abe-Ouchi University of Tokyo Norman H Sleep Stanford...