Bleachworld: Atmospheric Evolution of Water Worlds

Bleachworld, AGU Fall Meeting 2011
Bleachworld: Atmospheric Evolution of Water Worlds
Raymond T. Pierrehumbert
The University of Chicago
Eric Gaidos
University of Hawaii
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Bleachworld, AGU Fall Meeting 2011
The water loss scenario
• With sufficiently large insolation, get hot, moist upper atmosphere
• H2O exposed to UV, photolyzes to H and O
• H is lost to space, O2 reacts with rocks
• Then it’s game over for your ocean and life
But what if there are no rocks to serve as a sink for O2?
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Bleachworld, AGU Fall Meeting 2011
For this talk, a Waterworld is ...
A planet with an ocean sufficiently deep that its rocky core does not lead
to significant oxidative weathering.
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Bleachworld, AGU Fall Meeting 2011
We are interested because ...
• O2 back reaction could inhibit ocean loss from habitable-zone planets
orbiting M stars during, active early stage of M star evolution
• And we’d like to understand ...
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Bleachworld, AGU Fall Meeting 2011
The case of GJ1214b
• Old M-star, Tef f = 3026K
• Low density super-Earth. Water-dominated composition?
• Mass equivalent to 26,000 Earth oceans
• Close orbit. L~ = 22000W/m2 = 16L
• This puppy should be blasting out hydrogen like gangbusters!
• Transit IR spectroscopy →
No H2 envelope (Bean et al, w/ caveats)
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Bleachworld, AGU Fall Meeting 2011
Dramatis Personae
• The Radicals: OH, HO2,O1D
• The Moderates: O3P,H,O3
• The Conservatives: H2O,H2,O2,H2O2
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Bleachworld, AGU Fall Meeting 2011
How to re-form water?
H + HO2 → OH + OH → O3P + H2O
H + HO2 → O(3P) + H2O
H + HO2 → O2 + H2
OH + O3 → HO2 + O2
H + O2 + M → HO2
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Bleachworld, AGU Fall Meeting 2011
Ozone and its consequences
O(3P) + O2 + M → O3
O3 + hν(NUV) → O(1D) + O2
O1D breaks up H2 (and lots of other stuff)
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Bleachworld, AGU Fall Meeting 2011
A two-layer toy photochemistry model
EUV,FUV
NUV
H2O Photolysis layer
H2O
H
O3
H2O
O2
H2O
H2O2
H2
OH
O(3P) H
HO2
O3 O(1D)
OH
Reformation layer
O2
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Bleachworld, AGU Fall Meeting 2011
First look at photolysis layer (p = .004Pa)
EUV,FUV
NUV
H2O Photolysis layer
H2O
H
O3
H2O
O2
H2O
H2O2
H2
OH
O(3P) H
HO2
O3 O(1D)
OH
Reformation layer
O2
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Bleachworld, AGU Fall Meeting 2011
G star photon flux absorbed by H2O
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Absorbed Flux, photons/m /s
EUV
10
16
10
14
10
12
10
10
FUV
Ly-α
Flux Density
Abs. flux Blackbody
Abs. flux, Obs. Sun
8
10
0
50
100
150
wavelength (nm)
200
250
2
Bars H O per billion years
5
10
Cumulative Flux
4
10
1000
100
10
Cum. Flux, Blackbody
Cum. Flux, Obs. Sun
1
0.1
0
50
100
150
wavelength (nm)
200
250
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Bleachworld, AGU Fall Meeting 2011
For M stars, H2O photolysis dominated by Ly-α
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Total Mass GJ1214b
6
10
5
10
GJ1214b
insolation
4
10
Earthlike
insolation
1000
Earth Ocean
100
Young M
10
-2.5
-3
Sun
Lyman-α photolysis (bars/Gyr)
10
Old M
-3.5
-4
-4.5
Log(R-Ly)
-5
-5.5
-6
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Bleachworld, AGU Fall Meeting 2011
Not much chemistry goes on in the photolysis layer
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-3
Particle density ( cm )
2 10
1.5 10
H2O
O2
H
O3P
12
12
1 10
11
5 10
0
0
6
5 10
7
1 10
7
7
1.5 10 2 10 2.5 10
time (seconds)
7
7
3 10
3.5 10
7
7
4 10
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Bleachworld, AGU Fall Meeting 2011
The re-formation layer
EUV,FUV
NUV
H2O Photolysis layer
H2O
H
O3
H2O
O2
H2O
H2O2
H2
OH
O(3P) H
HO2
O3 O(1D)
OH
Reformation layer
O2
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Bleachworld, AGU Fall Meeting 2011
In presence of O2, H decays quickly in the reformation layer
Isolated
lower layer, p = 400 Pa, T = 300K, init [O ] = 10%
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2
1 10
15
-3
Particle density ( cm )
8 10
15
6 10
H
H2
15
dH2O
O2
H2O2
4 10
15
2 10
0
0
0.05
0.1
0.15
time (seconds)
0.2
0.25
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Bleachworld, AGU Fall Meeting 2011
Now couple to photolysis products from upper layer:G star case
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n =10 /cm
3
tot
3.5 10
10
10
7
10
9
7
10
8
7
7
10
10
6
10
5
10
4
3
H2 (molecules/cm )
2 10
1.5 10
H2
7
1 10
7
5 10
6
H2O2
O3
H
OH
HO2
H, O3, H2O2
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3 10
2.5 10
Lower Layer with G-star
photolysis source of O(3P) and 2H
1000
100
0
0
5 10
4
5
5
1 10
1.5 10
time (seconds)
2 10
5
5
2.5 10
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Bleachworld, AGU Fall Meeting 2011
M star: Young + Habitable Orbit or Old+ 1214b Orbit
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n =10 /cm
3
tot
1.5 10
13
Lower Layer with M-star Ly-α
photolysis source of O(3P) and 2H12
10
3
1 10
5 10
10
11
10
10
10
9
10
8
13
12
H2O2
O3
H
0
0
5000
4
OH
HO2
4
1 10
1.5 10
time (seconds)
2 10
4
H, O3, H2O2
H2 (molecules/cm )
H2
4
2.5 10
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Bleachworld, AGU Fall Meeting 2011
To Space
Reformation layer is a sink, limits H escape
EUV,FUV
NUV
H
O3
H2O
O2
H2O Photolysis layer
O(3P) H
Reformation layer
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Bleachworld, AGU Fall Meeting 2011
Conclusions
• Oxygen accumulated from losing 20% of initial ocean drives H and
H2 to low concentrations, and likely chokes off hydrogen escape and
irreversible water loss.
• Calculations at 400K aren’t much different
• Significant H escape still possible if it escapes from photolysis layer
faster than it gets transported to lower atm. Need to couple chemistry
to atmospheric escape model.
AGU next year ?
• We have not yet determined minimum O2 needed to cut off H escape.
• Actually, not much bleach on bleachworld!
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