Transcript Slide 1

EART164: PLANETARY
ATMOSPHERES
Francis Nimmo
F.Nimmo EART164 Spring 11
Dynamics Key Concepts 1
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•
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Hadley cell, zonal & meridional circulation
Coriolis effect, Rossby number, deformation radius
Thermal tides
Geostrophic and cyclostrophic balance, gradient winds
Thermal winds
u
Ro 
2 L sin 
du
1 P

 2 sin v  Fx
dt
 x
u
g T

z
fT y
F.Nimmo EART164 Spring 11
Dynamics Key Concepts 2
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Reynolds number, turbulent vs. laminar flow
Velocity fluctuations, Kolmogorov cascade
Brunt-Vaisala frequency, gravity waves
Rossby waves, Kelvin waves, baroclinic instability
Mixing-length theory, convective heat transport
Re 
uL

ul ~(e l)1/3
 NB
g   dT  g 
  

T   dz  C p 
2
 ~ uR / 
1/ 2
 dT
F ~ C p 
 dz
dT

dz
ad



3/ 2
1/ 2
g
 
T 
H2
F.Nimmo EART164 Spring 11
This Week - Long term evolution
& climate change
• Recap on energy balance and greenhouse
• Common processes
– Faint young Sun
– Atmospheric loss
– Orbital forcing
• Examples and evidence
– Earth
– Mars
– Venus
F.Nimmo EART164 Spring 11
Teq and greenhouse
Venus
Earth
Mars
Titan
Solar constant S (Wm-2)
2620
1380 594
15.6
Bond albedo A
0.76
0.4
0.15
0.3
Teq (K)
229
245
217
83
Ts (K)
730
288
220
95
Greenhouse effect (K)
501
43
3
12
Inferred ts
136
1.2
0.08
0.96
 S (1  A) 

Teq  
 4e 
1/ 4
 3 
Ts  T 1  t s 
 4 
4
4
eq

Recall that t   dz
So if =constant, then t =  x column density
So a (wildly oversimplified) way of
calculating Teq as P changes could use:
Example: water on early Mars
P
t 
g
F.Nimmo EART164 Spring 11
Climate Evolution Drivers
Driver
Period
Examples
Seasonal
1-100s yr
Pluto, Titan
Spin / orbit variations
10s-100s kyr
Earth, Mars
Solar output
Secular (faint young Sun);
and 100s yr
Earth
Volcanic activity
Secular(?); intermittent
Venus(?), Mars(?), Earth
Atmospheric loss
Secular
Mars, Titan
Impacts
Intermittent
Mars?
Greenhouse gases
Various
Venus, Earth
Ocean circulation
10s Myr (plate tectonics)
Earth
Life
Secular
Earth
Albedo changes can amplify (feedbacks)
F.Nimmo EART164 Spring 11
Common processes
• Faint young sun
• Atmospheric loss & impacts
• Orbital forcing
F.Nimmo EART164 Spring 11
1. Faint young Sun
• High initial UV/X-ray fluxes (atmos loss)
• Sun was 30% fainter 4.4 Gyr ago
Zahnle et al. 2007
F.Nimmo EART164 Spring 11
Faint Young Sun
• 4.4 Gyr ago, the Sun emitted 70% of today’s flux
• What would that do to surface temperatures?
Venus
Earth
Mars
Titan
Teq (K) at 4.4 Gyr B.P.
209
224
198
76
Assumed ts
136
1.2
0.08
0.96
Ts (K) at 4.4 Gyr B.P.
665
263
201
87
Ts (K) at present day
730
288
220
95
Albedos assumed not to have changed
• Effects most important for Earth and Titan
• Earth would be deep-frozen, and Titan would not have
liquid ethane
• Why might these estimates be wrong?
F.Nimmo EART164 Spring 11
Feedbacks
• Temperature-albedo feedback can positive or negative
– Clouds – negative feedback (T , A )
– Ice caps – positive feedback (T , A )
 S (1  A) 

Teq  
 4e 
1/ 4
4e T 4
Aeq  1 
S
A, Aeq
A, Aeq
ICE CAP
CLOUDS
A
A
Aeq
Aeq
T
T
F.Nimmo EART164 Spring 11
2. Atmospheric loss
• An important process almost everywhere
• Main signature is in isotopes (e.g. C,N,Ar,Kr)
• Main mechanisms:
–
–
–
–
–
–
Thermal (Jeans) escape
Hydrodynamic escape
Blowoff (EUV, X-ray etc.)
Freeze-out
Ingassing & surface interactions (no fractionation?)
Impacts (no fractionation)
F.Nimmo EART164 Spring 11
Jeans escape
• Thermal process (in exosphere)
• Important when thermal velocity of molecule
exceeds escape velocity (H,He especially)
• Leads to isotopic fractionation
  nvth (1   )e

2
esc
2
th
v
2GM / R


v
2RgT / 
 is flux (atoms m-2
s-1, n is number
density (atoms m-3)
F.Nimmo EART164 Spring 11
Hydrodynamic escape
• Other species can be “dragged along” as H is
escaping (momentum transfer)
• Important at early times (primordial atmos.)
• Process leads to isotopic fractionation
• Fractionation is strongest at
intermediate H escape rates –
why?
F.Nimmo EART164 Spring 11
Blowoff/sputtering (X-ray/UV)
• Molecules in the exosphere can have energy
added by photons (e.g. X-rays, UV etc.),
charged particles or neutrals (e.g. solar wind)
• This additional energy may permit escape
• Energy-limited mass flux is given by:
dma
 Rext Fext
e
dt
GM / R
2
Here Fext is the particle flux of interest, Rext2 is the relevant cross-section, M
and R are the mass and radius of the planet and e is an efficiency factor (~0.3)
E.g. hot Jupiters can lose up to ~1% of their mass by this process;
more effective for lower-mass planets
F.Nimmo EART164 Spring 11
Freeze-out
• Unlikely unless other factors cause initial reduction in
greenhouse gases (solar radiance increasing with time)
• But potentially important albedo feedbacks
• Can happen seasonally (Mars, Triton, Pluto?)
• Mars probably lost a lot of its water via freeze-out as its
surface temperature declined
Triton freeze-out (Spencer 1990)
F.Nimmo EART164 Spring 11
Ingassing and surface interactions
• Plate tectonics can take volatiles (e.g. water) and
redeposit them in the deep mantle
• Reactions can remove gases e.g. oxygen was efficiently
scavenged on early Earth (red beds) and Mars
• A very important reaction is the Urey cycle:
MgSiO3  CO2  MgCO3  SiO2
• This proceeds faster at higher temperatures and in the
presence of water (+ and - feedbacks)
• Causes removal of atmospheric CO2 on Earth and
maybe Mars (but where are the carbonates?)
• Reverse of this cycle helped initiate runaway
greenhouse effect on Venus (see later)
F.Nimmo EART164 Spring 11
Magma oceans
• Magma oceans can arise in 4 ways:
–
–
–
–
Close-in, tidally-locked exoplanets (hemispheric)
Extreme greenhouse effect (e.g. steam atmosphere)
Gravitational energy (giant impacts) (Earth)
Early radioactive heating (26Al) (Mars?)
• Some volatiles (e.g.
H2O, CO2) are quite
soluble in magma
• Magma oceans can
store volatiles for later,
long-term release
F.Nimmo EART164 Spring 11
Impact-driven loss
• Tangent plane appx.
• Runaway process
• Much less effective on
large bodies (Earth)
than small bodies
(Mars)
• Asteroids tend to
remove volatiles;
comets tend to add
• Does not fractionate
2
Vi
2
M  2 Ri 0 H
Vesc
M 1 Vi R

2
M
4 Vesc R
2
2
i
2
p
F.Nimmo EART164 Spring 11
Isotopic signatures
Solar N/Ne=1
Zahnle et al. 2007
F.Nimmo EART164 Spring 11
3. Orbital forcing
• Universal process, details vary with planet
• For Earth, Milankovitch cycle forcing amplitudes are
small compared to (observed) response – feedbacks?
F.Nimmo EART164 Spring 11
1. Earth
F.Nimmo EART164 Spring 11
Water on early Earth
• Hadean Zircons (4.4 Ga)
– Oxygen isotopes (higher than expected for mantle)
– Low melting temperatures (Ti thermometer)
• Isua Pillow Basalts (3.8 Ga)
– Indicates liquid water present
– Possible indication of plate tectonics (?)
F.Nimmo EART164 Spring 11
Faint Young Sun Problem
Rampino & Caldeira (1994)
• How were temperatures suitable for liquid
water maintained 4 Gyr B.P.?
• Presumably some greenhouse gas (CO2?)
• Urey cycle as temperature stabilizer F.Nimmo EART164 Spring 11
Bombardment
• Earth suffered declining impact flux:
• Moon-forming impact (~10% ME, ~4.4 Ga)
• “Late veneer” (~1% ME, 4.4-3.9 Ga)
• “Late Heavy Bombardment” (0.001% ME, 3.9 Ga)
• Atmospheric
consequences unclear –
chondritic material
added, but also blowoff?
• Comets would probably
have delivered too much
noble gas
Bottke et al. 2010
F.Nimmo EART164 Spring 11
Snowball Earth
Cap carbonate
Tillite
•
•
•
•
•
Ice-albedo feedback (runaway)
Several occurrences (late Paleozoic last one)
Abundant geological & isotopic evidence
Details are open to debate (ice-free oceans?)
How did it end?
F.Nimmo EART164 Spring 11
2. Mars
F.Nimmo EART164 Spring 11
Early Mars was Wet
Hematite
“blueberries”
(concretions?)
F.Nimmo EART164 Spring 11
Was early Mars “warm and wet” or
“cold and (occasionally) wet”?
• Usual explanation is to appeal to a thick, early CO2
atmosphere, allowing water to persist at the surface
• How much CO2 would have been required?
• Atmosphere was subsequently lost
• One problem was absence of observed carbonates
H2O
(Urey cycle)
• Possible solution
is highly acidic
waters (?)
Mars lower
atmosphere
F.Nimmo EART164 Spring 11
Transient early hydrosphere?
• Alternative – cold Mars, with subsurface water
occasionally released by big impacts
Segura et al. 2002. Most of water is from melting subsurface.
F.Nimmo EART164 Spring 11
Obliquity cycles on Mars
• Obliquity on Mars varies much more strongly than on
Earth (absence of a big moon, proximity to Jupiter)
• Long-term
obliquity is
chaotic
• Mars experienced
periods when
poles were
warmer than
equator
F.Nimmo EART164 Spring 11
Evidence for obliquity cycles?
Laskar et al. 2002
F.Nimmo EART164 Spring 11
Snowball Mars?
• Moderate obliquity
periods may have
allowed nearequatorial ice to
develop
Hellas quadrangle (mid-latitudes)
F.Nimmo EART164 Spring 11
Atmospheric loss – many choices
• Low surface gravity - easy for atmosphere to escape
• But further from Sun than Earth – lower solar flux
• Death of dynamo increased
atmospheric loss via
sputtering (?)
• Impact erosion probably
important – runaway
process (no fractionation)
• D/H and N isotope ratios
indicate substantial loss with
fractionation (see Week 3)
Melosh and Vickery 1989
F.Nimmo EART164 Spring 11
Long-term evolution
• Atmosphere certainly thicker (at least transiently) in
deep past, and then declined
• Large quantities of subsurface ice at present day
• Details poorly understood
Catling, Encyc. Paleoclimat. Ancient Environments
F.Nimmo EART164 Spring 11
MAVEN & MSL
• MAVEN launches late 2013
• Measures Martian upper
atmospheric composition
and escape rates
• MSL landed Aug 2012
• Measuring Martian rock and
atmospheric compositions
F.Nimmo EART164 Spring 11
3. Venus
F.Nimmo EART164 Spring 11
Background
• Venus atmospheric pressure ~90 bar (CO2),
surface temperature 450oC
• Earth has similar CO2 abundance, but mostly
locked up in carbonates
• If you take Earth and heat it up, carbonates
dissociate to CO2, increasing greenhouse effect
– runaway
• Will this happen as the Sun brightens?
F.Nimmo EART164 Spring 11
Another runaway greenhouses
• This one happened first, and involves H2O
• H2O in atmosphere lost via photodissociation
Once the water is lost, then CO2
drawdown ceases and
the CO2 greenhouse takes over
• Did Venus lose an ocean? (D/H evidence)
F.Nimmo EART164 Spring 11
Recent outgassing & climate
• Venus was resurfaced ~0.5 Gyr ago, probably
involving very extensive outgassing
• How has atmosphere evolved since then?
(Taylor Fig 9.8)
F.Nimmo EART164 Spring 11
Afterthought - Exoplanets
• Mostly gas giants
• Orbital parameters very different (tidal
locking, high eccentricity, short periods)
• In some cases, we can observe H loss
• Just starting to get spectroscopic information
• Inferred temperature structure can tell us about
dynamics (winds)
F.Nimmo EART164 Spring 11
Key Concepts
• Faint young Sun, albedo feedbacks, Urey cycle
• Loss mechanisms (Jeans, Hydrodynamic, Energylimited, Impact-driven, Freeze-out, Surface
interactions, Urey cycle) and fractionation
• Orbital forcing, Milankovitch cycles
• “Warm, wet Mars”?
• Earth bombardment history
• Runaway greenhouses (CO2 and H2O)
• Snowball Earth
F.Nimmo EART164 Spring 11
Key equations
 S (1  A) 

Teq  
 4e 
1/ 4
P
t 
g
MgSiO3  CO2  MgCO3  SiO2
 3 
Ts  T 1  t s 
 4 
4
4
eq
  nvth (1   )e

2
Vi
2
M  2 Ri 0 H
Vesc
dma
 Rext Fext
e
dt
GM / R
2
F.Nimmo EART164 Spring 11
End of lecture
F.Nimmo EART164 Spring 11
How about radar image of subsurface ice?
F.Nimmo EART164 Spring 11
Albedo feedback
• Main sources of albedo are clouds and ices
dT
1T
• Equilibrium gives:

dA'
4 A'
• On Earth, 1% change in albedo causes 1oC
temperature change – more than predicted. Why?
• Feedbacks can work both ways e.g.
– Ocean warming – more clouds form – albedo
increases (stable feedback). This feedback is the main
uncertainty in climate prediction models.
– Ice-cap growth – albedo increases (unstable feedback)
F.Nimmo EART164 Spring 11
N, Ne and Ar – atmospheric loss
14N/15N
36/38Ar
Solar
357
5.8
Venus
-
5.6
Earth
272
5.3
Mars
170
5.5
Jupiter
440
5.6
Titan
56
-
20Ne/36Ar
Why do we use isotopic ratios?
Planets (except Jupiter) have more heavy N and Ar – loss process
20/22 Ne and 36/40Ar tell us about radiogenic processes
F.Nimmo EART164 Spring 11
D/H – water loss
• Higher D/H suggests more water loss
• Not all loss mechanisms involve fractionation!
Titan (CH4)
Chondrites
Mars
Venus (0.016)
Hartogh et al. 2011
F.Nimmo EART164 Spring 11
Milkovich and Head 2005
F.Nimmo EART164 Spring 11