Coupling the atmosphere with interior dynamics:
Implications for the resurfacing of Venus
L. Noack, D. Breuer, T. Spohn
Icarus, 2012
Presentation by Ria Fischer
22.05.2014
Topics in Planetary Sciences
22.05.2014
Topics in Planetary Sciences
Overview
1. Introduction:
1.1 Venus, Earth’s Twin
1.2 Spacecraft missions to Venus
1.3 Venusian atmosphere
1.4 Venus surface features
2. The paper
3.1 Coupled Atmosphere and Mantle Model
3.2 Results
3.3 Conclusions
3. Discussion
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1. INTRODUCTION
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1.1 Venus: Earth’s twin
Venus
Earth
Distance
0.723 AU
1.000 AU
Radius
6052 km
6378 km
Mass
0.8150 ME
1 ME
Density
5.20 g/cm3 (4.0)
5.52 g/cm3 (4.1)
Both planets have
- similar size
- density
- active tectonics
- active volcanism
- a major atmosphere
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1.1 Venus: Earth’s twin
Venus
Earth
Distance
0.723 AU
1.000 AU
Radius
6052 km
6378 km
Mass
0.8150 ME
1 ME
Density
5.20 g/cm3 (4.0)
5.52 g/cm3 (4.1)
Orbital period
224.69 d
365.24 d
Sidereal day
243.02 d (retrograde)
23.9345 h
Axial tilt
177.36°
23.4°
Pressure
92 bar
1 bar
Atmosphere
96% CO2
N2 , O2
Clouds
Sulfuric acid, covered
Water, ragged canopy
Surface temperature
740 K
290 K
‘continents’
10% of surface
45% of surface
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1.2 Spacecraft missions to Venus
Venera 1-16 orbiter and lander
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1.2 Spacecraft missions to Venus
Venera 1-16 orbiter and lander
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1.2 Spacecraft missions to Venus
Venera 1-16 orbiter and lander
Pioneer
Magellan SAR (synthetic aperture radar)
~𝟕𝟎𝟎 𝒌𝒎
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1.2 Spacecraft missions to Venus
Venera 1-16 orbiter and lander
Pioneer
Magellan SAR (synthetic aperture radar)
Venus Express
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1.3 Venusian atmosphere
Venus
Earth
CO2 – 96.5%
N2 – 78%
CO2 → CO+O
N2 – 3.5%
O2 – 21%
SO2 + O → SO3
SO2 – 0.015%
Ar – 0.9%
H2O – 0.002%
H2O – 0.001% - 5%
SO3 + H2O → H2SO4
CO2 – 0.039%
→ Strong greenhouse effect due to CO2
and H2O
𝑇𝑐𝑎𝑙𝑐 = 325 𝐾 vs. 𝑇𝑜𝑏𝑠 = 740 𝐾
→ Full sulfuric acid cloud coverage
→ Planet only accessible by radar imaging
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1.4 Venus surface features
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1.4 Venus surface features: lowland plains
→
→
→
→
90% Rolling lowland plains of tholeiitic basalt
Younger surface
‘Ocean floor without water’
Marginally deformed
𝟑𝟕 𝒌𝒎
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Topics in Planetary Sciences
1.4 Venus surface features: lowland plains
→
→
→
→
→
90% Rolling lowland plains of tholeiitic basalt
Younger surface
‘Ocean floor without water’
Marginally deformed
Broken up by volcanic structures like:
 Coronae, novae, arachnoids
~𝟐𝟎𝟎 𝒌𝒎
22.05.2014
Topics in Planetary Sciences
1.4 Venus surface features: lowland plains
→
→
→
→
→
90% Rolling lowland plains of tholeiitic basalt
Younger surface
‘Ocean floor without water’
Marginally deformed
Broken up by volcanic structures like:




Coronae, novae, arachnoids
Calderas
Shield volcanoes
Pancake domes
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𝟐𝟓 𝒌𝒎
1.4 Venus surface features: lowland plains
→
→
→
→
→
90% Rolling lowland plains of tholeiitic basalt
Younger surface
‘Ocean floor without water’
Marginally deformed
Broken up by volcanic structures like:





Coronae, novae, arachnoids
Calderas
Shield volcanoes
Pancake domes
Lava flows up to 7000 km long
~𝟏𝟎𝟎 𝒌𝒎
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1.4 Venus surface features: ‘continents’
→
→
→
→
→
90% Rolling lowland plains of tholeiitic basalt
Younger surface
Marginally deformed
‘Ocean floor without water’
Broken up by volcanic structures like:





Coronae, novae, arachnoids
Calderas
Shield volcanoes
Pancake domes
Lava flows
→ 10% Highlands:
Aphrodite Terra, Ishtar Terra
→ Older surface age
→ Strongly deformed
→ Isostatically compensated
→ Tesserae (ridged terrain)
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1.4 Surface age and mantle convection
Venus
Earth
300 Ma – 1 Ga
oc. crust: < 200 Ma
cont. crust: up to 4 Ga
Determining surface ages on
Venus is generally harder
due to it’s thick atmosphere
→ Young surface for absent plate tectonics
→ Magmatic vs. non-magmatic features
→ Small patches, coronae, plains basins are of younger age than
crustal plateaus
→ Gravity/topography correlation  dynamic support from
mantle convection?
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1.4 How to renew a surface without plate tectonics?
1. Catastrophic resurfacing event at ~500 Ma
2. Local mobilisation of surface and continued renewal of
surface patches
Peclet number
𝑣𝐷
𝑣 ∙ 𝛻𝑇
𝐸𝐶𝑜𝑛𝑣𝑒𝑐𝑡𝑖𝑜𝑛
𝑃𝑒 ≔
≈
≈
κ
κ𝛻 2 𝑇
𝐸𝐷𝑖𝑓𝑓𝑢𝑠𝑖𝑜𝑛
𝑃𝑒 < 1
1 ≤ 𝑃𝑒 < 10
stagnant
highly sluggish
10 ≤ 𝑃𝑒 < 100
sluggish
𝑃𝑒 ≥ 100
mobile
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2. The Paper:
2.1 COUPLED ATMOSPHERE AND
MANTLE MODEL
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Surface temperature
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H2O degassing
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2.1 2D/3D Mantle convection model
→
→
→
→
GAIA in spherical annulus/shell
Standard hydrodynamic PDEs
Free-slip boundary conditions
Dry olivine-dominated non-Newtonian rheology
𝐸 + 𝑝ℎ 𝑉
−1/𝑛 (1−𝑛)/𝑛
η=𝐴
𝜀
exp
𝑛𝑅𝑇
→ Radiogenic and primordial heating
Coupling:
→ Partial melting  degassing of water
→ Dehydration  viscosity increase of 100 of residue
→ Set surface temperature as temperature boundary condition
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2.1 2D/3D Mantle convection model
Amount of water removed from mantle
Water in
mantle
Total melt
volume
∆𝑄𝐻𝑛2𝑂 = 𝐶𝐻𝑛2𝑂 𝐶𝑒𝑥𝑡𝑟 𝑉𝑛 (𝑡 𝑛 )
Removed
water
= 10%
Extrusive
volcanism coeff.
Remaining amount of water in the mantle
𝑉𝑛 (𝑡 𝑛 )
𝑛
𝑛−1
𝐶𝐻2𝑂 = 𝐶𝐻2𝑂 1 − 𝐶𝑒𝑥𝑡𝑟
𝑉𝑚𝑎𝑛𝑡𝑙𝑒
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2.1 Atmosphere model
→ Consider H2O, CO2, SO2, albedo, cloud effects, solar luminosity
→ radiative-convective atmosphere model (Bullock and Grinspoon, 2001)
1/4
𝑆0 (𝑡)
𝑇0 𝑡, 𝛼𝐻2𝑂 = 𝑇0 4.5 𝐺𝑎, 𝛼𝐻2𝑂
𝑆0 (4.5 𝐺𝑎)
→ Atmospheric loss of H2O: exponential decrease with characteristic lifetime 𝜏𝐻2𝑂
𝛼𝐻2𝑂 𝑡 𝑛 = 𝛼𝐻2𝑂,𝑖𝑛𝑖 𝑒𝑥𝑝 −
𝑡
𝜏𝐻2𝑂
Atmospheric loss
𝑛
𝑛
+
𝑘=1
∆𝑄𝐻𝑘2𝑂
𝑡𝑛 − 𝑡𝑘
𝑒𝑥𝑝 −
𝑀𝑎𝑡𝑚
𝜏𝐻2𝑂
Coupling:
→ H2O partial pressure  surface temperature
→ Set surface temperature as temperature boundary
condition
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Mantle degassing
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2.1 List of models and parameters
H2O degassing
Surface temperature
Coupled
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2.1 List of models and parameters
Decoupled
H2O degassing
Surface temperature
737 K
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2. The paper:
2.2 RESULTS
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2.2 Coupled vs. decoupled
decoupled
coupled
Reference case 1
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2.2 Coupled vs. decoupled: surface mobilisation
Profile B:
 Locally mobilised surface
 Lower viscosity contrast
coupled
Reference case 1, coupled
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2.2 Coupled vs. decoupled
Coupled model shows:
- Recovering surface temperature
after critical surface temperature
is reached
- Mobilisation of the surface
- Less outgassing due to cooling by
mobilisation
Case 14
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2.2 Formation of patches
Patches of varying
(a) age
(b) heat flux
(c) surface velocity
can be observed
Case 14-c
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2.2 Parameter study: Temperature
Lower mantle temperature leads to
- higher maximal surface temperature
- Higher maximal surface velocities
- Less degassing
Higher mantle temperature leads to
- Lower surface temperature
- Slow episodic resurfacing events
- More efficient degassing
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Parameter study: Critical surface temperature
𝑃𝑒 < 1
𝑃𝑒 ≥ 100
1 ≤ 𝑃𝑒 < 10
10 ≤ 𝑃𝑒 < 100
1 ≤ 𝑃𝑒 < 10
10 ≤ 𝑃𝑒 < 100
𝑃𝑒 ≥ 100
𝑃𝑒 < 1
Critical surface temperature:
- Depends on convective stresses
- Depends on vigour of mantle convection
- Depends on viscosity contrast (Mobilisation: < 105 )
- Is lower for lower Mantle temperature
- Is lower for non-Newtonian rheology
- Is lower in wet rheology
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Range: 750 𝐾 to 1000 𝐾
2. The paper:
2.3 CONCLUSIONS
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2.3 How to stabilize the Runaway Greenhouse Effect
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2.3 How to stabilize the Runaway Greenhouse Effect
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2.3 How to stabilize the Runaway Greenhouse Effect
𝟑𝟎𝟎 𝒌𝒎
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3. DISCUSSION
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Discussion
Model setup:
→ Why use Earth-like parameters for the interior?
Because we don’t know?
Is this realistic?
→ The model runs for 4.5 Ga. From when on can we assume that the planet was fully
formed and differentiated?
→ Crust formation is neglected. Is crust formation essential?
→ Melt model: solidus temperature is set mantle temperature after melt removal
→ Atmosphere model: only H2O considered
Geodynamics:
→ Possible surface expressions of surface mobilisation?
→ How have terranes formed in this model?
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