Transcript Tectonic and Thermal Evolution of Venus and the Role of

Tectonic and Thermal
Evolution of Venus and the
Role of Volatiles: Implications
for Understanding the
Terrestrial Planets
Suzanne E. Smrekar et al.
Discussed by John Hossain
October 6th, 2015
This paper explores the role of volatiles like water
in the evolution of three terrestrial planets: Venus, Earth, and
Mars
Size is the dominant factor controlling the geologic evolution of
a terrestrial planet: the larger the planet, the longer it takes to
cool.
This is true overall, but doesn’t explain many geological
differences.
Earth and Venus are considered “twins” because of similar sizes
and densities, but Earth is an “active lid” planet whereas Venus
and Mars are both “stagnant lid” planets.
Interior heat plays a role in the geological behavior of these
planets, as do volatiles.
Water as a volatile
Water Content
D/H Ratio
Earth
71%
149*10^6
Venus
0.003%
16,000*10^6
(~107-150 times Earth)
Mars
0.03%
780*10^6
(~5-6 times Earth)
D/H ratio = Deuterium to Hydrogen Ratio
Both Mars and Venus had more water early in their histories
(erosion and rock deposition on Mars; similar size of Venus to
Earth).
Tectonic History
 Earth is the only known body with plate tectonics,
which drives geologic deformations and volcanism.
 The motion of the plates is fueled by mantle
convection.
Plate motion may only be possible with a low
viscosity asthenosphere.
 *Origin of the asthenosphere: mineralogical
responses to increased pressure or the weakening
effect of water on bond strengths in olivine and
other mantle constituents.
 Mars and Venus have volcanism also, but not driven
by plate movement.
 No evidence for plate boundaries in the topography of
Venus.
 Geological deformations still exist for a number of
reasons, such as volcanism.
Plateaus form from mantle upwelling or downwelling;
volcanic rises form from upwelling.
85% of the surface is volcanic plains.
Average crater retention age is 700 m.y.; possible
range from 300 m.y to 1 b.y.
Only about 1000 craters, indicating major planetwide
resurfacing.
 The leading hypothesis for Venus’ geology: lack of water
resulted in a lithosphere rigid enough to allow a stagnant
lid. Any convective stresses are too weak to overcome
the yield strength of the lithosphere.
Venus Topography
Goddard Spaceflight Center, NASA
Red and white=
high elevation
 Mars is believed to have lost much of its interior heat relatively quickly
because its diameter is roughly half of Earth or Venus.
 This led to the loss of an engine for much geologic activity.
 Convection may still occur; stagnant lid could be thick, limiting the
cooling rate.
 May have once had plate tectonics; magnetic record of crust, indicative
of an active dynamo.
 Two main features on surface: global dichotomy and the Tharsis rise.
 Global dichotomy: topographic elevation change that separates the
smooth northern lowlands from the cratered southern highlands.
 Formation may be due to mantle convection or meteor impact basin.
 Tharsis rise: massive volcanic plateau.
 Formation may be due to mantle upwelling or an impact.
Mars Topography
Lithosphere Journal, GSA
Venus doesn’t have plate tectonics likely because of the
loss of volatiles (volatiles increase the strength of
convection, reduce the strength of the plates, and lower
the viscosity below the plates to form an asthenosphere)
and Mars doesn’t because of its small size and subsequent
cooling.
Effects of Volatiles on Rheology
 Water has a huge impact on mechanical properties
of rocks and deformation.
Pushes open fractures and faults
Acts as a lubricant.
Weakens rock along grain boundaries.
 On Earth, partial melting occurs below the midocean ridges  water partitions in the melt phase 
drier upper mantle incorporates into lithosphere and
wetter layer underneath becomes asthenosphere.
Strength Envelope Plot for Earth – strength of rock in
the oceanic lithosphere as a function of depth. Rocks
to the right of the curve will deform.
Depth (km)
Higher strains
leader to faster
deformation 
Temp (C)
 Venus has lost very significant quantities of water
(Re: D/H ratio)
 From this follows a very rigid crust and upper mantle
despite the high surface temperatures (~470 C)
 Difficult to constrain the rheology of Venus because
of uncertain parameters: crustal thickness, thermal
gradient, composition, volatile content, and mantle
viscosity.
 A model of the planetary interior’s strength
envelope follows:
Similar strength envelope plot for Venus’s lithosphere – drier and
higher temp; Rocks to the right of the curve will deform.
 Estimating volatiles on Mars can only be done indirectly, as by
studying Martian meteorites.
These are so old (hundred of m.y.+) that they provide only
snapshots.
 These meteorite suggest formerly high volatile content.
 Convection on Mars is not as vigorous as on Earth; may imply
that the mantle is more viscous than Earth’s mantle.
Earth’s mantle is also better mixed, as shown by a larger
isotopic range on Mars.
 Melt inclusions in some meteorites contain amphiboles
(hydroxyl groups!), which imply a significant previous water
content.
Small diameter, rapid cooling!
Volatiles and Interior Convection
 Resurfacing of Venus’ lithosphere was done through volcanic
activity within the past b.y. This could have occurred multiple
times, but it is unclear.
Occurred when convective stresses imposed on the
lithosphere reached the yield strength of the rock.
May be because the stagnant lid also may have triggered
resurfacing: it reduces the amount of interior heat loss,
which can result in large volumes of pressure release
melting.
Possible transition from active to stagnant lid.
 Possible positive feedback: low tectonic activity leads to
stagnant lid, which then leads to a reduction in tectonic
activity.
University of Maryland
 One possible reason plate tectonics may have
stopped on Mars: the formation of the dichotomy
was a result of increased mantle temperatures 
increased temps lowered the mantle’s viscosity 
lowered convective stress in the mantle, causing it
to drop below the lithospheric yield.
 For Venus and Mars, additional conditions would be
necessary to obtain mobile-lid movement like on
Earth.
Narrow fault zones with “low” friction, low viscosity
asthenosphere, water and other volatiles, etc.
Overarching point: convecting stresses imposed on
the lithosphere must be capable of reaching the
yield strength of the lithosphere.
Geochemistry and the Stagnant Lid
 Despite no plate tectonics, lithospheric instabilities on Venus
may be due to density variations caused by mantle melting
and magma rise, moving iron into parts of the lithosphere.
 The planet’s interior likely contains some fraction of water,
carbon dioxide, and other volatiles.
 These incompatible elements encourage melting in material
sinking from the lithosphere. As dense material sinks, it is
warmed and volatiles can be released from the sinking
material into the surrounding upper mantle:
 Thus, volcanic activity can take place sans an active lid.
 Melt products have a wide range of shapes: pancake
domes, festoon flows, long (hundreds of km) and narrow
channels.
Pancake Domes
Lunar and Planetary Institute
Festoon Flows
Solarviews
Stagnant Lid Convection and Mantle Plumes
 A hotter mantle creates a thinner lithosphere, leading to more
melting and volcanism.
 On the other hand, a dry interior would have a higher
viscosity and thicker lithosphere.
 Convection on Venus is driven by heat from radiogenic
decay, cooling from above, and the presence of a hot core.
 A liquid core is not proven, but may be there:
 Similarity in size between Earth and Venus, suggesting
quick segregation of iron into the core.
 A planet as small as Mars still has a liquid iron core.
 A hot thermal boundary is necessary to explain the
prominent plumes and volcanoes.
 Without a hot thermal boundary, there would only be
“cold” plumes forming below the crust, and so convection
would suffer.
How to find number of plumes?
Lambda = wavelength of the plumes
Delta = thickness across a boundary layer
Delta-Mu = Viscosity ratio across a boundary layer
 Core surface area divided by the square of the
wavelength gives an estimate for the number of plumes.
 The number of plumes is similar to plumes on Earth,
suggesting that the conditions at the core-mantle
boundary are similar on the two planets.
Geological Society,
London
Lambda (h) =
Viscosity ratio across
boundary layer
Earth: Strong cooling because of
subduction  large viscosity
variations at the core-mantle
boundary  plumes with large
heads and persistent tails
Jellinek et al., Geophysical Research Letters
 On Earth, subduction of a cold lithosphere occurs,
leading to low-viscosity thermal plumes.
Low viscosity = more likelihood of asthenosphere.
 Analogue on Venus:
~100 m.y.
University of Maryland
 On Mars, both the dichotomy and Tharsis rise (very long
wavelength plumes) may be explained by two
hypotheses:
A) Endothermic phase transition at the core-mantle
boundary between spinel and perovskite:
Decreases heat flow
Decreases mantle temperature
Decreases number of upwelling plumes
Barrier to all but the longest wavelength convections.
B) A viscosity contrast between the upper and lower
mantles, with the upper mantle having a lower viscosity.
Magma builds up for long periods and then spills
over as massive flows.
Mars Topography
Lithosphere Journal, GSA
Interactions Between Climate and the Solid
Body
 Degassing of volatiles to the atmosphere affects the
bulk viscosity of the mantle.
 Can also contribute to the greenhouse effect.
 Temperature changes from this can cause thermal
contraction/expansion on the planet’s surface,
leading to fractures.
 With enough time, these effects can penetrate into
the mantle and affect melting and volcanism.
As an example, climate change might initiate a
transition from an active lid to a stagnant lid (or
vice versa?)
Eclogites are important because they
can play a role in driving convection.
Main Points
 Volatiles have a major impact on rheology, possibly
more than interior temperature differences.
 The transition between active and stagnant lid
convection occurs when convective stresses are no
longer able to strongly deform the lithosphere.
 Therefore, the paper assumes that the high
temperatures on early Venus and Mars allowed for an
active lid, but there is not yet direct evidence.
 The transition to a stagnant lid on Mars happened
because of rapid cooling. On Venus, it may have
happened because of a large loss of water.
 Future surface missions will clear up many of our
outstanding questions.