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 6 th, 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 ContentD/H Ratio Earth71%149*10^6 Venus0.003%16,000*10^6 (~ times Earth) Mars0.03%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 mid- ocean 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. Higher strains leader to faster deformation  Depth (km)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.