Avi Stein
6 Oct 2015
E&ES 371
Notes in Planetary Evolution
In the visible, Venus has a very high albedo and cloud cover. However, we can see
through the clouds with radar.
Venus is the most visited planet in the solar system. Soviets decided to go to Venus, and
they did. They kept launching Venera missions every 13 months, starting in 1961.
Although they insulated them from the heat and the pressure, each of the landers lasted 2
hours or less.
The landing sites on Venus have plate-y rocks, and some soil. Erosion comes from
impact cratering and mass wasting (rocks falling), but not water, which is the most
effective weathering mechanism on Earth.
The Venera missions landed on basalt flows. You can see volcanoes and flows from
radar.
Surface of Venus is 90 bars (90x Earth) and 450 degrees C.
The only surface-based mineralogy measurements come from the seven Venera missions.
Shows basalts, but not a big surprise: we see lots of volcanoes, and this is what we expect
from the solar system.
Craters were the biggest surprise. The crater density gives the age of 500 million years,
which predicts a large resurfacing event.
Earth’s oldest rock is about 4 billion years old, and the oldest ocean plate is about 200
million years old. Venus should have some old parts, and a lot of younger parts.
Earth has a lot of water that lets plates slide, but Venus doesn’t, so it would have a
massive resurfacing event.
John Hossain presenting Smrekar paper:
Tectonic and Thermal Evolution of Venus and Role of Volatiles.
Size matters (it’s a first degree approximation) but is not everything.
-Interior heat matters, but so do the volatiles.
-Earth has 71% H20, Venus has .003%.
-Venus has a very high D/H ratio (implies it had more water in the past).
-We expect Mars had more water as well.
Earth is the only place with plate tectonics because fluid asthenosphere. From stress?
Because water?
Venus has tectonics, from upwellings and downwellings in the mantle. The lack of water
keeps its rigid lid.
Venus does have high and low elevation parts, but no linear systems (like mountain
chains). Mars doesn’t either, but it’s also half the size. Mars has a global dichotomy
(smooth lowlands and cratered/old highlands) and the Tharsis Rise (due to upwelling).
Mars doesn’t have plate tectonics because it’s small and cold.
Rheology is the study of flow of water. Volatiles help the crust fracture, lubricate, and
weaken. On Earth, this mechanism is partial melting which helps the asthenosphere.
The boundary between the lithosphere and the asthenosphere: because water lowers the
differential stress necessary to deform asthenosphere, this boundary defines where the
lithosphere gets fluid.
Venus has lost a lot of water. The lithosphere is very rigid. We don’t know too much
about Venus, though. Based on what we know, we can draw a strength envelope plot
(differential stress versus depth) for only the lithosphere.
We know some about the lithosphere of Mars because of Martian meteorites. Convection
on Mars isn’t great.
Volatiles and Interior Convection:
-The stagnant lid keeps the heat in, then hits critical point and overturns.
-We don’t have models for how this happened.
-Maybe Venus used to have plate tectonics, but not now. Ran out of water, because water
isn’t happy on the surface with its heat and pressure.
Convecting stresses must be strong enough to break through lid to get plate tectonics.
Olatiles melt materials, bringing it down in the mantle, being released, and coming back
to the surface. This creates mantle movement.
Convection and Plumes in Stagnant Lid:
-Convection comes from cooling on surface, heat from core. Not as fluid as Earth.
-We can figure out the number of plumes based on thickness of core/mantle boundary,
viscosity, and heat.
-High wavelength of plumes makes huge plumes. On Venus, it’s low, creating lots of
plumes going up and dripping back down.
On Mars, ideas for global dichotomy and Tharsis Rise:
A) Endothermic phase transitions, which is a barrier for short wavelength convections
B) Upper mantle has lower viscosity.
Interaction of Climate and Solid Body:
-Degassing of volatiles creates greenhouse effects.
-This could actually lead to more volcanism.
Recap:
-Volatiles are more important than heat differentiation (maybe)
-Venus didn’t always have a stagnant lid (possibly)
-Venus lost a lot of water, and Mars cooled off, so neither of them have plate tectonics
now.