PDA

View Full Version : Is plate tectonics normal?



Tom Mazanec
2018-Jun-18, 01:23 PM
Is Earth's continental drift unusual? Will most planets between, say, half and twice the Earth's mass have the same system?
Or would such a planet 2/3 covered by water have a bunch of little islands, or one big supercontinent, all the time?

Swift
2018-Jun-18, 02:29 PM
50% of the planets we have data for, that are between 0.5 and 2.0 Earth masses, have plate tectonics. ;)

The short answer is, of course, we have no idea. People have been modeling exoplanets, but there is no consensus, as best as I can tell.

One reference (https://www.sciencedirect.com/science/article/pii/S003206331300161X)

Several numerical studies have been published in the past years speculating about the existence of plate tectonics on large exoplanets. They focus on aspects like the mass of a planet, the interior heating rate and the occurrence of water in the mantle. Different trends in the propensity for plate tectonics have been observed in particular when varying the planetary mass: with increasing mass the surface mobilization is found to be either more, equally or less likely than on Earth. These studies and their implications are, however, difficult to compare as they assume different initial conditions and parameter sets, and either neglect the pressure effect on the viscosity or assume a rather small influence of the pressure on the rheology. Furthermore, the thermal evolution of the planets (i.e. cooling of core and decrease in radioactive heat sources with time) is typically neglected.

In our study, we use a 2D finite volume code and apply a pseudo-plastic rheology. We investigate how a strong pressure-dependence of the viscosity influences not only the convective regime in the lower mantle, but also in the upper mantle and hence the likelihood to obtain plate tectonics. We examine how our results change when either assuming a wet or a dry rheology or when employing different initial conditions, focussing on the initial temperature in the lower mantle and at the core-mantle boundary. We find that the initial temperature conditions have a first-order influence on the likelihood of plate tectonics on large exoplanets. For standard literature values for initial temperatures of super-Earths, surface mobilization is less likely than on Earth, for warm initial temperature the result is vice versa.

Simulations that neglect the time-dependence of internal heat sources on the other hand tend to lead to an increasing likelihood of plate tectonics with increasing mass. Finally, our investigations suggest that a wet rheology does not necessarily favor plate tectonics, but – depending on the reference viscosity – may rather lead to a stagnant-lid regime.

Tom Mazanec
2018-Jun-18, 04:46 PM
Quote: either more, equally or less likely than on Earth
Very decisive ;-)

The Backroad Astronomer
2018-Jun-18, 07:09 PM
There will be probably be a spectrum of plate tectonics on planets. Rocky planets the size of Mercury and Mars would probably have none. Planets the closer to the Earth would have plate tectonics. Some icy moons like Europa, Titan and even icy planets would have there own version based off of ice flows,

Swift
2018-Jun-18, 07:16 PM
There will be probably be a spectrum of plate tectonics on planets. Rocky planets the size of Mercury and Mars would probably have none. Planets the closer to the Earth would have plate tectonics. Some icy moons like Europa, Titan and even icy planets would have there own version based off of ice flows,
I don't think that is a good model - if it was, then Venus would have plate tectonics, and I do not believe there is any evidence for it. If you look at the reference I linked to, the models are based on a lot more than just planet size and rocky vs. icy.

The Backroad Astronomer
2018-Jun-18, 07:22 PM
There are other variables such as if the planet has radioactive decay, how well the planet holds the heat and what the planet is made of. The underlying idea there will be a range not just all planets this size will have plate tectonics or not. Plus we need more data on exoplanet to determine what is going on each planet.

CJSF
2018-Jun-18, 09:28 PM
Isn't it also part of the giant impact Moon hypothesis that much of Earth's crust was stripped away (later coalescing into the Moon) to make its crust thin enough for plate tectonics as we have it? Or has that been discounted at this point?

CJSF

The Backroad Astronomer
2018-Jun-18, 11:25 PM
Didn't of that but I don't think getting hit by large objects would be rare in the universe.

John Mendenhall
2018-Jun-19, 03:19 AM
The Wiki article is here:

https://en.wikipedia.org/wiki/Plate_tectonics

I would give the article 5 stars, and the graphics are great!

cjameshuff
2018-Jun-19, 03:41 AM
There will be probably be a spectrum of plate tectonics on planets. Rocky planets the size of Mercury and Mars would probably have none. Planets the closer to the Earth would have plate tectonics. Some icy moons like Europa, Titan and even icy planets would have there own version based off of ice flows,

The evidence isn't conclusive, but Mars has signs of past plate tectonics.

The Backroad Astronomer
2018-Jun-19, 01:47 PM
The evidence isn't conclusive, but Mars has signs of past plate tectonics.
Yes but Mars cooled off and the plat tectonics has ceased

cjameshuff
2018-Jun-19, 10:59 PM
Yes but Mars cooled off and the plat tectonics has ceased

Small planets not having active plate tectonics for as long as larger planets, or active plate tectonics having ceased on one particular small planet, are both quite different from small planets not having plate tectonics.