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Swift
2015-Mar-11, 12:49 AM
From Nature Geoscience (http://www.nature.com/ngeo/journal/v7/n11/full/ngeo2271.html) - "Nitrogen speciation in upper mantle fluids and the origin of Earth's nitrogen-rich atmosphere"

Abstract:

Volatile elements stored in the mantles of terrestrial planets escape through volcanic degassing, and thereby influence planetary atmospheric evolution and habitability. Compared with the atmospheres of Venus and Mars, Earth's atmosphere is nitrogen-rich relative to primordial noble gas concentrations1, 2, 3. The compatibility of volatile elements in mantle minerals versus melts and fluids controls how readily these elements are degassed. However, the speciation of nitrogen in mantle fluids is not well constrained4, 5, 6. Here we present thermodynamic calculations that establish the speciation of nitrogen in aqueous fluids under upper mantle conditions. We find that, under the relatively oxidized conditions of Earth's mantle wedges at convergent plate margins7, 8, 9, nitrogen is expected to exist predominantly as N2 in fluids and, therefore, be degassed easily. In contrast, under more reducing conditions elsewhere in the Earth's upper mantle and in the mantles of Venus and Mars, nitrogen is expected predominantly in the form of ammonium (NH4+) in aqueous fluids. Ammonium is moderately compatible in upper mantle minerals10, 11 and unconducive to nitrogen degassing. We conclude that Earth's oxidized mantle wedge conditions—a result of subduction and hence plate tectonics—favour the development of a nitrogen-enriched atmosphere, relative to the primordial noble gases, whereas the atmospheres of Venus and Mars have less nitrogen because they lack plate tectonics.

Interesting idea, that the lack of plate tectonics lead to the lack of nitrogen in Venus' and Mars' atmospheres.

malaidas
2015-Mar-11, 12:56 AM
Ok I give you mars, seeing as it has no molten core, but how do we know Venus has no tectonics?

malaidas
2015-Mar-11, 01:24 AM
Ignore the above, I have my answers.

Noclevername
2015-Mar-11, 03:26 AM
So the old sci-fi standard of terrestrial planets with Earthlike atmospheres isn't a big fluke and I can write about them without looking like a chump? Good to know. ;)

publiusr
2015-Mar-14, 07:52 PM
This would seem to support Rare Earthers. You need a big Moon (or just a Moon-forming impact) after all, just to keep things stirred up.

Swift
2015-Mar-14, 09:17 PM
You need a big Moon (or just a Moon-forming impact) after all, just to keep things stirred up.
I've never heard this. Do you have a reference?

publiusr
2015-Mar-15, 07:21 PM
Just a guess on my part--something to start the tectonics on worlds not around gas giants.

You might remember some recent stories saying life may not need a Moon after all. If a Thea hit added just enough disruption to heat things up enough to get tectonics going--with nitrogen coming from that--I would guess non-protest life might be more rare after all.

starcanuck64
2015-Mar-17, 06:25 AM
Just a guess on my part--something to start the tectonics on worlds not around gas giants.

You might remember some recent stories saying life may not need a Moon after all. If a Thea hit added just enough disruption to heat things up enough to get tectonics going--with nitrogen coming from that--I would guess non-protest life might be more rare after all.

Did it require a large impactor to initiate plate tectonics?

My understanding is radioactive decay provides enough heat to power the process over this length of time. And the presence of water is necessary at plate boundaries to provide the lubrication needed. This is likely why Venus' plate tectonics ended after the early water was lost.

Eclogite
2015-Mar-17, 11:20 AM
The importance of the thickness and composition of the crust and athenosphere, variables that are controlled by several disparate factors, are also important in initiating and sustaining plate tectonics. There was considerable debate in this area. I'm unsure of what consensus was arrived at. I am sure that it would be decidedly provisional.

The idea that the moon, through its tidal effects, may have been important in the origin of life has never had status as an essential requirement, though some individual researchers may have viewed it that way. The recent fashion of looking to sub-sea vents as the locale for abiogenesis completely removes its importance. However, if tidal effects do play a key role then consider that simulations of planetesimal interactions produce large moon-forming episodes in a significant number of instances - 10% to 15% IIRC.

Darrell
2015-Mar-17, 12:44 PM
Did it require a large impactor to initiate plate tectonics?

My understanding is radioactive decay provides enough heat to power the process over this length of time. And the presence of water is necessary at plate boundaries to provide the lubrication needed. This is likely why Venus' plate tectonics ended after the early water was lost.

Additionally, a significant amount of the heat budget of the Earth is primordial heat from its original formation, or so it is thought. How quickly the heat of formation of a planetary body takes to dissipate is dependent on mass and, much more complicated, on the rate of heat flow from the core through to the surface.

Regarding Venus, I seem to remember that a recent mapping mission found evidence of massive, as in globe spanning, vulcanism in the past. It is thought that on a cyclic basis there are massive active volcanic periods where the entire surface of Venus is resurfaced. This is thought to be an example of advective transport of heat from the core in contrast to how heat transfer from the core occurs on Earth. On Earth the heat transfer from the core is largely via mantle convection which is a driver of plate tectonics. Venus does not have the properties necessary for plate tectonics and therefore can not dissipate heat in that way.

Eclogite
2015-Mar-17, 02:31 PM
Additionally, a significant amount of the heat budget of the Earth is primordial heat from its original formation, or so it is thought. Do not overlook the role of radioactive elements within the mantle. Lord Kelvin, assuming a completely molten Earth and noting the current rate of heat loss, computed an age for the Earth of no more than 400 million years. The order of magnitude error was because he was then unaware of radioactivity and of the presence of such elements in the mantle. But your central point, that we do not require a giant impact to furnish the thermal energy for plate tectonics, is valid.


How quickly the heat of formation of a planetary body takes to dissipate is dependent on mass and, much more complicated, on the rate of heat flow from the core through to the surface.It is potentially misleading to refer exclusively to heat flow from the core. While the temperature of the core is much greater than that of the mantle, the much larger volume of the mantle, coupled with the radioactivity noted above, means that a substantial part of the Earth's heat 'capital' is resident in the mantle.


Regarding Venus, I seem to remember that a recent mapping mission found evidence of massive, as in globe spanning, vulcanism in the past.This is not recent. The primary evidence arises from crater counting, which allows an approximate age of any surface to be determined. From such studies it appears that the entire surface of Venus was made over about 600 million years ago. The idea was first proposed as a consequence of the Magellan probe mapping in the early 1990s, so it is around twenty years old. (Geologically yesterday, I agree.)
.


Venus does not have the properties necessary for plate tectonics ...............Although nothing is ever that simple. For example Eastern Aphrodite Terra has been interpreted as follows:
• Lithosphere thinning => isostatic uplift => volcanism (Phillips R. J. and Malin M. C. (1983) Venus, 159-214.)
• Crustal divergence analogous to mid-ocean ridges (Head J. W. and Crumpler L. S. (1987) Science,1380-1385.)
• Circum-global rift zone separating two plates (Suppe J. andConnors C. (l992)LPSC XXIII, 1389-1390.)
• Crustal shortening and subduction (McKenzie D. et al. (1992) JGR)

Note that three of the four hypotheses invoke some aspect of plate tectonics, but note also, that these ideas pre-date the Magellan mapping results.

Darrell
2015-Mar-17, 04:22 PM
I'm not. Merely pointing out that it is a significant factor in addition to radioactive elements.

It seems like yesterday.

Thank you for the references and the clarifications.

starcanuck64
2015-Mar-17, 07:58 PM
Regarding Venus, I seem to remember that a recent mapping mission found evidence of massive, as in globe spanning, vulcanism in the past. It is thought that on a cyclic basis there are massive active volcanic periods where the entire surface of Venus is resurfaced. This is thought to be an example of advective transport of heat from the core in contrast to how heat transfer from the core occurs on Earth. On Earth the heat transfer from the core is largely via mantle convection which is a driver of plate tectonics. Venus does not have the properties necessary for plate tectonics and therefore can not dissipate heat in that way.

This sounds similar to what happens on Earth when the continental plates form a single super-continent which results in the buildup of heat and magma under the lithosphere which is then released in massive fissure flows such as the Siberian Traps.

starcanuck64
2015-Mar-17, 08:13 PM
Then again the Siberian Traps could have been the result of a mantle plume based on the isotopic evidence.

http://www.sciencedirect.com/science/article/pii/001670379390149Q


We present a tightly controlled and comprehensive set of analytical data for the 250-Ma Siberian flood-basalt province. Consideration of major- and trace-element compositions, along with strontium, lead and neodymium isotopic compositions, strongly supports earlier Russian subdivision of this magmatism into three magmatic cycles, giving rise to three assemblages of eleven basalt suites in the ascending order Ivakinsky-Gudchikhinsky, Khakanchansky-Nadezhdinsky and Morongovsky-Samoedsky. Geochemical and isotopic discontinuities of varying magnitude characterize most of the boundaries between the eleven recognized basalt suites in the Noril'sk area.

Although we conclude that the dominant volume of erupted magma originated from an asthenospheric mantle plume, none of the lavas is interpreted to directly represent asthenospheric melts, which would have been far more magnesian. On the basis of thermal considerations, we consider it unlikely that vast volumes of basaltic melt were produced directly from the continental lithospheric mantle beneath the Siberian craton.

Tectonics and mantle convection and heating are fairly complex.