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kaberettConference conference conference! Start of session two. As always, please feel encouraged to ask whatever questions you have, and I will do my best to answer them. It is also worth noting that there is an annual Lunar & Planetary Sciences conference, and that this meeting at the Royal Society took place in that context - and seems by all accounts to have been incredibly valuable!
After Kona [first big conference on Moon stuff] predominant view was that Moon was predominantly projectile. Now we believe that the Moon is largely Earth-like. [Let's add another clarifying note, actually: the Giant Impact Hypothesis is generally accepted, and assumes that something Really Big (Mars-sized) hit the Earth at some point - though as you've seen/will see from some of the other notes, exactly when is a matter of serious debate... anyway, so you've got this Huge Lump o' Rock hurtling in, and it smashes into the Earth, and largely vapourises both itself *and* some of the Earth, forming a mixture of the two. The question is: in that vapour, are the majority of the atoms from Earth or are the majority of the atoms from this Big Lump o' Rock From Outer Space? Increasingly, the geochemistry has us convinced that they have to be mostly from Earth - or that the projectile, via some mechanism, had a nearly identical isotope chemistry to the Earth. The latter is currently considered highly unlikely (or, er, grasping at straws), because what we know of Mars (one of the other rocky/terrestrial planets) is that its composition is MARKEDLY different to Earth's, whereas the Moon and the Earth are in most cases as close-as-damn-it identical. This is some of why everyone is so keen to get data from Mercury and Venus - it will let us know whether Mars is an anomaly among the terrestrial planets (i.e. Mercury/Venus/Earth all have similar compositions) or whether it's indicative of the general state of play (i.e. M/V/E all have different compositions). If the former, a projectile with a composition that matches the Earth's exactly is suddenly much more plausible - but the data we've got so far says Erm, Nope. Of course, instruments continue to improve... and there's a third option, which is very attractive, that the impact managed to effectively homogenise the projectile's matter with the proto-Earth's matter. This would neatly solve the problem of identical compositions, but we have no models at all that come anywhere close to being able to account for this.]
======================================
M ALEX HALLIDAY -- EARTH-MOON ISOTOPIC SIMILARITIES & DIFFERENCES
Starts out by telling us that YO DAVE did most of the work :-)
Bits and bobs to think about, that he actually thinks are a thing:
The Moon formed very late (70-150Ma after start of solar system) (probably later than that...):
- Sr isotopic composition - very well defined: the Moon has very little Rb, almost no in-growth.
- the earliest time that the Moon can have formed can be derived from the Rb-Sr model ages relative to a CI chondritic solar system. This gives 9-13Ma - it's not an early object.
- Hf-W gives earliest time Moon can have formed is 37Ma (Halliday 2013) - doesn't have lots of radiogenic tungsten
- actual age of the Moon can be dduced from its initial Sr ratio assuming it formed from the Earth with Rb/Sr = 0.03. Gives 90 +- 20Ma (Halliday 2008). This model can be questioned! How well do we know Rb/Sr of Earth? Recently been questioned.
- or actual age of the Moon can be deduced from its initial Sr ratio assuming it formed from an impact-eroded Earth with Rb/Sr = 0.021. 125 +- 25Ma... (Halliday 2013)
-- Saturn completed by 5Ma, Earth by 15Ma after creation of solar system
The Moon's atoms came from the Earth:
- oxygen isotopes: diversity across whole solar system, except Earth and Moon agree (Wiechert 2001) - no EM difference within 5ppm
- same results found for W and Si between Silicate Earth and Moon (Armytage 2012...)
- and Xhang 2012 says no difference in nucleosynthetic Ti isotopic composition between silicate Earth and Moon (everything else EVERYWHERE ELSE)
- Si isotope data for the Earth's mantle and hence BSE are heavy by 0.1-0.2 per mill. "Silicon in the Earth's Core" (Letters)
-- higher-pressure core formation? Earth bigger than Mars, segregation of Si into Earth's core...?
There are major Earth-Moon differences in chemical composition:
- "this is very striking and obvious - we haven't been saying so much about that"
- K/U! Rb/Ba! Difference in moderately-volatile elements. (U, Ba refractory.)
- if the Moon really did come from the Earth, we have to deplete it in volatile elements. So GI scenario has to involve loss of moderately-volatile elements...
- also differences in terms of Fe from basalts. Mars, Vesta [mother of the moon], Moon have much higher Fe in basalts. For a long time argued that if the Moon did come from the Earth, must change iron content or change way iron is removed during partial melting. Earth's core carried on forming after giant impact --> loss of more iron to core???
- "... I haven't seen Jeff. Is ħe here?" "Jeff can't come", responds the audience...
- soooooooooort of relationship between heliocentric distance and Mantle FeO in wt%. Different degrees of core formation? Different degrees of oxidation? What does the heliocentric distance here MEAN? Solar nebula involved? (FeO in mantle increases w/ heliocentric distance)
Accretion after the giant impact has to have been very small:
- no difference in W isotopic composition between silicate Earth and Moon. SO incredibly tight - subsequent mass of late veneer no more than ~0.8%
Earth's major volatiles predate the Moon and imply significant loss processes:
- Origin of water in the terrestrial planets - Drake 2005 - one of the people arguing of water from late veneer
- Enhanced atmospheric loss on protoplanets at the giant impact phase in the presence of oceans...
- if PGEs from late veneer, it must have been a volatile-depleted one
- 14N is SERIOUSLY depleted in BSE - way morethan can be explained by a late veneer - lots of work recently
- noble gases even more extreme than 1H, 14N, 13C - HUGE spread. If these things all came from CI carbonaceous chondrites - have to explain the variability
- H, C, N partially siderophile so variability has them getting sucked into core? But that doesn't explain why N more depleted than C, because lab experiments show C more siderophile than N...
- composition of meteorites C/N-H/N very well-defined trend - but the Earth DOES NOT FIT (much higher H) so does. not. fit. late-veneer model.
Neon isotopes do not provide much evidence of loss during the giant impact:
- atmospheric 20Ne/22Ne does not match interior - but no evidence that we've actually undergone loss, plots pretty well between CI and solar (via solar wind...)
- see v heavy Xe relative to every reservoir in solar system - but it's found in the Earth's interior, as well as atmosphere, plus some on Mars - so probably not Xe loss
Venus-Earth differences in volatiles might reflect giant impact, but could ALSO reflect amorphous ices:
- maybe depletion is not to do with loss, but to do with mixing of different kinds of components?
- successful modelling! Produce Earth's budget for (heavy) noble gases with a tiny amount of cometary material plus CI carbonaceous chondrites
Discussion:
- merge of cores of impactor and impactee happens so rapidly that we never see equilibration of impactor core w/ mantle? Well, hopefully, BUT "experiments show that it ain't that simple" - when see planetary objects colliding, some of the material merges directly with the core, but not all of it does
- "I guess the volatiles blow off somehow, ask Dave" ("how do you explain the volatile depletion if we form the Moon via high-energy impact?")
After Kona [first big conference on Moon stuff] predominant view was that Moon was predominantly projectile. Now we believe that the Moon is largely Earth-like. [Let's add another clarifying note, actually: the Giant Impact Hypothesis is generally accepted, and assumes that something Really Big (Mars-sized) hit the Earth at some point - though as you've seen/will see from some of the other notes, exactly when is a matter of serious debate... anyway, so you've got this Huge Lump o' Rock hurtling in, and it smashes into the Earth, and largely vapourises both itself *and* some of the Earth, forming a mixture of the two. The question is: in that vapour, are the majority of the atoms from Earth or are the majority of the atoms from this Big Lump o' Rock From Outer Space? Increasingly, the geochemistry has us convinced that they have to be mostly from Earth - or that the projectile, via some mechanism, had a nearly identical isotope chemistry to the Earth. The latter is currently considered highly unlikely (or, er, grasping at straws), because what we know of Mars (one of the other rocky/terrestrial planets) is that its composition is MARKEDLY different to Earth's, whereas the Moon and the Earth are in most cases as close-as-damn-it identical. This is some of why everyone is so keen to get data from Mercury and Venus - it will let us know whether Mars is an anomaly among the terrestrial planets (i.e. Mercury/Venus/Earth all have similar compositions) or whether it's indicative of the general state of play (i.e. M/V/E all have different compositions). If the former, a projectile with a composition that matches the Earth's exactly is suddenly much more plausible - but the data we've got so far says Erm, Nope. Of course, instruments continue to improve... and there's a third option, which is very attractive, that the impact managed to effectively homogenise the projectile's matter with the proto-Earth's matter. This would neatly solve the problem of identical compositions, but we have no models at all that come anywhere close to being able to account for this.]
======================================
M ALEX HALLIDAY -- EARTH-MOON ISOTOPIC SIMILARITIES & DIFFERENCES
Starts out by telling us that YO DAVE did most of the work :-)
Bits and bobs to think about, that he actually thinks are a thing:
The Moon formed very late (70-150Ma after start of solar system) (probably later than that...):
- Sr isotopic composition - very well defined: the Moon has very little Rb, almost no in-growth.
- the earliest time that the Moon can have formed can be derived from the Rb-Sr model ages relative to a CI chondritic solar system. This gives 9-13Ma - it's not an early object.
- Hf-W gives earliest time Moon can have formed is 37Ma (Halliday 2013) - doesn't have lots of radiogenic tungsten
- actual age of the Moon can be dduced from its initial Sr ratio assuming it formed from the Earth with Rb/Sr = 0.03. Gives 90 +- 20Ma (Halliday 2008). This model can be questioned! How well do we know Rb/Sr of Earth? Recently been questioned.
- or actual age of the Moon can be deduced from its initial Sr ratio assuming it formed from an impact-eroded Earth with Rb/Sr = 0.021. 125 +- 25Ma... (Halliday 2013)
-- Saturn completed by 5Ma, Earth by 15Ma after creation of solar system
The Moon's atoms came from the Earth:
- oxygen isotopes: diversity across whole solar system, except Earth and Moon agree (Wiechert 2001) - no EM difference within 5ppm
- same results found for W and Si between Silicate Earth and Moon (Armytage 2012...)
- and Xhang 2012 says no difference in nucleosynthetic Ti isotopic composition between silicate Earth and Moon (everything else EVERYWHERE ELSE)
- Si isotope data for the Earth's mantle and hence BSE are heavy by 0.1-0.2 per mill. "Silicon in the Earth's Core" (Letters)
-- higher-pressure core formation? Earth bigger than Mars, segregation of Si into Earth's core...?
There are major Earth-Moon differences in chemical composition:
- "this is very striking and obvious - we haven't been saying so much about that"
- K/U! Rb/Ba! Difference in moderately-volatile elements. (U, Ba refractory.)
- if the Moon really did come from the Earth, we have to deplete it in volatile elements. So GI scenario has to involve loss of moderately-volatile elements...
- also differences in terms of Fe from basalts. Mars, Vesta [mother of the moon], Moon have much higher Fe in basalts. For a long time argued that if the Moon did come from the Earth, must change iron content or change way iron is removed during partial melting. Earth's core carried on forming after giant impact --> loss of more iron to core???
- "... I haven't seen Jeff. Is ħe here?" "Jeff can't come", responds the audience...
- soooooooooort of relationship between heliocentric distance and Mantle FeO in wt%. Different degrees of core formation? Different degrees of oxidation? What does the heliocentric distance here MEAN? Solar nebula involved? (FeO in mantle increases w/ heliocentric distance)
Accretion after the giant impact has to have been very small:
- no difference in W isotopic composition between silicate Earth and Moon. SO incredibly tight - subsequent mass of late veneer no more than ~0.8%
Earth's major volatiles predate the Moon and imply significant loss processes:
- Origin of water in the terrestrial planets - Drake 2005 - one of the people arguing of water from late veneer
- Enhanced atmospheric loss on protoplanets at the giant impact phase in the presence of oceans...
- if PGEs from late veneer, it must have been a volatile-depleted one
- 14N is SERIOUSLY depleted in BSE - way morethan can be explained by a late veneer - lots of work recently
- noble gases even more extreme than 1H, 14N, 13C - HUGE spread. If these things all came from CI carbonaceous chondrites - have to explain the variability
- H, C, N partially siderophile so variability has them getting sucked into core? But that doesn't explain why N more depleted than C, because lab experiments show C more siderophile than N...
- composition of meteorites C/N-H/N very well-defined trend - but the Earth DOES NOT FIT (much higher H) so does. not. fit. late-veneer model.
Neon isotopes do not provide much evidence of loss during the giant impact:
- atmospheric 20Ne/22Ne does not match interior - but no evidence that we've actually undergone loss, plots pretty well between CI and solar (via solar wind...)
- see v heavy Xe relative to every reservoir in solar system - but it's found in the Earth's interior, as well as atmosphere, plus some on Mars - so probably not Xe loss
Venus-Earth differences in volatiles might reflect giant impact, but could ALSO reflect amorphous ices:
- maybe depletion is not to do with loss, but to do with mixing of different kinds of components?
- successful modelling! Produce Earth's budget for (heavy) noble gases with a tiny amount of cometary material plus CI carbonaceous chondrites
Discussion:
- merge of cores of impactor and impactee happens so rapidly that we never see equilibration of impactor core w/ mantle? Well, hopefully, BUT "experiments show that it ain't that simple" - when see planetary objects colliding, some of the material merges directly with the core, but not all of it does
- "I guess the volatiles blow off somehow, ask Dave" ("how do you explain the volatile depletion if we form the Moon via high-energy impact?")
(no subject)
Date: 2013-09-29 11:34 pm (UTC)(no subject)
Date: 2013-09-30 03:58 pm (UTC)(no subject)
Date: 2013-09-30 04:16 am (UTC)(no subject)
Date: 2013-09-30 01:10 pm (UTC)I continue to be quite fascinated by how much debate there actually is on this subject. :D