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kaberettM H. J. MELOSH -- NEW APPROACHES TO THE MOON'S ISOTOPIC CRISIS
Two new theories have come up, that make plausible attempts at explaining the isotopic crisis - but he isn't happy. When he agreed to give this talk two years ago he hoped one would come up... but he's intending to criticise everybody in the room.
Giant Impact Hypothesis 1984. Favourite theory of Moon's origin. Explains such things as the lack of metallic iron in the Moon.
- NEARLY accounts for major chemical differences for bulk Earth and Moon (except 2xlarger GeO/MgO) - but Moon isn't enhanced in refractory elements
- ... lunar crustal thickness thinner than previously thought, so Al + Ca content has come down
- ... but then the geochemists got better machines. And Things Fell Apart.
- so we thought Moon was 70% impactor... but oxygen gave that the lie, and now e see similar similarities in Mn-Cr, K, W, Si, Ti ("everyone has problems with eutectites")
- Pahlevan & Stevenson (2007) attempted to resolve oxygen similarity by proposing volatile exchange between orbiting debris disc and Earth
- Ti is highly refractory and the precision is very high - it's not as volatile as oxygen, yet the difference is tiny compared to difference between Earth and carbonaceous chondrites, so HOW do you get Ti homogenised?
-- need LOTS of material to get exchanged to achieve this much equilibration (plus the Earth is big, yo)
-- nearly 5 disc masses of material must be echanged to get 1% similarity of composition. OH DEAR.
--- ... but that would also exchange angular momentum, which would bring the lunar disc down, as total angular momentum of E-M system is not large enough to sustain the lunar disc (rotation period necessary to shed mass from equator: 1.5 hours, vs period of proto-Earth w/ all angular momentum of 4 hours) [nb this calculation doesn't include evection resonance, but it... shouldn't matter anyway)
- "Wada et al (2006) performed high-resolution simulations of a gaseous disc and showed that it collapsed onto the proto-Earth ona time scale of days. Material only remains in orbit if a liquid or solid phase condenses out and no material exchange occurs" - need gas phase for material exchange
Canup et al (2012) showed that the difficulty does not lie in the use of SPH - a conventional Eulerian hydrocode gives nearly identical results.
- so we REALLY DO need a condensed-matter disc.
- Reufer et al (2012) proposed a fast, steep hit-and-run impactor that may also possess at ice mantle - but at best disk contained about 73% of Earth mantle, but not quite a Moon mass was lofted. These computations were constrained to match the present angular momentum of the Earth-Moon system - so maybe some more work is called for
- Cuk & Stewart (2012) suggest evection resonance could drain large amounts of angular momentum from a tidally-evolving Moon - but Wisdom (2013 preprint) argues that a more reliastic model does not lead to large angular momentum loss
-- this extra freedom allows high-speed retrograde impact with a fast-spinning initial Earth could inject mainly Earth mantle material into orbit - but again, it would be mostly gaseous - hence unstable on a short timescale to tidal wossnames?
Equation of State?:
- Kraus (2012) and Kurosawa (2013) found that silicates are much easier to vapourise by impact than we had previously believed
- these new EoS have yet to be incorporated into any work
MAny of the recent simulations can, under the right conditions, produce a Moon. However, many trial conditions do not, and I am still uneasy about these models... there are a _lot_ of models, and only a small fraction succeed (i.e. yield mostly Earth material in orbit)
- models are contrived - not in the negative sense, but - special pleading for very special conditions of our lunar formation
- can we find a solution in which isotopic similarity is a normal outcome? Or are we actually living in a contrived world...?
-- "our current situation is reminiscent of pre-Apollo days, when capture COULD put a Moon in orbit, but only under very special initial conditions" ("quadruple precision in FORTRAN")
- ... and isotopic constraints are still only upper limits - could shrink further, making Moon and formation conditions even more ~special~
- ... back to Darwin...? i.e. fission - but what about volatile depletions, FeO enrichment, small core, etc?
"A giant impact still looks very attractive - we now accept that large catastrophic impacts were the natural final episode of terrestrial planet formation, and only large impacts can inject the large amounts of energy and momentum implied by the sudden formation of a Mooon as large as ours"
What is needed?:
- some proces that adds large amounts of energy and momentum to the Earth's mantle, without leaving a trace of its own material
- a projectile composed of 100% ice (unlikely chemically)
- a solid iron projectile that then vanishes from the Moon-forming disc (Slightly less unlikely...? hit-and-run collision stripping mantle from projectile?)
- a projectile identical in composition to the Earth (unliekly according to current accretion models)
- ... or something we haven't thought of yet...
"Our Moon is there! It's a wonderful accessory to our planet, a necessity for life, but we still have no idea about its origin..."
Discussion:
- "impactor probably formed in Earth-like orbit? why are you rejecting this?" -- "I base my scepticism on the basis of the current models of planetary accretion, in which it appears that all of the major planets originally accreted from bodies that came from very different parts of the solar system... even though the Grand Tack, if it happened, would have packed all of the embryos and planetesimals into a small space, they'd still have [been all over]" -- i.e. unlikely to have identical compositions
- "let's just reject the idea of retrograde impact on rapidly spinning Earth - perhaps the way to reject it is to consider what would happen to the energy of rotation of this rapidly-spinning Earth - would have been large enough to vapourise the Earth" -- "Er, well, I think that's an... extreme vision..." YES GOOD I LIKE YOU YOU ARE BEING VERY DEFENSIVE OF SARAH BEING COMPETENT I LIKE YOU
- "I may be the sole surviving proponent of capture here" - OH DEAR. - "elliptic, not circular, three-body problem - and then it's a stochastic problem."
- [different questioner] .......... "there was recently an article in the New Scientist, about a month ago" NO NO NO NO NO -- oh wait, you're talking about the March 2013 paper on atomic bombs around the core
- okay, so we should probably actually think about moons of Jupiter and Saturn: irregular moons look captured, but regular satellites formed from disc? Yet Mars moons are regular, but hideously compositionally different...
- but what if we transport atoms faster than overall material exchange? could that sort out Ti?
- assertion that we need to know more about Venus and Mercury before we claim that all terrestrial planets are very different compositionally, just because Mars is different
- could it be an onion-like Moon created from multiple events? -- constraints on Moon's interior not that strong; lateral heterogeneity not actually that big. But depth heterogeneity...? We probably need a seismic network on the Moon, lololol we don't have one of those.
- to what extent do we actually know how much the Moon is homogeneous (elementally and isotopically)
- what if the Grand Tack had caused REALLY THOROUGH homogenisation? ... then what the fuck is Mars doing, basically, but otherwise it would be *nice* (lets us go back to the canonical model)
Two new theories have come up, that make plausible attempts at explaining the isotopic crisis - but he isn't happy. When he agreed to give this talk two years ago he hoped one would come up... but he's intending to criticise everybody in the room.
Giant Impact Hypothesis 1984. Favourite theory of Moon's origin. Explains such things as the lack of metallic iron in the Moon.
- NEARLY accounts for major chemical differences for bulk Earth and Moon (except 2xlarger GeO/MgO) - but Moon isn't enhanced in refractory elements
- ... lunar crustal thickness thinner than previously thought, so Al + Ca content has come down
- ... but then the geochemists got better machines. And Things Fell Apart.
- so we thought Moon was 70% impactor... but oxygen gave that the lie, and now e see similar similarities in Mn-Cr, K, W, Si, Ti ("everyone has problems with eutectites")
- Pahlevan & Stevenson (2007) attempted to resolve oxygen similarity by proposing volatile exchange between orbiting debris disc and Earth
- Ti is highly refractory and the precision is very high - it's not as volatile as oxygen, yet the difference is tiny compared to difference between Earth and carbonaceous chondrites, so HOW do you get Ti homogenised?
-- need LOTS of material to get exchanged to achieve this much equilibration (plus the Earth is big, yo)
-- nearly 5 disc masses of material must be echanged to get 1% similarity of composition. OH DEAR.
--- ... but that would also exchange angular momentum, which would bring the lunar disc down, as total angular momentum of E-M system is not large enough to sustain the lunar disc (rotation period necessary to shed mass from equator: 1.5 hours, vs period of proto-Earth w/ all angular momentum of 4 hours) [nb this calculation doesn't include evection resonance, but it... shouldn't matter anyway)
- "Wada et al (2006) performed high-resolution simulations of a gaseous disc and showed that it collapsed onto the proto-Earth ona time scale of days. Material only remains in orbit if a liquid or solid phase condenses out and no material exchange occurs" - need gas phase for material exchange
Canup et al (2012) showed that the difficulty does not lie in the use of SPH - a conventional Eulerian hydrocode gives nearly identical results.
- so we REALLY DO need a condensed-matter disc.
- Reufer et al (2012) proposed a fast, steep hit-and-run impactor that may also possess at ice mantle - but at best disk contained about 73% of Earth mantle, but not quite a Moon mass was lofted. These computations were constrained to match the present angular momentum of the Earth-Moon system - so maybe some more work is called for
- Cuk & Stewart (2012) suggest evection resonance could drain large amounts of angular momentum from a tidally-evolving Moon - but Wisdom (2013 preprint) argues that a more reliastic model does not lead to large angular momentum loss
-- this extra freedom allows high-speed retrograde impact with a fast-spinning initial Earth could inject mainly Earth mantle material into orbit - but again, it would be mostly gaseous - hence unstable on a short timescale to tidal wossnames?
Equation of State?:
- Kraus (2012) and Kurosawa (2013) found that silicates are much easier to vapourise by impact than we had previously believed
- these new EoS have yet to be incorporated into any work
MAny of the recent simulations can, under the right conditions, produce a Moon. However, many trial conditions do not, and I am still uneasy about these models... there are a _lot_ of models, and only a small fraction succeed (i.e. yield mostly Earth material in orbit)
- models are contrived - not in the negative sense, but - special pleading for very special conditions of our lunar formation
- can we find a solution in which isotopic similarity is a normal outcome? Or are we actually living in a contrived world...?
-- "our current situation is reminiscent of pre-Apollo days, when capture COULD put a Moon in orbit, but only under very special initial conditions" ("quadruple precision in FORTRAN")
- ... and isotopic constraints are still only upper limits - could shrink further, making Moon and formation conditions even more ~special~
- ... back to Darwin...? i.e. fission - but what about volatile depletions, FeO enrichment, small core, etc?
"A giant impact still looks very attractive - we now accept that large catastrophic impacts were the natural final episode of terrestrial planet formation, and only large impacts can inject the large amounts of energy and momentum implied by the sudden formation of a Mooon as large as ours"
What is needed?:
- some proces that adds large amounts of energy and momentum to the Earth's mantle, without leaving a trace of its own material
- a projectile composed of 100% ice (unlikely chemically)
- a solid iron projectile that then vanishes from the Moon-forming disc (Slightly less unlikely...? hit-and-run collision stripping mantle from projectile?)
- a projectile identical in composition to the Earth (unliekly according to current accretion models)
- ... or something we haven't thought of yet...
"Our Moon is there! It's a wonderful accessory to our planet, a necessity for life, but we still have no idea about its origin..."
Discussion:
- "impactor probably formed in Earth-like orbit? why are you rejecting this?" -- "I base my scepticism on the basis of the current models of planetary accretion, in which it appears that all of the major planets originally accreted from bodies that came from very different parts of the solar system... even though the Grand Tack, if it happened, would have packed all of the embryos and planetesimals into a small space, they'd still have [been all over]" -- i.e. unlikely to have identical compositions
- "let's just reject the idea of retrograde impact on rapidly spinning Earth - perhaps the way to reject it is to consider what would happen to the energy of rotation of this rapidly-spinning Earth - would have been large enough to vapourise the Earth" -- "Er, well, I think that's an... extreme vision..." YES GOOD I LIKE YOU YOU ARE BEING VERY DEFENSIVE OF SARAH BEING COMPETENT I LIKE YOU
- "I may be the sole surviving proponent of capture here" - OH DEAR. - "elliptic, not circular, three-body problem - and then it's a stochastic problem."
- [different questioner] .......... "there was recently an article in the New Scientist, about a month ago" NO NO NO NO NO -- oh wait, you're talking about the March 2013 paper on atomic bombs around the core
- okay, so we should probably actually think about moons of Jupiter and Saturn: irregular moons look captured, but regular satellites formed from disc? Yet Mars moons are regular, but hideously compositionally different...
- but what if we transport atoms faster than overall material exchange? could that sort out Ti?
- assertion that we need to know more about Venus and Mercury before we claim that all terrestrial planets are very different compositionally, just because Mars is different
- could it be an onion-like Moon created from multiple events? -- constraints on Moon's interior not that strong; lateral heterogeneity not actually that big. But depth heterogeneity...? We probably need a seismic network on the Moon, lololol we don't have one of those.
- to what extent do we actually know how much the Moon is homogeneous (elementally and isotopically)
- what if the Grand Tack had caused REALLY THOROUGH homogenisation? ... then what the fuck is Mars doing, basically, but otherwise it would be *nice* (lets us go back to the canonical model)
(no subject)
Date: 2013-10-22 09:05 pm (UTC)(no subject)
Date: 2013-10-22 11:16 pm (UTC)