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kaberettCompletely unproofed. All notes typed up during the talk they were on (I'm going to actually post them one-by-one, though, to reduce information overload). This is actually, yes, how I take notes during lectures without handouts. V happy to attempt to answer any questions you have about any of this!
MISC:
- many more jeans and t-shirts than I expected
- lots of long-haired dudes!
- from where I'm sat it looks to be about 70% male but that might be pessimistic? [SL] thinks so, but I know what I think... -- no, I just back-of-enveloped it, and I make it ~31% not-dudes. 54 definitely-not-dudes, possibly been a bit pessimistic about names I couldn't gender, and didn't count full list *dead*-on, but.
=====================================
M WELCOME
-- enh. Reading out a description of what the RS does. Also everyone is dudes.
-- though the poor lad is of course having a small panic because he has stepped in at the last minute because the person who was supposed to be doing this has been delayed
=====================================
M DAVID STEVENSON -- REASSESSING THE GIANT IMPACT AND ITS ALTERNATIVES
"Playing Devil's advocate" - "Nullius in verba" loosely translated as "Don't believe what they tell you"!
Kona 1984 - several people independently decided that giant impacts was the way to go
Monterey 1998 - again organised by Alex Halliday. Realisation that thinking about origin of the Moon requires thinking about the origin of the Earth.
London 2013 - is 14 years later, so the cadence is correct...
Uses Cree Indian image of Sun - Moon - Earth as single system. :-)
Where are we?
- giant impact very widely believed to explain both geochm and dynamical constraints. It's "natural" (giant impacts common) and "it works" (can make a moon)
- but PROBLEMS: will it make what we see?
- ... democracy is the worst form of governance invented, except all the other ones
Need to explain:
- mass (~1% of Earth)
- angular momentum budget
- Moon is like Earth's mantle (volatile depletion (potassium))
- Moon started hot: not natural for object of its size
- striking isotopic similarity of Earth & Moon
- striking age relationship (late event, as was completion of Earth formation - concretion of Earth was late in solar system set-up)
Ideas:
- intact capture
- disintegrative capture and accretion (forming a disk)
- fission
- collisional ejection and accretion (giant impact is of this type)
- binary accretion
Capture:
- dynamically possible (Triton) but doesn't explain angular momentum
- would need to carry out same high energy events that are required for Moon composition & hot starting state elsewhere
- would need to attribute isotopic similarity to a common reservoir (not including Mars) from which terrestrial bodies formed (Si, W)
- no natural explanation for timing
- main argument against capture is geochemical -- and is VERY STRONG. Dynamics alone can't deal fix it.
Binary accretion:
- mysterious initial condition - can possibly do if start w/ 2 bodies, but.
- offers no prospect of explaining hot initial state or bulk composition
- same problems as capture in terms of geochem
Fission
- George Darwin!
- spin up Earth to rotational break-up
- 2012 version uses giant impact on fast-spinning Earth (Cuk & Stewart)
- attractive for geochemistry, timing: nearest model to just plucking Moon from Earth
- dynamical problem: may require angular momentum extraction larger than that permitted - need to get rid of more than 1/2 of angular momentum would inherit
Giant Impact:
- impact "splashes" material into Earth orbit, mostly from projectile? (in standard model) - but subsequent turbulent mixing
- Moon forms from a disk in perhaps ~100-~1000 years (thermally-determined)
- one Moon, nearly equatorial orbit, near Roche limit (beyond corotation) - tidally evolves outward
SPH model - and Canup:
"standard" case
- creates hydrostatic state on timescale order days
- thermodynamics of disc? Phase diagram liquid/liquid + vapour/vapour... infer PT conditions in disc
-- entropy turns out roughly constant with radius
-- convert that to PT with hydrostatic equation
-- get to mostly liquid + vapour (via Lever rule!)
- nb not actually clear that we completely melt Earth in giant impact - less compressible perovskite in deep mantle
- do not dry out disc in a straightforward manner - hydrodynamic escape (poss how deplete potassium?) - radiative cooling
- fundamental origin of the differences between Earth, Mars and meteorites is NOT UNDERSTOOD (oxygen isotopes)
-- "identity" of Earth and Moon taken to imply same "source"OR may be a consequence of mixing between Earth & lunar forming disc
-- Ti isotopes make mixing a less likely story...
Isotope mixing model (Pahlevan & Stevenson 2007):
- deep Earth will not have time to homogenize with the disk during lunar formation!
-- made worse: entropy distribution in Post-Giant Impact mantle is STABLY STRATIFIED - convectively stable
- can meet ALL the criteria (angular momentum met by standard model only - can we do away with the requirement?) EXCEPT "striking isotopic similarity" (and maybe volatile depletion)
Problem with isotopes...:
- how do we get a more Earth-like disc? "hit and run" (Rueufer et al 2012); "fast spinning Earth" (Cuk & sStewart 2012); SubEarths (Canup 2012)
-- but these don't have enough homogenization, or make use of "questionable angular momentum extraction mechanism" (evection resonance)
-- requires very narrow range of dynamical parameters
- so maybe try to make Earth & impactor more isotopically similar? -- need to explain similarity of Si and W, which requires material that participated in core formation
-- Venus is key...?
Venus: why doesn't it have a Moon?
- never had one? BUT THEN why does it have such low spin - standard story for this is questionable - giant impacts that make moons are commmon
- had one and lost it early on? Escaped & subsequently crashed/Spiraled in & crashed?
Earth-Moon differs from enus-Neith in that it was the lsat major event in the terrestrial zone -- supported by isotopic evidence
Some other recent developments...
- additional isotopic evidence of Earth-Moon similarities
- Water in the Moon. But this might come from impacts? Not clear if it constrains the giant impact origin
- Part of Earth survived the giant impact? Touboul et al 2012
- Earth is non-chondritic? Nd story
Conclusions:
- is the giant impact story in doubt? -- No, well, "Doubt may be uncomfortable but certainty is absurd" - Voltaire
- Has the giant impact story met the challenges of the data?
-- no. Limitations of our understanding of the data? Or that we haven't found the right story/scenario?
- what we need: better understanding of Earth & Moon; improved models; Venus! - says Yo Dave More complete understanding of terrestrial planet formation.
Discussion:
- What about Mercury? - It's small. Worry about reaching conclusions just by looking at Mercury, but for /process/ Mercury is important. Dave focussing on Venus because oxygen isotopes can be done "even from the atmosphere". Mercury small size might be due to giant impact?
- is it special pleading to say that the final major event in the solar system was the giant impact that set up the Earth-Moon system? Dave replies: could the Earth have had a Moon before our current one? Yes, probably - and it would likely die! So the most likely way to end up with us having a Moon is for it to be the last major event. The actual probability of our particular outcome is low, but as a class of outcomes it's reasonable. The Moon-forming event is singular ONLY in that it was the last event, and therefore leaves a record.
- radius of the Earth after impact - does statement about stably-stratified wrt convection reach into the core? What is the state of the core at the end of Earth accretion? Can't model it on the same graph for entropy - need to keep same composition on one graph. Dave thinks core probably stratified stably both in terms of composition & entropy, because later-added material will be hotter. Earth's magnetic field? Probably was one at time of formation - because MOLTEN MANTLE CAN ACT AS DYNAMO. Core dynamo, though? Need to rethink core dynamo, though, says Dave...
- at least some of the material that went into the Moon saw core formation, saw high PT, because of the Si & T isotopics
- what do you think the single most important observation could be? - VENUS, says Yo Dave.
MISC:
- many more jeans and t-shirts than I expected
- lots of long-haired dudes!
- from where I'm sat it looks to be about 70% male but that might be pessimistic? [SL] thinks so, but I know what I think... -- no, I just back-of-enveloped it, and I make it ~31% not-dudes. 54 definitely-not-dudes, possibly been a bit pessimistic about names I couldn't gender, and didn't count full list *dead*-on, but.
=====================================
M WELCOME
-- enh. Reading out a description of what the RS does. Also everyone is dudes.
-- though the poor lad is of course having a small panic because he has stepped in at the last minute because the person who was supposed to be doing this has been delayed
=====================================
M DAVID STEVENSON -- REASSESSING THE GIANT IMPACT AND ITS ALTERNATIVES
"Playing Devil's advocate" - "Nullius in verba" loosely translated as "Don't believe what they tell you"!
Kona 1984 - several people independently decided that giant impacts was the way to go
Monterey 1998 - again organised by Alex Halliday. Realisation that thinking about origin of the Moon requires thinking about the origin of the Earth.
London 2013 - is 14 years later, so the cadence is correct...
Uses Cree Indian image of Sun - Moon - Earth as single system. :-)
Where are we?
- giant impact very widely believed to explain both geochm and dynamical constraints. It's "natural" (giant impacts common) and "it works" (can make a moon)
- but PROBLEMS: will it make what we see?
- ... democracy is the worst form of governance invented, except all the other ones
Need to explain:
- mass (~1% of Earth)
- angular momentum budget
- Moon is like Earth's mantle (volatile depletion (potassium))
- Moon started hot: not natural for object of its size
- striking isotopic similarity of Earth & Moon
- striking age relationship (late event, as was completion of Earth formation - concretion of Earth was late in solar system set-up)
Ideas:
- intact capture
- disintegrative capture and accretion (forming a disk)
- fission
- collisional ejection and accretion (giant impact is of this type)
- binary accretion
Capture:
- dynamically possible (Triton) but doesn't explain angular momentum
- would need to carry out same high energy events that are required for Moon composition & hot starting state elsewhere
- would need to attribute isotopic similarity to a common reservoir (not including Mars) from which terrestrial bodies formed (Si, W)
- no natural explanation for timing
- main argument against capture is geochemical -- and is VERY STRONG. Dynamics alone can't deal fix it.
Binary accretion:
- mysterious initial condition - can possibly do if start w/ 2 bodies, but.
- offers no prospect of explaining hot initial state or bulk composition
- same problems as capture in terms of geochem
Fission
- George Darwin!
- spin up Earth to rotational break-up
- 2012 version uses giant impact on fast-spinning Earth (Cuk & Stewart)
- attractive for geochemistry, timing: nearest model to just plucking Moon from Earth
- dynamical problem: may require angular momentum extraction larger than that permitted - need to get rid of more than 1/2 of angular momentum would inherit
Giant Impact:
- impact "splashes" material into Earth orbit, mostly from projectile? (in standard model) - but subsequent turbulent mixing
- Moon forms from a disk in perhaps ~100-~1000 years (thermally-determined)
- one Moon, nearly equatorial orbit, near Roche limit (beyond corotation) - tidally evolves outward
SPH model - and Canup:
"standard" case
- creates hydrostatic state on timescale order days
- thermodynamics of disc? Phase diagram liquid/liquid + vapour/vapour... infer PT conditions in disc
-- entropy turns out roughly constant with radius
-- convert that to PT with hydrostatic equation
-- get to mostly liquid + vapour (via Lever rule!)
- nb not actually clear that we completely melt Earth in giant impact - less compressible perovskite in deep mantle
- do not dry out disc in a straightforward manner - hydrodynamic escape (poss how deplete potassium?) - radiative cooling
- fundamental origin of the differences between Earth, Mars and meteorites is NOT UNDERSTOOD (oxygen isotopes)
-- "identity" of Earth and Moon taken to imply same "source"OR may be a consequence of mixing between Earth & lunar forming disc
-- Ti isotopes make mixing a less likely story...
Isotope mixing model (Pahlevan & Stevenson 2007):
- deep Earth will not have time to homogenize with the disk during lunar formation!
-- made worse: entropy distribution in Post-Giant Impact mantle is STABLY STRATIFIED - convectively stable
- can meet ALL the criteria (angular momentum met by standard model only - can we do away with the requirement?) EXCEPT "striking isotopic similarity" (and maybe volatile depletion)
Problem with isotopes...:
- how do we get a more Earth-like disc? "hit and run" (Rueufer et al 2012); "fast spinning Earth" (Cuk & sStewart 2012); SubEarths (Canup 2012)
-- but these don't have enough homogenization, or make use of "questionable angular momentum extraction mechanism" (evection resonance)
-- requires very narrow range of dynamical parameters
- so maybe try to make Earth & impactor more isotopically similar? -- need to explain similarity of Si and W, which requires material that participated in core formation
-- Venus is key...?
Venus: why doesn't it have a Moon?
- never had one? BUT THEN why does it have such low spin - standard story for this is questionable - giant impacts that make moons are commmon
- had one and lost it early on? Escaped & subsequently crashed/Spiraled in & crashed?
Earth-Moon differs from enus-Neith in that it was the lsat major event in the terrestrial zone -- supported by isotopic evidence
Some other recent developments...
- additional isotopic evidence of Earth-Moon similarities
- Water in the Moon. But this might come from impacts? Not clear if it constrains the giant impact origin
- Part of Earth survived the giant impact? Touboul et al 2012
- Earth is non-chondritic? Nd story
Conclusions:
- is the giant impact story in doubt? -- No, well, "Doubt may be uncomfortable but certainty is absurd" - Voltaire
- Has the giant impact story met the challenges of the data?
-- no. Limitations of our understanding of the data? Or that we haven't found the right story/scenario?
- what we need: better understanding of Earth & Moon; improved models; Venus! - says Yo Dave More complete understanding of terrestrial planet formation.
Discussion:
- What about Mercury? - It's small. Worry about reaching conclusions just by looking at Mercury, but for /process/ Mercury is important. Dave focussing on Venus because oxygen isotopes can be done "even from the atmosphere". Mercury small size might be due to giant impact?
- is it special pleading to say that the final major event in the solar system was the giant impact that set up the Earth-Moon system? Dave replies: could the Earth have had a Moon before our current one? Yes, probably - and it would likely die! So the most likely way to end up with us having a Moon is for it to be the last major event. The actual probability of our particular outcome is low, but as a class of outcomes it's reasonable. The Moon-forming event is singular ONLY in that it was the last event, and therefore leaves a record.
- radius of the Earth after impact - does statement about stably-stratified wrt convection reach into the core? What is the state of the core at the end of Earth accretion? Can't model it on the same graph for entropy - need to keep same composition on one graph. Dave thinks core probably stratified stably both in terms of composition & entropy, because later-added material will be hotter. Earth's magnetic field? Probably was one at time of formation - because MOLTEN MANTLE CAN ACT AS DYNAMO. Core dynamo, though? Need to rethink core dynamo, though, says Dave...
- at least some of the material that went into the Moon saw core formation, saw high PT, because of the Si & T isotopics
- what do you think the single most important observation could be? - VENUS, says Yo Dave.
(no subject)
Date: 2013-09-24 11:37 pm (UTC)(no subject)
Date: 2013-09-25 10:13 am (UTC)(no subject)
Date: 2013-09-24 11:39 pm (UTC)(no subject)
Date: 2013-09-25 10:12 am (UTC)(no subject)
Date: 2013-09-25 12:49 pm (UTC)I am reminded of Emily Lakdawalla's repeated point that acronyms in talks impede understanding.
Seeking Knowledge for the Crater Good of Humanity
Date: 2013-09-25 07:05 am (UTC)Re: Seeking Knowledge for the Crater Good of Humanity
Date: 2013-09-25 10:11 am (UTC)Re: Seeking Knowledge for the Crater Good of Humanity
Date: 2013-09-25 12:50 pm (UTC)!
Though I'm more accustomed to thinking about the Late Heavy Bombardment in relation to crater counts than in relation to chemical composition.
Re: Seeking Knowledge for the Crater Good of Humanity
Date: 2013-09-26 04:54 pm (UTC)Young planets get bolide in their formative years
Date: 2013-09-26 06:37 pm (UTC)Re: Young planets get bolide in their formative years
Date: 2013-10-01 07:30 am (UTC)Geek Pizza sounds like it could be quite a fun thing!