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kaberettYou are still encouraged to ask questions :-)
F SARAH STEWART -- A SEQUENCE OF GIANT IMPACTS LEADING TO THE ORIGIN OF THE EARTH AND MOON
(I like her. She actually credits her students on the starting slide.)
Earth's mantle was not completely mixed at the end of the accretion - the Moon formation did not homogenize the Earth. (W heterogeneity in old rocks, Xe between upper and lower mantle).
- Earth had multiple magma ocean & atmospheric loss events.
- def not FULL mantle magma oceans, but... partial ones?
Evidence for TErrestrial Magma Oceans:
- He/Ne ratio in Earth is higher than solar. Need to find a way to preferentially lose neon! Deep Earth (deep mantle plumes) is 2.5, solar & meteorites is 1.5, and depleted mantle has values of at least 10. No long-term process on Earth can raise the ratio from 1.5/2.5 to 10! So some other event has to do that...
Noble gas data and multiple magma oceans
1. Nebular ingassing. On all embryos, radiogenically-melted magma oceans. Nebular atmospheres ingas - dissolve into magma. He is 1.5-2 times more soluble in magma than Ne, so will end up with slightly enriched frozen magma (2.5)
2. Atm blowoff & reequilibration - (likely) impact event blows off substantial fraction of atmosphere, and remelts part of ocean - so more outgassing, with Ne outgassing more than He.
3. Atm blowoff and re-equilibration - the last one of these is likely to be the Moon-Forming Giant Impact, possibly more in between
- complicated sequence & complicated story - but might it have happened?
A sequence of events leading to EM origin.
- how is nebular ingassing preserved through multiple giant impacts (in lower mantle)?
- what types of giant impacts lead to substantial atmospheric lost
- what types of giant impacts (Also) make a Moon?
- at least 3 steps required! If we don't blow off the atmosphere completely in one of the steps, need MORE steps.
How to preserve nebular gas signature?
- don't giant impacts melt & mix a planet?
- impact kinetic energy larger than latent heat o fmelting IF distributed evenly, which CAN do when we have a complete magma ocean - Rayleigh number large, turbulent mixing
- if we trap the ratio by not melting the entire mantle...
- impact energy is deposited heterogeneously! Antipode hemisphere experiences much less shock heating.
-- what happens to Antipode material? Least shocked, --> coldest, most dense. So in hydrodynamic simulations it sinks to the lower mantle... material that makes disc is sampled from near the impact point
-- does disc material completely melt?
- hah, here we get the bit where she wants Simon to write an equation of state for perovskite - hee, and then she grins at Simon about it
- ... and then models the lower mantle as MgO
- if you begin with a solid lower mantle, it is VERY hard to melt the whole mantle except with an ENORMOUSLY energetic impact (more so than anything in serious consideration)
-- though if initial T is higher and impacts are closer together, then maybe - but currently time for refreezing the mantle shorter than expected time between impact events
-- can this layer retain a nebular gas signal?
What types of impact remove the atmosphere?
- atm loss depends on shock pressure field, ocean, rotation
- actually quite hard to do!
- depends on surface velocity generated by impact shock
Earth volatile budget with atm loss:
- Earth's volatile ratios differ from chondritic
- bulk ejection of atmosphere separates atmophile elements from those that are soluble in water or silicates
- ejection of steam atm or ocean separates hydrophile from magmaphile elements
What types of giant impacts could make the Moon?
- can we go back to the Moon-forming giant impact establishing the present-day angular momentum?
- argues that even if Venus oxygen does match Moon and Earth, the canonical impact scenario does NOT remove the atmosphere, which she reckons we need somehow!
Constraints on the last giant impact:
- present angular momentum, ~2-Moon mass disc, high angular momentum, blow off atmosphere... looking at spaces between overlap of last three
Minimum sequence of events:
- nebular ingassing
- nebula dissipates
- planetesimals impact (because atmosphere no longer protects)
- at 30-60Ma, see Moon formation with last giant impact
nb NOT a unique sequence of events; similar to Canup.
Assessing hte Moon-formation scenarios:
- modelling catching up with geochemistry!
- geochemical data cannot be satisfied with one giant impact
- need multiple atm loss and partial mantle magma oceans. Likely that the mantle was not completely melted by impacts
- atm loss requires high velocity, high spin, and/or an ocean. Canonical impact scenarios do not remove atmosphere. Volatiles are fractionated from chondritic ratios.
Discussion:
- when can atmosphere loss occur? - noble gases are constraint on when the last blow-off had to happen (Robin asked this. MY LIFE JUST PASSED THE BECHDEL TEST.)
- other ways to lose the atmosphere? -- no, hydrodynamic escape is MUCH too slow (one full loss by 50Ma, but the Moon's probably formed by them).
- doesn't believe in magma ocean freezing from middle out
- oh my word dude who has been working on his own slides ALL MORNING is now asking questions. WHY. ... it is being all "did both impactor and planet get completely destroyed!!!"
- most wanted: noble gas isotopes from Venus!
F SARAH STEWART -- A SEQUENCE OF GIANT IMPACTS LEADING TO THE ORIGIN OF THE EARTH AND MOON
(I like her. She actually credits her students on the starting slide.)
Earth's mantle was not completely mixed at the end of the accretion - the Moon formation did not homogenize the Earth. (W heterogeneity in old rocks, Xe between upper and lower mantle).
- Earth had multiple magma ocean & atmospheric loss events.
- def not FULL mantle magma oceans, but... partial ones?
Evidence for TErrestrial Magma Oceans:
- He/Ne ratio in Earth is higher than solar. Need to find a way to preferentially lose neon! Deep Earth (deep mantle plumes) is 2.5, solar & meteorites is 1.5, and depleted mantle has values of at least 10. No long-term process on Earth can raise the ratio from 1.5/2.5 to 10! So some other event has to do that...
Noble gas data and multiple magma oceans
1. Nebular ingassing. On all embryos, radiogenically-melted magma oceans. Nebular atmospheres ingas - dissolve into magma. He is 1.5-2 times more soluble in magma than Ne, so will end up with slightly enriched frozen magma (2.5)
2. Atm blowoff & reequilibration - (likely) impact event blows off substantial fraction of atmosphere, and remelts part of ocean - so more outgassing, with Ne outgassing more than He.
3. Atm blowoff and re-equilibration - the last one of these is likely to be the Moon-Forming Giant Impact, possibly more in between
- complicated sequence & complicated story - but might it have happened?
A sequence of events leading to EM origin.
- how is nebular ingassing preserved through multiple giant impacts (in lower mantle)?
- what types of giant impacts lead to substantial atmospheric lost
- what types of giant impacts (Also) make a Moon?
- at least 3 steps required! If we don't blow off the atmosphere completely in one of the steps, need MORE steps.
How to preserve nebular gas signature?
- don't giant impacts melt & mix a planet?
- impact kinetic energy larger than latent heat o fmelting IF distributed evenly, which CAN do when we have a complete magma ocean - Rayleigh number large, turbulent mixing
- if we trap the ratio by not melting the entire mantle...
- impact energy is deposited heterogeneously! Antipode hemisphere experiences much less shock heating.
-- what happens to Antipode material? Least shocked, --> coldest, most dense. So in hydrodynamic simulations it sinks to the lower mantle... material that makes disc is sampled from near the impact point
-- does disc material completely melt?
- hah, here we get the bit where she wants Simon to write an equation of state for perovskite - hee, and then she grins at Simon about it
- ... and then models the lower mantle as MgO
- if you begin with a solid lower mantle, it is VERY hard to melt the whole mantle except with an ENORMOUSLY energetic impact (more so than anything in serious consideration)
-- though if initial T is higher and impacts are closer together, then maybe - but currently time for refreezing the mantle shorter than expected time between impact events
-- can this layer retain a nebular gas signal?
What types of impact remove the atmosphere?
- atm loss depends on shock pressure field, ocean, rotation
- actually quite hard to do!
- depends on surface velocity generated by impact shock
Earth volatile budget with atm loss:
- Earth's volatile ratios differ from chondritic
- bulk ejection of atmosphere separates atmophile elements from those that are soluble in water or silicates
- ejection of steam atm or ocean separates hydrophile from magmaphile elements
What types of giant impacts could make the Moon?
- can we go back to the Moon-forming giant impact establishing the present-day angular momentum?
- argues that even if Venus oxygen does match Moon and Earth, the canonical impact scenario does NOT remove the atmosphere, which she reckons we need somehow!
Constraints on the last giant impact:
- present angular momentum, ~2-Moon mass disc, high angular momentum, blow off atmosphere... looking at spaces between overlap of last three
Minimum sequence of events:
- nebular ingassing
- nebula dissipates
- planetesimals impact (because atmosphere no longer protects)
- at 30-60Ma, see Moon formation with last giant impact
nb NOT a unique sequence of events; similar to Canup.
Assessing hte Moon-formation scenarios:
- modelling catching up with geochemistry!
- geochemical data cannot be satisfied with one giant impact
- need multiple atm loss and partial mantle magma oceans. Likely that the mantle was not completely melted by impacts
- atm loss requires high velocity, high spin, and/or an ocean. Canonical impact scenarios do not remove atmosphere. Volatiles are fractionated from chondritic ratios.
Discussion:
- when can atmosphere loss occur? - noble gases are constraint on when the last blow-off had to happen (Robin asked this. MY LIFE JUST PASSED THE BECHDEL TEST.)
- other ways to lose the atmosphere? -- no, hydrodynamic escape is MUCH too slow (one full loss by 50Ma, but the Moon's probably formed by them).
- doesn't believe in magma ocean freezing from middle out
- oh my word dude who has been working on his own slides ALL MORNING is now asking questions. WHY. ... it is being all "did both impactor and planet get completely destroyed!!!"
- most wanted: noble gas isotopes from Venus!