Cosmochemistry and geochemistry

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Cosmochemistry and geochemistry
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Cosmic Abundances

• Relative numbers of atoms of all elements
  characterizing the Solar System
• Main proxies
• - The solar atmosphere
• - Chondrites
• (each has problems)

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The most common elements
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Abundance groups

• (a) Hydrogen and helium dominate completely
• (b) Oxygen, carbon, nitrogen and neon contribute
  about 90 of the rest
• (c) Magnesium, silicon, iron and sulfur
  contribute about 90 of the rest
• (d) Argon, aluminum, calcium, sodium and nickel
  contribute about 90 of the rest

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Chemical peculiarities

• The cosmic abundances are NOT universal!
• Galactic chemical evolution has gradually
  increased the metal abundances
• Variations of 30 exist between stars of
  similar age
• The interstellar medium has roughly similar
  abundances

(N. Christlieb)
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The Interstellar Medium
Interstellar clouds Neutral hydrogen (HI) and
molecular (H2) clouds In both cases, submicron
grains contribute 1 of the total mass Molecular
clouds are larger and much denser than HI clouds,
hence much more massive
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Cloud opacities

• Center-surface extinction of a HI cloud 0.1
  magnitude
• Ratio of cloud radii ? 2
• For each H atom, the number of grains is the same
  in both clouds
• Ratio of grain number densities ? 330

The center-surface extinction in a molecular
cloud is about 70 magnitudes!
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Dark clouds

• Coalsack dark nebula Horsehead nebula
• nebula in the southern in Orion
• Milky Way

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Molecular clouds
The extreme darkness leads to very low
temperatures!

• H atoms combine into H2 molecules on grain
  surfaces (some other simple molecules too)
• No UV flux ? no photodissociation
• Other molecules are formed by chemical reactions
  in the gas phase
• Strongly non-equilibrium chemistry

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Interstellar molecules
(a few examples)

• Observed in dark clouds by submm or radio
  emission (the clouds are transparent at such
  wavelengths)
• Hundreds of molecules have been detected

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Icy mantles

• At very low temperatures, the molecules freeze
  out on grain surfaces ? icy mantles
• Ices have been observed spectroscopically in the
  IR domain H2O is a dominant constituent

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Grain structure

• The icy mantles contain very reactive radicals
• As the cloud is dispersed, the remaining grains
  are heated ? explosive chemistry as the radicals
  combine ? organic refractories (yellow stuff)
• Original grains are made of silicates (stellar
  atmospheres), but organic mantles are added in
  molecular clouds
• In the final prestellar cloud, an icy mantle is
  added

Greenberg and Hage (1990)
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Solar nebula chemistry
assume Chemical Equilibrium
K equilibrium constant p partial pressure
Molecule formation
f mole fraction P total pressure
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Chemistry of major elements
Reactions in a cooling nebula
N atoms consumed
C atoms consumed
water
methane
ammonia
water ice
ammonia monohydrate
methane clathrate hydrate
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HCNO phase diagram
Notes 1. Uncertainties about the relative
amounts of CO and H2O 2. Methane and ammonia
formation in the outer Solar System may be
kinetically inhibited 3. Is H2O ice crystalline
or amorphous?
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Amorphous ice

• Deposition of H2O on a cold plate at very low
  temperature leads to an amorphous structure of
  the ice
• This is extremely rich in micropores and can trap
  large quantities of ambient gases
• The amorphous ice crystallizes irreversibly at a
  rate that grows exponentially with the
  temperature
• Upon crystallization, trapped gases are released

Bar-Nun and Kleinfeld (1988)
(CH4, CO, Ar, N2)
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Questions about ice

• Were the icy mantles of presolar grains made of
  amorphous ice?
• Did such mantles crystallize or even vaporize as
  they entered the solar nebula?
• Did condensation in the solar nebula lead to
  amorphous or crystalline ice?
• Did clathrate hydrates play any role in pristine
  materials?
• Are cometary nuclei made of amorphous ice?

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Rock-forming chemistry
iron metal
forsterite
enstatite
troilite
olivines, pyroxenes
Hydration at Tlt500 K
serpentine
Olivines and pyroxenes dominate in
chondrites carbonaceous chondrites contain
abundant hydrated minerals Those were the first
water-bearing solids in the solar nebula
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Condensation sequence

• The staircase separates gases from solids
• Elements are shown in order of decreasing
  volatility
• Chondrites are used as cosmic thermometers

Cooling solar nebula
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Geochemical classification

• Siderophiles - metal-loving elements free,
  reduced metal
• - e.g., Fe, Ni, Co, Cu, Au
• Lithophiles - rock-loving elements oxides or
  silicates
• - e.g., O, Si, Fe, Mg, Ca, Al, Ti, Mn, V, F
• Chalcophiles - sulfur-loving elements sulfides
• - e.g., S, Fe, Zn, Pb, Ge, Se

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Earths differentiation

• Core 32.5, Mantle 67, Crust 0.5
• Mantle silicates Mg dominates over Fe pyroxene
  dominates over olivine
• Incompatible elements ions cannot substitute for
  Mg2 in mantle silicates ? driven into the crust
  (Na, K, Ti)

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Important silicate minerals
Olivines (dark and/or green)
Feldspars (Fe-free, light)
Pyroxenes
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Rocks
Composed of assemblages of different minerals

• Primitive formed directly from solar nebula
  condensates
• Igneous formed from cooling magma in partially
  molten objects
• Metamorphic altered by chemistry, temperature or
  pressure
• Sedimentary formed by accumulation of mineral
  grains or organics

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Igneous rock classification
Parameters silica (SiO2) content, grain size
mafic
felsic
volcanic
plutonic
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Common igneous rocks

• Granite plutonic, felsic (typical of the Earths
  continental crust)
• Basalt volcanic, mafic (typical of the oceanic
  crust and common on other planets)

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Ice sublimation
equilibrium
sublimation
Rate of sublimation into vacuum rate of
condensation from a saturated vapour
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Cometary activity
Energy balance of an icy surface
absorbed
radiated
latent heat
Dominates far away
Dominates near the Sun
H2O is the least volatile of the ices Its
profile fits with observed cometary activity