Lecture 3 – Planetary Migration, the Moon, and the Late Heavy Bombardment Abiol 574.
Lecture 4: Origin of Earth’s Volatiles Abiol 574.
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Transcript of Lecture 4: Origin of Earth’s Volatiles Abiol 574.
Lecture 4: Origin of Earth’s Volatiles
Abiol 574
“Excess volatiles”• Term coined by
William Rubey (circa 1955)
• Definition: Compounds present at Earth’s surface that were not derived from converting igneous rock to sedimentary rock
• Rubey and other geologists presumed that the atmosphere and oceans were derived from outgassing by volcanoes
H2O is one important excess volatile Others include CO2, N2, S, and Cl
Impact degassing• We now think that
many of Earth’s volatiles, including water, may have been released directly to the surface by impacts
• Large impacts are predicted by models of planetary accretion
• The process of volatile release during impacts is called impact degassing
• If Earth’s atmosphere was predominantly formed from impacts, we can learn more about it by looking at meteorites..
Two basic types of meteorites
http://www.cpither.freeserve.co.uk/sporadic_meteors.htm
• Made of (you guessed it..)
• Made of silicates
Iron meteorites
http://www.cpither.freeserve.co.uk/sporadic_meteors.htm
• These objects formed when the differentiated cores of large planetesimals were subsequently disrupted by collisions
Ordinary chondrites
• Ordinary chondrites are a type of stoney meteorite that (usually) contain chondrules
• Definition: chondrules—millimeter-sized inclusions in some meteorites that formed (somehow) within the solar nebula
Ref.: J. K. Beatty et al., The New Solar System, Ch. 26
Stoney meteorite classifications
Ref.: J. K. Beatty et al., The New Solar System, Ch. 26
Carbonaceous chondrites
Ref.: J. K. Beatty et al., The New Solar System, Ch. 26
ALH77307
Compositions fromAllende meteorite
• CI Carbonaceous chondrites are considered to be the most similar in composition to the solar nebula• They do not have chondrules!
Volatiles in meteorites
• Carbonaceous chondrites are rich in water and other volatiles– Up to 20 wt.% H2O (although some of this
may be absorbed by the meteorite after it hits the Earth)
– Approximately 3.5 wt% organic C– Nitrogen and noble gases are trapped within
the organic carbon matrix• Ordinary chondrites are much less
volatile-rich– Roughly 0.1 wt% H2O
Is the Earth formed from chondrites?
• Mass of Earth: 61024 kg• Mass of oceans: 1.41021 kg• Ordinary chondritic planet:
61024 kg (0.001) = 61021 kg = 4 oceans
• Carbonaceous chondritic planet: 61024 kg (0.15) = 91023 kg
= 600 oceans!– So, we only need a few carbonaceous-type
planetesimals to get Earth’s water– Alternatively, we could build the Earth from ordinary
chondrites. But then we run into the problems mentioned last time, i.e., it is difficult to form hydrated silicates in the inner Solar System
Asteroid belt
• Range: 2-3.5 AU– Mars: 1.5 AU– Jupiter: 5.2 AU
• Inner belt (2-2.5 AU)– “S-type” asteroids
• Outer belt (2.5-3.5 AU)– “C-type” asteroids– These ones are
thought to be carbon-rich, like carbonaceous chondrites
http://content.answers.com/main/content/wp/en/thumb/8/80/350px-InnerSolarSystem-en.png
Kirkwood Gaps
http://en.wikipedia.org/wiki/Kirkwood_gap
• So, could water-rich planetesimals from the outer asteroid belt region have hit the Earth during accretion?
Yes!
Accretion of volatiles
Raymond et al., Icarus (2006)
• Yes, it is possible for planetesimals to migrate in from the outer asteroid belt region during accretion• The planet formed at 1 AU in this particular simulation is extremely water-rich: oceans would be 10’s of kilometers deep!
Stochastic volatile delivery
Raymond et al., Icarus (2004)
• Outcomes of 11 different simulations• Some planets formed near 1 AU are wet, others are dry
SolarSystem
• Another way to approach the problem of delivery of volatiles/formation of the atmosphere and oceans is to use noble gases
• Why?
• The noble gases occupy the rightmost column of the periodic table• Their outer shell of electrons is completely filled
• Another way to approach the problem of delivery of volatiles/formation of the atmosphere and oceans is to use noble gases
• Why?
• Answer: Because they are chemically unreactive (except possibly for Xe) and, hence, they should just tend to sit in a planet’s atmosphere, if they don’t escape
Solar noble gases (non-radiogenic)
Ref: H. D. Holland, Chemical Evolution of the Atmosphere and Oceans (1984), p. 33. (After Anders and Owen, 1977)
• The lighter noble gases are most abundant in the Sun, and presumably in the solar nebula, as well
Noble gases in Earth’s atmosphere
g/g planetGas Concentration (ppmv) g/g solar
20Ne 16.5 4.510-9
36Ar 31.5 3.910-7
84Kr 0.65 2.910-5
132Xe 0.0234 1.110-4
Ref.: Holland, H. D., The Chemical Evolution of the Atmosphere and Oceans (1984), p. 30
• • But the lighter noble gases are depleted in Earth’s atmosphere relative to solar abundances• What does this tell us?
What does this tell us?
1. Earth’s atmosphere did not form primarily from gravitational capture of gases from the solar nebula
– Or, if there was a captured atmosphere, it must have been nearly entirely lost, perhaps during the Moon-forming impact
2. Whatever process brought in the noble gases delivered the heavy ones more efficiently than the light ones
Planetary noble gas abundances
• Venus has ~100 times more noble gases than Earth, while Mars has ~100 times less
• Venus, Earth, and Mars all have roughly the same pattern of elemental abundances
• Meteorites have more Xe than does Earth (or Venus or Mars) “Missing xenon”
problem
Ref.: T. Owen et al., Nature (1992), Fig. 1
• One can also learn things by looking at isotopic ratios…
Terrestrial xenonisotopes
Ref.: R. O. Pepin, Icarus (1991)
• Linear fractionation pattern could be explained by hydro- dynamic escape of hydrogen dragging off Xe (Pepin, 1991)• Dragging off Xe, however, would entail dragging off everything else• It should also fractionation Kr isotopes very strongly, and this is not observed --So, Pepin assumes that Kr is replaced later, while Xe is not (?)
Neon isotopes• 3-isotope plots can be used
to distinguish gases coming from different sources
• Data shown are neon isotope ratios in MORBs (midocean ridge basalts)
• Earth’s atmosphere is depleted in 20Ne relative to 22Ne– 21Ne is radiogenic and is
simply used to indicate a mantle origin
• Mantle Ne resembles solar Ne– Ne is thought to have been
incorporated by solar wind implantation onto dust grains in the solar nebula
• The atmospheric 20Ne/22Ne ratio can be explained by rapid hydrodynamic escape of hydrogen, which preferentially removed the lighter Ne isotope
Ref: Porcelli and Pepin, in R. M. Canup and K. Righter, eds., Origin of the Earth and Moon (2000), p. 439
Volatiles from comets?• The comet model can
successfully explain the relative ratios of Ar, Kr, and Xe (thereby solving the “missing Xe problem”)– This can be simulated in
the lab by looking at low-temperature adsorption of gases onto amorphous ice
• Terrestrial planets fit on a mixing line between an indigenous source and comets
• Ne has to come in by another route, as mentioned previously
Ref.: Owen et al., Nature (1992), Fig. 2b
D/H Ratios• But the comet model
fails to account for the D/H ratio of the oceans (or, at least, it used to fail…)
• Cometary D/H is at least twice that of seawater (but see below)
• D/H in carbonaceous chondrites is scattered, but its average is close to that of Earth’s oceans
• Could Earth’s noble gases and its water have come from different sources?
Ref: Owen and Bar-Nun, in R. M. Canup and K. Righter, eds., Origin of the Earth and Moon (2000), p. 463
D/H from a Kuiper Belt comet
• Comet Hartley 2 is a short-period comet that appears to have a terrestrial D/H ratio (P. Hartogh et al., Nature, 2011)
• Characteristics– 2.250.6 km– Albedo = 0.028
• Orbital characteristics– P = 6.46 yr– e = 0.694– i = 13.6”
• Passed 0.12 AU from Earth on Oct. 20, 2010
Hartley 2 from EPOXI (formerly DeepImpact)
D/H from a Kuiper Belt comet
Hartogh et al., Nature (2011)
• If this is indeed a former Kuiper Belt object, then we may need to re-evaluate our models for how D/H varied in the early Solar System