For thought: 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 H 2 O is one important excess volatile Others include CO 2, N 2, S, and Cl Credit: AFP - Merapi Volcano, Indonesia **After notes by J. Kasting**
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. Iron Meteorite Stoney Meteorite
Iron meteorites 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 contain chondrules Definition: chondrules millimeter-sized inclusions in some meteorites that formed within the solar nebula Ref.: J. K. Beatty et al., The New Solar System, Ch. 26
Carbonaceous chondrites Compositions from Allende meteorite Carbonaceous chondrites are considered to be the most similar in composition to the solar nebula Ref.: J. K. Beatty et al., The New Solar System, Ch. 26
Volatiles in meteorites Carbonaceous chondrites are rich in water and other volatiles Up to 20 wt.% H 2 O (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% H 2 O
Is the Earth formed from chondrites? Mass of Earth: 6 10 24 kg Mass of oceans: 1.4 10 21 kg Ordinary chondritic planet : 6 10 24 kg ( 0.001) = 6 10 21 kg = 4 oceans Carbonaceous chondritic planet : 6 10 24 kg ( 0.15) = 9 10 23 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.
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
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!
Accretion of volatiles Movie Raymond, Quinn & Lunine (2006, Icarus, 183, 265-282)
Stochastic volatile delivery Solar System Raymond et al., Icarus (2006) Outcomes of 11 different simulations Some planets formed near 1 AU are wet, others are dry
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 and, hence, they should just tend to sit in a planet s atmosphere, if they don t escape
Solar noble gases 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 planet Gas Concentration (ppmv) g/g solar 20 Ne 16.5 4.5 10-9 36 Ar 31.5 3.9 10-7 84 Kr 0.65 2.9 10-5 132 Xe 0.0234 1.1 10-4 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 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
What does the previous slide tell us? Venus, Earth, and Mars all have roughly the same pattern of elemental abundances they all received their noble gases from the same type of source (probably either comets or asteroids)
What does the previous slide tell us? Venus has ~100 times more noble gases than Earth, while Mars has ~100 times less Was Venus hit with one large volatile-rich planetesimal? Mars is easier to explain: It loses atmosphere from sputtering by the solar wind
What does the previous slide tell us? Meteorites have more Xe than does Earth (or Venus or Mars) Is Xe hidden somewhere within the Earth? (e.g., adsorbed onto shales?) No.. Did Earth lose Xe to space? Maybe the Xe was not delivered to Earth by meteorites, but by comets instead (Owen et al., 1992)
D/H Ratios But the comet model fails to account for the deuterium/hydrogen ratio of the oceans! Cometary D/H is at least twice that of seawater Ref: Owen and Bar-Nun, in R. M. Canup and K. Righter, eds., Origin of the Earth and Moon (2000), p. 463
Implications of the D/H ratios Comets account for no more than ~10% of Earth s volatiles Most of Earth s volatiles come from the asteroid belt region Dynamical models * predict that large, icy planetesimals are responsible for much of the volatile delivery Corollary: Not all terrestrial planets need to receive the same initial inventories of water or other volatiles * Ref: Morbidelli et al., Meteoritics and Space Science (2000)
Terrestrial xenon isotopes Linear fractionation pattern could be explained by hydrodynamic escape of hydrogen dragging off Xe (Pepin, 1991) Dragging off Xe, however, would entail dragging off everything else.. Ref.: R. O. Pepin, Icarus (1991)
Martian xenon isotopes (from SNC meteorites) Martian Xe looks like terrestrial Xe Is this because of escape, or did both planets get their Xe from previously fractionated sources? Venus Xe could provide the answer (when eventually measured) Ref.: R. O. Pepin, Icarus (1991)
Neon isotopes Ref: Porcelli and Pepin, in R. M. Canup and K. Righter, eds., Origin of the Earth and Moon (2000), p. 439 Earth s atmosphere is depleted in 20 Ne relative to 22 Ne Mantle Ne looks more like solar Ne neon was dragged off by hydrodynamic escape of hydrogen
Volatiles from comets? The comet model can successfully explain the relative ratios of Ar, Kr, and Xe 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 Ref.: Owen et al., Nature (1992), Fig. 2b