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 Credit: AFP - Merapi Volcano, Indonesia EAS 4803/8803 - CP **After notes by J. Kasting** 23:1
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 EAS 4803/8803 - CP 23:2
If Earth s atmosphere was predominantly formed from impacts, we can learn more about it by looking at meteorites. Iron Meteorite EAS 4803/8803 - CP Stoney Meteorite 23:3
Iron meteorites These objects formed when the differentiated cores of large planetesimals were subsequently disrupted by collisions EAS 4803/8803 - CP 23:4
Ordinary chondrites Ordinary chondrites are a type of stoney meteorite that contain chondrules Definition: chondrules millimetersized inclusions in some meteorites that formed within the solar nebula EAS 4803/8803 - CP Ref.: J. K. Beatty et al., The New Solar System, Ch. 26 23:5
Carbonaceous chondrites Compositions from Allende meteorite Ref.: J. K. Beatty et al., The New Solar System, Ch. 26 Carbonaceous chondrites are considered to be the most similar in composition to the solar nebula EAS 4803/8803 - CP 23:6
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 EAS 4803/8803 - CP 23:7
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. EAS 4803/8803 - CP 23:8
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 EAS 4803/8803 - CP 23:9
So, could water-rich planetesimals from the outer asteroid belt region have hit the Earth during accretion? Yes! EAS 4803/8803 - CP 23:10
Accretion of volatiles EAS 4803/8803 - CP 23:11
Accretion of volatiles Movie Raymond, Quinn & Lunine (2006, Icarus, 183, 265-282) EAS 4803/8803 - CP 23:12
Stochastic volatile delivery EAS 4803/8803 - CP 23:13
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 EAS 4803/8803 - CP 23:14
Solar noble gases EAS 4803/8803 - CP 23:15
Noble gases in Earth s atmosphere EAS 4803/8803 - CP 23:16
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 EAS 4803/8803 - CP 23:17
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 EAS 4803/8803 - CP 23:18
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) EAS 4803/8803 - CP 23:19
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 EAS 4803/8803 - CP 23:20
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) EAS 4803/8803 - CP 23:21
EAS 4803/8803 - CP 23:22