1. Lecture 4: Origin of Earth’s Volatiles
Abiol 574

2. “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

3. 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

4. If Earth’s atmosphere was predominantly formed from impacts, we can learn more about it by looking at meteorites..

5. Two basic types of meteorites
Made of (you
guessed it..)
Made of silicates

6. Iron meteorites These objects formed when the differentiated
cores of large
planetesimals
were subsequently
disrupted by
collisions

7. 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

8. Stoney meteorite classifications
Ref.: J. K. Beatty et al., The New Solar System, Ch. 26

9. Carbonaceous chondrites
ALH77307
Compositions from
Allende meteorite 
CI Carbonaceous chondrites are
considered to be the most similar
in composition to the solar nebula
They do not have chondrules!
Ref.: J. K. Beatty et al., The New Solar
System, Ch. 26

10. 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

11. Is the Earth formed from chondrites?
Mass of Earth: 61024 kg
Mass of oceans: 1.41021 kg
Ordinary chondritic planet:
61024 kg (0.001) = 61021 kg
= 4 oceans
Carbonaceous chondritic planet:
61024 kg (0.15) = 91023 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

12. Asteroid belt Range: 2-3.5 AU Inner belt (2-2.5 AU)
Mars: 1.5 AU
Jupiter: 5.2 AU
Inner belt (2-2.5 AU)
“S-type” asteroids
Outer belt ( AU)
“C-type” asteroids
These ones are thought to be carbon-rich, like carbonaceous chondrites
thumb/8/80/350px-InnerSolarSystem-en.png

13. Kirkwood Gaps

14. So, could water-rich planetesimals from the outer asteroid belt region have hit the Earth during accretion?
Yes!

15. 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!

16. Stochastic volatile delivery
Solar
System
Raymond et al., Icarus (2004)
Outcomes of 11 different simulations
Some planets formed near 1 AU are wet, others are dry

17. Another way to approach the problem of delivery of volatiles/formation of the atmosphere and oceans is to use noble gases
Why?

18. The noble gases occupy the rightmost column of the periodic table
Their outer shell of electrons is completely filled

19. 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

20. 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

21. Noble gases in Earth’s atmosphere
g/g planet
Gas Concentration (ppmv) g/g solar
20Ne 10-9
36Ar 10-7
84Kr 10-5
132Xe 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?

22. What does this tell us?
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
Whatever process brought in the noble gases delivered the heavy ones more efficiently than the light ones

23. 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

24. One can also learn things by looking at isotopic ratios…

25. Terrestrial xenon isotopes 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 (?)
Ref.: R. O. Pepin, Icarus (1991)

26. 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

27. 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

28. 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

29. 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
2.250.6 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 Deep
Impact)

30. 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