What is our current understanding about the origin of the atmosphere?submitted by Cindy Shellito, University of Northern Colorado; Linda Sohl, Columbia University; and Jim Kasting, The Pennsylvania State University
Why is this question important?This question has implications for when we think life became established on Earth.
What we know...Many textbooks currently give an older view regarding the origin of the atmosphere. A common idea involves volcanic outgassing of volatiles from Earth's interior. The premise of this hypothesis is Earth was formed by gradual accretion of cold rocky materials. Over time as Earth grew larger frictional heating lead to the melting of these proto-earth materials. These molten materials allowed differentiation to occur. Over 500 million years the Earth cooled gradually. Heavier elements ended up near the center of the earth lighter elements near the surface and gases burst forth from volcanoes forming the atmosphere. It was thought that the Earth rocks did not have sufficient water to form the oceans and that the oceans formed gradually through bombardment by comets.
What are some of the new ideas? Computer simulations by Raymond et al. (2006) suggest that the accretion of many small (1-10 km) water-rich planetesimals via impact with larger (~1000-km) "planetary embryos" are capable of creating Earth-like worlds that have at least the same water budget as the modern Earth. Asteroids are a more likely source of this water since the mean value of D/H ratios of carbonaceous chondrites is similar to that of Earth's oceans (Morbidelli et al. 2000). Raymond et al.'s requirement for planetesimals containing 0.006% weight volume of water is consistent with an independent estimate for the total amount of water accreted onto the Earth (4 x 1020 to 2 x 1022 kg) (Dauphas et al. 2000) as well as a simple back-of the envelope calculation taking into account the current volume of the oceans and assuming a similar volume of water in the mantle.
How to link this topic to the classroom
To illustrate the importance of impacts in creating the Earth's atmosphere and oceans ask students to consider the following questions (these may be incorporated into lecture—provide students with background information and ask them to work in groups of 2-3 to obtain answers).
1. Determine the current volume of water on Earth as a weight percentage of the total Earth mass assuming that the volume of water in the mantle is equivalent to that of the oceans. (As a follow-up to this compare the results to the weight percentage of water in meteorites—up to 10%):
Mass of Earth: 6x1027
Mass of Oceans: 1.4x1024 (assume an equivalent amount is also in the mantle)
Weight Percentage of water (compared to mass of earth) = 0.05%
2. Calculate the kinetic energy required to melt volatile material carried in on planetesimals.
Heat of vaporization (silicates): 13MJ/kg
Heat of vaporization (ice): 3 MJ/kg
Velocity of arriving planetesimal ~ 20 km/s (must be greater than the escape velocity) KE=1/2*m*v**2 = 2x108 J/kg = 200MJ/kg (this is 15 times the amount of energy needed for vaporization)
References and other Resources
- Abe Y. Matsui T. 1986. Early evolution of the Earth: accretion atmosphere formation and thermal history. Journal of Geophysical Research 91: E291-302.
- Ahrens T.J. 1993. Impact erosion of terrestrial planetary atmospheres. Ann Rev. Earth Planet. Sci. 21 25-55.
- Dauphas N. Robert F. and Marty B. 2000. The late asteroidal and cometary bombardment of Earth as recorded in water deuterium to protium ratio. Icarus 148: 508-512.
- Morbidelli A. Chambers J. Junine J.I. Petit J.M. Robert F. Valsecchi G.B. and Cyr K.E. 2000. Source regions and timescales for the delivery of water to the Earth. Meteoritics & Planetary Science 35: 1309-1320.
- Owen T.C. and Bar-Nun A. 2001. Contributions of icy planetesimals to the Earth's early atmosphere. Origins of Life and Evolution of the Biosphere 31: 435-458.
- Raymond S.N. Quinn T. Lunine J.I. 2004. Making other earths: Dynamical simulations of terrestrial planet formation and water delivery. Icarus 168: 1-17 (RQL04).
- Raymond S.N. Quinn T. Lunine J.I. 2006. High-resolution simulations of the final assembly of Earth-like planets I. Terrestrial accretion and dynamics. Icarus 183: 265-282.
References on older ideas about the origin of the atmosphere:
- Fischer A.G. 1965. Fossils early life and atmospheric history. Proceedings of the National Academy of Sciences 53: 1205-1213.
- Holland H.D. Engel A.E.J. et al. 1962. Model for the evolution of the Earth's atmosphere. Petrologic Studies: A Volume to Honor A.F. Buddington. New York Geol. Soc. Am. 447-477.