Ranges of atmospheric mass and composition of super-earth exoplanets

1. Ranges of atmospheric mass and composition of super-earth exoplanets Elkins-Tanton & Seager 2008 Raquel Martinez EES 579 October 2, 2012

2. How do planets get their atmospheres? • Three primary opportunities • Nebular gas capture • Degassing during accretion • Degassing from tectonic processes http://www2.astro.psu.edu/users/niel/astro1/slideshows/class43/slides-43.html www.geo.ku.edu/programs/tectonics/

3. Basic characteristics of chondrites • Least processed, most primitive meteorite class (de Pater & Lissauer 2001) • Contain silicate components • Chondrules • Olivine • Pyroxene • Silicate Glass • Water in the form OH • Up to 20 mass% (Wood 2005) • Low oxygen chondrites contain metallic iron and nickel • Up to 50 mass% (Hutchison 2004) http://www.geokem.com/meteorites.html

4. Classes of Chondritic meteorites • CI (Ivuna Type) – Lack chondrules and refractory inclusions, metallic Fe. • CM (Mighei Type) – Contain chondrules of 0.1 to 0.3mm in diameter • CR (Renazzo Type) – Rich in metallic Fe-Ni, chondrules of 1mm • CO (Ornans Type) – Experienced very little aqueous alteration, 30% matrix • CV (Vigarano Type) – mm inclusions, abundant refractory materials, dark matrix • CK (Karoonda Type) – fewer inclusions than CV • H – High total Fe and high metallic Fe • L – Low total Fe content, most common type of meteorite to fall on Earth • LL – Low total Fe and low metal content • EH (High Enstatite) – small chondrules, high ratios of siderophiles to Si • EL (Low Enstatite) – larger chondrules, low ratios of siderophiles to Si

5. Basic characteristics of achondrites • Lack metallic iron, water, chondrules • Up to 3 mass% water (Jarosewich 1990) • Represent silicate remnants of planetesimals that differentiated into metallic iron cores and silicate mantles http://www.daviddarling.info/encyclopedia/A/achon.html

6. Modern Accretion Theories • Planets form from differentiated planetary embryos (radii ~a few x103 km) that move radially • Planets are a mixture of inner and outer material • Results in a wide range of bulk compositions • Rocky, iron metal-bearing, no water • Mixtures of metallic iron, silicate rock, unconstrained volatiles and ices Raymond et al. 2006

7. Atmospheres of Earth, venus, and mars • Present-day atmospheres indicate water and carbon dioxide have remained since accretion • Bulk of volatiles delivered toward the end accretion • Not all water reacted with iron since the planets have a metallic core and a volatile-rich atmosphere • Reactions are limited to the mantle and atmosphere and do not involve the core for later stages of accretion

8. Elemental Composition of Earth http://burro.astr.cwru.edu/stu/advanced/venus.html http://en.wikipedia.org/wiki/File:Atmosphere_gas_proportions.svg http://burro.astr.cwru.edu/stu/advanced/mars.html

9. Models and assumptions • Case 1A – Primitive Material Alone • Water oxidizes metallic iron • Hydrogen is degassed • Case 1B – Primitive Material with Added Water • Water added so that all metallic iron is oxidized • Used the average composition of 13 classes of chondrites from Hutchison (2004) • Compositions divided into silicate, metallic iron, and water fraction

10. Case 2A – Differentiated Material Alone • Achondritic material accretes onto protoplanet with existing core • Assumed whole-mantle magma ocean • Case 2B – Differentiated Material with Added Water • Assumed differentiation like that of our terrestrial planets • Core about ½ of planet’s radius • Silicate mantle • Achondritic water contents used by Jarosewich (1990) • No loss of volatiles during accretion • Modeled planets of 1, 5, 10, 20, and 30 Earth masses • Each with 0.5, 1, 5, and 10 mass% water in the magma ocean • Above 3 mass% water means added later

11. Magma ocean freezes from the bottom up Magma ocean freezes from the bottom up. As the magma ocean cools, the magma ocean adiabat moves toward lower temperature. Because the slope of the mantle liquidus is shallower than that of the magma ocean adiabat, it intersects with the magma ocean adiabat at the bottom first, where solidification begins. (Li & Agee 2000)

12. Other assumptions (Case 2) • Once volatiles are plentiful enough to exceed saturation capacity of the magma, degassing occurs • Partition according to their equilibrium partial pressures (Papale 1997)

13. results

15. 1A results • CI – about 23 mass% water • EH - 0.4 mass% H, .1 mass% C • Initial iron fraction controls the atmospheric composition • Not all planets formed from these meteorite classes will have iron cores • Can’t escape oxidation • Results in atmosphere of H, H2O • If iron is abundant, less water • Core forms, dissociates all water • Atmosphere forms without H2O

16. 1B results • All metallic iron is oxidized • Produces significant hydrogen atmospheres • Especially EH (high-iron chondrites) • Carbon content of atmospheres can range from 0.1 to 5 mass% of the planetary mass • Assumes that all carbon is volatilized

17. 2A results • Water content of atmospheres are much lower than in Case 1A • Even so, the resulting water composition is sufficient to produce Earth-like water budget

18. Case 2A results continued • Between 70% and 97% of volatiles are degassed • Remainder are contained within mantle silicates • More carbon will be degassed than water • The larger the planet, the lower the initial water fraction and the higher the water fraction retained in the mantle

19. Case 2B results • Volatiles would migrate inward from the outer solar system • Massive water atmospheres can be created on rocky planets • Above a certain initial water content of the magma ocean a fluid ocean can exist on the surface of the planet

20. Planet has formed, What next? • 3 main processes before final atmosphere attained • Atmospheric escape • Photolysis of molecules • Chemical kinetics • 3 end-members considered • All hydrogen has escaped • Hydrogen remains but methane and ammonia form slowly • Hydrogen doesn’t escape but methane and ammonia can form

21. HYDROGEN HAS ESCAPED • Photolysis has destroyed H-bearing molecules • No liquid water ocean ensures that CO2 dominates the atmosphere • A la Venus • If liquid oceans exist on the surface, some CO2 would reside within

22. SLOW METHANE AND AMMONIA FORMATION • Slow reaction rates for CH4 and NH3 require low temperatures and low pressure • Chemical kinetic “bottleneck” • Photolysis would break up the molecules and prevent them from reforming • This results in dominant molecules of H2, CO, H2O, N2 • Early Earth and Venus atmosphere would be dominated by carbon species • METHANE AND AMMONIA CAN FORM • Larger planet mass and amount of radiation received by star • Dominant molecules of H2, H2O, CH4, and NH3 • Some atmospheres were also 10% of the planet’s mass suggesting that methane and ammonia could be stable • These planets were dominated by water • Little nitrogen would be delivered by chondritic meteorites

23. Helium does not incorporate itself into silicates significantly • In this paper’s models, He is not present which implies that large atmospheres of He formed via other processes • If astronomers can observe He in exoplanet atmospheres can differentiate those created by outgassing or nebular capture • Jupiter: ~24 mass% He (NASA Planetary Fact Sheet) • Saturn: ~3% by Volume (NASA Planetary Fact Sheet) • Since not all iron was oxidized in terrestrial planets, this implies that they formed in reducing conditions • This could mean that the innermost planetary nebula may have lacked oxygen • Some people favored no water remaining after accretion • Authors assert that planets form with a range of final water contents • Hydrogen-outgassed atmosphere relies on metallic iron being oxidized by reaction with water • End-member of iron oxidation in planetary embryos results in no core terrestrial planet

25. Conclusions • Considered mass and composition range of super-Earth exoplanet atmospheres • From 1 to 30 Earth masses • Outgassing only • Different class of meteorites • These models predicted something not seen in the Solar System • Coreless terrestrial planets • Terrestrial planet with deep liquid oceans • Since He is not significantly outgassed, if observed outside our Solar System, one could distinguish between outgassing and nebular capture • Final atmosphere masses: 1% to 20% of planet’s total mass • Initial atmosphere compositions dominated by either carbon compounds, hydrogen, or water

26. Caution! • This paper relied on Earth’s own meteorite database • Exosolar planetary systems may have a larger range of planet building blocks to choose from

27. references • Harper, C. L., & Jacobsen, S.B. 1996, Science, 273, 1814 • Hashimoto, G. L., Abe, Y., & Sugita, S. 2007, J. Geophys. Res., 112, E05010 • Hutchison, R. 2004, Meteorites: A Petrologic, Chemical, and Isotopic Synthesis (Cambridge: Cambridge Univ. Press) • Jacobsen, S. B. 2005, Ann. Rev. Earth Planet. Sci., 33, 351 • Jarosewich, E. 1990, Meteoritics Planet. Sci., 25, 323 • Li J., & Agee, C. B. 1996, Nature, 381, 686 • O’Brien, D. P., Morbidelli, A., & Levison, H. F. 2006, Icarus, 184, 39 • Papale, P. 1997, Contrib. Min. Pet., 126, 237 • Raymond, S. N., Quinn, T., & Lunine, J. I. 2006, Icarus, 183, 265 • Wood, J. A. 2005, in ASP Conf. Ser. 341, Chondrites and the Protoplanetary Disk, ed. A. N. Krot, E.R.D. Scot, B. Reipurth (San Francisco: ASP), 953