Skip to main content Accessibility help

The influence of galactic cosmic rays on ion–neutral hydrocarbon chemistry in the upper atmospheres of free-floating exoplanets

  • P. B. Rimmer (a1), Ch. Helling (a1) and C. Bilger (a2)


Cosmic rays may be linked to the formation of volatiles necessary for prebiotic chemistry. We explore the effect of cosmic rays in a hydrogen-dominated atmosphere, as a proof-of-concept that ion–neutral chemistry may be important for modelling hydrogen-dominated atmospheres. In order to accomplish this, we utilize Monte Carlo cosmic ray transport models with particle energies of 106 eV<E<1012 eV in order to investigate the cosmic-ray enhancement of free electrons in substellar atmospheres. Ion–neutral chemistry is then applied to a Drift–Phoenix model of a free-floating giant gas planet. Our results suggest that the activation of ion–neutral chemistry in the upper atmosphere significantly enhances formation rates for various species, and we find that C2H2, C2H4, NH3, C6H6 and possibly C10H are enhanced in the upper atmospheres because of cosmic rays. Our results suggest a potential connection between cosmic-ray chemistry and the hazes observed in the upper atmospheres of various extrasolar planets. Chemi-ionization reactions are briefly discussed, as they may enhance the degree of ionization in the cloud layer.


Corresponding author


Hide All
Aikawa, Y., Umebayashi, T., Nakano, T. & Miyama, S.M. (1999). Evolution of molecular abundances in proto-planetary disks with accretion flow. Astrophys. J. 519(2), 705.
Bean, J.L., Miller-Ricci Kempton, E. & Homeier, D. (2010). A ground-based transmission spectrum of the super-Earth exoplanet GJ 1214b. Nature 468, 669672.
Bilger, C., Rimmer, P. & Helling, C. (2013). Small hydrocarbon molecules in cloud-forming brown dwarf and giant gas planet atmospheres. Mon. Not. R. Astron. Soc. 435, 18881903.
Borucki, W.J., Levin, Z., Whitten, R.C., Keesee, R.G., Capone, L.A., Summers, A.L., Toon, O.B. & Dubach, J. (1987). Predictions of the electrical conductivity and charging of the aerosols in Titan's atmosphere. Icarus 72, 604622.
Capone, L.A., Dubach, J., Whitten, R.C., Prasad, S.S. & Santhanam, K. (1980). Cosmic ray synthesis of organic molecules in Titan's atmosphere. Icarus 44, 7284.
Capone, L.A., Dubach, J., Prasad, S.S. & Whitten, R.C. (1983). Galactic cosmic rays and N2 dissociation on Titan. Icarus 55, 7382.
Dehn, M. (2007). PhD Thesis, University of Hamburg.
Demory, B.-O. et al. (2011). The high Albedo of the hot Jupiter Kepler-7 b. Astrophys. J. 735, L12.
Frenklach, M. & Feigelson, E.D. (1989). Formation of polycyclic aromatic hydrocarbons in circumstellar envelopes. Astrophys. J. 341, 372384.
Gredel, R., Lepp, S., Dalgarno, A. & Herbst, E. (1989). Cosmic-ray-induced photodissociation and photoionization rates of interstellar molecules. Astrophys. J. 347, 289293.
Gurnett, D.A., Shaw, R.R., Anderson, R.R. & Kurth, W.S. (1979). Whistlers observed by Voyager 1 – detection of lightning on Jupiter. Geophys. Res. Lett. 6, 511514.
Harada, N., Herbst, E. & Wakelam, V. (2010). A new network for higher-temperature gas-phase chemistry. I. A preliminary study of accretion disks in active galactic nuclei. Astrophys. J. 721, 15701578.
Helling, C. et al. (2008). A comparison of chemistry and dust cloud formation in ultracool dwarf model atmospheres. Mon. Not. R. Astron. Soc. 391, 18541873.
Helling, C., Jardine, M. & Mokler, F. (2011). Ionization in atmospheres of brown dwarfs and extrasolar planets. II. Dust-induced collisional ionization. Astrophys. J. 737, 38.
Hoyle, F. & Wickramasinghe, N.C. (2000). Astronomical Origins of Life: Steps towards Panspermia. Kluwer Academic Publishers, Dordrecht.
Krestinin, A.V. (2000). Detailed modeling of soot formation in hydrocarbon pyrolysis. Combust. Flame 121(3), 513.
Lavvas, P., Yelle, R.V. & Vuitton, V. (2009). The detached haze layer in Titan's mesosphere. Icarus 201, 626633.
Liang, M.-C., Yung, Y.L. & Shemansky, D.E. (2007). Photolytically generated aerosols in the mesosphere and thermosphere of Titan. Astrophys. J. 661, L199L202.
Lodders, K. (2004). Jupiter formed with more tar than ice. Astrophys. J. 611, 587597.
MacGregor, M. & Berry, R.S. (1973). Formation of HCO+ by the associative ionization of CH+O. J. Phys. B At. Mol. Phys. 6, 181196.
McElroy, D., Walsh, C., Markwick, A.J., Cordiner, M.A., Smith, K. & Millar, T.J. (2013). The UMIST database for astrochemistry 2012. Astron. Astrophys. 550, A36.
Millar, T.J., Farquhar, P.R.A. & Willacy, K. (1997). The UMIST database for astrochemistry 1995. Astron. Astrophys. Suppl. 121, 139185.
Miller, S.L. & Cleaves, H.J. (2006). Prebiotic chemistry on the primitive earth. Syst. Biol.: Vol. I: Genom.: Vol. I: Genom. 1, 1.
Molina-Cuberos, G.J., López-Moreno, J.J., Rodrigo, R. & Lara, L.M. (1999a). Chemistry of the galactic cosmic ray induced ionosphere of Titan. J. Geophys. Res. 104, 2199722024.
Molina-Cuberos, G.J., López-Moreno, J.J., Rodrigo, R., Lara, L.M. & O'Brien, K. (1999b). Ionization by cosmic rays of the atmosphere of Titan. Planet. Space Sci. 47(10–11), 1347.
Moses, J.I., Bézard, B., Lellouch, E., Gladstone, G.R., Feuchtgruber, H. & Allen, M. (2000). Photochemistry of Saturn's atmosphere. I. Hydrocarbon chemistry and comparisons with ISO observations. Icarus 143, 244298.
Moses, J.I., Fouchet, T., Bézard, B., Gladstone, G.R., Lellouch, E. & Feuchtgruber, H. (2005). Photochemistry and diffusion in Jupiter's stratosphere: constraints from ISO observations and comparisons with other giant planets. J. Geophys. Res. (Planets) 110, 8001.
Moses, J.I. et al. (2011). Disequilibrium carbon, oxygen, and nitrogen chemistry in the atmospheres of HD 189733b and HD 209458b. Astrophys. J. 737, 15.
Orgel, L.E. (1998). The origin of lifeâ€'a review of facts and speculations. Trends Biochem. Sci. 23(12), 491495.
Pont, F., Knutson, H., Gilliland, R.L., Moutou, C. & Charbonneau, D. (2008). Detection of atmospheric haze on an extrasolar planet: the 0.55–1.05 μm transmission spectrum of HD 189733b with the HubbleSpaceTelescope. Mon. Not. R. Astron. Soc. 385, 109118.
Porco, C.C. et al. (2005). Imaging of Titan from the Cassini spacecraft. Nature 434, 159168.
Prasad, S.S. & Tarafdar, S.P. (1983). UV radiation field inside dense clouds – its possible existence and chemical implications. Astrophys. J. 267, 603609.
Rages, K. & Pollack, J.B. (1983). Vertical distribution of scattering hazes in Titan's upper atmosphere. Icarus 55, 5062.
Rimmer, P. & Helling, C. (2013). Ionization in atmospheres of Brown Dwarfs and extrasolar planets IV. The Effect of Cosmic Rays. ArXiv e-prints.
Rimmer, P.B., Herbst, E., Morata, O. & Roueff, E. (2012). Observing a column-dependent ζ in dense interstellar sources: the case of the Horsehead nebula. A&A 537, A7.
Showman, A.P., Fortney, J.J., Lian, Y., Marley, M.S., Freedman, R.S., Knutson, H.A. & Charbonneau, D. (2009). Atmospheric circulation of hot Jupiters: coupled radiative-dynamical general circulation model simulations of HD 189733b and HD 209458b. Astrophys. J. 699, 564584.
Shull, J.M. & Hollenbach, D.J. (1978). H2 cooling, dissociation, and infrared emission in shocked molecular clouds. Astrophys. J. 220, 525537.
Shumilov, O.I., Kasatkina, E.A., Henriksen, K. & Vashenyuk, E.V. (1996). Enhancement of stratospheric aerosols after solar proton event. Ann. Geophys. 14, 11191123.
Sing, D.K. et al. (2011). Hubble Space Telescope transmission spectroscopy of the exoplanet HD 189733b: high-altitude atmospheric haze in the optical and near-ultraviolet with STIS. Mon. Not. R. Astron. Soc. 416, 14431455.
Sittler, E.C., Hartle, R.E., Bertucci, C., Coates, A., Cravens, T., Dandouras, I. & Shemansky, D. (2010). Energy Deposition Processes in Titan's Upper Atmosphere and Its Induced Magnetosphere, p. 393.
Skilling, J. & Strong, A.W. (1976). Cosmic ray exclusion from dense molecular clouds. Astron. Astrophys. 53, 253258.
Stark, C.R., Helling, C., Diver, D.A. & Rimmer, P.B. (2013). ApJ. 776, 11.
Velinov, P.I.Y. & Mateev, L.N. (2008). Improved cosmic ray ionization model for the system ionosphere atmosphere – calculation of electron production rate profiles. J. Atmos. Sol.-Terres. Phys. 70, 574582.
Velinov, P.I.Y., Mishev, A. & Mateev, L. (2009). Model for induced ionization by galactic cosmic rays in the Earth atmosphere and ionosphere. Adv. Space Res. 44, 10021007.
Venot, O., Hébrard, E., Agùndez, M., Dobrijevic, M., Selsis, F., Hersant, F., Iro, N. & Bounaceur, R. (2012). A chemical model for the atmosphere of hot Jupiters. ArXiv e-prints.
Vidotto, A.A., Fares, R., Jardine, M., Donati, J.-F., Opher, M., Moutou, C., Catala, C. & Gombosi, T.I. (2012). The stellar wind cycles and planetary radio emission of the τ Boo system. Mon. Not. R. Astron. Soc. 423, 32853298.
Visscher, C. & Moses, J.I. (2011). Quenching of carbon monoxide and methane in the atmospheres of cool brown dwarfs and hot Jupiters. Astrophys. J. 738(1), 72.
Wakelam, V., Selsis, F., Herbst, E. & Caselli, P. (2005). Estimation and reduction of the uncertainties in chemical models: application to hot core chemistry. Astron. Astrophys. 444, 883891.
Wakelam, V. et al. (2012). A KInetic Database for astrochemistry (KIDA). Astrophys. J. Suppl. 199, 21.
Whitten, R.C., Borucki, W.J., O'Brien, K. & Tripathi, S.N. (2008). Predictions of the electrical conductivity and charging of the cloud particles in Jupiter's atmosphere. J. Geophys. Res. (Planets) 113, 4001.
Wilson, E.H. & Atreya, S.K. (2003). Chemical sources of haze formation in Titan's atmosphere. Planet. Space Sci. 51, 10171033.
Witte, S., Helling, C. & Hauschildt, P.H. (2009). Dust in brown dwarfs and extra-solar planets. II. Cloud formation for cosmologically evolving abundances. Astron. Astrophys. 506, 13671380.
Woitke, P. & Helling, C. (2004). Dust in brown dwarfs. III. Formation and structure of quasi-static cloud layers. Astron. Astrophys.414, 335350.
Woitke, P., Kamp, I. & Thi, W.-F. (2009). Radiation thermo-chemical models of protoplanetary disks. I. Hydrostatic disk structure and inner rim. Astron. Astrophys. 501, 383406.
Woods, P.M. & Willacy, K. (2007). Benzene formation in the inner regions of protostellar disks. Astrophys. J. Lett. 655(1), L49.
Zahnle, K., Marley, M.S., Freedman, R.S., Lodders, K. & Fortney, J.J. (2009). Atmospheric sulfur photochemistry on hot Jupiters. Astrophys. J. 701, L20L24.
Recommend this journal

Email your librarian or administrator to recommend adding this journal to your organisation's collection.

International Journal of Astrobiology
  • ISSN: 1473-5504
  • EISSN: 1475-3006
  • URL: /core/journals/international-journal-of-astrobiology
Please enter your name
Please enter a valid email address
Who would you like to send this to? *



Altmetric attention score

Full text views

Total number of HTML views: 0
Total number of PDF views: 0 *
Loading metrics...

Abstract views

Total abstract views: 0 *
Loading metrics...

* Views captured on Cambridge Core between <date>. This data will be updated every 24 hours.

Usage data cannot currently be displayed