Cosmic Rays enhance Prebiotic Chemistry on Sunless Worlds

In the International Journal of Astrobiology, Camille Bilger, Christiane Helling and Paul Rimmer (2014) presented a proof-of-concept on the potential effect of cosmic rays in the upper atmospheres of exoplanets (atmospheric pressures between 0.000001 bar and 0.000000001 bar, where 1 bar is the pressure of Earth’s atmosphere at sea level). We model the cosmic ray transport through the atmosphere of a planet with elemental composition and surface gravity similar to (but not the same as) Jupiter, if Jupiter were much farther away from the sun. This model atmosphere was produced using the Drift-Phoenix code (an upcoming post will be available soon on the Drift-Phoenix code).

Cosmic rays are charged particles (electrons, protons, bare nuclei) hurled through our galaxy at relativistic speeds by supernovae. When they strike the upper atmosphere of a planet, found to change its chemistry.

Cosmic Ray air shower

Figure 1. Cosmic Ray air shower

The combination of ultraviolet light from a star and cosmic ray ionization involves a delicate interplay between physics and chemistry, and is a hard problem to solve. It is simpler to consider cosmic ray chemistry on planets without daylight. These planets are often gas giants far from their host star, or rogue planets without a host star at all. These gas giants, like Jupiter, have an atmosphere made up not of mostly oxygen and nitrogen, but of mostly hydrogen, with a significant amount of nacient atomic hydrogen.

This is therefore a reducing atmosphere, and provides an ideal environment for making molecules believed to be important for the origin of life. Cosmic rays would ionize the molecular hydrogen, and this would make the atmosphere even more reducing.

Artistic impression of cosmic rays entering Earth's atmosphere. (Credit: Asimmetrie/Infn via CERN).

Figure 2. Artistic impression of cosmic rays entering Earth’s atmosphere. (Credit: Asimmetrie/Infn via CERN).

A reducing atmosphere is the standard initial chemical environment used in “origin-of-life” experiments, such as the Urey-Miller experiment. In the Urey-Miller experiment, an ionizing source in the form of an electrical discharge is initiated in a molecular gas, and so long as the atmosphere is reducing, prebiotic molecules are formed, including the twenty common amino acids found in living systems. If a reducing atmosphere, such as one dominated by oxygen and nitrogen, is used, the experiment produces no organic compounds.

The ion-neutral reactions made possible by cosmic ray ionization allow more of the hydrogen to be liberated from its molecular form, and increases the rate of reducing reactions. These reactions are found to be responsible for much of the prebiotic chemistry. Specifically, cosmic rays help to make biologically important molecules such as ammonia and acetylene. How much do cosmic rays help? They increase the abundance of some of these species by 10x or 100x in some cases.

Figure 3. Volume fraction of various species as a function of the gas pressure, p [bar] for the model atmosphere of a free-floating giant gas planet. The results assuming chemical quenching of C2H2 and C2H4 at height at ∼10−3 (dashed), and the results with cosmic ray ionization (solid) are all presented in this plot. A thick black horizontal line indicates the pressure above which termolecular (??) reactions may dominate.

Figure 3. Volume fraction of various species as a function of the gas pressure, p [bar] for the model atmosphere of a free-floating giant gas planet. The results assuming chemical quenching of C2H2 and C2H4 at height at ∼10−3 (dashed), and the results with cosmic ray ionization (solid) are all presented in this plot. A thick black horizontal line indicates the pressure above which termolecular reactions may dominate. (Rimmer et al. 2014, Fig. 4.)

The third picture in this article is from our paper, showing how much various molecules are enhanced or reduced because of cosmic rays. Some of the molecules that are enhanced, like ammonia, are key ingredients in the formation of the amino acid Glycine. These ingredients often must overcome a reactive barrier in order to form the amino acid, and overcoming this barrier may be made possible by electrostatic activation of ammonia (see our previous post here).

An image of the HR8799 planetary system from the December 2010 press release.

Figure 4. An image of the HR8799 planetary system from the December 2010 press release. (Credit: NRC-HIA, C. Marois, and Keck Observatory)

Our work may be relevant for directly imaged exoplanets orbiting HR 8799. Since these planets are ten times farther from their host star than Jupiter is from the sun, they are forever shrouded in the dark of night. It will be good to see how a star would change the chemistry, but this is a difficult problem. In order to find out what a star does in the upper atmosphere, it will be necessary to consider how the starlight passes through the atmosphere, and how that will affect thetemperature. The temperature will change the chemistry, and the chemistry will in turn affect how the starlight passes through theatmosphere. Much work still needs to be done to solve the problem for planets closer to their sun.

For more details check out the original paper on arXiv:

P. B. Rimmer, Ch. Helling and C. Bilger 2014, IntJAstrobio, 13, 173

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4 thoughts on “Cosmic Rays enhance Prebiotic Chemistry on Sunless Worlds

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