What chemical effects does lightning have on other planets? How does life affect the atmospheres of other worlds? The ground-breaking experiments of Miller and Urey showed that in some cases, lightning can produce the chemicals necessary for the origin of life. Did lighting succeed in starting off the chemistry that resulted in life on the Early Earth? Does it do so for other planets? These are the questions that a paper by Paul Rimmer & Christiane Helling (2016) wrestle with.
Chemistry leaves a very distinctive footprint on exoplanet atmospheres, thanks to the quantum mechanical precision of spectroscopy. Generally, the spectra of exoplanets can be determined in two ways. The most straightforward way to determine the spectrum of an exoplanet is to look directly at the object, and to observe its emitted light. This can be done only for exoplanets that are bright and far away from their host star, so that the starlight can be blocked out, leaving the planet’s light for astronomers to study. The light from the exoplanet is emitted at characteristic wavelengths, mostly in the infrared. The other way to obtain a spectrum is to look at a transiting planet, a planet that, from our perspective, passes in front of its host star, blocking out the star’s light. At some wavelengths, more light is blocked and the planet looks larger. At other wavelengths, less light is blocked and the planet looks smaller. These wavelengths are determined to exceptional precision by the quantum mechanical properties of the chemical species within the exoplanet’s atmosphere. Physics influences the chemistry, and changing the chemistry changes the light that the exoplanet emits and absorbs. Chemistry gives us a window into the lives of these alien worlds.
Some of the most interesting things in the universe are far outside equilibrium. Equilibrium is a way of describing a system by certain bulk quantities, such as temperature, pressure, volume, and available elements. If you know these properties, you have learnt a lot about the system. You know its chemical composition and its colour. But most of the interesting places in the universe exist outside equilibrium. Take Earth’s atmosphere, for example. It’s comprised of 78% molecular nitrogen, 21% molecular oxygen, and then small amounts of carbon dioxide, methane, hydrogen and water. If the carbon held within the biomass were present in the atmosphere, then our atmosphere would be made up of almost entirely nitrogen and carbon dioxide, with virtually no oxygen or methane. Even if the biomass were not liberated into the atmosphere, all the methane would be destroyed. Methane shouldn’t exist in our atmosphere at all at equilibrium. Life keeps the methane locked out of equilibrium. Additionally, there would be no ozone layer at all. Ozone is produced when molecular oxygen reacts with ultraviolet light from the sun.
Life and light are both written into our planet’s chemistry. And not just our planet’s. Directly imaged planets have chemical features that cannot be explained without invoking strong winds carrying equilibrium mixtures of molecules into places where they are out of equilibrium, and where they are replenished at a rate much faster than they are destroyed. Some hot Jupiters indicate abundances of acetylene and ammonia that are far from equilibrium. The search for life on other worlds has just started, and is focused entirely on the detection of just the kind of non-equilibrium chemistry that indicates the presence of life. There have been a variety of chemical models used to explain this non-equilibrium chemistry, either for Earth and Earth-like planets, or for hot Jupiters or directly imaged exoplanets. The models that work for one object generally cannot be applied to an object with a completely different composition or temperature, and none of these models considers the detailed chemistry of lightning.
An object like Jupiter is dominated by hydrogen, with trace amounts of carbon, oxygen and nitrogen, among other elements. Earth, on the other hand, is dominated by nitrogen and oxygen, with trace amounts of carbon in the atmosphere. The exceedingly thin atmosphere of Mars is primarily composed of carbon dioxide and nitrogen. Jupiter, Earth and Mars all would have formed from the same disk of gas and dust, with the gas mostly made of hydrogen. Why do they look so different? Why does Jupiter have so much hydrogen and the Earth and Mars don’t? The most important reason for this is that Mars and Earth are too light to retain hydrogen. They probably did have a hydrogen atmosphere very early in their lifetimes, but lost it long ago. Super-Earths, rocky exoplanets that are more massive than the Earth, have been discovered in abundance, and would have sufficient mass to retain much more hydrogen.
Paul Rimmer and Christiane Helling have developed a comprehensive chemical network called STAND, that incorporates hydrogen, carbon, nitrogen and oxygen chemistry, complete up to glycine, as well as simple chemistries for a variety of other species, including titanium, silicon, sodium, calcium, and potassium. It is robust for a wide range of temperatures and compositions, so that it can be applied to both hot Jupiter planets and directly imaged planets far from their host star, to rocky planets like Mars, Earth, or even to moons like Titan. Additionally, it includes UV and X-ray photochemistry, and chemistry induced by cosmic ray ionization or by interaction with other charged particles. The combination of these tools makes it so STAND can model in detail the chemistry of lightning, to determine both what observational effect it might have on exoplanet atmospheres and to determine whether lighting on different worlds might effectively open up the pathways to prebiotic chemistry.
In combination with ARGO, a photochemistry and diffusion model, STAND has been tested against various planets and exoplanets to find out whether it achieves results consistent with observation and with other models. STAND itself was also tested against laboratory tests similar to those of the Miller experiment, and found the same general trend concerning the hydrogen bonds of carbon and nitrogen, measured in the paper by a quantity called Rr. When they form mostly hydrogen bonds (Rr is small), they produce significant concentrations of glycine; and when carbon and nitrogen are bonded to oxygen or to each other instead of hydrogen (Rr is big), they produce small concentrations of glycine. This is true up until a certain point, when virtually all the nitrogen and carbon share hydrogen bonds, and none are bound to oxygen (when oxygen is bound either to itself as molecular oxygen or is in the form of water). In this case, interestingly, virtually no glycine is formed at all. This result from the paper is shown here in colour (Fig. 2)
The STAND network is now being compared to work from leading origin of life research groups, and is also being applied to explain the observed chemical impact of aurorae on brown dwarfs. Rimmer & Helling’s 2015 paper show how important it is to understand the connection between physics and chemistry in exoplanet atmospheres, because often chemistry is the only thing that can be determined very precisely or in much detail. They also provide the community with a new tool to model this chemistry at its most interesting, when it’s far from equilibrium.
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