Work in Progress: Exotic Chemospheres
Friday 17 June 2022
In last week’s newsletter I discussed, among other matters, the possibility of worlds that are chemically distinct from our world, though still an expression of the familiar laws of chemistry. In that brief sketch of chemical diversity I failed to note that not all isotopes are stable, and unstable isotopes decay into stable isotopes, so that even if a planet is formed in the context of exotic chemistry, it will only maintain this chemical exoticism if the isotopes are stable. Thus stable isotopes represent a natural bound to the chemical possibilities of our universe, though there is plenty of interesting chemistry that takes place while radioactive elements are decaying.
Lord Kelvin once attempted to show that Earth was not old enough for natural selection to have taken place, and these objections were taken quite seriously until the discovery of radioactivity showed the age of Earth to be on the order of billions of years. Thus even unstable elements can play an important role in the history of a planet. The amount and kinds of radioactive matter — most of the heavier elements of which are produced almost instantaneously in supernova events — that enrich a protoplanetary disk will play a role in the formation and development of planets.
I can’t recall where I read this, but I believe that I read that natural nuclear reactors like that at Oklo were a function of the natural level of enrichment present in uranium on Earth more than a billion years ago, whereas this kind of natural nuclear reactor could not happen today. In order for us to get fission out of uranium, we have to process it to levels of enrichment no longer naturally present in Earth’s crust — but these levels of enrichment were known in Earth’s past.
A planet with a significantly greater amount of uranium in its early history might reveal a plethora of naturally formed fission reactors, while a planet with significantly less uranium might not have enough to mine and refine, and it might not even have enough to prevent it from rapidly cooling and thus being too cold to have a biosphere. There is, then, a Goldilocks Zone for the presence of radioactive materials in a planet, with too much or too little radioactivity being filters for an emergent complexity regime such as we know on Earth.
Another natural bound to the chemical possibilities of planets are the possible geochemical processes to which matter is subject, and which produce mineral species. Earth is rich in mineral species because of the many geophysical processes taking place — not only the rock cycle driven by vulcanism (which can and does take place on other worlds in the solar system), but also the water cycle, and many biogeochemical cycles.
There are a number of mineral species present throughout the solar system, and presumably common also in other planetary systems, but there are a great many more mineral species on Earth because of the complexity of the geophysical processes that Earth represents. As far as I know, there is no available metric to signify the complexity of a given planet, but we need to recognize that some planets are much more complex than other planets, and planetary complexity is likely a driver of other forms of emergent complexity that supervene upon geochemical complexity. Earth’s geochemical complexity eventually resulted in life, and life in turn drives further geochemical complexity.
“Stagnant lid” planets without plate tectonics have fewer geochemical processes than planets with plate tectonics, and so likely have fewer mineral species. There is a question as to the habitability of stagnant lid planets — cf., e.g., Habitability of Earth-like Stagnant Lid Planets: Climate Evolution and Recovery from Snowball States, by Bradford J. Foley. Given that human beings are contemplating the prospects for settling Mars, which is a stagnant lid planet, we should make the distinction between a planet complex enough to experience an origins of life event, and a planet complex enough for life to exist on it, whether or not life could originate there. The idea of habitability serves as a kind of rough proxy for planetary complexity, and we could even distinguish between habitable (consistent with the existence of life) and inhabited (consistent with life and which has experienced an origins of life event and so it is actually inhabited) in order to preserve this distinction.
There are already conceptions of the origins of life that make this distinction, and I have discussed some of these papers in other writings. For example, in Boundary Conditions for Emergent Complexity Longevity I discussed the paper The origin of RNA precursors on exoplanets, by Paul B. Rimmer, Jianfeng Xu, Samantha J. Thompson, Ed Gillen, John D. Sutherland, and Didier Queloz, which posits an “abiogenesis zone” distinct from a habitable zone, showing that the two zones may overlap but do not necessarily coincide.
Comets are not large enough to have plate tectonics, but they do experience a number of geochemical processes that are not found elsewhere in the solar system (especially cyclical processes of heating and cooling over cosmological scales of time), and they are likely enriched in mineral species as a result of these processes. Accordingly, in some recent presentations I have focused on comets as possible places for emergent complexity to arise. In a planetary system with very large comets, or a very large number of comets (recall that one of the earliest possible explanations for the unusual lightcurve of Boyajian’s Star was a “swarm” of comets), comets could be the most complex bodies in that systems and thus the best bet for a location for further emergent complexity to supervene on earlier forms of complexity.
Thus a metric of planetary complexity would be one metric for the relative complexity of a planetary system, and the overall complexity of a planetary system would take into account the complexity of planets within the planetary system, the number and complexity of comets and asteroids, and generally speaking the complexity of any large feature. Could a ring system reach a level of complexity that it resulted in a greater number of mineral species? Perhaps so.
Some measures of complexity would be pretty straight-forward: number of planets, number of elements and isotopes, number of mineral species present, and so on. Insofar as some of these metrics could be determined or inferred from distant observations, we might be able to form an estimate of the overall complexity of a planetary system before we have the wherewithal to reliably conduct exoplanet atmospheric spectroscopy.
At the earliest stages of the development of a planetary system the study of protoplanetary disks can be a proxy for the elements present at planetary formation, while at the end stages of the development of planets — now called necroplanetology — the breakup of planets is being studied. It turns out that the “infall of formerly orbiting planetary objects” into the atmosphere of a white dwarf can be observed (cf. “Polluted white dwarfs reveal exotic mantle rock types on exoplanets in our solar neighborhood” by Keith D. Putirka and Siyi Xu). In the discussion portion of the paper the authors write:
“Our results verify that PWDs record the accretion of rocky exoplanets, but they also show that those exoplanets associated with PWDs have compositions that are exotic to our Solar System — sufficiently so to require new rock classification schemes to describe their mineral assemblages.”
This then at least partially validates my speculation that the chemistry of other worlds could be quite different from terrestrial chemistry. The final citation of this paper is “Mineral ecology; chance and necessity in the mineral diversity of terrestrial planets,” by Robert M. Hazen, Edward S. Grew, Robert T. Downs, Joshua Golden, and Grethe Hystad. This paper states:
“…stars can differ significantly from the Sun in relative abundances of rock-forming elements, which implies that bulk compositions of some extrasolar Earth-like planets likely differ significantly from those of Earth, particularly if the fractionation processes in evolving stellar nebulas and planetary differentiation are factored in.”
This paper concludes with the following observation:
“Were Earth’s history to be replayed, and thousands of mineral species discovered and characterized anew, it is probable that many of those minerals would differ from species known today.”
This sounds a lot like a mineralogical parallel of Stephen J. Gould’s famous thought experiment of rewinding the tape of life. Be that as it may, it appears that even given the natural boundary conditions of stable elements and the finite number of processes that generate mineral species, that planets can vary considerably in their mineralogical composition, and this in turn means distinct chemospheres. Distinct chemospheres, in turn, could be the basis upon which distinct forms of emergent complexity could supervene. This is the point of view of emergent complexity pluralism, and, so far as I understand matters, contemporary planetary science suggests the possibility of this.