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Discord Invitation
Planetary Interfaces and the Origins of Life
As a locus of emergent complexity – in the forms of life, consciousness, intelligence, technology, and civilization – Earth has been, as a planet, sufficiently complex that these further emergent complexities could supervene upon prior complexities. What are these prior complexities? What makes Earth a complex planet that can give rise to further complexity?
Earth seems to be more geologically complex than other planets because of the many processes occurring on its surface (as well as in the mantle, etc.). Mars may have had water at one time, but Mars freeze dried itself, so processes based on water (the hydrological cycle) stopped. Also, Mars is not tectonically active, and has a very weak magnetic field compared to Earth. This is only one example of another planet, but other planets in our planetary system are likely to be even less geologically complex (with some exceptions noted below). However, we are certain to learn a lot of interesting things when we get to study the moons of the gas giant planets, where there are probably processes occurring that don’t occur on Earth.
The natural geological structures of Earth are complex because Earth is geologically active: it builds mountains, oceans grow and shrink depending upon the present location of the continents, and these things occur because of plate tectonics and the supercontinent cycle. And because of these processes (think of the cycling of rocks from igneous to sedimentary to metamorphic, and so on) minerals are produced that are not produced in less complex environments. So the geological complexity and the minerological complexity are linked.
Note: on geological and minerological complexity, I cannot highly enough recommend the works of Robert Hazen, especially his book The Story of Earth: The First 4.5 Billion Years, from Stardust to Living Planet, which goes into some detail on what Hazen calls “mineral evolution.” Minerals evolve to a greater degree of complexity in more geologically complex environments. I also highly recommend Hazen’s Great Courses lectures The Origin and Evolution of Earth: From the Big Bang to the Future of Human Existence.
It has become a familiar motif of contemporary pedagogy to teach the Earth sciences in terms of Earth “spheres,” which usually includes, at a minimum, the lithosphere, the hydrosphere, the atmosphere, and the biosphere. This list can be extended in several ways. The lithosphere can be further subdivided into inner core, outer core, mantle, upper mantle, asthenosphere, and the crust. The atmosphere can also be further subdivided into the troposphere, stratosphere, mesophere, thermosphere, and exosphere. The hydrosphere can be divided in the hydrosphere and the cryosphere, and the whole of Earth can be placed within the magnetosphere. Sometimes “geosphere” is used to indicate the lithosphere, hydrosphere, and atmosphere as distinct from the biosphere.
This motif of Earth “spheres” is one way in which we can think about the complexity of Earth as a planet. Mars, with a lithosphere and an atmosphere, but no hydrosphere, is a simpler physical system than Earth. (Mars seems to have had oceans in the distant past, and even has some fragment of a cryosphere, but the oceans of Mars have been gone for about as long as life has existed on Earth.) Mars, with only a lithosphere and a atmosphere, has only one interface between these two: the planetary surface where land and air meet.
Earth, with an atmosphere, hydrosphere, and lithosphere, has three kinds of interfaces between Earth systems: the atmosphere-hydrosphere interface on the surface of the oceans, the hydrosphere-lithosphere interface where water and land meet, and the lithosphere-atmosphere interface, where air and land meet. It is interesting to note that there is an origins of life theory that roughly corresponds to each of these interfaces of Earth systems, where one can expect the highest degree of complexity due to the interactions of these systems, as well as origins of life theories that involve life exclusively originating within one of these Earth systems.
Darwin’s idea of a “warm little pond” can be taken as either a hydrosphere-atmosphere interface or as a lithosphere-atmosphere interface (the latter if the water in the shallow pond is considered negligible). The hydrothermal vent idea for the origins of life is a hydrosphere-lithosphere interface. The approach of the Miller-Urey experiment could be considered to take place entirely within the atmosphere, or as an example of hydrosphere-atmosphere interface, or even as an atmosphere-lithosphere interface. The Oparin ocean idea (also generically known as the “primordial soup”) can be understood either as origins of life entirely within the hydrosphere or as a hydrosphere-atmosphere interface. The idea of clay as a medium for the formation of the macro-molecules involved in life can be understood as a mechanism confined entirely to the lithosphere, or as a lithosphere-atmosphere interface.
Given the relatively recent expansion of our conception of habitability to include the possibility of life in subsurface ocean environments, it is interesting to note that on such worlds (many of which worlds are moons) the cryosphere would be a major component of the geological complexity of that world. At minimum, subsurface ocean worlds would, like Earth, have two interfaces: the hydrosphere-cryosphere interface and the hydrosphere-lithosphere interface. This would be the case if the subsurface ocean completely separated the icy crust from the rocky world. If some features of the lithosphere, however, penetrated upward through the ice, there would also be a lithosphere-cryosphere interface, in which case a subsurface ocean world would have three interfaces, like Earth.
Even if we find no life in the subsurface oceans of the moons within our solar system, it will still be interesting to learn about the complex geological interactions between cryosphere, hydrosphere, and lithosphere. In the case of Saturn’s moon Enceladus, which spews out water into space from its subsurface ocean through cracks in the ice, this seepage may further interact with Saturn’s considerable magnetosphere, for additional complexity. We have a lot yet to learn within our solar system, and what we learn will have applications to the complexity we know on Earth, as well as to complexities we do not find on Earth but which we may find implicated in further forms of emergent complexity elsewhere.
