Alien Earth

It's almost funny that a molecule as simple and ubiquitous as water, which for obvious reasons has evolved as a standard compound for the all sorts of scientific reference, is actually one of the more defiant little buggers when it comes to high-level quantitation. Water is highly polar, meaning that a lot of partial electron charge huddles on the oxygen side of it, leaving the little Mickey Mouse ears of the hydrogens to wiggle around with their pants half down, showing a little proton as it were, and correspondingly getting all positively charged. In liquid water, these protons are attracted to their neighbors' oxygens and the whole crew is rather amorous. As they slosh and stir, it can be hard to tell who is with whom, those protons hugging their own oxygen, but stealing kisses from the one next door. It's that extensive hydrogen bonding that causes the unusual crystallization, necessitates fudge factors in the equations of state, requires fairly complicated descriptions of its solvating behavior--when water dissociates itself (or something else), does a bare proton end up floating around? A hydronium ion? H-O***H networks alternating themselves several layers deep until the effect of the charge fizzles out? It depends on how you need to look at it. How about polywater? Nah, let's not get carried away.

Each of the elements, especially as you go along the second row of the periodic table, loves hydrogen in its own special way. Oxygen, fluorine and nitrogen are the elements that really get into the hydrogen bonding game. You can make some good analogies with liquid ammonia, which like water, will autodissociate (now into ammonium and amide ions), and which will dissolve ionic species. Ammonia doesn't love itself quite as much and will stabilize (most) ions less aggressively. In the symphony of ionic transport, it plays a duller beat, but could ammonia be a viable solvent for some alien biology? I don't see any obvious reason why not.

I'm fond of these sorts of analogies--swapping media can offer windows onto the universalities of the chemistry and physics but make the whole thing look drastically alien. From another planet. The taken-for-granted chemical balance on earth--what with the free oxygen, water, and the oxidized carbon and silicon--is in a way arbitrary, some dynamic product of temperature and composition, evolved to some steady state. If we limit our speculation to the low-temperature, low-gravity conditions where "chemistry" matters, where life processes still could fit within the usual speculations, then a tour around the solar system shows familiar features carved out of exotic substances. Even on the small handful of dense, rocky balls within a rocket blast, atmospheric and geological dynamics are surprisingly familiar, even though they occur at forbidding temperatures and pressures, circulating toxic compounds.



The surface of Venus evidently lacks plate tectonics, and a going theory is that the planet heats cyclically, the crust periodically weakens and floods with magma every few hundred million years. The atmosphere is denser than we're familiar with, reaching supercritical conditions for the predominant CO₂, contributing to massive greenhouse heating like a great big heat shield. Most of the action is in the turbulent skies, precipitating sulfuric acid, with possible pockets of oxygen and moderate temperatures in the higher reaches. It's not like sulfuric acid is unusual or poorly understood--you could probably even distill it in the lab if you were so maniacally inclined--but looking at whole weather systems of the stuff is really something. The system is water-deficient, but may not have always been so. Presumably the lighter water long since boiled and buoyed itself to the upper atmosphere only to be beaten away by the solar wind.



The upper reaches of Venus may even be more hospitable than the darling neighbor Mars. We're familiar by now with images of dusty vistas of the red planet. That photo could almost be a back-porch view of a quiet Arizona evening, if you could only breathe. The planet has many intriguing features, but if Mars had a dynamic surface liquid layer, then it's also been scrubbed over the eons by the solar wind. (It helps to have a big magnet in the core of your planet, and just a little more gravity too.) After the landscapes and the sunrises, my favorite images of the Martian surface have shown the aggregate soil, small wind-eroded BBs and sand.



Unlike our planetary neighbors, Jupiter's moon Europa does exhibit plate tectonics, evidently with lateral spreading and subduction. The surface of the moon is smooth, and believed to be only a hundred million years young. The plates are ice, and the mantle is slush, which turgidly convects the plates along like a cold analogue of earth's silicate-based geological mechanics. Volcanoes occasionally cough up water and other cryogenic gases. In Europa's case, the driving energy for its geology* appears to come orbit-induced stresses, other bodies pulling it this way and that.



As hopeful celestial bodies go, Saturn's moon Titan is pretty great. It has a dense nitrogen atmosphere, with temperatures ranging from "dry ice" in the upper reaches to "liquid nitrogen" in its colder altitudes, which admittedly are worrisome conditions for any potential microbes, but on the other hand, there's tons of higher molecular weight organic material, for hypothetical organisms to compose themselves from. Titan is the only other well-known body (I believe) to be hydrologically* active at the surface, with surface systems of hydrocarbon rivers, lakes, and clouds that mimic our earth's water cycle. It's basically raining lighter fluid, but don't worry about lighting a match, there's no oxygen. Like Europa, Titan is believed to have a solid or slushy mantle of water, and possibly a liquid layer which would probably be stinking with ammonia, if it exists.

It's fascinating, but the truth is, I can only pore over this stuff for so long. After a while the images start to look like inanimate wastelands: cold, poisonous, and, as it comes down to the human experience, arbitrary. You could as easily be getting worked up over the deeper meaning of the ice deposits in your fridge, of the formations of schmutz that get caked on the bottom of a lab beaker or something, and I guess those things can be fascinating too, not so depressingly lonely. Ever since Schiaparelli believed he saw canals on Mars, the optimistic view of these hostile orbs--where nooks of biocompatibility exist or existed--has been the hopeful go-to narrative for scientists. Did oceans on Mars and Venus disappear, and could it happen to Earth? Are Europa and Titan teeming with cold, slow biological seas? It's the way we have always liked to tell our stories.

The lesson, really, is a sense of context. We're comfortable speculating on the geological* and climatological histories of other celestial bodies (there aren't too many consequences of this), and these scant data points to provide insight on How Things Work on planetary scales. If you go far enough back, earth was one of these sulfurous alien hellholes too. Our oxygen-rich atmosphere was a geological late-comer, and the older earth was smoking with nitrogen, methane, carbon dioxide and water, and metals just kind of hung around in their reduced forms. The change in the chemical environment on the surface was enormous and comprehensive. It precipitated out enormous quantities of iron from the ocean (providing handy concentrations of ore for the talking monkeys that'd inhabit the place two and a half billion years later), ate up the methane, and changed the temperature dynamics. The biology of the place completely changed at the same time, and it was all incredibly fast, in the scheme of these things.

The Great Oxidation Event may have been the most significant "moment" in the planet's history, enabling a whole lot of new surface weathering processes, including rampant biological diversification. There had been anaerobic bugs for a while on the planet, not doing much but metabolizing CO₂ and pooping methane, and it is unclear just how many billions of years ago the small population of oxygenic organisms showed up. What is clear is that they eventually won out after a long period of peaceful coexistence. One theory has it that the earth's mantle eventually cooled enough to significantly reduce the geological production of nickel, which is vital in the methanogens' metabolism. In that scenario, the blue-green algae, or cyanobacteria, won out because their competitors just weren't getting their vitamins, and O₂ filled the seas and skies with all their constant breathing. Cyanobacteria is pond scum among other things, and it's still everywhere. Methanogens can still be found in the deep dark places.

Other authors have suggested that either a general decrease in the generation of chemical reducing agents (not just nickel) or an increase in photosynthetic oxygen production could sufficiently affect the global electrochemistry balance to a more oxygen-rich condition. They model the existence of two stable steady state oxygen concentrations for those chemical dynamics, and a perturbation of either of these factors could have sent the atmosphere into the high mode. No doubt it took time for the available iron (and nickel) to absorb it all, for the atmospheric and temperature equilibria to establish, and for the green bugs to really take over the place, but Goldblatt et al., believe the conditions were present to suddenly switch 2.4 Gyr ago, and the transition itself may have happened in as short as a few tens of thousands of years (from now to cave art days, by means of comparison).

They also suggested that the upper steady state of oxygen concentration was much less sensitive to perturbation, and had the switch downward been instigated in those days, it'd have taken a few million years to go anoxic again. Worrying about losing our atmosphere is about as wise as waiting for the sun to explode. On the other hand, it does illustrate a surprising sensitivity within the planetary system and its biosphere, and there are certainly other fluctuating phenomena (such as glaciation), which are faster, and which may also be bistable, with (relatively) rapid dynamics in between. Even there, there's a touch of hubris about suggesting where our activity will ultimately get the planet, and we should worry more about the perturbation than the million-year equilibrium. It's true that human beings are affecting the carbon cycle and the biosphere faster than has usually occurred, but big-time biology, in its general sense, has recovered even from big extinction events. I'm really thinking more of the context right now, what we think of as alien. Biological species have transformed the composition of the outer layers of the globe in a drastic way during the planet's long history. In the case of cyanobacteria and oxidation, it was probably driven by geology, and the bugs got the job done as a response, grew as they grow, a matter of cause and effect. For some reason, we don't think of people this way.



*wrong words, but "planetology" or "ethanological" or whatever sounds even dumber and more confusing.