Tuesday, October 4, 2016

The Solar System: Why Earth?

HEADS UP!  My blog is moving to Wordpress from now on, at thegreatatuin.wordpress.com, because Blogger is proving to be VERY difficult to work with especially when dealing with my inevitable images and graphs.  To be determined if I continue in both places or switch over there entirely.  I will be putting up notification of this on the home page as well.
So after another entirely too long hiatus, I'm back.  This time even more intense work on my PhD ate my time, along with a number of personal things.  In whatever case, I didn't exactly have much time to write for myself over the last few months.  I'm almost glad though - the last year or so has seen a positive EXPLOSION of amazing origin of life research and I can't wait to pick through it on the record.
But for now before jumping into origin of life research I think I have to instead talk about the Earth itself for a while.  So far I've talked about the position of Earth's biosphere in the extremely large context of the history of the universe, our position in time relative to the formation of stars and planets and our position in space relative to different galaxies.  This has been informative, but we are necessarily working with very little information and coarse scales here.  By comparison within our own solar system there a massive wealth of information that we have nonetheless only barely started to look at.  By looking at the history of Earth and comparing it to other objects in our own solar system, I think some very important principles driving the appearance of biospheres like ours become clear and some very important questions we do not know the answers to fall out.

Earth is a very special place.  It is a world utterly out of chemical equilibrium.
That is not to say that other worlds in our solar system are in equilibrium - equilibrium is stasis and death, the lack of any energy flow from concentrated sources to diffuse sinks.  The atmospheres of Venus and the gas giants act as vast heat engines, transferring the energy of their daylit sides to the nightside and the poles creating winds and clouds in the process.  Geology driven by radioactive decay and primordial heat of formation deep inside solid worlds turns over rock through volcanoes and drives chemical cycling of elements into the atmosphere and back into rock.  But Earth is a world apart in its disequilibrium.  Our atmosphere and upper geosphere are charged up with energy, all this oxygen mixed with methane just itching to oxidize on decadal timescales.  The ground itself is full of geologically dredged-up iron and other substances constantly sucking oxygen out of the air on rather longer timescales.  And that is to say nothing of all the highly flammable carbon, biomass, ripped out of CO2 and reduced from that highly oxidized form to all kinds of substances and then left laying around or buried.  Anyone with a good enough spectrometer, looking at the Earth across the solar system or even from another star if their instruments were good enough could tell something strange was going on here.  A very dynamic, active process transducing lots of energy and utterly remaking the atmosphere and geochemistry for billions of years, creating an extraordinarily reactive atmosphere and oxidizing the crust to great depth.
It's this complete transformation of Earth and only Earth that I think can be explained much more simply than most people think.
Proponents of the 'rare-Earth hypothesis' like to point to everything unique about Earth in our solar system and suggest that unless every single aspect of our world were recapitulated elsewhere, you wouldn't get a big happy biosphere like ours.  I'm not a big fan of this approach.  Every body in the solar system is the product of a unique history and every body in the universe will be unique if you look close enough.  If you look at enough properties, eventually you will convince yourself that the particular combination is unlikely to be found elsewhere and you will probably be right.  Be it the large moon birthed in a collision that may have temporarily vaporized a significant fraction of the proto-Earth's mantle, plate tectonics smoothing the carbon cycle and delivering fresh nutrients to the surface constantly, or its particular amount of water, Earth has many unique properties.  But with exactly one known biosphere to go on, you run a very real risk of overfitting your models to that one data point.  We really cannot say with any certainty which of these properties might be necessary to create a biosphere in another solar system, or how common such sets of properties could be.
I think we can be more confident when we compare Earth to other objects in our solar system where we have more information.  The conditions in this cluster of worlds are diverse, but there is exactly one known biosphere orbiting the Sun.  Emphasis on 'known' - I will argue in a moment that our knowledge in this area is absurdly incomplete and there could be half a dozen biospheres in our own solar system that we would never have noticed with the science done so far.  Still, ultimately, I would say Earth is unique in exactly one way that matters for any biology and has driven its evolution for at least 3.5 gigayears:
Earth is the only place in the solar system where solvents and clement temperatures meet diverse small-molecule feedstocks and minerals in the presence of large amounts of sunlight.
There's several things going on here.  The first few are what most people think of when they talk about the presence of life on a planet: an environment in which something we would call 'life' is chemically possible.  The question of what exactly 'life' is is a complicated one and one I'll address when talking about its origin at some point, but for now I'm calling it complicated organic chemistry that can carry heredity and dissipate energy in its environment to do its business.
To allow the kinds of organic chemistry needed by life as we know it, you need a few things in the environment.  Firstly, temperature and solvent - it needs to be warm enough for reactions to proceed with some speed and for solvents to be liquid at least some of the time, but not so hot that large complex molecules you need for catalysis and heredity break down into their constituent parts.  Life as we know it seems to have a hard limit of ~120 Celsius on the high side, and rather below freezing in the presence of specialized antifreeze chemicals on the low side even if living things can lay dormant doing nothing when they're much colder.  Secondly, small-molecule feedstocks.  All life on Earth ultimately builds its biomolecules from simple, inorganic molecules - H2O, CO2, and N2 for the vast majority of biomass on Earth.  Anything that eats organic molecules ultimately, if you follow the food webs, will get them from something fixing carbon into biomass from CO2, and very nearly all nitrogen in protein ultimately derives from the pool of N2 in the same air.  Third, minerals.  No living thing on Earth can do without mineral nutrients.  Our genetic material itself contains huge amounts of phosphate, an inorganic ion leached from rocks, and almost every functional energy metabolism on the planet depends completely on electrons hopping along beautifully orchestrated chains of iron, sulfur, copper, and molybdenum ions clutched by proteins that are really just acting as scaffolds for them rather than enzymes.  Even if we didn't need all that calcium in our bones, minerals are needed as catalysts and components of basic biomolecules.
Last comes what I think is far too often overlooked: energy.  All this complex chemistry doesn't just happen.  A reaction that is thermodynamically favorable - releasing energy in the form of heat or introducing entropy by moving something from high concentration to low - will happen spontaneously, but before too long everything that can happen has happened and everything stops.  To drive complex chemistry and the production of biomolecules, there has to be energy flux into the system, a disequilibrium to be tapped.  And this is where Earth shines.  Other places in our solar system have liquid solvents, small molecule feedstocks, minerals, or all three.  But there is nowhere else in the solar system that has all those material requirements, PLUS the flux of over a kilowatt of energy per square meter the sun pours down onto the same surface.
The surface of Mars and the cloudtops of venus get plenty of sunlight, but Mars is nearly vacuum-dessicated and UV-radiation-blasted while Venus's clouds are lacking in anything resembling a mineral surface within 70 kilometers even though sulfuric acid might be a workable biological solvent for something that evolved in it.  The icy moons of the outer solar system have plenty of liquid water underground at clement temperatures full of dissolved minerals and feedstocks, but these clement environments are all under up to kilometers of ice.  Even on Titan, where you might imagine some kind of very low-temperature reactions happening in hydrocarbon solvents, the sunlight at the top of the clouds is something like 1% as bright as here and under the cloud deck it's more like 0.1%.
But on Earth, you have solvent falling from a sky made out of every small molecule feedstock you could want onto mineral surfaces bathed in a kilowatt per square meter of energy.  And photosynthesis capturing this vast energy flux is ultimately what makes the Earth's biosphere what it is, splitting water into oxygen and building biomass by combining the resultant hydrogen with CO2 and N2.  It is what has transformed the planet so completely you could tell it was alive from light-years away if you could get a quick glimpse of its atmosphere.

I promised when I started this blog to talk about actual observables, what we can know and what we can't with the information actually at hand.  In that spirit, after talking about how Earth is so special when it comes to biology in our solar system I need to insert a vital caveat.
There could be half a dozen active biospheres in this solar system right now and we would never know it with the information at hand.
A distinction has to be made between the presence of life, and the presence of a huge high-energy high-biomass biosphere like that of the Earth that chemically transforms a world.  There are plenty of places on Earth that have every attribute I talked about above except sunlight.  Fifty years ago you might have assumed that without sunlight to drive the primary producers of biomass there would be no life there - and as we have seen recently, you would have been wrong.
Sunlight is a vast, concentrated energy source as biological energy sources go: 170,000 terawatts hitting the Earth at all times is NOT shabby, and even if photosynthesis often only captures a small fraction of that it drives the vast majority of the metabolism on Earth.  But in recent decades, the discovery of organisms creating biomass from CO2 using energy not derived from the sun has shown that photosynthesis is not the only biological energy source on Earth.  Microbes can be found kilometers under the surface of the continents and the ocean, and a large fraction of the biomass of the planet may be living at very low cell densities and metabolic rates kilometers underground where they have no interaction with sunlight or energy derived from it at all.  To quote Jan Amend, a geochemist at the University of Southern California, "We keep digging and digging and digging deeper and have not hit the bottom of the biosphere."
Photosynthesis and related activities (i.e. us) on the surface of the Earth definitely represents the majority of the metabolic activity of the planet.  But it is NOT all there is, and perhaps more importantly, given how complicated a process photosynthesis in all its forms is it can not have been how the first life on Earth got its energy.  Something else has to have been the energy source for the earliest life.
Chemolithotrophs, organisms that fix CO2 into biomass like plants but using energy from small chemical disequilibria within rock to do it, have been found kilometers deep under the continents.  Methanogens and acetogens, organisms that take H2 and oxidize it with CO2 into methane and acetate for their energy (and may have a key role to play in the origin of life on Earth - more on that in a future post) are found anywhere biological processes create H2 but also deep on the bottom of the ocean, living off  geological processes that make H2.  The total wattage available to these organisms is constrained.  I am unsure how to quantify it precisely, but given sheer thermodynamics it probably has to be comfortably less than the estimated geological heat flux of the Earth (about 47 terawatts, ~0.03% of what the planet gets from the Sun) plus a little extra for chemical reactions that occur at the mouths of certain types of hydrothermal vents, but the discoveries in this new sector of the biosphere just keep coming.
Pictured above is Desulforudis audaxviator, image taken from the Microbe Wiki (https://microbewiki.kenyon.edu/index.php/Desulforudis_audaxviator).  This bacterium was discovered kilometers underground in an anoxic aquifer at a pH of 9.3 and a temperature of 60 C.  It dies from the tiniest whiff of oxygen, and carries out all the reactions necessary to take rock and N2 and CO2 and turn it into biomass, functions that can be spread out across a dozen species in a surface ecosystem.  It is believed to live off the energy of radioactive decay in the rocks around it that split water and create the tiniest of chemical disequlibria for it to insert itself into as a middleman in their dissipation.
This planet has been crawling with scientists for a few hundred years, and swarming with pretty-damn-smart people for two hundred thousand years before that.  D. audaxviator was discovered in 2005.  These non-solar-driven living things are very easy to miss.  They can be detected with the latest molecular biology techniques because we know exactly what we are looking for, chemicals that are shared by all terrestrial living things that we have learned to detect at the single-molecule level, and because of recent advances in telling apart geochemistry from biochemistry which is not always as straightforward as it appears.
What have we done on other worlds in our own solar system compared to the massive, systematic exploration that was needed to find these things on  Earth?  It's very hard to do science somewhere by dropping a few kilograms of automated scientific instruments from the sky every few years that cost well over their weight in gold and have severe bandwidth constraints.  Big things are being missed all the time.  It took until 2008 to figure out a basic, extremely important fact about the Martian soil - the fact that it contains up to 0.5% extremely reactive (and toxic) perchlorate created by the interaction of radiation and ancient salt deposits.  Every result from every instrument that probed and sampled the Martian soil from the Viking landers in the 1970s onwards had to be reinterpreted in the light of this finding and it was only conclusively discovered in the first place because one of the Phoenix lander's instruments basically malfunctioned in the presence of the unexpected substance.  The geysers of Enceladus, discovered in 2004 when the Cassini probe reached Saturn, spew the ocean of that tiny geologically active world into space where it can be sampled.  The instruments on Cassini are not sensitive enough to resolve the composition in all but the most general of terms - I have been to a talk in which one of the head scientists of the mission shrugged while pointing at a graph from the mass spec and said "there are molecules with at least three carbons in there somewhere."
Would we ever detect the presence of D. audaxviator deep underground at Mars, or microbes living in hydrothermal vents within Europa or Enceladus from this kind of data?  At this point, I really don't think so. By my count, there are at least 6 places in our solar system with everything necessary for a low-biomass biosphere and the origin of life (again more on this later) that we simply couldn't detect with the data at hand - underground at Mars, the oceans of Europa, the oceans of Ganymede, the deep underground water oceans of Titan, the surface hydrocarbon lakes of Titan, and the oceans of Enceledus.  Count me in  halfway for more speculative ideas about the cloudtops of Venus (where the limiting factor would be availability of minerals and not getting plunged down into the sterilizing depths of the atmosphere) and potential oceans within other icy moons and Kuiper belt objects.  We don't know if there are biospheres in these places, and with current data we can't know.  The answer is just insufficiently constrained.
We have only just gotten to the good part of solar system exploration.  It wasn't that many decades ago that the planets were only small discs through telescopes to us with rudimentary maps if any.  Now they are worlds, and we finally know enough about them and about Earth itself to actually start asking the really interesting questions and to start looking for things that are hard to find.  The presence or the absence of another biosphere in our solar system would be extremely informative.  A second origin of life in our solar system would suggest that biospheres are extremely common, even if you need special circumstances for it to explode in scale and remake a world. Only one origin in this system would be a little data in favor of the origin of life being tougher, but we need a LOT of information before we would be able to say that.  Proving a negative is difficult to say the least, and proving that there was never a biosphere elsewhere in our solar system that died out later is even harder.
Apparently every time I write for this blog, I wind up producing huge essays.  Up next, I want to talk about the history of life on Earth and its main developments over time and what they may (or may not) tell us about the types of biospheres that could exist elsewhere.