Water - the (apparently) really rare magic elixer that turns chemistry into biology: Many planets and moons harbour bizarre bodies of water – but that alone wont make them as life-friendly as Earth, say two planetary scientists WATER, water everywhere. As the keen eyes of our telescopes probe the cosmos, they see more of one molecule than anything else. Water in huge clouds around distant quasars, water shot from faraway stars, water in the atmospheres of giant planets. That gives us hope we might soon find one thing we have long sought, but not seen: a beautiful blue water world quite like Earth. Given the sheer number of worlds we now know to be orbiting other stars, finding one like ours would seem to be just a matter of time. Is it that simple? The more we learn about how Earth acquired and retained its water, the more it seems the situation was incredibly fortuitous. And as we discover how water is stored elsewhere in our solar system, our planet is starting to seem like an outlier. Even in a water-filled cosmos, Earth might still be one of a kind amid water worlds far weirder – and more hostile to life – than our own. We might be in possession of an extraordinarily precious, rare jewel: our oceans. Water is the most common compound in the universe, the combination of the most abundant element, hydrogen, and the third most abundant, oxygen. In space, it usually takes the form of tiny ice grains, or lumps or coatings on interstellar dust particles, or of a very thinly dispersed gas. To make liquid water, that enabler of life on Earth, gaseous water molecules at the right temperature need to be pressed together by gravity. That generally requires the neighbourhood of a star – not always a nice place to be. Our own suns violent incandescence as it lit up more than 4.5 billion years ago segregated the components of a disc of gas and dust that whirled around it, much of it far older stuff that had drifted in from interstellar dust clouds. Close in, those substances with the highest melting points condensed, forming the raw material for the rocky planets. The rest, including water vapour, are thought to have been swept away to condense farther out in colder climes. Only beyond a snow line around where Jupiter now orbits did water form icy planets and comets. Here we encounter our first mystery. Earth formed well inside the solar systems snow line, and remains there still. So how did it get its water? Perhaps even amid the violence of this process the colliding rocks that formed it retained some water that was later exhaled through volcanoes as steam, to condense as rain in the thick primordial atmosphere. Or perhaps the oceans came from beyond the skies, as comets and meteorites from outside the snow line crashed onto early Earth. No matter: the oceans arrived. They have been in place for at least 3.8 billion years, to judge by the oldest rock strata apparently laid down in water. How Earth managed to hold on to its oceans for so long is a second mystery. It is particularly curious considering how little water it has. The image of ours as a blue planet is a little misleading. Earths centre is some 6400 kilometres down, but its oceans are on average just 4 kilometres deep, and their deepest point some 11 kilometres. Its waters are a moist surface film far thinner, comparatively, than the skin on an apple. A look at our neighbours Mars and Venus shows how lucky Earth has been. They too had surface water in the early days, perhaps even large oceans. On frozen Mars today we see ancient shorelines more than 3 billion years old, and detect clays formed in water. Soon, though, Mars lost most of its atmosphere and protective magnetic field, and its water vapour leaked away. Venus is an inferno surrounded by suffocating clouds of sulphuric acid now, but probe measurements show it too once had abundant liquid water, until rising levels of water vapour and carbon dioxide led to a runaway greenhouse effect that boiled it off. What made Earth different? The key is probably plate tectonics. The movement of segments of Earths uppermost layer is unique, we think, among the rocky planets of the solar system. They crash against each other, buckling, rising or driving down into the planets hot mantle. There is some evidence such tectonics tried to start up on Mars, but if so it didnt last long. On Earth, it has created natural depressions: ocean basins, underlain by dense newly forming crust, that hold deeper waters; and shallow seas on the lighter, more ancient crust of the continents. The bottom of these containers is cracked at the subduction zones where water-soaked plates slide down into the mantle. That water is mostly wrung back out to emerge as volcanic steam in mountain ranges. This constant cycling of water, and the unlikely coexistence of wet and dry surfaces is, it turns out, crucial. Water evaporating from the oceans condenses as rain and chemically attacks the land, modulating atmospheric composition and global temperature. The atmosphere thus formed has a lid – a cold trap made by the chill of the stratosphere – that freezes water vapour out and stops it escaping into space. Below this lid, almost uncannily, all three phases of water – solid, liquid and gas – coexist almost all of the time: the only planetary surface known where this has been sustained for any long period. To complete this remarkable planetary machine, plate tectonics itself needs water to function: water lubricates descending tectonic plates and softens mantle minerals so they melt more easily. Geochemist Francis Albarède of the Ecole Normale Supérieure in Lyon, France, thinks that waters arrival from outer space kick-started the plate-tectonic motor 3 billion years ago. If so, water may itself have formed the unique conditions for its persistence on Earth. Our special brand of plate tectonics has also allowed the regulation of salt in the oceans. Salts are washed from weathered rocks on land into the sea, but also, from time to time, extracted from the sea in gargantuan amounts. The most recent of these salinity crises was 6 million years ago, when the Strait of Gibraltar locked tectonically. Isolated, the Mediterranean Sea evaporated away over a million years, laying down a 3-kilometre-thick stratum of solid salt on a blinding, baking, toxic desert. By the time the water gushed back, about 5 per cent of the global ocean salt had disappeared into the rock. Tortuous evolution So Earths oceans have probably become a little less salty over geological time, rather than evolving through continual salt input into an ever-stronger brine. Lucky for us – such a strengthening brew would have played havoc with the osmotic mechanisms of biological cells, stunting the growth of life that, for 90 per cent of its time on Earth, was essentially a thing of the oceans. As it was, the oceans and life co-evolved in a long, tortuous process. The advent of photosynthesis meant that a dangerous but energy-packed element, oxygen, slowly spread through the oceans, cleaning them of billions of tonnes of dissolved iron by forming insoluble oxides. At one stage, dubbed the boring billion, the oceans became packed with sulphide, as microbes stripped the oxygen atoms from sulphate ions in the still-stagnant depths; the process locked up micronutrients, and evolution slowed to a crawl. This was a minor hiccup. Oxygenation proceeded, and evolution picked up once more. Eventually, multicellular life appeared. Did life take its first steps on our now ocean-less neighbours, too? Every half billion years or so Venus replaces its crust through huge magma outpourings that would have obliterated any early fossils. On Mars, we wait and see as the rovers explore, their senses sharper with every generation. Perhaps they will find some fossil evidence for life, or even a few hardy microbes still lurking in the damp patches near the surface. This brings us to the solar systems other oceans. Far-travelling missions have told us that bodies of liquid water greater than Earths do exist – but, contrary to what we might expect, they are far beyond the snow line, among the extraordinary array of moons around Jupiter and Saturn. These are no dull globes of inert rock or ice as once thought. These distant, subsurface oceans form by the same mechanism as the maelstroms feared by ancient mariners: tides. Nearly half a billion miles away from us, the heat generated by Europas gravitational interaction with the huge bulk of Jupiter melts ice to maintain an ocean perhaps 100 kilometres deep, beneath a crazy-paving carapace of ice that might even support something like plate tectonics. Europas sister moon Callisto, too, might have an inner ocean, though buried deep below thicker, ancient, cratered ice – as might another moon, Ganymede (see diagram). Titan, which circles Saturn, is the only place in the solar system other than Earth with abundant surface liquid. It has seas of hydrocarbons such as methane, propane and ethane that can stay liquid at surface temperatures below -170 °C. They are large enough to make an oil executive drool: the wonderfully named Kraken MareMovie Camera is 1000 kilometres across. But it also has a deep-lying, tidally generated water ocean, shielded from the cold outside beneath a layer of ice. Enceladus, another Saturnian moon, probably has hidden oceans too. Certainly as Saturns tides flex its ice crust, water fountains out from fractures near its south pole as a spray that freezes instantly. Just last month, analysis of data from NASAs Cassini mission suggested a further moon, Mimas, might also host an internal ocean. Farther out still, there is a lot of water – but almost all as ice, often laced with nitrogen and ammonia. The last of our star systems liquid water oceans might, just possibly, lie deep below the thick ice crust of Neptunes moon Triton, with ammonia acting as an antifreeze. Otherwise, when the ice of those bodies warms sufficiently, it doesnt quite melt, but it can extrude upwards as ice volcanoes. So counting two on Titan, thats anything up to nine oceans in our solar system, most of them distant, dark and hidden. Could they harbour life? Possibly. But the subsurface locations of the oceans would mean life metabolising without oxygen in the cold darkness. Many Earthly bacteria do the same: for the first billion years of Earths biosphere, there was no oxygen. If theres anything weve learnt about microbes it is that they are very good at coping with difficult conditions and scarce nutrients. But how salty are those deep-lying waters, what kind of currents flow in them and how do they interact with adjoining rock and ice? Do sedimentary strata pile up on their floors? These are large unknowns, important for life, and direct measurements are impossible. Earth itself might provide some answers. For all their stability, its oceans were at times ice-covered, ice-free, sluggish, racing and hotter, perhaps, than a good cup of tea. Fossil records of these earlier oceans might be a better guide to how they work elsewhere than our seas today with their ultra-sophisticated lifeforms. The fact that life persisted in Earths oceans through all these earlier states seems a good omen for life elsewhere. But how much can our solar system tell us about others? The past two decades have seen a plethora of new planets and solar systems discovered, and a succession of surprises. There are hot Jupiters, gas giants orbiting bizarrely close to their stars, and super-Earths, rocky planets far more massive than ours. Many planets have freakish, looping orbits quite unlike the near-circularity of our solar system. Our view is skewed, because it is easier to detect big planets close to their stars, but its already clear that our solar system is not standard. So the pattern of oceans across the universe is likely to be different, too. There are a few candidates for water worlds. The super-Earth 55 Cancri e has a density consistent with a water envelope, but orbits so close that it is likely to be supercritical water – not quite liquid water and not quite steam, but more like the superheated steam used to decaffeinate coffee beans. Another large, hot, low-density planet, GJ 1214 b, might be a true water world, with the pressure in its thousand-kilometre ocean depths transforming the hot water into hot ice. Neither scenario seems quite suitable for life as we know it. Beginning of the end But as for ocean planets such as Earth, sitting pretty in the Goldlilocks zone near to their snow line, there is no close match yet. Perhaps NASAs James Webb Space Telescope, due for launch in 2018, will detect some. Whether it will see smaller, tidally warmed moons that are likely to abound is another matter. We are living in an era of steadily growing, but still imperfect information, but all the indications are that Earths long-lived, stable surface oceans are the exception, rather than the rule. In the normal order of things, they should last about another billion years before the sun heats up enough to vaporise them, puncture the stratospheric cold trap, and whisk them out into space. Then Earth will be dry and stone-dead, and probably, once shorn of its lubricant, with a stalled plate tectonic mechanism. Perhaps the frozen and buried seas of Mars might also thaw and return in the future, but Earth without oceans would be barren indeed. We may force things sooner. The final billion years of Earths oceans is off to a sticky start, with over-fishing, choking plastic debris and rapidly looming global warming and acidification. If we are not careful, well see the dawn of the Myxocene, biologist Daniel Paulys nightmare vision of an increasingly anoxic ocean, home only to bacterial slime and jellyfish – a sad beginning to the end of a long-lived, lucky cosmic jewel. This article appeared in print under the headline Just add water? Jan Zalasiewicz and Mark Williams are planetary scientists at the University of Leicester, UK, and the authors of Ocean Worlds: The story of seas on Earth and other planets (Oxford University Press, 2014) newscientist/article/mg22429930.600-weird-wet-worlds-why-earth-is-lucky-to-have-oceans.html?full=true#.VFJ8bqDF-Uc
Posted on: Thu, 30 Oct 2014 18:02:23 +0000
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