Completing the recipe

Last week, I wrote a piece on the peculiarities of Jupiter's moon Io -- surely one of the most inhospitable places in the Solar System, with hundreds of active volcanoes, lakes of liquid sulfur, and next to no atmosphere.  But there's a place even farther out from the warmth of the Sun that is one of our best candidates for an inhabited world -- and that's Saturn's icy moon Enceladus.

It's the sixth largest of Saturn's eighty-some-odd moons, and was discovered back in 1789 by astronomer William Herschel.  Little was known about it -- it appeared to be a single point of light in telescopes -- until the flybys of Voyager 1 and Voyager 2 in 1980 and 1981, respectively, and even more was learned by the close pass in 2005 by the Cassini spacecraft.  

One of Cassini's spectacular photographs of Enceladus [Image is in the Public Domain courtesy of NASA/JPL]

Enceladus, like Io, is an active world.  It has a thick crust mostly made of water ice, but there are "cryovolcanoes" -- basically enormous geysers -- that jet an estimated two hundred kilograms of water upward per second.  Some of it falls back to the surface as snow, but the rest is the primary contributor to Saturn's E ring, 

Where it gets even more interesting is that beneath the icy crust, there is an ocean of liquid water estimated to be ten kilometers deep (just a little shy of the depth of the Marianas Trench, the deepest spot in Earth's oceans).  Like Io's wild tectonic activity, the geysers of Enceladus are maintained primarily by tidal forces exerted by its host planet and the other moons.  But that's where any resemblance to Io ends.  Chemically, it could hardly be more different.  Analysis of the snow ejected by the cryovolcanoes of Enceladus found that dissolved in the water was ordinary salt (sodium chloride), with smaller amounts of ammonia, carbon dioxide, methane, sulfur dioxide, formaldehyde, and benzene.

What jumped out at scientists about this list is that these compounds contain just about everything you need to build the complex organic chemistry of a cell -- carbon, nitrogen, oxygen, hydrogen, and sulfur.  I say "just about" because one was missing, and it's an important one: phosphorus.  In life on Earth, phosphorus has two critical functions -- it forms the "linkers" that hold together the backbones of DNA and RNA, and it is part of the carrier group for energy transfer in the ubiquitous compound ATP.  (In vertebrates, it's also a vital part of our endoskeletons, but that's a more restricted function in a small subgroup of species.)

But just last month, a paper was presented at the annual meeting of the American Geophysical Union describing the research that finally found the missing ingredient.  There is phosphorus in Enceladus's ocean -- in fact, it seems to have a concentration thousands of times higher than in the oceans of Earth.

This is eye-opening because phosphorus is a nutrient that is rather hard to move around, as vegetable gardeners know.  If you buy commercial fertilizer, you'll find three numbers on the package separated by hyphens, the "N-P-K number" representing the percentage by mass of nitrogen, phosphorus, and potassium, respectively.  These three are often the "limiting nutrients" for plant growth -- the three necessary macronutrients that many soils lack in sufficient quantities to grow healthy crops.  And while the nitrogen and potassium components usually (depending on the formulation) "water in" when it rains and spread around to the roots of your vegetable plants, phosphorus is poorly soluble and tends to stay pretty much where you put it.

The fact that the snow on Enceladus has amounts of phosphorus a thousand times higher than the oceans of Earth must mean there is lots down there underneath the ice sheets.

This strongly boosts the likelihood that there's life down there as well.  Primitive life, undoubtedly; it's unlikely there are Enceladian whales swimming around under the ice.  But given how quickly microbial life evolved on Earth after its surface cooled and the oceans formed, I feel in my bones that there must be living things on Enceladus, given the fact that all the ingredients are there.  (The oceans on Earth formed on the order of 4.5 billion years ago, and the earliest life is likely to have begun on the order of four billion years ago; given a complete recipe of materials and an energy source, complex biochemistry seems to self-assemble with the greatest of ease.)

Maybe I'm being overly optimistic, but the discovery of phosphorus in the snows of Enceladus makes me even more certain that extraterrestrial life exists, and must be common in the universe.  If we can show that there are living things down there, on a mostly frozen moon 1.4 billion kilometers from the Sun, then it will show that life can occur almost anywhere -- as long as you have all the ingredients for the recipe.

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Roots of the problem

It's natural enough to think that humans are the only organisms that damage their own habitat.  We certainly seem to be doing a damn good job of it.  But there have been other times living things have sown the seeds of their own destruction.

One good example is the Great Oxidation Event -- sometimes, justifiably, nicknamed the "Oxygen Holocaust."  It occurred just over two billion years ago, and hinges on one rather surprising fact; oxygen is a highly reactive, toxic gas.

There's good evidence that aerobic respiration -- the set of biochemical reactions that allows us to burn the glucose in our food, and which provides us with the vast majority of the energy we use -- evolved first as a mechanism for detoxifying oxygen, and only afterward got co-opted into being an energy pathway.  The problem was that prior to the Great Oxidation Event, all of the organisms had been anaerobes, which are capable of releasing energy without oxygen.  To the vast majority of anaerobes, oxygen is a deadly poison.  That's why when there was a sudden, massive injection of oxygen into the Earth's atmosphere a couple of billions of years ago, the result was that just about every living thing on Earth died.

The tipping point came with the evolution of yet another energetic pathway: photosynthesis.  Photosynthesis was a tremendous innovation, as it allowed organisms to harness light energy instead of chemical energy, but it had one significant downside.  The first part of the reaction chain of photosynthesis breaks up water molecules and releases oxygen.  So when the first photosynthesizers evolved -- probably something like modern cyanobacteria -- oxygen gas began to pour into the oceans and atmosphere.

Something like 99% of life on Earth died.

The survivors fell into three groups: (1) the handful of organisms that had some early form of aerobic respiration as a detoxification pathway; (2) anaerobes that had a way of hiding from the oxygen, like today's methanogens that live in anaerobic mud; and (3) the photosynthesizers themselves.

From the organisms that survived that catastrophic bottleneck came every living thing we currently see around us.

So we're far from being the only organisms that cause ecological problems.  The reason the topic comes up, in fact, is because of another example I'd never heard of until I bumped into a paper in the Geological Society of North America Bulletin last week; the Devonian mass extinctions, which are one of the "Big Five" extinction events that have struck the Earth.  This particular series of cataclysms wiped out an estimated seventy percent of marine species, but it may have been triggered by the evolution of something that seems innocuous, even benevolent.

Tree roots.

Plants had only colonized the land during the previous period, the Silurian, enabled to do so by yet another innovation; the evolution of vascular tissue.  The internal plumbing vascular plants have (the xylem and phloem you probably remember from your biology classes) allow plants to move water farther and faster, so they were no longer so tied to living in ponds and lakes.  Plus, vascular tissue in many plants doubles as support tissue, so this facilitated growing taller (a significant advantage when you're competing with your near neighbors for light).

But if you're taller, you're also more likely to topple when it's windy.  So then there's selection for who's got the best support system.  The winners: plants with roots.

Devonian Forest by Eduard Riou (ca. 1872) [Image is in the Public Domain]

Like vascular tissue, roots are multi-purpose.  They not only provide support and anchoring, they're good at creating lots of absorptive surface area for water and nutrients.  (Some roots are also evolved to store starch -- carrots come to mind -- but that's an innovation that seems to have come much later.)  So now we have a competition between plants for who's got the best supports, and who can access nutrients from the soils the fastest.

Roots very quickly became good at twisting their way into rocks.  You've undoubtedly seen it; tree roots clinging to, and breaking up, rocks, asphalt, cement, pretty much any barrier they can get a foothold into.  When that happened, suddenly there's an erosive force breaking up bedrock and transporting nutrients (especially phosphorus) into plant tissue.  Phosphorus began to leach out of the rock into the soil, and when the plants died all the phosphorus in the tissue was released into rivers, streams, and lakes.

The result was a massive influx of nutrients into bodies of water.

Have you ever seen what happens when chemical fertilizers get into a pond?  It fosters algal blooms, and when the algae dies and decomposes, the oxygen levels plummet and the entire pond dies.

That's what happened during the late Devonian Period -- but planet-wide.

The huge reef-building rugose and tabulate corals and stromatoporid sponges were wiped out en masse.  Other groups, such as trilobites and brachiopods, which depended on the reefs for habitat and food, got knocked back hard as well.

All, the authors claim, because of a nifty innovation in the structure of land plants.

It's tempting to think that the environment is stable; we look around us and think things have always been this way, and will always be this way.  What more of us need to understand is that while the global ecosystem is resilient up to a point, there is always a tipping point.  The scary part is we can pass that point suddenly, without even realizing it.  Then before we're even aware of what's happened, the last chance to turn things around is gone.

The difference between what happened during the Great Oxidation Event and the Devonian Mass Extinctions, and what's happening now, is that back then there was no conscious awareness on the part of the organisms who created the problem and those that were affected.  Now, we have (or should have) the awareness to see what is happening, and enough knowledge to make some smart decisions and halt the self-destructive path we're on.

Let's hope that it's not too late.

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