Crusty jigsaw

Imagine a planet composed of liquids and gases. Close to its surface, the radiation of energy into the deep chill of space results in the crystallization of material. Varying compositions result in different solids forming, the vagaries of phase changes and atomic lattice arrangement produce a huge array of forms. Gravity keeps all this crystalline stuff tightly packed, a thin sheath around the planetary sphere, cracking here and there, floating and bobbling on top of the liquid interior. This is, of course, the nature of the Earth, and the presumed nature of any substantial rocky world with radiogenic internal heating, or enough latent heat of formation combined with youth.

It's quite sobering to be reminded that our picture of how all this crusty stuff operates, how the great plates of rock shift and slide around, is still very, very new. Fifty years ago and the idea of plate tectonics was only just taking proper shape. Fast forward and we now talk about the inevitability of plate tectonics on super-Earth's - rocky planets several times the mass of ours. We care about this because it seems that active plate tectonics and volcanism play a critical role in the long-term regulation of the Earth's climate - forcing surface temperatures into the regime where liquid water can exist.

It's intriguing therefore to see that our understanding of the physics behind plate tectonics is still a matter of intense debate and study. A couple of new results bubbled up during the summer. One is the claim of a new understanding of how the Earth's crust shifts and wiggles - based on essentially the same physics that explains how objects move through viscous fluids. In this picture then the way the planetary crust moves is very much a function of that crust itself, a bit like how your bobsled run is determined to a great extent by the mass and slipperiness of your ride, not just by the ice underneath. This runs against many previous models, where deep interior processes in the liquid part of the planet effectively determine what you see up top. Another work, employing state-of-the-art computer simulation has made recent claims to tie the deep ebb and flow of the Earth's interior to the frosty bump and drift of the outer plates. Rather nicely, this grand simulacrum also suggests that precisely how all the surface cracks and gaps, the faults and fault zones, fit together plays a critical role in determining how the overall plate tectonics of the planet operate.

Just like a fiendish jigsaw, you can't see the big picture until you know how all the small pieces go together. Perhaps not surprisingly, our crusty surface is a hugely non-linear system, hard to predict ab initio. This should raise some concerns for making claims about the nature of plate tectonics on distant exoplanets, but it also indicates a possible opportunity. It may well be that there are distinct types of crustal activity that can occur on rocky planets, from the kind of plate motions that we see on the modern Earth to more fractured styles, or more global styles. At some level these will all link into climate, history, and chemistry. Detecting the presence of atmospheric gases associated with geochemical processes - such as sulfur dioxide - could actually help tell us about the arrangement of continents (or not) on these distant worlds.

Stepping stones

Maybe it's the flurry of new planets this week, or something else, but the subject of interstellar exploration has been bouncing around more than usual. A discussion that sometimes crops up when talking to others engaged in exoplanetary science is firmly in the speculative, but intriguing, category. It goes like this; let's suppose we find a terrestrial-type planet around a relatively nearby star (read less than 30 light years away), perhaps even around one of the Alpha Centauri members. Let's further suppose that - possibly with the James Webb Space Telescope, or a next-gen ground-based super 'scope - we garner evidence for an atmosphere and several big chemical clues that there could readily be a biosphere on this world. What do we do next?

There are somewhat mundane answers - build better instruments, get better statistics - that may be the most realistic, but there's also that nagging idea that the next thing to do would be to find a way to study such a planet up close. If enough coffee has been consumed then it's a matter of finding a handy Tony Stark, willing to sink hundreds of billions into a robotic interstellar probe, on a long-shot for glory. There's a problem though, unless you intend a very long round trip, how do you get the information back? While we are now pretty good at picking up signals from distant spacecraft - even from Voyager 2 at over 100 AU from the Earth - getting data back from a few light years is going to be hugely difficult. The required transmitter power, as well as interstellar scintillation, is a major hurdle.

A solution, that has cropped up in various guises, even in the idea of von Neumann probes, and the interplanetary internet, is that you don't just send one probe. Rather, you send a chain of probes - pearls on a string - capable of communicating between themselves even if not individually directly back to Earth. It would take a long time, but as the furthest end of the chain crept towards a target stellar system we'd have ongoing feedback, the continuous relay of data as we crept through interstellar space. It might be optimal to build the biggest receiver and transmitter at the outermost practical limits of our solar system - the equivalent of an internet 'backbone' - with a clear line back to Earth. So how many probes would you need to get to somewhere like Alpha Centauri?

This system is about 278,000 astronomical units (AU) away. If we optimistically think we could build probes capable of to-and-fro communication over a few hundred AU then we're talking about a thousand or more devices. This sounds awfully challenging, but remember that we (as some hypothetical sublimely patient species) don't expect probe-1 to reach Alpha Centauri for a few tens of thousands of years. We only have to launch every ten years or so. Even if each probe cost 10 billion dollars (allowing for lowered cost after the first few models) that's peanuts over this timescale. In the meantime we have an ever extending tendril out into interstellar space. Being an innovative species we would undoubtedly think of ever more wonderful things to add to the probes, increasing the scientific return.

Powering transmitters and receivers, as well as sizing their antennae or dishes, is still a problem. Given the timescale to reach the target star then even radioisotopes are going to peter out (fission reactors are a no-go, the fuel burns out too fast). Chemical energy might actually be the best option; a store of redox components, mix them periodically and recharge the batteries, the ultimate fuel-cell.

All over-caffeinated speculation. But if we ever get serious about stepping beyond, then making sure we don't drop the signal is going to be a very real issue.

Here we go...

The danger with spending time on a careful post (see below) is that something hits the waves while you're at it. Take a normal G-dwarf star, stare at it with an ultra-high-precision spectrograph, wait, and find perhaps as many as 7 planets. This is the incredible announcement for the system of HD 10180 coming out of the European Southern Observatory.

5 definite 'Neptune' class worlds. One possible 'Saturn' and one itty-bitty 1.4 Earth mass object lurking on the hairy statistical edge of confirmation...

Not only that, but mostly close to circular orbits from almost on top of the star out to a few astronomical units.

It's a beautiful bit of astronomy, it's also a real eye-opener to the potential richness of planetary systems...there's planets in them there hills.....

Seeing red

Seventy percent of all energy consumed by life on Earth is in the form of solar photons. Interestingly this consumption is by a minority of organisms, those exploiting the mechanisms of photosynthesis. It amounts to a globally averaged energy intake of some 100 terawatts, but even this is peanuts compared to the solar input at the Earth's surface, which is about 90 petawatts. By comparison, all available geophysical energy - thermal and chemical - is a paltry 30 terawatts.

If you want energy, then photons are the way to go. Catching and transducing photons into chemical energy in biological systems is accomplished via the extraordinary chlorophyll pigments. In oxygen producing organisms four types of chlorophyll had been known, each tuned to slightly different wavelengths. For example, chlorophyll 'a' grabs photons at around 465 nanometers (bluish visible light) and at around 665 nanometers (reddish visible light) - leaving behind the familiar green photons that we enjoy in our foliage. The other, rarer, chlorophylls absorb at similar wavelengths. Now, in a neat paper that appeared in Science last week, Chen et al. have identified a fifth chlorophyll 'f' - extracted from organisms lurking in modern day stromatolite formations.

Why get excited about this? Chlorophyll 'f' does something not seen before in photosynthesis - it slurps up photons from the near-infrared. At about 706 nanometers we're into a regime just beyond the visible. Obviously other pigments and structures can, and do, absorb photons in this regime, but only 'f' is known to actually use the photons to split water molecules - the key step in oxygenic photosynthesis.

To my mind this raises a number of fascinating connections to questions of life on Earth and beyond. It suddenly connects the dots to earlier indications of organisms exploiting infra-red photons around deep-ocean hydrothermal vent systems - or at least offers a molecular solution. It also, and here I'm heading out on a limb, might connect to an issue that's long bothered me. We've talked before about how our Sun was as much as 30% fainter three or four billion years ago. With this faintness comes a small shift in the peak output of a star's spectrum (being a blackbody to first order) towards the red. For the young Sun this would have only been a few tens of nanometers - but it would have meant a slightly better flux of these near infrared photons. In addition, a different atmospheric composition and chemistry on a youthful Earth could have altered the typical range of photons making it down to the surface. It seems not unreasonable to suspect that chlorophyll 'f' could have given ancient microbial life a leg-up over the competition. Indeed, did chlorophyll 'f' come along first?

It's pretty startling evidence of nature's capacity to find molecular machinery to exploit even low energy infra-red photons. Now picture a world around a low-mass star, with a drastically redder spectrum. Chlorophyll 'f' points the way to how organisms there might manage all the great tricks of photosynthesis that we see here on Earth.

Protein universe

A couple months ago a rather stunning paper slipped into the journal Nature. It presents a sophisticated investigation of how quickly the genetic codes for proteins are evolving - across the 3.5 billion years or so of life on Earth.

The idea is that the specific genetic codes, the sequences of amino acids, that describe proteins - the workhorses of molecular biology - cannot withstand big changes, otherwise the complex structures formed will just not do their job properly. Swap in a different amino acid somewhere in a chain of a thousand and you'll no longer fold this big molecule up into the right shape, and you've gone from a 1/2 inch wrench to a pair of tweezers.

Over time though small changes can, and do, occur. As long as the final outcome permits the same job to be done by the protein, all is well. Now, let's suppose that a whole clutch of modern organisms share a common ancestor (something we've touched on before in these pages). We should be able to see just how different the protein coding has become since that time, and we should be able to tell whether this type of gentle evolution has stopped or not.

Povolotskaya and Kondrashov apply to proteins exactly the same methodology that Edwin Hubble did to the measurement of the expansion of the universe. They look to see how fast the coding is changing as a function of how different those proteins are - just as Hubble looked at recession velocities versus the physical distance between galaxies. What they find is that after about 3.5 billion years the protein universe here on Earth is still, slowly, diverging and expanding - it's not yet reached a true optimal state. They also point out that while 98% of locations in a protein sequence can't deal with quick tampering (change an amino acid there and the whole thing ceases to work), over billions of years you could more or less re-write the code for a given protein and get the same function. That's a bit like changing Hamlet by one word every new print run, until you have a totally different script, but the same outcome.

So, what says all this for the nature of life in the universe? These proteins plays roles in things like  metabolic processes that have remained unaltered for billions of years - solutions for how life extracts energy from its environment that are pretty close to optimal. Yet here we see a universe of slowly diverging, expanding, molecular structures - the very fabric of the biological cosmos on Earth. To my mind this might present a huge challenge to the notion of convergent evolution - the idea that there are a limited number of molecular or physiological solutions that life can use. Take a different planet, with a biosphere a couple billion years old. The stately evolution of its protein universe would almost certainly have taken a path unlike that here, exploring this vast multi-parameter space of molecular structures along alien paths. It both supports the notion of life as a potentially extraordinarily robust phenomenon, and as a hugely diverse one.

The panspermia paradox

A brief hiatus and a lot of stuff happens - the oldest rocks on Earth, 3 million year old human tools, and future directions for astronomy. So, ignore all of that and bring focus to bear on a persistent idea. The notion of panspermia - the transferral of viable organisms between planets, and even between star systems and further.

There is no doubt that planetary surface material is continually being shipped around between rocky planets and moons in our solar system. Ejected by asteroidal or cometary impacts, chunks of stuff follow a range of orbital trajectories that result in both eventual return to their origins or transferral to the surfaces of other worlds. Increasing evidence suggests that a variety of (typically microbial) organisms could be carried along, surviving both the extremes of pressure and acceleration, as well as exposure to thousands to millions of years of interplanetary space. There is a real possibility for life to both cross-infect, and even to be 'seeded' from planet or moon to planet or moon.

Enthusiasts for panspermia go further, and have been known to invoke this as a mechanism for galaxy-wide dispersal of life - taking one rare occurrence of life and spreading it. There is however a factor that to my knowledge is rarely considered, and that is natural selection. You or I, fluffy bunnies, and daffodils are all unlikely candidates for interplanetary or interstellar transferral. The sequence of events involved in panspermia will weed out all but the toughest or most suited organisms. So, let's suppose that galactic panspermia has been going on for the past ten billion years or so - what do we end up with?

Although it's a complex problem, it seems likely that life driven by cosmic dispersal will end up being completely dominated by the super-hardy, spore-forming, radiation resistant, rock-eating (endolithic) type of critters. There will be no advantage to a particularly diverse gene pool. Billions of years of galactic transferral will have whittled it down to only the most indelicate and non-fussy microbes - super efficient, super persistent, and ubiquitous - the galactic top dogs.

Now, we might argue that there are many organisms on Earth that could fit the bill, and could be the links to these ancestral interlopers. The problem, and the potential paradox, is that if galactic panspermia is real then the type of life it will evolve should be everywhere. There would be stuff on the Moon, Mars, Europa, Ganymede, Titan, Enceladus. Every nook and cranny in our solar system would be a veritable paradise for these ultra-tough lifeforms. It may be too early to rule this out, but if life is sparse in our neighborhood then it would seem to argue strongly against the possibility of Galactic panspermia.