Discussion and news about the modern effort to understand the nature of life on Earth, finding planets around other stars, and the search for life elsewhere in the universe

Saturday, July 31, 2010

Summer reading

It's not the usual post. A conversation about interstellar travel reminded me of a short, whimsical, piece of fiction I'd noodled with a while back. Nothing to do with the search for life in the universe, but much to do with the nature of long-distance travel, human enterprise, and that cosmic force; irony. So here goes, a little reading for the celestial beach chair.

All personnel. This is Mother, attention please. As you know we have now entered the Alpha Centauri AB system. The Santa Maria is adjusting for final insertion into a wide survey orbit. All navtechs and astros are to report to workstations and are reminded to follow United Federation survey protocols. Reactors three and four will begin shutdown as plasma drives are brought offline. Personnel generations six and seven are reminded that it is critically important to assist final generations with adjustment from interstellar environment. Counselors will be available to mediate. All children are to continue school on the normal schedule.

All personnel. This is Mother, attention please. Initial survey indicates two terrestrial mass inner planets around Alpha Centauri A, one with significantly non-equilibrium atmosphere, indicating potential biosphere. Gas giant b harbours two large moons, inner moon appears oceanic with high specular reflection. Access to Santa Maria spin axis recreational arenas and hydroponic fields will be limited, personnel are encouraged to remain on outer decks for g-acclimation. Colonization refresher courses will be available on deck nine. Exceptions will be made for centenarians and older.

All personnel. This is Mother, attention please. The Santa Maria will switch internal clocks to local frame in twelve hours. Elapsed journey time will then be 1,015 Earth years, eight Earth months and three Earth days precisely. Wide band modulated transmissions have been detected from terrestrial planet c, precise nature of transmissions has not yet been established. All anthropol personnel are to report to workstations and follow contact protocols. Counselors will be available on decks seven through ten.

All personnel. This is Mother, attention please. Contact has been established with planet c. Personnel of the United Federation interstellar cruiser Vixen have established a colony there during the past ten years. Colonists report that a quantum-entanglement drive was developed during year 950 of Santa Maria journey time. Vixen departure from Earth was during year 980 of Santa Maria journey time. Counselors will be available on all decks until further notice. Anti-depressants will be made available following consultation with assigned medical personnel.

All personnel. This is Mother, attention please. Arrival two hours ago of the United Federation interstellar scout-class Sabre was responsible for strong electromagnetic pulse. All primary systems are fully functional, secondary systems will be back online shortly. The singularity worm-drive onboard the Sabre was developed during year 995 of Santa Maria journey time. The Sabre is now deploying its colonist transports to gas giant b ocean moon. Personnel are advised to join counseling groups on all decks.

All personnel. This is Mother, attention please. The results of the Santa Maria referendum on proposition 1A have been tabulated. Santa Maria will be refurbished with a singularity worm-drive and upgraded habitat resources. Cryochambers will be added to deck five for optional suspension and all interior living areas will be resurfaced and modernised. Generations six through eight will disembark and join Vixen and Sabre colonists on planet c and the gas giant b ocean moon. Remaining generations will retrain for the journey to Barnard’s Star, anticipated to take only fifteen years. Congratulations as you begin this historic voyage as the next colonists to another star. Legal counselors are available on decks two through five to assist in waiver statements with regard to the United Federation’s limited liability for your first arrival status at Barnard’s Star.

(c) C. Scharf (2010).

Tuesday, July 27, 2010

Planetary Prometheus

Imagine, if you will, a planet with atmosphere, oceans, rocks and life. On this planet then most chemical reactions are either slow and geophysical, or quick and biological but very localized. There is however an exception. Because of the particular nature of this world there is the ever-present potential for a type of chemical reaction that is not only fierce and destructive, but self-propagating. Once triggered it can spread across hundreds, even thousands, of square miles. It preferentially attacks and transforms living material - leaving behind a fragile deposit, stripped of most biomatter. It can only stop by either exhausting the supply of fresh reactants, or when its chemical energy is sucked away by an un-reactive medium.

This is a tricky planet. It forever teeters on the edge of letting this chemical storm get a grip, but its climate and varied topography help to confine outbreaks. The very compounds responsible are themselves critical ingredients for much of the life on this world, and cannot be eliminated. Indeed, the reaction itself serves a number of key roles in stabilizing populations, cycling elements between air, ground, and oceans, and is ancient enough to have been incorporated into the survival strategies of large numbers of species.

Imagine we could visit this world. Entering orbit we would scan it with our telescopes. Curiously, at any given time, we would observe tens of thousands of these intense chemical maelstroms dotted across the globe. Their signatures would be quite distinct, and we might be quite astonished that life existed in such a perilous environment.

Of course, this is no hypothetical planet. It is the Earth. The chemical reaction we know as fire is a strange and intriguing, and often overlooked, aspect of life here. The young Earth of 3 billion years ago, with little or no oxygen in its atmosphere, or flammable biomatter, would have probably only seen fire in volcanic settings. Somewhere along the line, maybe a billion or two years later, with enough free oxygen, perhaps some dried up mat of plant life on a tidal shore was the first victim of arson - possibly a result of lightning. Today, fires cover the globe. Satellite imagery, or remote sensing, tells the story. The image in the upper left shows thousands of fires scattered across south-central Africa, seen by the MODIS instrument on NASA's Aqua satellite. Many of these have been set by humans, following an ancient pattern of land-use. Humans have learnt to exploit this chemical fragility.

I think we tend to underestimate the chemical reactivity of our homeworld. Fire is an excellent example - it's so familiar to us that we (well, I) even have to pause to remember that it's something chemical, fiercely exothermic. It raises a number of interesting questions. Is a phenomenon like fire simply a consequence of the kind of chemical reactivity needed for a planet to harbor life? Life on Earth needs a lot of reduction-oxidation pathways. Can you propel a biosphere to the kind of richness we see today without taking this walk on the wild side - risking destruction for the chance to make hay with oxygen?

Evidence suggests that, for example, around 270 million years ago atmospheric oxygen levels were significantly higher than today - and that fire was much more frequent on a global scale. More oxygen and it becomes hard to avoid burning all flammable materials, clearly there could be a feedback mechanism at play - complicated by geography and climate. Just how fire-prone can a planet become before it wipes out its surface biosphere?

Anyone got any marshmallows?

Monday, July 26, 2010

Exploding moss/Aquatic Mars

Trawling the scientific news and literature can be an obsession - there's just so much incredible stuff being discovered all the time. It's also hard to choose favorite items, whatever ability to prioritize that evolution has endowed humans with, it's not always very effective. So, with that by way of an excuse, there were two utterly unrelated items that popped up last week that I cannot choose between - your vote.

The first involves something I'd never thought of before. Terrestrial moss (yes moss), produces spores to propagate, since it is a flowerless and seedless plant. Spores are tiny capsules a few tens of micrometers across and are perfect for being dispersed in the air. The puzzle has been that when the wind blows on Earth there is usually a small layer of atmosphere, about 10 cm wide, that serves as an interface between the planet surface and the free-flowing air. If you want to be carried off by the wind you need to get above this layer that's a result of friction, fluid dynamics, and topography. Most mosses lurk no more than about a centimeter above the ground - so how do they get their spores into the mossy equivalent of the jet-stream?

A paper by Whitaker & Edwards in Science examines the problem, and with ultra-high speed photography they find that some mosses use pressurized, explosive, capsules of spores to launch them upwards with g-forces of as much as 30,000. Although this is impressive, as lilliputians it results in a roughly 30 mile-an-hour Vespa-like launch velocity. The puzzle is that even this shouldn't get the spores high enough. The new study provides the answer - the exploding capsules are such that they produce a spore-laden vortex ring that burrows its way up to the magical 10 cm altitude. It's very much like a smoke ring, a complex structure that maintains integrity. In effect, natural selection has endowed moss with both a knowledge of the bigger picture of terrestrial atmospheric science, and some fluid mechanics that Boeing would envy. It's a further reminder that even 'simple' life is seldom just that.

One then wonders if a planet like Mars once had spore launching organisms littering its surface. A new paper by Morris et al. fills in one of the blanks. With all the geological evidence that Mars was, at least for a time, a wet place there has remained a bit of a conundrum. The simplest way for Mars to have been more temperate was for it to harbor a thicker, carbon-dioxide rich, atmosphere - with a strong greenhouse effect. If that had ever happened then there should be massive amounts of carbonate rich rocks laying around from that era. Until now the evidence for this had been scant. By studying the walls of Gusev crater with the Spirit rover Morris et al. get a look at the geological history - and they have now identified significant, and ancient, carbonate deposits from about 3.5 billion years ago. Checkmark the box.

It's not the end of the story, but it adds significantly to the evidence for a warm, wet, young Mars. The next question is how long did it retain this climate - long enough for life?

Wednesday, July 21, 2010

On the iceberg

Although incredible progress has been made in the search for exoplanets, we're still teetering on the peak of the cloud enshrouded mountain, with little real knowledge of what lies beneath. It's instructive to take a look at where things are at today. The first image here (go on, click on it) is a plot of estimated planetary mass (typically a lower limit owing to the nature of the detection techniques used) versus the estimated distance of the stellar parent. A total of 448 planets are pulled out of the excellent exoplanet catalog. Not bad going for 15 years of effort - although it is sobering to realize that our galaxy alone almost certainly contains more than a hundred billion objects we'd term planets - and likely many more. So far the majority of our detected planetary swarm is within about 100 parsecs - about 330 light years - of the Sun. A keen eye will spot that the lowest mass planets, a few hundredths of the mass of Jupiter - the unit used on the Y-axis - also lurk preferentially in some of the closer systems. This is an artifact of the choices made in investigating systems and the nature of planet detection methods.

This 2nd plot (minus a couple of systems) reveals some more of that skewness. The low mass planets at the bottom also correlate with stars that have a slightly lower average mass than those higher up in the plot. Lower mass stars are more strongly perturbed by the gravitational tug of orbiting planets, so the telltale signs of small planets are easier to find - but they're also fainter, and so they are better targets when they're not too far away.

This lower left corner is one of the most intriguing places to go looking for planets. An astonishing 70% of all stars in our Galaxy are less than half the mass of the Sun. There are at least 300 such objects within 10 parsecs of us. They're the real dwarfs, some are a thousand times fainter than the Sun, but they can burn their nuclear fuel for a trillion years. If planets - at least the smaller rocky ones - can form efficiently around these stars, then most worlds in our galaxy, and indeed the universe as a whole, will be bathed in their reddish light. The next few years should reveal more about the closest examples, and their alien environments.

Wednesday, July 14, 2010

Altogether now

The issue of multi-cellular life is very, very interesting. A number of years ago Carl Zimmer wrote a very nice piece discussing how, when, and why microbial life on Earth got to acting in a multi-cellular mode. Interestingly, the organism he talks about is the self-same alga - Volvox - that was the subject of the genetic detective work I mentioned in the last post. What I find so fascinating about all this (hence the sudden flurry of posts), are the broader implications for life elsewhere in the universe. This is also motivation for the kind of hare-brained social gaming experiment I suggested last time.

In astrobiology there is ongoing debate about just how unusual, or not, so-called 'complex' life might be out there in the cosmic deeps. A lot of the discussion got rounded up in Ward & Brownlee's book Rare Earth, where they tried to argue the case for our homeworld, and its biosphere, being particularly special. I couldn't help but feel at the time, and more so now, that this line of reasoning was overly mired in the increasingly archaic view that evolution is this rather inflexible progression towards 'complex' life. I think it underestimates the sheer opportunistic side of biology. We also get so blinkered by the historical and sociological context of the development of evolutionary biology that we come to see 'complex' life as a veritable house of cards. One wrong step, one asteroid too many, one poor mutation, and a planet will be forever bereft of multi-cellular life.

As Zimmer discusses, the transition between single-celled, microbial organisms and multi-cellular life has happened multiple times across different phyla during the Earth's history. It occurs as and when the advantages of being 'big', outweigh the advantages of being 'small' - jolly old natural selection. Now, here's the next bit of wide-eyed speculation [and yes, I know, countless sci-fi tales got there first...]. Does multi-cellularity necessarily mean physical connection? While there is certainly a lot of physics that can be exploited by getting together on a microscopic scale (even quantum entanglement in photosynthesis), there may equally be advantages to remote connection. And, while I really don't want to go all starry eyed here, an informationally nutritious medium like the Internet, could provide just the kind of connectivity such life would require - hooking together the individually sophisticated human microbes. Is the internet part and parcel of the evolution of life on Earth? Well, I might not go quite that far, but could a mechanism of long-distance, but intimate, connection occur in a purely biological system?

Obviously if you're James Cameron and you live on an allegorical moon called Pandora then the answer is yes. But for astrobiology it may not be crazy to relax the rules for what is meant by 'complex' life. The next question would be whether the planet-wide signatures of dispersed multi-cellular life would look any different than those from the types of life we know about.

Tuesday, July 13, 2010

Microbes..who, us?

Stewing in heat-waves, rain, and who knows what else. Summertime in the northern hemisphere of the planet is a good time to wallow in a bit of speculative thinking. A couple of posts ago I talked about a fabulous piece of work on what could be a 2.1 billion year old example of emerging multicellular life here on Earth. Hot on the heels of this is another study of the origins and mechanisms of multi-cellular organisms. The paper, by Prochnik and collaborators, examines genetic evidence for the history of multi-cellular behavior in a type of algae.

The bottom line is that the algae seems to have very, very similar genetic building blocks to its most similar single-celled, microbial relatives. Specifically, the protein toolset doesn't alter much between the single guys and the multi-cellular descendants. What this means is that it wasn't some giant leap to move to a multi-cellular, or communal, existence - at least for these species. To use the terminology of the study, these organisms are not particularly innovative - with a bit of an evolutionary shrug of 'oh what the heck' they segued into a new multi-cellular mode of life. Of course I exaggerate, but it does seem that this was not some earth-shattering, Charlton Heston type, moment in ancient biology. The same tools can be used to fashion a new type of life.

Which brings me to the summer speculation. Here we all sit gazing at our screens, our little brains and hands pawing at the pixels and keys. Each of us is quite uni-cellular in that sense, self-contained, sending out feelers and sensors, pulling in environmental information. The medium that we are immersed in may be a virtual one, packets of electrons whizzing to and fro across the world, but it's arguably just as good as a nice bit of pond water, or agar gel.

Then something happens. Perhaps it's an earthquake, a political event, a particularly enthralling YouTube video of cats licking themselves. Whatever it is, it can serve to motivate a sudden shift of mode for thousands, if not millions of individuals. Instead of solitary action we can form a mob, whether by just pinging the same page, sharing on Facebook, emailing everyone we know, or actually hopping on a bus and going to a demonstration. The internet has made this type of behavior far, far, easier. Each of the cells may not think they're part of a larger organism, but by most standards they are. Social gaming is another excellent example - 'help me grow my carrots by clicking here' - if that isn't inter-cellular communication to perform function then I don't know what it is.

So, here's the proposal, for all you bored but brilliant programmers. How about making a social game that is actually a cunningly disguised attempt to build a virtual multi-cellular lifeform? All of us single-celled organisms will just use our familiar toolsets to participate, but in doing so we will create and evolve something new, much bigger than the sum of the parts. The point of this - other than lowering office productivity - would be to perform a scientifically useful experiment to learn about the types of networks and 'global' mechanisms involved in shifting from uni-cellular life to multi-cellular. We might, just might, gain insight to this transition...

[oh, and I'm sure someone else has thought about this before...]

Friday, July 9, 2010

Our leaky solar system

We've all grown up with the notion that we're a long way from the nearest stars. This isolationist mentality extends to the way we think about all the stuff in our solar system; it's ours, all ours! However, our Sun almost certainly formed together with many other stars from the same cloud of molecular hydrogen, with the usual sprinkling of heavier elements and interstellar dust. Long since dispersed by the inexorable slewing of material as it lumbers around the Galaxy - every 210 million years or so at our placement from the galactic center - our stellar brothers and sisters may be well scattered by now.

There was a time though, some 4-6 billion years ago, when other stars, likely with their complements of dusty planet-forming material, could have swept by. The absolute outermost reaches of our local gravitational well are populated by the cold residue of planet formation, the objects we know as cometary nuclei. This postulated fuzz of stuff is the Oort cloud, and should extend outwards from us to as much as 100,000 astronomical units distance - almost halfway to the closest stars of Alpha Centauri.

An intriguing new work by Levinson et al in this weeks Science re-opens an investigation into the precise origin of the Oort cloud. From what we know about long-period comets, sailing in from this distant population, there appear to be some serious discrepancies about the likely number of objects out there in our deepest reaches. Their basic argument is that there are too many objects in the Oort cloud to be accounted for by their origin around the giant planets - from where they were gravitationally flung outwards during planet formation 4.5-5 billion years ago.

The rather shocking alternative, carefully studied with state-of-the-art computer simulations, is that as much as 90% of the Oort cloud has actually been gravitationally captured from other stars, during close passes billions of years ago. Equally, we've shed plenty of icy lumps that are now stuck with different stars - somewhere along a common path around the galaxy.

With a wave of the wand our solar system is less of an isolated specimen than a separated sibling, carrying along the familial baggage of our stellar birth cluster. This means that some of the long period cometary bodies that spew their guts as they dip towards the Sun are truly alien - born around another star altogether. As a small habitable planet, we get the occasional wash from this material, and the occasional collision. Whatever the chemistry and isotopic mix is of these ancient objects, it's slowly getting mixed into the inner solar system. Remarkably this means that there is a pathway that could conceivably take a molecule formed around a giant planet in another star system and eventually deposit it in your Pina Colada. Anyone for another round?

Friday, July 2, 2010

Original multicellular life

We are cellular clumps. Great big, sticky, masses of about ten trillion cells, plus our microbial passengers. Although they all operate in unison, our cells are each quite self-contained. Membranes hold together our 3.3 billion base-pairs of information packaged up tight, and all manner of other molecular structures that allow the cell to hum away like a tiny machine. On the other end of the spectrum are the microbes, where an entire living organism is encapsulated in one single cell.

Somewhere in-between is a fascinating and tricky regime. Most microbial life - bacteria and the ancient archaea - live in colonies, typically of many, many different species. They communicate, they glue together in biofilms, they both support and rely upon each other. It is so very tempting to look at this and see how life on Earth went from these single-celled characters to multi-cellular grandeur. It's just a matter of joining the dots - literally.

Fossil evidence for when and how multi-cellular life on Earth first reared up has been awfully tough to come by though. Small squishy things do not take well to billions of years of mineralization. It does seem to have coincided rather broadly with the time at which Earth's atmosphere began to load up with significant amounts of oxygen about 2.4 billion years ago - courtesy of photosynthetic organisms like cyanobacteria. Available oxygen, while toxic to much of microbial life, opens up a whole new, energetically favorable, chemical network - a big tipping point towards large and complex lifeforms. A fabulous paper appeared this week in Nature, by El Abani and collaborators, along with a nice opinion piece by Donoghue & Antcliffe, describing perhaps the closest thing yet to our multi-cellular ancestors - hot on the tail of the oxygenating Earth.

In this paper they report fossil remains of curious looking, bumpy, swirly, scallopy characters as big as 12 centimeters, from what is now Gabon. At 2.1 billion years old they land on the cusp of atmospheric transformation. Carbon and sulfur isotopes seal the deal on these being biological in origin (see my other post). Everything about these objects screams cellular colony. Does that make them multi-cellular? Well, incredibly, the patterns in these fossils strongly suggest that the type of cell-to-cell communication that is the hallmark of creatures like ourselves was at play.

So here we are, meet your great-to-the-power-of-nine-aunt. In the search for life elsewhere in the universe we often come up against the question of whether 'complex', multicellular life is going to be rare or not. Certainly its longevity on planets probably comes down to fine details of environmental stability. However, if microbial life can gentle slither into this mode of operation - like our friends from Gabon seem to be doing - I think it ups the odds that for at least part of a biosphere's history, multi-cellular type life will be around. Perhaps we should think of life like us more as a desert bloom - it may happen only briefly when conditions are right, but the potential is always there.