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

Monday, June 28, 2010

Favorite nuclear flavors

One of the signposts of life - and something that will likely be exploited in the study of potential Martian critters - is isotopic fractionation. The general wisdom is that life prefers light isotopes. Given the choice between hydrogen and deuterium (always present due to its construction 13.7 billion years ago during primordial nucleosynthesis) living things will tend to choose the lighter hydrogen. The same is true for carbon-12 and carbon-13, the light guy wins and is preferentially incorporated into the chemical doings of cells. When the extraordinary plumes of atmospheric methane were confirmed on Mars a couple years ago it was clear that the next step would be to try to pin down the isotopic composition of the gas. Here on Earth one can quite readily distinguish methane produced by bacteria or archaea and that produced by abiotic processes (most is biological), by tasting the isotopic composition.

So why is this so, why does life like particular isotopes. Ask a biologist and they will typically say that it's all about enzymes. Processes inside cells that work with enzymes - fabulous catalysts - happen faster with light isotopes, and bingo, you naturally sieve out the heavy guys and leave them in the dust. At least that's the line I've heard again and again. I was fascinated then when I stumbled across a very interesting work by Calsciotti. In this and subsequent papers the discovery of cellular processes involving nitrogen are described that actually prefer the heavy isotope nitrogen-15 rather than the lighter nitrogen-14. It appears that in some cases the heavier atoms provide a more energetically lucrative reaction route, et voila, like the fickle thing it is, life spits out the lighter variants in favor of something meatier.

The light isotope rule is not even that. Biochemistry again demonstrates that after a few billion years of fine-tuning it can exploit just about any trick in the book to get ahead. What I find amazing is that only about 0.4% of nitrogen on the Earth is the weightier 15, yet here we have organisms that will make use of the better chemical throughput of that isotope. One's first reaction is 'why bother?'. Obviously it may just be an unintended consequence of the chemical network, but I think the odds are good that somewhere along the line that tiny advantage will have been important. Here's the big question though, would a different biosphere - a martian one perhaps - with a presumably different evolutionary history, make all the same biochemical choices? To throw a final spin on this; Mars has a higher nitrogen-15 abundance than the Earth, a relative enrichment of about 60%. How much would this environment have swayed things?

Wednesday, June 23, 2010

Planetary fumigation

Where do you go on Mars to look for life ? Some pretty big decisions are looming for NASA's next Mars rover - the plutonium powered, SUV-scale beast known rather delicately as Curiosity. The choice of landing sites is currently down to four, and much depends on clues to the past history of these regions. Some show signs of a watery era, with the possibility of rich sedimentary rock - the kind of places where the detritus from an ancient martian biosphere would have ended up. Curiosity will be armed with an array of instruments, some capable of analyzing the detailed organic chemistry being trampled underfoot.

There has been considerable debate in the community about where to land, how to avoid chemical and even biological contamination of where you land, and exactly what instruments to carry - given the very real limitations to hauling every gram of gear to Mars. There's no doubt that Curiosity represents the next great stage in exploring another planet, and looking for the signs of extinct, or even extant, life. We'd like to get it as right as possible.

Thinking about all this raises something else, that gets less attention. There is a supposition that if Mars once had a watery surface environment - even if only fleeting - and some kind of biosphere, then there must surely be traces left. Obviously the geochemical alteration of the surface is likely to be noticeable, but why do we assume it is hard to erase the signs of life from a planet ? It may be an Earth-centric bias.
What would it take to erase life from the Earth, and perhaps also any sign that it was ever there ?

For everything on the surface then we can imagine burning, exploding, acidifying, and irradiating until we had just an amorphous, crispy, residue that would be entirely sterile and uninteresting. However, then there's the subsurface - kilometers deep rock, oceanic sediments and porous material, isolated subterranean water. All of this is chock full of slow-living microbial life. A solution would be to 'go Venus', melt the surface and keep up strong volcanism for billions of years - heat and pressure over time will transform just about anything. Time alone might do it. If you could wait another five billion years or so (at which point the Sun will have engulfed the Earth anyway) then perhaps enough of the molecular handprint of life will have fizzled away to be unrecognizable. In short, without a cataclysm then it's hard to imagine a way to remove all traces of past or present life on Earth.

But what about Mars ? If a planet never gets a fully entrenched, deeply embedded biosphere does it retain the same stain ? Suppose for a while, in the distant past, Mars did have a wetter and more obviously nurturing surface environment. Perhaps microbial life got going across a number of optimal regions, but was then cut off - as if life on Earth had stopped 3 billion years ago. It seems it would be much easier for time and chemistry, not to mention volcanism and asteroid impact, to lay down the white-out. It sounds pessimistic, but it's really more about tackling the fundamental problems of molecular paleontology - you need to find an understanding of what might be blocking your view in order to reconstruct what was once there...

Monday, June 21, 2010

Swings and roundabouts

The hunt for terrestrial-type planets is hugely challenging, although steady progress is being made. Last week NASA's Kepler mission finally began to release data on some of its candidate planet-hosting stars. In a bit of controversial, but to some extent understandable maneuvering, the Kepler team has kept what are probably the juiciest systems to themselves. The New York Times had an excellent piece on this tale. Regardless of what data any of us get our sticky paws on there is still a big hurdle to deal with. Kepler is seeking the tiny dips in starlight when planets transit their stellar hosts, but this alone is not enough to either confirm with absolute certainty the presence of a planet, or to determine the mass and detailed orbital characteristics of new worlds. The next stage involves painstaking followup with ground-based telescopes equipped with exquisite spectrometers, seeking the wispy signs of Doppler shifting starlight due to the gravitational offsets induced by planets.

It's quite a bottleneck. It's also pushing the limits of experimental sensitivity as we seek planets equivalent to the Earth - where the stellar 'wobble' can be mere centimeters per second, the speed of a royal wave. Kepler hasn't even got to those candidates yet, with orbits close to 12 months in length we need the next several years to witness multiple  transits. There is no doubt that it will take a great amount of scientific cooperation and effort to finally nail down the number of Earth-equivalent worlds that Kepler sees.

Even then, what we'll end up with is really just (although 'just' is relative) a well defined estimate of how many such planets should exist galaxy-wide - together with some idea of their typical compositions and orbits. Bottom line is that the newspaper banner in three or four years will read something like '15 %' - this being the fraction of normal stars hosting small rocky worlds. Is that worth the $600 million for Kepler, plus many more millions for all the followup work ? Such is the nature of modern fundamental science. The Large Hadron Collider costs well in excess of $6 billion, that's about a billion per physics question that it might help answer. The overall cost of the Human Genome Project was about $3 billion - roughly a dollar a base-pair.

I'd argue that by comparison Kepler is positively efficient, but it does raise an interesting question. Just how much are we as a species willing to invest in the search to find other worlds and to seek out signs of other life in the universe ? It's not a cheap enterprise. For me though it is no less important than understanding fundamental physics, or our genetic blueprints. In fact I'd stick my neck out and suggest that unless we find new worlds, new biospheres, we will never have a way to place the Earth and ourselves in proper scientific context. The answer to our origins will come from seeing the bigger picture, and that is surely cheap at any price.

Tuesday, June 15, 2010

Seven years in deep space

The falcon (Hayabusa) has landed. A few days ago the Japanese space agency's extraordinary craft re-entered the Earth's atmosphere and successfully deposited its sample return capsule in the Australian outback. This was an incredible technological feat, and apart from Apollo and Luna samples from the Moon and the solar particle and cometary dust return missions of Genesis and Stardust, the only other time a spacecraft has made it back to the homeworld possibly carrying scraps of pristine extraterrestrial material. Pristine is the key here. Hayabusa gently bounced off the asteroid Itokawa back in 2005, and may have captured grains of this ancient body that traces an elliptical orbit looping inside that of the Earth before reaching some 70% further out.

Thanks to Hayabusa we know that Itokawa is an extraordinary 'rubble pile' half a kilometer long - not a solid body but rather a loose collection of rocks from tiny pebbles to much larger. Weakly held together by its own gravity this is an amazing snapshot of one episode of planet formation; the agglomeration of solids. Figuring out the precise chemical constituents of a body like this could yield clues to the pathways by which material coalesces in the early stages of a baby star system. It could also add a big piece to the jigsaw puzzle of carbon chemistry in our solar system, and ultimately the origins of terrestrial chemistry.

There's another aspect to Hayabusa that is perhaps even grander. The image of this cleverly fabricated robot burning up across the night sky evokes some powerful emotions. Here is one of our pioneer voyagers of the deeper universe that lies all about us. A persistent machine, nurtured and nursed through a variety of problems by its smart operators. Seven years on it returns, carrying - we hope - a precious sample that will expand our view of nature. Seven years is a long time these days, Hayabusa has come back to a different world. This is a glimpse of our future in the solar system. The meteor-like streaks of returning probes, and eventually astronauts, lighting our skies. New mariners, returning to harbor, bringing exotica that change everything, just as they find a world changed by time.

Monday, June 14, 2010

Guest post at Scientific American

Scientific American has been good enough to put a post up from the Life, Unbounded coffers. You can catch it at their guest blog spot. In it I present a short discussion about some of the practicalities of looking for 'habitable' planets - those showing evidence of an entrenched biosphere. Just to stoke things a little I make the suggestion that if we abandon the strategy of studying a few worlds in great detail (with great difficulty) that we might be able to address the global question of whether there is any life out there. We sacrifice pesky details, like knowing exactly which planets have biospheres, but maybe succeed in answering whether any in a particular class have organisms lolling about. Of course I paper over the enormous difficulties in doing even this, but it's fun to throw the idea out there.

Friday, June 11, 2010

Gratuitous self-advertising

Although clearly if you're reading this blog then you already have a brain the size of a planet and are possibly a pan-dimensional hyper-being of unlimited intellect, but I was happy to find out that Life, Unbounded is on the list of 'hottest science blogs' over at the UK broadsheet The Guardian. Not bad for a baby only a couple months old. Thank you Guardian.

Thursday, June 10, 2010

The (chilled) broth of life

A couple of interesting new results on Titan's atmospheric composition and chemistry have popped up and been quickly swamped by discussions of life on this frigid moon. The story goes; photochemically produced molecules of hydrogen and acetylene have been 'mapped' at different layers of the Titanian atmosphere, including close to, and on, the surface. Intriguingly the hydrogen distribution suggests a 'flow' towards the moon's surface followed by its disappearance. However, there is no obvious way for the gas to be incorporated into surface materials, so it's being transformed or used up by something as it reaches these low altitudes. Similarly, acetylene appears to drop off in abundance and is not present on the surface.

Measurements like these are notoriously tricky, based as they are on remote spectroscopic data from the Cassini mission and require careful consideration of the incompletely understood chemistry of Titan's atmosphere. Indeed, there are basic chemical mechanisms that could be responsible for yanking hydrogen and acetylene out of the atmosphere without putting a trace on the surface. Photochemistry - driven by ultraviolet photons trickling in from the distant Sun - can carry on transforming molecules in the chill 90 Kelvin atmospheric soup. New organic compounds can then just drift to the surface with a different spectroscopic signature. It requires a tough bit of scientific disentangling.

What's got some people excited is an earlier discussion by McKay and Smith in 2005. They pointed out that if some hypothetical organisms existed on Titan then they could derive plenty of chemical energy by reacting hydrogen with acetylene (along with ethane and some other organics). The apparent flow of hydrogen down to Titan's surface would seem to supply plenty of this chemical broth, more than sufficient for a considerable biosphere. It would also seemingly explain the lack of surface acetylene - it's just being gobbled up.

Is this just enthusiasm from a 'pro-life' brigade ? Well, it's certainly premature to suggest that this is a sign of a Titanian biosphere. Just because a chemical route to pulling energy from the environment exists doesn't mean anything is exploiting it. Without liquid water then methane might be the bio-solvent of choice, but that has a lot of challenges - it's a far less versatile solvent than good old H20. Nonetheless, it is pretty amazing that these results point towards exactly the kind of effect you might see if low-temperature, alien, life was slurping around on the shoreline of this enigmatic moon. Clearly Titan is an even more interesting place than we knew.

Monday, June 7, 2010

Road trip

A cute topic, allowing for hours of happy speculation, is how exactly to best explore other planets or moons in our Solar System when we can't be there in person. The armchair engineer might almost be tempted to jump to their feet - it's a juicy problem. I was reminded of it the other day when I read a short news item on a design study being undertaken to evaluate the best way to build a 'tumbleweed rover' for Mars. As the name suggests, you drop something like a spherical cage onto Mars, equipped with surfaces to catch the breeze, your favorite sensors and cameras, and let the wind take it where it will.

It sounds wonderfully elegant, until you start to worry about the details. Topography is the most obvious problem - even the bounciest tumbleweed, with the occasional blast of stormy weather - is eventually going to get stuck, wedged, or dropped into a spot that traps it. A solution is of course to drop lots of tumbleweed rovers onto a planet like Mars - certainly an appealing picture, dozens or more sweeping across Elysium Mare in a great Terran invasion. However, it's not entirely clear whether the type of data you could accumulate from such a campaign would balance out the cost.

Another idea that has done the rounds is to have a long-duration solar powered glider or aircraft, or even airship. At high altitude, away from anything but the tallest mountains, and above much of the debilitating martian dust storms, such semi-autonomous craft could provide in-atmosphere monitoring of chemistry, weather, and potentially higher resolution surface imaging than space-based platforms. Unlike a surface rover, the light-delay time to Earth would be less of an issue. Setting the airborne machine on a course for hundreds of miles could be done without the need for constant guidance. Similar ideas have also cropped up for exploring a far more alien environment on Titan. With a frigid but much thicker atmosphere, Titan could be a fantastic place for aerial exploration. With about a 90 minute light travel time - depending on the Earth-Saturn configuration - this may be the only real way to explore without a highly sophisticated AI calling the shots.

Many of these ideas take their inspiration from nature. The only question mark for me is that exploration or migration strategies used by organisms - be they bacteria or humans - may be optimal for survival or resource gathering, but not necessarily for gathering the best data.

Friday, June 4, 2010

Paradox Earth: II

A few weeks ago I wrote about one of the most slippery topics in understanding the evolution of environment on the Earth, with huge implications for other terrestrial-type planets. The so-called 'Faint Young Sun Problem' is that 3-4 billion years ago the Sun was about 30% fainter than it is today - a consequence of really quite well understood physics in stellar cores; stars get hotter and brighter as they age. The Earth teeters close to a distance from the Sun beyond which the planet would freeze up, yet multiple lines of evidence point towards a temperate environment back in our youth - despite a significantly less lush solar output. What kept things warm on the planet?

Stuffing more greenhouse gases, like carbon dioxide or methane, into the atmosphere could be one solution. Problems arise though in matching what you'd need to keep things toasty to the geological record of atmospheric composition billions of years ago. Now here comes another possible answer. A new paper by Wolf and Toon in this week's Science (together with an opinion piece by Chyba) suggests that a hydrocarbon rich atmosphere would form a fairly delicious photochemical haze. A haze would shield the lower atmosphere from destructive ultra-violet photons and allow for very potent greenhouse gases like ammonia to survive for far longer than they would otherwise.

The idea of soupy hazes in the young Earth is not new. The problem with hazes is that they end up reflecting away more sunlight altogether, which can completely negate any enhanced greenhouse effect. Wolf and Toon provide a clever solution to this. They point out that everyone has assumed haze particles - greasy little microscopic things - were spherical. More realistically they are probably 'fluffy' or fractal in nature - like malformed snowflakes. Remarkably, this transforms the optical properties of the haze - ultraviolet photons are still blocked, but lower energy visible and infrared photons are scattered and still make it through. So, nasty ultra-violet light is kept out, strong greenhouse gases like ammonia can form, and the warming rays of visible and infrared light can get to work and keep the young Earth at a nice temperature.

It's another interesting take on the problem, with implications for when we start coming across Earth-analogs around other stars that are young compared to us.

Tuesday, June 1, 2010

Star Flavors

As the effort to find new planets around stars barrels along, some intriguing characteristics are beginning to reveal themselves. We know that stars range from just about a tenth of the mass of our Sun all the way up to some crazy big balls of plasma hitting over 100 times more massive. The environment for any planets orbiting in these systems will be radically different. Small stars are dim, as much as a thousand times fainter than the Sun, with a red hue. The most massive stars are tens of thousands of times more luminous, blue-hot, barely held together by gravity, belching forth material. In  keeping with their outward appearances, low mass stars can sustain a gentle fusion burn of their core material for a trillion years. The big guys follow the live-fast-die-young business plan, and rip through their semi-stable, rock and roll, lifestyle in a mere million years or so. Pity the planets in those systems.

Some new work adds support for another layer of flavor by finding evidence for a fundamental shift in the nature of the planetary populations around stars of different masses. Stars just a little heftier than our Sun, by 50-100%, appear to play host to many more giant planets - objects larger than Jupiter - than do stars like the Sun. While we don't yet know what's going on with the smaller, more terrestrial-type, planets that may or may not exist in these systems, it suggests an acute planetary dependence on the size of the stellar parent (among other things).

Our still fledgling ideas about how planets form, out of the great disks of gas and dust that surround young stars, are more or less consistent with an upturn in the numbers of giant planets around bigger stars, but the details are hard to come by. What's really surprising is just how little a change in the type of star is needed to see a pretty dramatic shift in the numbers of giant planets - whatever is going on is a sensitive business.

This adds another lever into the machinery for trying to evaluate systems as potential harbors for life. Fast burning, massive, stars may make certain types of planets more efficiently, but what matters the most - longevity or numbers? Do we even care about these types of systems if small terrestrial-type planets get muscled out? Maybe it's the moon systems of these giant planets that are of interest?  A star twice the mass of the Sun is perhaps 5 times more likely to harbor giant planets and their moons, but will burn through its hydrogen core in a 'mere' two billion years. Is that a second class habitat or a premium one?