Sometimes space news really manages to strike a chord with people. Yesterday, it seems, was one of those moments, with the announcement that amino acids had been discovered in comets for the very first time. Within hours, half the news sites on the internet picked up on the story, from the BBC to Scientific American. Understandably so, as this is a pretty big discovery -- with even bigger implications!

Roughly 5 years after NASA's Stardust mission brought home the first ever sample of cometary dust, it's been officially announced. Amongst the chemicals that make up comet Wild 2 is glycine, the simplest amino acid. Looking at the carbon isotopes found in the glycine (it's enriched with carbon-13) confirmed it was formed in space, and likely very old. To quote NASA's Mike Zolensky “We see in this comet that amino acids were forming at the earliest time in our solar system.”

Now amino acids are pretty big news for astrochemical folks like me. To be perfectly honest, the discovery that comets can harbour amino acids didn't come as any surprise to me. It was more of a reassurance that all is right in the Universe. After all, we know amino acids can form in meteorites. We've even found nucleobases in those! Amino acids have been shown to form favourably through good old fashioned thermodynamics, and even chemical theorists not unlike myself have been in on the act of trying to find them in space.

But in comets? The fact that now we know for sure* opens up a few interesting thoughts. One is the old concept of panspermia. And when I say old, I really do mean it. The idea goes back hundreds of years, with various philosophers and scientists pondering whether life could've fallen from the heavens. The most prominent recent proponent of the idea, though, was Sir Fred Hoyle. Hoyle was a firm believer that life itself formed in space, before it arrived to blossom here on Earth. Panspermia tells us that comets could be the seeds from which life grew. Just like any other seed, all they needed was plenty of water and the right amount of sunlight.

Comets really are a hotbed of chemical activity**, containing, in particular, plentiful carbon, oxygen and nitrogen. Reactions between these chemicals could be driven by any number of things. Ultraviolet light, either from the Sun or from other stars can drive chemistry in the outer reaches of the solar system where comets commonly lurk. Alternatively radioactive elements in the comets themselves would create heat as they decayed, causing pockets of warm liquid water for chemistry to take place (coupled with ionising radiation to initiate the chemical reactions therein). No surprise then, that we should find amino acids in them. Considering all of this, it doesn't even seem unreasonable that some kind of primitive life might have formed inside a comet before being delivered to a planet later to fend for itself.

At risk of descending into flagrant speculation (which is all too easily done on such a fascinating topic), the Sun is believed to be surrounded by billions and billions of comets. Some of these comets are occasionally flung straight out of the solar system, to sail off aimlessly into the night. And the Sun is just one of many billions of stars in the Milky Way. Even if only a small fraction of these comets contain the fundamental ingredients of life, it certainly lends a lot of weight to the idea of panspermia. Maybe David Darling was right. Perhaps life really is everywhere.



*And believe me, science is all about knowing things for sure. Don't mind my idle speculations, by the way. That's what a blog's for, after all.

**As I've written about before actually, a lot of molecules were discovered for the first time, anywhere, ever in comets. The brightest optical emission line of the C₃ molecule lies somewhere in the violet part of the visible spectrum and, as a result of how it was discovered, is still commonly referred to as the "Comet Head Band!"



Image from spacetelescope.org (with thanks to Ian O'Neil for the heads up!)

Comets, let's face it, have rather a bad name. Traditionally, they're been seen as bad omens. Harbingers of doom, fortelling disaster. So ingrained into our consciousness is this line of thought, that the very word "disaster" stems from words meaning "bad star" (from dis and astro)! Comets have been featured in movies (ranging from the kitsch to the reasonably well received) and authors have written about them for some time. But are they really killers? Maybe comets are just misunderstood...

In all fairness, it's widely accepted that there was a giant impact around 65 million years ago, and it's widely (albeit not universally) believed that this event killed off the dinosaurs in the Cretaceous–Tertiary extinction. While it's been hypothesised that other impacts may have been responsible for certain other mass extinctions in Earth's history, Nathan Kaib and Thomas Quinn of Washington University are evidently not so sure.

The two have run some numerical simulations to investigate long period comets. Long period comets are those which come from the very edge of the solar system. Loosely bound by gravity, the Sun is thought to be surrounded by the Oort cloud -- a shell of icy objects. A nearby star passing too close can perturb these cosmic snowballs, causing them to fall towards the Sun and flare into brilliant comets as they start to warm up. If these errant oort cloud objects aren't flung into interstellar space, they can fall into huge elliptical orbits, orbiting the Sun on timescales of thousands to millions of years.



Although it was long believed that the outer reaches of the oort cloud were the main source of such long period comets, Kaib and Quinn's simulations suggest that the actual source is the inner oort cloud. There's a big difference there. A couple of light years of difference, in fact (the outer oort cloud is believed to stretch as far as three light years away from the Sun!). A close encounter with another star can still cause a shower of comets from here -- though even without such a gravitational nudge, it was still found that most long period comets come from the inner and not the outer oort cloud.

The upshot of their simulations, very simply, it's unlikely that more than two or three of these objects could have struck Earth over the past 500 million years, and that's assuming the maximum possible number of oort cloud objects. The (minor) Eocene-Oligocene extinction about 40 million years ago has been suggested before to be the result of a comet shower. If Kaib and Quinn are right, it must have been the most intense cometary shower to have occurred since the Cambrian period! This reduces the chance that extinctions are caused by comet showers. To use Kaib's own words, "...comet showers are probably not likely causes of mass extinction events."

As Jupiter's recent bruise will attest, the giant planets are very helpful in shielding Earth from cometary impacts, deflecting comets or simply bearing the brunt of their impacts so that we don't have to. Interestingly, this means that whatever giant impact occurred to cause the infamous Cretaceous–Tertiary extinction was a purely random event. A fluke. Frankly, I'm not entirely sure how comfortable I am with that...


Image: Comet McNaught, reproduced with kind permission from Chris Picking of Starry Night Skies Photography. His pictures are great. You should go and look at some more!

Reference: Reassessing the Source of Long-Period Comets - Kaib & Quinn (2009)

Those with sharp enough eyes, who know where to look, should be able to see Comet Lulin in the sky even as I type this. It should be somewhere in the constellation Virgo, with a magnitude of about +6 (you may need binoculars if you're a city dweller). It reaches its closest approach to Earth on Tuesday 24th, at about 0.41 AU away.

Comets are fascinating things. Even more so when, like Lulin, it's on its first visit to the inner solar system. Comets are, after all, full of ices. As it draws near to the Sun, all of those ices start to sublime into a huge extended atmosphere called a coma. This coma is huge, but not very dense. The end result is a tenuous glowing gas cloud that can easily be as big as Jupiter. The solar wind interacts with all of this, puffing it in the opposite direction to the Sun and causing the comet to sprout a (sometimes magnificent) tail.

Lulin is also a striking shade of green. That comes from a couple of particular molecules that it's particularly rich in. C₂ (dicarbon) and CN (cyanogen). Probably C₃ too -- a molecule which was actually first discovered by looking at comets. In spectroscopy, the brightest C₃ band is still commonly known as the "comet head" band.

Other comets, like Halley, contain different ratios of these molecules because each pass near the Sun causes ices to ablate away or react together, chemically changing the comet. The result is that they glow in different colours.

Seemingly, objects that spend a lot of time in the outer solar system pick up a lot of cyanogen, so they often glow green. I'm just guessing, but it's probably why the rarely seen Aurigid meteors a couple of years ago also glowed green. Meteor showers, after all, come from comets. All comets leave a trail of dust in their wake, leaving more and more on each orbit. Every time the Earth passes through one of this loops of cometary debris, we get a meteor shower -- the more times the comet's passed by, the bigger this dust cloud will be, and so the more meteors we'll see from down here on Earth. The Aurigids are rare because the comet that leaves them is on a huge orbit, spending centuries deep in the Oort cloud before eventually falling back in towards the Sun. As a result, it doesn't spend a lot of time in our neighbourhood...



This image, incidentally, must've been taken around February 6th, because that double star in the background would be Zubenelgenubi (also known as Alpha Librae).


Image beautifully photographed by Mike Broussard. Go and look at his other photographs. They're rather lovely -- especially all the comets!

Our solar system is hypothesised to be surrounded entirely by the Oort cloud -- A vast spherical shell of icy dust that extends almost one quarter of the way to proxima centauri. Past the Sun's heliosphere and exposed to the raw interstellar wind, the Oort cloud is believed to be the true outer edge of the solar system. Believed, that is, because it's never been directly observed. Nonetheless, it's postulated to contain billions of comets. Tenuous bodies of icy clathyrate held together by a fine filigree of silicate. So fine, that if all the ice was to melt away, the remains would be as fragile as cigarette ash. The oort cloud is believed to be the source of all long period comets in the solar system. Objects with such wildly eccentric orbits that they can pass from the outermost edge of the solar system to within the orbit of Mercury, with a single orbit that can last for hundreds or even thousands of years. Some, known as sungrazers, can pass within a few thousand kilometres of the Sun's photosphere.

But many of these strange creatures, lurking in the dark edges of the solar system, only take up a temporary home in the oort cloud. Many approach the Sun with hyperbolic trajectories, passing our star once, before being flung out to the far reaches of interstellar space. In fact, some theories predict that around 65% of all long and short period comets are still being lost to the interstellar medium. So where are they all? Some believe there's an inner oort cloud "reservoir" where comets are unlikely to be perturbed into interstellar space -- assuming that only Jupiter has powerful enough gravity to fling objects out of the solar system altogether. The truth may lie somewhere between these two extremes, but the trouble with comets is that until they start to outgas and form their brilliant tails, they're very hard to spot. They reflect only 4% of light that hits their surfaces. Coal, for comparison reflects around 8% of light. Looking for something that's half as luminous as coal in the blackness of space is, understandably, no easy feat. Whatever the case, true interstellar comets will be travelling with velocities comparable to the relative velocities of other stars (around 38,000 miles per hour). Some scientists postulate that as many as 4 comets every century may be bound for interstellar space.

With every other star in the Milky Way (at least 200 billion) and every star that's ever been born in the Milky Way, our interstellar medium should be awash with comets, shouldn't it? So what happens to them all? Do they just stay in the ISM, in a galactic orbit? Do they get tangled up in the oort clouds of other stars? Many astronomers believe that as stars pass through giant molecular clouds (GMCs), oort cloud objects can be stripped away, and be replaced by material captured from the GMC. Perhaps most of the comets we've ever seen didn't originally come from our Solar system. Maybe they've been captured from interstellar space by the Sun.

Who knows... maybe some comets even get thrown between galaxies.