: it was the photo that defined a generation’s relationship with our planet. Taken by the Apollo 8 crew in lunar orbit on Christmas Eve 1968, the vision of a blue, cloud-speckled planet shimmering into view in a hostile darkness brought home to us how lonely our planet is. Only inside Earth’s fragile blue bubble do atoms come together, bit by bit, to form the molecular basis of a chemical wonder: life.
Or so we thought. Gazing in the other direction, we have recently begun to see things that challenge this view of Earth as a lonely bubble. Not life itself, to be sure, but abundant evidence of the complex carbon chemistry that creates it – small hydrocarbons, sugars, alcohols and even, perhaps, the odd amino acid. That could alter our world view just as fundamentally as Earthrise did. “It completely changes your perspective of what’s happening on Earth and what happens in the universe,” says , an astrochemist at the University of Arizona in Tucson. And it raises a big question: how in the universe was all that stuff made out there?
It’s not that chemistry doesn’t happen in outer space. We’ve known for the best part of a century that carbon, oxygen and other elements forged in the nuclear furnaces of stars can come together in interstellar space to form simple molecules such as water, carbon monoxide and ammonia. The resulting molecular clouds, which condense and contract to form further generations of stars, can be whoppers: Sagittarius B2, at the centre of our Milky Way, is 150 light years in diameter.
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Alien interlopers
But even molecular clouds are unlikely arenas for the intricate chemical dances needed to make more complex molecules. For a start, they are sparsely populated. And they are frigidly cold – far too cold to supply the energy for most of the chemical processes we know to work on Earth. These conditions make it implausible for atoms to meet, greet and react in interstellar space.
Signs that this might not be the full story were brought by an alien interloper just a few months after the Earthrise photo was taken. On 28 September 1969, a bright fireball was seen over the town of Murchison in Victoria, Australia, as one of the biggest meteors ever observed exploded overhead.
Analysis of the Murchison meteorite material in the decades since has turned up a series of surprises: a slew of relatively complex organic molecules, including amino acids, the molecules that come together to make proteins, and . Similar findings have been made in other meteorites. That is significant because these meteorites are thought to have a primitive chemical composition similar to that of the material that went into making the solar system. “Meteorites are like messengers from the molecular cloud in which our solar system formed,” says Ziurys.
It is only with a new generation of powerful infrared and radio telescopes that we have begun to peer into these molecular clouds in any detail. Such telescopes can measure the low-energy electromagnetic waves given out by molecules as they rotate in a gas cloud. According to the rules of quantum mechanics, molecules emit waves of set frequencies as they spin. The spectrum of light coming from different regions of space thus hides subtle signs of the chemistry it contains.
As we’ve got better at interpreting these signs, we have been hit with a barrage of surprises. The largest molecules seen in space are the fullerenes, the class of weighty carbon molecules that includes the 60-carbon-atom buckyball. These were spotted in the environment of an ageing star by . But there is also a panoply of smaller stuff, ranging in size from just two to 13 atoms – including molecules a few heartbeats away from those that make life.
For example, in 2008 aminoacetonitrile, a precursor of glycine, the simplest of the amino acids, was found (). The NASA discovered glycine itself on the surface of the comet 81P/Wild 2 in 2009, although .
Other unambiguously identified molecules include sugar-like glycolaldehyde. The , a monster collection of radio telescopes brought into service in the Chilean Andes in 2011, most recently pinpointed it near a sun-like star just 400 light years away from Earth, suggesting that this molecule might have been present in the molecular cloud that to our own solar system. This year ethanimine, a precursor of one of the bases of DNA, was found in .
Now stands at 180 (see diagram). And we can assume they are just the tip of a very large iceberg. The more complex a molecule, the more difficult it is to spot.
“We have radio telescopes with such high sensitivity now that we are obtaining spectra with wall-to-wall spectral lines: lines on top of lines on top of lines.” says Ziurys. “It gets very difficult to identify a complex molecule because their lines are weak and they are hidden in this forest.”
“The bigger a molecule, the harder it is to spot – what we have seen is probably just the tip of a very large iceberg”
The bigger a molecule is, the floppier it becomes – which makes it difficult to pin down its expected spectrum. Part of the controversy with something like glycine, says Ziurys, is that even the lab researchers can’t agree on what its characteristic spectrum looks like.
Even so, from all we have found so far, relatively big molecules clearly do form in space, and those molecules are surprisingly similar to the ones we have on Earth. But how does this happen? On Earth, an amino acid is formed in a complex series of reactions in a plant or animal, mediated by enzymes that have evolved for this purpose over millions of years.
The sparseness and coldness of space means that, regardless of what the processes responsible for forming big molecules are, it must happen extremely slowly. An observer with a telescope would have to wait thousands of years before seeing any change. “The rate of chemical changes is just too small to observe in real time,” says of the University of Hawaii at Manoa.
Kaiser is one researcher taking things into his own hands. By condensing compounds such as water, carbon dioxide and ammonia in a vacuum chamber at just 10 degrees above absolute zero, he and his colleagues have been recreating something like interstellar conditions in the lab. Speeding up reaction times is then a case of putting more stuff into the chamber, so that the atomic encounters of millions of years in space happen within a few hours. “Your laboratory is like some sort of time machine,” says Kaiser.
Using this set-up, the team that glycine in space probably forms through a , a reaction that has been used to synthesise amino acids in industrial chemical plants since the 19th century, but which we thought did not occur in nature.
On Earth, you put together exactly the right ingredients – an aldehyde, ammonia and cyanide – heat it to the right temperature and then mix the brew with some acid. In space, it seems, the industrial chemist’s job is performed by two things: high-energy radiation and dust. Fierce ionising radiation zinging about space provides the energy to induce all sorts of reactions to get the Strecker mechanism going.
Cosmic dust – solid debris expelled by stars, covered with an icy coat of carbon dioxide, ammonia and hydrocarbons – helps the reaction in three ways. First, it provides a surface: molecules adsorbed onto a surface generally need much less energy to react. Second, it takes up excess energy produced in the reactions. Third, its icy layer provides protection for the molecules formed in the initial stages of the reactions – if they were nakedly exposed to cosmic radiation, they would be destroyed almost as soon as they were made.
Most recently, Kaiser has been working with researchers from the University of California, Berkeley, to find out whether space chemistry might even go beyond amino acids. “We wanted to see whether amino acids get linked on model ice grains,” he says. If two amino acids bind together, they form a dipeptide, a very small protein. The discovery of proteins on model ice grains in the lab would suggest that the ultimate building blocks of life do indeed form in space.
The experiments proved fiddly: only tiny amounts of the products were ever formed. But after three years, 40 repetitions of the experiment and countless cross-checks and unravelling of spectra, the team finally had their answer in March this year. – and possibly even more complex structures such as tripeptides can too. The European Space Agency’s , due to land on the comet 67P/Churyumov-Gerasimenko in mid-2014, might be able to confirm whether that is indeed the case – at least on a comet.
But there are still some things in space whose presence takes more than a sprinkling of stardust and some tanning by ionising radiation to explain. in interstellar space. These small molecules form when one hydrogen atom is removed from methanol, the simplest alcohol. In Earth’s atmosphere this occurs when methanol encounters a hydroxyl radical, which consists of just one hydrogen and one oxygen atom. But lab experiments showed that in the conditions found on dust grains in an interstellar cloud, the wrong hydrogen atom gets removed, and the reaction forms a different molecule, known as a hydroxymethyl radical. Equally, it didn’t seem possible that the methoxy radical could be formed from methanol and hydroxyl floating around in free space: there simply wouldn’t be the energy to get them to react.
This year, from the University of Leeds, UK, and his colleagues solved the paradox – unintentionally. “Our original goal was to study reactions of the hydroxyl radical in our own atmosphere,” says Heard. But their apparatus just happened to let them cool down the reaction. When they reached around -200 °C, it suddenly became 100 times as fast.
“There was this huge enhancement of the reaction rate and we just thought ‘Gosh! Something else must be going on here!’,” says Heard.
Quantum assist
And so it turned out: the impish ways of quantum mechanics. Low temperatures calm things down so that the molecules stick together long enough for quantum-mechanical tunnelling to take place. Theoretical chemist from the University of Stuttgart, Germany, likens this effect to throwing a ball at a wall and seeing it fly straight through instead of bouncing back. Similarly, an atom involved in a chemical reaction sometimes does not need to overcome an energy barrier – the uncertainty about its position inherent in quantum mechanics gives it a small probability of tunnelling through the barrier.
The smaller the atom involved, the more easily this occurs. “This is why tunnelling is most important in reactions involving hydrogen,” says Kästner. It turns out that exactly this quantum assist allows the hydrogen atoms in the reaction between methanol and the hydroxyl radical to reposition themselves and – at low temperatures, and in free space.
This might be the beginning of a completely new interstellar reaction network. All you need to do is remove a hydrogen atom from a methoxy radical to make formaldehyde. Formaldehyde could react again and again and eventually grow into a complex organic molecule.
Whether this precise reaction occurs in free space is as yet speculation, but recent discoveries of organic molecules in space have restarted the long-running debate about whether Earth truly was the place where life’s precursor molecules formed. One theory of how life got started was that the first peptides on Earth consisted of three to eight amino acids, and could have triggered the formation of bigger molecules, which eventually led to big proteins and enzymes and other life-essential molecules. Speculations on how those small peptides could have formed on Earth are pretty vague and filled with inconsistencies. Earth’s early atmosphere, for example, doesn’t seem to have had the composition to allow the right chemistry to occur.
It now seems, however, that the chemistry underpinning life is not exclusively Earth’s preserve – the sun and its attendant planets were born into an environment that probably contained organic molecules at least as big as dipeptides. Perhaps these were transported to Earth by comets once Earth’s chemical conditions were right. Some time between 4 and 3.5 billion years ago, Earth went through a period of heavy bombardment by comets and meteorites, says Ziurys. “Shortly after this period the first evidence of life has been found.”
That could be coincidence, or there could be some alternative explanation. A study published last month showed, for example, that amino acids can be produced in the impact of an icy comet with a such as Earth’s. And of course, even if the molecules Earth made were not entirely its own invention, our planet still took the chemistry far further in the cosmos than anywhere else – as far as we know. That’s reason enough to cherish our unique blue bubble – while pausing to think, as we gaze out into space around us, that our molecules at least are not alone.
This article appeared in print under the headline “Close encounters”
