Last week, a slew of scientific papers told the story of comet lander Philae's bumpy touchdown, comet 67P's surprisingly fluffy surface, and — most exciting — the discovery of life's building blocks there. We haven't found life. But we may have found part of life's origin story, buried on this icy rock.
When scientists say they have found the building blocks of life, what they really mean is that there are organic molecules on comet 67P. This is a big deal because the more places that we find these molecules — which we believe evolved into life on Earth — the more evidence we have that life could exist beyond our planet. Comets are especially intriguing because of the popular 'panspermia' hypothesis that suggests life arose when microorganisms used space rocks to hitch rides between planets. So what are these "building blocks" and how are they related to life as we know it?
You'd be forgiven if you're not totally sure what organic means — the word is thrown around in so many different contexts, from food marketing to science fiction, that its actual scientific meaning has become a bit obscure. So, first things first: In the chemistry sense, the term 'organic' is simply used to describe any compound with carbon atoms bonded to other carbon atoms or to hydrogen atoms. In reality, this means 'organic' refers to thousands of very different and diverse chemicals, from simple gases like methane (CH4) to large, complex proteins and DNA molecules. Some organic compounds are only built by living organisms, but many simple organics can be formed by abiologically. Others, such as polyethylene (of supermarket bags) and polystyrene (of packing peanuts) aren't found in the natural world at all.
Organic: Basically, anything that contains carbon bonded to more carbon. Picture: Wikimedia
So, organic: Humans, grocery store bags, cow farts. But what about outer space? Turns out, there's plenty of organic chemistry floating about in the cosmic void, as well. Take, for instance, methane. There are literally oceans of the stuff on Saturn's frigid moon Titan. As we recently learned, methane-rich ice sheets coat the surface of Pluto. Our space probes have sniffed methane in the Martian crust (though still highly debated) and in Mercury's atmosphere, and we have good reason to believe there's methane forming geochemically beneath the frozen surface of Saturn's icy moon Enceladus.
Another simple organic molecule, formaldehyde (CH2O), has been spotted in interstellar dust clouds throughout the galaxy. Formaldehyde forms when carbon monoxide reacts with hydrogen gas, and cosmic radiation seems to help that process along. Even amino acids, the nitrogen-rich building blocks of proteins, are now believed to assemble spontaneously in deep space with the aid of UV radiation. In 2009, NASA's Stardust mission sniffed out the first amino acid on a comet, and a newly proposed NASA mission would do the same for the ice geysers of Enceladus.
What Did We Find on 67P?
The more we look for it, the more we're finding organic chemistry all over our cosmic backyard. So, what did we find on comet 67P? On the one hand, lots of interesting things! On the other, a bunch of stuff we might have sort of expected.
Panspermia: The hypothesis that life rained on the ancient Earth from above. Picture: Wikimedia
In a first-of-a-kind experiment, Philae's Cometary Sampling and Composition (COSAC) instrument collected molecules from 10 kilometers above the comet's surface, shortly after the initial touchdown, and at the lander's final resting site. Analysing the composition of these samples with COSAC's mass spectrometer, Fred Goesmann and colleagues identified 16 different organic compounds, four of which — methyl isocyanate, acetone, propionaldehyde, and acetamide — had never been spotted on a comet before.
In a related study, Ian Wright and colleagues used Philae's Ptolemy instrument to sample ambient gas above the lander after touchdown. This analysis revealed some of the main components of the coma — that nebulous cloud of gas surrounding a comet — including water vapour, carbon dioxide and carbon monoxide. Ptolemy also sniffed a surprisingly large amount of polyoxymethylene, a formaldehyde-family compound.
These studies have shown us, for the first time, just how diverse cometary organic chemistry can be. But life on a comet? Not so much. All of the organic molecules discovered on 67P can be formed abiologically, by the interaction of comet ice with UV and cosmic radiation. Goessmann et al. explain:
The COSAC molecules form a consistent set related by plausible formation pathways. A nitrogen source such as NH3 must originally have been abundant to form the many N-bearing species, but could since have mostly evaporated or been used up in reactions. All the COSAC organics can be formed by UV irradiation and/or radiolysis of ices due to the incidence of galactic and solar cosmic rays: alcohols and carbonyls derived from CO and H2O ices, and amines and nitriles from CH4 and NH3 ices. Hydrolysis of nitriles produces amides, which are linked to isocyanates by isomerization .
But despite their abiological alibis, these molecules are exciting from an astrobiology perspective. As the researchers point out, many of the organic molecules we're finding are precursors to the sugars, amino acids, and nucleobases that form DNA and proteins — the building blocks of the building blocks, if you will. What's more, while the Earth has evolved considerably over the history of the solar system, the interiors of comets are considered pristine record-keepers from the very beginning.
"I think this absolutely bears on origin of life studies," Wright told Gizmodo over the phone. "We all want to understand the origin of life but we can't really do it on Earth, because the processes of biology and geology have completely reset the record. We have to look at the other side of the timeline — at the things that might have contributed abiological material to the process. I believe there are abiological compounds in comets that, if they were to have rained down on the surface of the Earth, and given the right conditions, could have helped life get started."
Which is to say, understanding the organic profile of comets can help researchers zero in on which molecules, in particular, might have helped kickstart things here on Earth.
"I think what we're doing here with these results from Philae is putting some names to the organics on comets, which will help refine the theoretical and laboratory work that goes into studying the origin of life," Wright said. "It's great to have that hard information about how these processes could have gotten started."