Earlier this month, I ran into more than a few articles on news websites with headlines something like these:
“NASA’s Curiosity Rover finds potential building blocks of life on Mars” (CBS News)
“Chasing Signs of Life, Curiosity Rover Discovers Organic Building Blocks on Mars” (Popular Mechanics)
If I hadn’t read the research papers behind the press announcement, I would have thought NASA had just found aliens, or at least promising signs of past life on the red planet! Words like “organic molecules” and “building blocks of life” are thrown around a lot in popular science articles, and are often misleading since the lay reader probably uses these phrases very differently than scientists do. Finding these molecules often has little or nothing to do with life at all, despite their names.
To scientists, an organic molecule is any compound which contains the element carbon. Sometimes people use the word to mean “only molecules that are found in living things,” or which include long chains or rings made of linked carbon atoms, but a molecule doesn’t necessarily have to satisfy either of those criteria to be “organic.” It doesn’t have to be pesticide-free, grass fed, or free-range either (bad joke, I know). It is true that the vast majority of molecules which are found in cells contain carbon, but many of these molecules can be made without life’s help.
Take methane, for instance: this gas which, figured prominently in NASA’s recent Mars announcements, is an organic molecule. One carbon atom bonded to four hydrogen atoms, methane is made by certain bacteria on Earth that live in soils and in the guts of animals, and some scientists think that the first life on Earth might have produced it in a similar way to these microbes. This is why methane is sometimes called a biosignature – finding it on an alien world could be a hint that it supports life – but like most organic molecules, methane can be produced without biology in many ways including volcanic activity and chemical reactions in the atmosphere. In fact, the methane observed on Mars recently which made all those headlines about “signs of life” and “building blocks of life” is thought to come from natural planetary processes unrelated to life. This geologically produced methane is just as “organic” as methane that is produced by life – the word “organic” refers to the chemical structure, not the source of the molecule.
Organic molecules are sometimes called the “building blocks of life” because large molecules like DNA and proteins which make up living things are made of many repeated, smaller organic molecules which snap together like LEGO bricks to build up larger structures. However, calling simple organic molecules the “building blocks of life” is about as correct as calling water molecules the “building blocks of clouds.” Yes, you need water to make a cloud, but there are plenty of non-cloud places where one could go looking for water, and many non-cloud functions which water performs. Organic molecules are the same way: you need them for life, but they are found throughout the universe in places where there’s no life at all. We’ve found small organic molecules on comets, asteroids and inside meteorites, in the atmosphere of Saturn’s freezing moon Titan, and even in the giant clouds of dust and gas which take up the space between stars. The presence of small carbon-containing molecules in these places can be explained by well-understood chemistry and geology, nobody is suggesting that we’ve found life in these places.
So are organic molecules the building blocks of life? Yes – but only sometimes. Everywhere we’ve found organic molecules so far, except for the Earth, they aren’t produced by living things. Rather, they make up giant interstellar dust clouds, atmospheric hazes around moons and other bodies, volcanic gasses, and the surfaces of comets. Perhaps one day we’ll discover organic molecules on an alien world whose presence or properties we can’t explain without life. Until then, life remains the last-resort hypothesis when it comes to explaining the existence of organic molecules throughout our universe. Trust me, if scientists ever do find even questionable evidence of alien life, you can bet that writers won’t have to use misleading buzzwords like “organics” and “building blocks” to get the point across.
The two research papers which prompted the June 7, 2018 NASA press conference on organic molecules on Mars describe the discoveries of seasonal variations in methane abundance in Mars’ atmosphere and organic material preserved in ancient rocks through a process called sulfurization. The papers are published in the journal Science and can be found here and here, respectively.