Discovery of organic molecules in Enceladus’ plumes with possible origin in deep ocean

This article is a summary of the research paper “Macromolecular organic compounds from the depths of Enceladus” which was published in the journal Nature on June 27, 2018. Only the abstract (short summary) is available for free, but many libraries and learning institutions offer resources for accessing scientific publications. 

Large organic molecules were discovered in water ejected from the ice-covered ocean of Saturn’s moon Enceladus. These molecules are macromolecules, meaning that they are made of repeated small molecules that link together.  Familiar macromolecules include DNA, proteins, and many plastics. Until this point, only simple molecules containing small numbers of carbon atoms and weighing less than 200 atomic units (1 au = 1/12 weight of carbon atom) had been discovered in Enceladus’s water plumes.

The newly discovered molecules were discovered by using data from the Cassini spacecraft, which orbited Saturn for over a decade before its mission ended with a controlled crash into Saturn in September of 2017. During its many orbits of Saturn, Cassini flew through the plumes of water which Enceladus shoots into space from under its icy crust to take samples for analysis by two onboard instruments called mass spectrometers. Data from mass spectrometers (“mass spectra”) reveal the weights of molecules in a sample, as well as the fragments those molecules often break down into during measurement. Since Cassini flew through Enceladus’ plumes at high speed, any large molecules that hit the sensor were likely to break up into fragments. Fragmentation also occurs because of how the measurements work: typical mass spectrometers can only make measurements of samples which have been ionized, or given an electrical charge, which involves stressing molecules in ways that often result in fragmentation.

Since fragments often have the same weights as simple molecules, it can be hard to tell if a mass spectrum shows evidence of broken up large molecules or many smaller molecules. Patterns in the Cassini mass spectra allowed scientists to figure out that they were observing large organic molecules broken into fragments. This hypothesis was supported by data from Cassini’s second mass spectrometer, which observed more small organic molecules in the plume when Cassini was flying through higher speeds; since the instrument can only detect small molecules it was able to see more organics during fast flybys, since higher speeds increase the chance of a large organics fragmenting into smaller, measurable ones.

Additionally, it was determined that at least some of observed large organic molecules had ring structures and included oxygen and/or nitrogen atoms in addition to the expected carbon and hydrogen. These insights came from analysis of “peaks”in the mass spectra. Peaks occur when many molecules/fragments in a sample share the same weight. Both the locations of and distances between peaks provide information about the structure of fragments and their parent molecules.

Since the composition of ice grains in the plumes depends on where they formed, the paper authors were able to use the chemical makeup of the ice where organic molecules were found to determine that the organics were not dissolved in Enceladus’ ocean. Rather, they probably came from a thin film of organic material at the ocean-ice boundary. At this boundary, popping bubbles would spray tiny drops of organic material into the plume as an aerosol. The Earth’s ocean has an organic film on its surface which produces aerosol droplets in a similar way. On Earth, organic molecules ride bubbles from deep waters to the organic film at the surface. If this process occurs on Enceladus as well, the organics observed in the moon’s plumes could have originated in the deep ocean.

This summary is intended to explain the actual content of the research paper without biasing the reader in favor of any particular interpretations beyond those presented in the work. Readers who wish to learn more about potential implications and interpretations may find the following articles good places to start:

1. Scientists find evidence of complex organic molecules from Enceladus (Science Daily)
2. Saturn Moon Enceladus is First Alien “Water World” with Complex Organics (Space.com)
3. Complex organic molecules discovered in Enceladus’ plumes could hint at life (Astrobiology Magazine)

Behind the Buzzword: “Building Blocks of Life” and “Organic Molecules”

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.