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)