A comet's tale

Were the molecular building blocks of life on Earth seeded by comets? The evidence is mounting – but there has been an important missing piece; ribose. Here we find out how making a comet in a lab has provided tantalising clues to the understanding of life on Earth.

The evolutionary origin of the genetic material employed by all living organisms on Earth has been impossible to elucidate. Ribonucleic acid, RNA, has been assumed to precede deoxyribonucleic acid but the evolutionary origin of RNA itself – and thereby the origin of life – remained unknown. We have now shown for the first time that ribose, the sugar that provides the central molecular subunit of the RNA genetic material in all living organisms, may have formed in cometary ices. This finding does not explain the molecular origins of the appearance of life on Earth, but it provides important molecular insights into the underlying chemical and astrophysical processes.

As a first step of the new spectacular finding, an artificial comet was produced at the Institut d'Astrophysique Spatiale in Paris-Orsay. By placing a representative mixture of water (H2O), methanol (CH3OH) and ammonia (NH3) in a high vacuum chamber at -200°C, the astrophysicists of the international team, Pierre de Marcellus and Louis le Sergeant d’Hendecourt, simulated the formation of dust grains coated with ice, the raw material of comets. This material was irradiated with UV photons, analogous to the molecular clouds where these grains form. The sample of the artificial cometary ice was then warmed to room temperature, analogous to comets when they approach the Sun. Its composition was analysed at the Institut de Chimie de Nice (CNRS/Université Nice Sophia Antipolis), optimising an extremely sensitive and accurate method called multidimensional gas chromatography coupled with time-of-flight mass spectrometry.

[caption id="attachment_54059" align="alignnone" width="600"] Ultraviolet processing of pre-cometary ices reproduces the natural evolution of interstellar ices observed in molecular clouds, leading to the formation of sugar molecules. Credit: L. Le Sergeant d'Hendecourt (CNRS) / NASA, ESA, and the Hubble Heritage Team (STScI/AURA).[/caption]

Previous experiments on artificial cometary ices of similar type provided information on the interstellar formation of a variety of amino acids¹ and aldehydes². By optimising the analytical procedure, the new experiments allowed for the detection of several sugars, including ribose and sister aldopentoses – arabinose, xylose, and lyxose³. The diversity and relative abundances of the aldopentoses and the related molecules suggest that they were formed via an interstellar formose-type reaction from formaldehyde, a molecule found in space and on comets that forms in large quantities from methanol and water. The ANR-, CONACYT- and CNES-funded collaboration involves scientists at the Institut de Chimie de Nice, the Institut d'Astrophysique Spatiale (CNRS/Université Paris-Sud), the SOLEIL synchrotron, the Aarhus University (Denmark), and the Universidad Autónoma del Estado de Morelos (Mexico). Our international collaboration proposes the first realistic scenario for the formation of this key compound, ribose, which had never been detected in meteorites or cometary ices until now. Our findings, which shed new light on the emergence of life on Earth, were published in the journal Science dated 8 April 2016.

As a first step of the new spectacular finding, an artificial comet was produced at the Institut d'Astrophysique Spatiale in Paris-Orsay.

The genetic material of all living organisms on Earth, as well as of viruses, is made up of nucleic acids, DNA and RNA. RNA, which is considered more primitive, is thought to have been one of the first molecules characteristic of life to appear on Earth. Scientists have long wondered about the origin of these biological compounds. Some of them believe that the Earth was seeded by comets or asteroids that contained the basic building blocks needed to form such molecules. And indeed several amino acids (the components of proteins), nitrogenous bases (one of the components of nucleic acids), and aldehydes have already been found in meteorites, as well as in artificial comets produced in the laboratory.

[caption id="attachment_54060" align="alignnone" width="620"] Simulation chamber for pre-cometary ices at the Institut d’Astrophysique Spatiale (IAS), Paris-Orsay. Ribose (and related molecules such as arabinose, lyxose and xylose) have been detected in pre-cometary ice analogs using multidimensional gas chromatography. Ribose forms in the icy mantles of dust grains from simple precursor molecules (water, methanol and ammonia) under high-energy radiation. Ribose makes up the backbone of ribonucleic acid (RNA), thought to be the genetic material of the first living organisms.
Credit: Uwe Meierhenrich.[/caption]

However, ribose, the other key component of RNA, had never yet been detected in extraterrestrial material or created in the laboratory under 'astrophysical' conditions. Now, by simulating the evolution of the interstellar ice making up comets, we have successfully obtained ribose, a key step in understanding the origin of RNA – and therefore of life. Although the existence of ribose in real comets remains to be confirmed, this discovery completes the list of the molecular building blocks of life that can be formed in interstellar ice. It also lends further support to the theory that comets are the source of the organic molecules that made life possible on Earth, and perhaps elsewhere in the Universe.

We have now shown for the first time that ribose, the sugar that provides the central molecular subunit of the RNA genetic material in all living organisms, may have formed in cometary ices

The detection of aldehydes in pre-cometary ice analogs is furthermore coherent with our data received from the Rosetta mission? that successfully deposited the Philae Lander on the nucleus of comet 67P/Churyumov-Gerasimenko in November 2014?. The COSAC instrument, which is a GC-MS device specifically designed for the in situ characterisation of chiral organic molecules, identified 16 organic species including aldehydes and N-bearing molecules in the cometary ice?. Refractory organic species such as amino acids and aldopentoses including ribose were not identified by Rosetta’s COSAC instrument because this instrument employed, after landing on the cometary nucleus, its ‘sniffing’ mode that was sensitive to volatile molecules only. Refractories such as amino acids and sugars are not volatile; for their detection by Philae-COSAC, a sample of the cometary nucleus would have been required. This sample was impossible to obtain due to the unexpected ‘vertical’ landing of Philae. The complexity of comet nucleus chemistry implies that early solar system chemistry fostered the formation of prebiotic material in significant concentrations.

Authors:

Uwe J. Meierhenrich is Professor at the University Nice Sophia Antipolis. After completing his Ph.D. at the University of Bremen, he identified amino acids in artificial comets at the Max Planck Institute for Solar System Research in preparation for the cometary Rosetta mission.

Cornelia Meinert is a Chargé de Recherche at the CNRS. Her current research focuses on the origin of the chirality of biomolecules.

References: 1. Muñoz Caro G.M., Meierhenrich U.J., Schutte W.A., Barbier B., Arcones Segovia A., Rosenbauer H., Thiemann W.H.-P., Brack A., Greenberg J.M.: Amino acids from ultraviolet irradiation of interstellar ice analogues. Nature 416 (2002), 403-406. 2. de Marcellus P., Meinert C., Myrgorodska I., Nahon L., Buhse T., Le Sergeant d’Hendecourt L., Meierhenrich U.J.: Aldehydes and sugars from evolved precometary ice analogs: Importance of ices in astrochemical and prebiotic evolution. Proc. Natl. Acad. Sci. USA 112 (2015), 965-970. 3. Meinert C., Myrgorodska I., de Marcellus P., Buhse T., Nahon L., Hoffmann S.V., Le Sergeant d’Hendecourt L., Meierhenrich U.J.: Ribose and related sugars from ultraviolet irradiation of interstellar ice analogs. Science 352 (2016), 208-212. 4. Meierhenrich U.J.: Comets and their Origin - The Tool to Decipher a Comet. Wiley-VCH, Weinheim (2015). 5. Myrgorodska I., Meinert C., Martins Z., Le Sergeant d’Hendecourt L., Meierhenrich U.J.: Molecular chirality in meteorites and interstellar ices, and the chirality experiment on board the ESA cometary Rosetta mission. Angewandte Chemie International Edition 54 (2015), 1402-1412. 6. Goesmann F., Rosenbauer H., Bredehöft J.H., Cabane M., Ehrenfreund P., Gautier T., Giri C., Krüger H., Le Roy L., MacDermott A.J., McKenna-Lawler S., Meierhenrich U.J., Munoz Caro G.M., Raulin F., Roll R., Steele A., Steininger H., Sternberg R., Szopa C., Thiemann W., Ulamec S.: Organic compounds on comet 67P/Churyumov-Gerasimenko revealed by COSAC mass spectrometry. Science 349 (2015), 497.