Glycine, an Amino Acid and Other Prebiotic Molecules in Comet 67P/Churyumov-Gerasimenko

Kathrin Altwegg 

Author: Kathrin Altwegg and the ROSINA Team 

Affiliation: Physikalisches Institut, University of Bern, Switzerland.

By now it is an established fact that comets contain the most primitive material of all solar system bodies. In situ results from the Giotto flyby at comet Halley in 1986 and remote sensing in various wavelength ranges established the presence of many organic molecules in the coma of comets. The importance of comets for the origin of life on Earth has been the topic of many discussions in the past [1]. Among the key ingredients for life as we know it are amino acids and phosphorous. Many primitive meteorites contain amino acids. However most of them are formed by aqueous alterations [2,3]. Traces of amino acids were detected in samples from the Stardust mission to comet Wild 2 [4]. However, there always remained some doubts that these amino acids were actually cometary as the extraction and analysis of the material from the aerogel and aluminium frame always involves either hot water or even acids, which could then readily form amino acid on Earth.

In order to investigate the composition of the organics and to assess the importance of comets for the origin of terrestrial life the European Space Agency ESA launched in 2004 the spacecraft Rosetta towards comet 67P/ Churyumov-Gerasimenko . This comet belongs to the Jupiter family with an aphelion at 5.5 AU and a perihelion at 1.25 AU. Its orbital period is 6.5 y. In order to match the comet’s orbit Rosetta had to flyby three times the Earth and once Mars to arrive at the comet more than 10 years after launch in August 2014. Subsequently, Rosetta flew with the comet around the Sun from 3.6 AU to perihelion and out again to 3.8 AU, sometimes as close as 10 km from the comet centre. This allowed a thorough investigation of the comet from up close during the different phases of its orbit.

On board Rosetta was the ROSINA (Rosetta Orbiter Sensor for Ion and Neutral Analysis) suite consisting of two mass spectrometers (DFMS and RTOF) and the cometary pressure sensor COPS [5]. These sensors were built to analyse the composition of the cometary atmosphere along its path around the Sun, encountering very low densities for large heliocentric distances to much more violent outgassing during perihelion.

ROSINA measured almost continuously during more than two years. Most of the time, water was the dominant component of the cometary coma. However, due to its peculiar shape and the tilted rotation axis of the comet, the coma was very heterogeneous and varying along the orbit. Apart from water, ROSINA identified many more simple molecules like CO, CO₂, HCN, CH₄ and NH₃. But it detected also many complex organics like aliphatic carbon chains, alcohols with up to five C atoms and amines [6]. Most surprisingly it detected abundant O₂ [7]. O₂ is very reactive and was believed to have been non-existent in the protosolar nebula. Very few detections of O₂ outside the Earth have been made so far. The very good correlation with water led to the finding, that O₂ was most probably formed in the presolar stage due to radiolysis of water ice and that the water ice survived the solar system formation unchanged.

In March 2015 Rosetta performed a close flyby over the comet surface of just 15 km from the comet centre. During this flyby, dust production was high. In the mass spectra of ROSINA DFMS from this flyby an analysis of the mass spectrometry data of ROSINA DFMS revealed two mass peaks at mass 75 Da, one of which was identified as coming from the amino acid glycine. The exact mass of glycine is 75.0315 Da. There are several isomers on the exact same mass. In mass spectrometry, where ionization of the neutrals is done by electron impact, isomers can be distinguished by their fragmentation pattern as molecules are not only ionized to yield the parent ion, but also dissociate into ionized fragments according to the structure of a molecule. All of the isomers of glycine could be ruled out by looking at the specific fragmentation pattern from the electron impact ionisation in the ion source of DFMS.

Figure 1: (click on the image to view with higher resolution) a mass spectrum of mass 75 Da, taken by ROSINA/DFMS on March 28, 2015, integrated over 160 s at a distance of ~20 km from the comet.

Figure 1 shows a sample mass spectrum at 75 Da. The number of ionized particles registered on the detector is given as a function of the position on the detector which corresponds to m/z. The mass resolution of DFMS m/Δm is ~9000 at FWHM for mass 28 and decreases with increasing mass. Also on mass 75 Da we find C₃H₇O₂ (75.0441 Da) which might be a fragment of propylene glycol (C₃H₈O₂) or any of its isomers or/and of even heavier species like butanediol (C₄H₁₀O₂). Only a thorough analysis of all fragments can identify the parent of C₃H₇O₂. Details on the data analysis for ROSINA DFMS can be found in Ref.[6].

To detect glycine in the coma of 67P was quite surprising as glycine has a sublimation temperature of 140°C (8), a lot higher than the comet surface (9). Analysis of the flyby revealed that the density of glycine did not follow the expected 1/r² behaviour, which led to the conclusion that glycine sublimated from dust grains in the coma, which can become much hotter due to their small sizes of a few μm and their low albedo of a few % [10].

The way to form glycine on dust grains has been investigated by [11-13]. It can be formed from the precursor molecule methylamine which was also found in the mass spectra of DFMS together with CO₂. It is up to now the only amino acid where a path for formation is known without involving liquid water. It is therefore not surprising that a search for other amino acids like e.g. alanine was unsuccessful, as the comet most probably never had liquid water.

As glycine is probably mostly on dust grains and its sublimation temperature is high, it is not possible to determine the glycine abundance in the nucleus. The abundance in the coma varies relative to water between 0 and 0.0025. Glycine is not always detected in the spectra of DFMS. We preferentially see a signal from glycine during the perihelion passage between spring 2015 and September 2015 and only if the spacecraft was close enough to the comet.

The precursor molecules methylamine and ethylamine are seen in the mass spectra only when glycine also is detected. The three molecules seem to be closely related which is not surprising. Chemical models show that glycine could form on dust grains via three radical-addition mechanisms at temperatures from 40-120 K [11] which is compatible with temperatures in hot cores. Glycine can also be formed involving photochemistry and CO₂ [13]. In both cases methylamine is part of the process.

Another important species for living organisms is phosphorous found in adenosine triphosphate (ATP), in the backbone of DNA and RNA, and in cell membranes. The phosphorous atom with a mass of 30.9732 Da was detected by DFMS already in October 2014. However, the search for the parent (PH3, PO, PN or HCP) was unsuccessful although these species have been detected in the interstellar medium [14-17]. This is mostly due to overlaps in the mass spectra with other species, very often with abundant sulphur bearing species.

Many of the molecules found by ROSINA DFMS in the coma of comet 67P are compatible with the idea that comets delivered key molecules for prebiotic chemistry throughout the solar system and in particular to the early Earth [18] increasing drastically the concentration of life-related chemicals by impact on a closed water body. The fact that glycine was most probably formed on dust grains in the presolar stage also makes these molecules somehow universal, which means that what happened in the solar system could probably happen elsewhere in the Universe.

This article is based on our work published in 'Science Advances', 2016 [19].

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