The importance of comets for the origin of life on Earth has been advocated for many decades. Amino acids are key ingredients in chemistry, leading to life as we know it. Many primitive meteorites contain amino acids, and it is generally believed that these are formed by aqueous alterations. In the collector aerogel and foil samples of the Stardust mission after the flyby at comet Wild 2, the simplest form of amino acids, glycine, has been found together with precursor molecules methylamine and ethylamine. Because of contamination issues of the samples, a cometary origin was deduced from the ¹³C isotopic signature. We report the presence of volatile glycine accompanied by methylamine and ethylamine in the coma of 67P/Churyumov-Gerasimenko measured by the ROSINA (Rosetta Orbiter Spectrometer for Ion and Neutral Analysis) mass spectrometer, confirming the Stardust results. Together with the detection of phosphorus and a multitude of organic molecules, this result demonstrates that comets could have played a crucial role in the emergence of life on Earth.

The provenance of water and organic compounds on Earth and other terrestrial planets has been discussed for a long time without reaching a consensus. One of the best means to distinguish between different scenarios is by determining the deuterium-to-hydrogen (D/H) ratios in the reservoirs for comets and Earth’s oceans. Here, we report the direct in situ measurement of the D/H ratio in the Jupiter family comet 67P/Churyumov-Gerasimenko by the ROSINA mass spectrometer aboard the European Space Agency’s Rosetta spacecraft, which is found to be (5.3 ± 0.7) × 10 ⁻⁴ —that is, approximately three times the terrestrial value. Previous cometary measurements and our new finding suggest a wide range of D/H ratios in the water within Jupiter family objects and preclude the idea that this reservoir is solely composed of Earth ocean–like water.

Several sulphur-bearing species have already been observed in different families of comets. However, the knowledge on the minor sulphur species is still limited. The comet's sulphur inventory is closely linked to the pre-solar cloud and holds important clues to the degree of reprocessing of the material in the solar nebula and during comet accretion. Sulphur in pre-solar clouds is highly depleted, which is quite puzzling as the S/O ratio in the diffuse interstellar medium is cosmic. This work focuses on the abundance of the previously known species H₂S, OCS, SO, S₂, SO₂ and CS₂ in the coma of comet 67P/Churyumov–Gerasimenko measured by Rosetta Orbiter Spectrometer for Ion and Neutral Analysis/Double Focusing Mass Spectrometer between equinox and perihelion 2015. Furthermore, we present the first detection of S₃, S₄, CH₃SH and C₂H₆S in a comet, and we determine the elemental abundance of S/O in the bulk ice of (1.47 ± 0.05) × 10⁻². We show that SO is present in the coma originating from the nucleus, but not CS in the case of 67P, and for the first time establish that S₂ is present in a volatile and a refractory phase. The derived total elemental sulphur abundance of 67P is in agreement with solar photospheric elemental abundances and shows no sulphur depletion as reported for dense interstellar clouds. Also the presence of S₂ at heliocentric distances larger than 3 au indicates that sulphur-bearing species have been processed by radiolysis in the pre-solar cloud and that at least some of the ice from this cloud has survived in comets up the present.

Comets are considered to be some of the most pristine and unprocessed Solar system objects accessible to in situ exploration. Investigating their molecular and elemental composition takes us on a journey back to the early period of our Solar system and possibly even further. In this work, we deduce the bulk abundances of the major volatile species in comet 67P/Churyumov-Gerasimenko, the target of the European Space Agency’s (ESA) Rosetta mission. The basis are measurements obtained with the ROSINA instrument suite on board the Rosetta orbiter during a suitable period of high outgassing near perihelion. The results are combined with both gas and dust composition measurements published in the literature. This provides an integrated inventory of the major elements present in the nucleus of 67P/Churyumov-Gerasimenko. Similar to comet 1P/Halley, which was visited by ESA’s Giotto spacecraft in 1986, comet 67P/Churyumov-Gerasimenko also shows near-solar abundances of oxygen and carbon, whereas hydrogen and nitrogen are depleted compared to solar. Still, the degree of devolatilization is lower than that of inner Solar system objects, including meteorites and the Earth. This supports the idea that comets are amongst the most pristine objects in our Solar system.

Comets contributed to Earth's atmosphere: Models of xenon's origin in Earth's atmosphere require an additional, unknown source that has been a mystery for several decades. Marty et al. measured isotopic ratios of xenon released from comet 67P/Churyumov-Gerasimenko and found that they match the heretofore unknown source. The xenon appears to have been trapped in ice within the comet since before the solar system formed. Comets contributed about a quarter of the xenon on Earth, which constrains the amount of other materials (such as water) delivered to our planet by comets. Science , this issue p. 1069