Cometary nuclei contain the least modified material from the formative epoch of our planetary system, and their compositions reflect a range of processes experienced by material prior to its incorporation in the cometary nucleus. Dynamical models suggest that icy bodies in the main cometary reservoirs (Kuiper Belt, Oort Cloud) formed in a range of environments in the protoplanetary disk, and (for the Oort Cloud) even in disks surrounding neighboring stars of the Sun's birth cluster. Photometric and spectroscopic surveys of more than 100 comets have enabled taxonomic groupings based on free radical species and on crystallinity of rocky grains. Since 1985, new surveys have provided emerging taxonomies based on the abundance ratios of primary volatiles. More than 20 primary chemical species are now detected in bright comets. Measurements of nuclear spin ratios (in water, ammonia, and methane) and of isotopic ratios (D/H in water and HCN; 14N/15N in CN and HCN) have provided critical insights on factors affec...

The Chemical Composition of Comets—Emerging Taxonomies and Natal Heritage

Determining the source(s) of hydrogen, carbon, and nitrogen accreted by Earth is important for understanding the origins of water and life and for constraining dynamical processes that operated during planet formation. Chondritic meteorites are asteroidal fragments that retain records of the first few million years of solar system history. The deuterium/hydrogen (D/H) values of water in carbonaceous chondrites are distinct from those in comets and Saturn’s moon Enceladus, implying that they formed in a different region of the solar system, contrary to predictions of recent dynamical models. The D/H values of water in carbonaceous chondrites also argue against an influx of water ice from the outer solar system, which has been invoked to explain the nonsolar oxygen isotopic composition of the inner solar system. The bulk hydrogen and nitrogen isotopic compositions of CI chondrites suggest that they were the principal source of Earth’s volatiles.

https://science.sciencemag.org/content/sci/337/6095/721.full.pdf

The Provenances of Asteroids, and Their Contributions to the Volatile Inventories of the Terrestrial Planets

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.

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67P/Churyumov-Gerasimenko, a Jupiter family comet with a high D/H ratio

Saturn's icy moon Enceladus is emitting a plume of water vapour and ice particles from warm fractures near its south pole known as tiger stripes. This plume material is thought to originate either from subsurface liquids or through the decomposition of ice. Postberg et al. report the first measurements of the compositions of freshly ejected particles, carried out by Cassini's dust detector during plume crossings. Salt-rich ice particles are found to dominate the total mass flux of ejected solids (>99%), which suggests that a salt-water reservoir with a large evaporating surface provides nearly all of the matter in the plume. The discovery of a plume of water vapour and ice particles emerging from warm fractures (‘tiger stripes’) in Saturn's small, icy moon Enceladus1,2,3,4,5,6 raised the question of whether the plume emerges from a subsurface liquid source6,7,8 or from the decomposition of ice9,10,11,12. Previous compositional analyses of particles injected by the plume into Saturn's diffuse E ring have already indicated the presence of liquid water8, but the mechanisms driving the plume emission are still debated13. Here we report an analysis of the composition of freshly ejected particles close to the sources. Salt-rich ice particles are found to dominate the total mass flux of ejected solids (more than 99 per cent) but they are depleted in the population escaping into Saturn's E ring. Ice grains containing organic compounds are found to be more abundant in dense parts of the plume. Whereas previous Cassini observations were compatible with a variety of plume formation mechanisms, these data eliminate or severely constrain non-liquid models and strongly imply that a salt-water reservoir with a large evaporating surface7,8 provides nearly all of the matter in the plume.

A salt-water reservoir as the source of a compositionally stratified plume on Enceladus

For decades, the source of Earth's volatiles, especially water with a deuterium-to-hydrogen ratio (D/H) of (1.558 ± 0.001) × 10^(−4), has been a subject of debate. The similarity of Earth’s bulk composition to that of meteorites known as enstatite chondrites suggests a dry proto-Earth with subsequent delivery of volatiles by local accretion or impacts of asteroids or comets. Previous measurements in six comets from the Oort cloud yielded a mean D/H ratio of (2.96 ± 0.25) × 10^(−4). The D/H value in carbonaceous chondrites, (1.4 ± 0.1) × 10^(−4), together with dynamical simulations, led to models in which asteroids were the main source of Earth's water, with ≤10 per cent being delivered by comets. Here we report that the D/H ratio in the Jupiter-family comet 103P/Hartley 2, which originated in the Kuiper belt, is (1.61 ± 0.24) × 10^(−4). This result substantially expands the reservoir of Earth ocean-like water to include some comets, and is consistent with the emerging picture of a complex dynamical evolution of the early Solar System.

Ocean-like water in the Jupiter-family comet 103P/Hartley 2

Jets of water ice from surface fractures near the south pole of Saturn's icy moon Enceladus produce a plume of gas and particles. The source of the jets may be a liquid water region under the ice shell-as suggested most recently by the discovery of salts in E-ring particles derived from the plume-or warm ice that is heated, causing dissociation of clathrate hydrates. Here we report that ammonia is present in the plume, along with various organic compounds, deuterium and, very probably, Ar-40. The presence of ammonia provides strong evidence for the existence of at least some liquid water, given that temperatures in excess of 180 K have been measured near the fractures from which the jets emanate. We conclude, from the overall composition of the material, that the plume derives from both a liquid reservoir (or from ice that in recent geological time has been in contact with such a reservoir) as well as from degassing, volatile-charged ice. As part of a general comprehensive review of the midsize saturnian satellites at the conclusion of the prime Cassini mission, PI McKinnon and co-I Barr contributed to three review chapters.

Liquid water on Enceladus from observations of ammonia and 40Ar in the plume

/pdf/liquid-water-on-enceladus-from-observations-of-ammonia-and-3k2860evfi.pdf

M. J. Nguyen

Papers

Liquid water on Enceladus from observations of ammonia and 40Ar in the plume

Liquid water on Enceladus from observations of ammonia and 40Ar in the plume (Nature (2009) 460, (487-490))

Ammonia, radiogenic argon, organics, and deuterium measured in the plume of Saturn's icy moon Enceladus