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Detailed mapping of the distribution of water and semiheavy water in the coma of Halley-type comet 12P implies a D/H ratio that is consistent with that of Earth’s water, indicating a common heritage.

In situ measurements from the Rosetta spacecraft reveal the presence of atomic emissions close to comet 67P’s nucleus. Such emissions are due to dissociative excitation of molecules by the interaction with the solar wind, identifying them as a form of aurora.

Centaur 29P/Schwassmann–Wachmann 1 exhibits a fascinating outgassing pattern in JWST observations, with compositional heterogeneities that may be related to the bilobate nature of its nucleus. The detection of CO and CO₂ isotopologues is also reported.

High-resolution ultraviolet and optical spectra of a large sample of comets show that Fe i and Ni i lines are ubiquitous, even when the comets are far from the Sun.

Observations of water ice on the surface of comet 67P/Churyumov–Gerasimenko show the ice appearing and disappearing in a cyclic pattern that follows local illumination conditions, providing a source of localized activity and leading to cycling modification of the ice abundance on the surface.

When the Philae lander bounced on the surface of comet 67P/Churyumov–Gerasimenko, it exposed primitive icy-dust material within cometary boulders; the intrinsic strength and porosity of this material is reported.

The largest asteroid of the Solar System, (1) Ceres, has been thought to have an icy surface; here it is observed to be emitting water vapour.

A report of the chemical composition of two distinct fragments of comet 73P/Schwassmann–Wachmann 3. The fragments are similar in composition, in contrast to the chemical diversity of comets and contrary to the expectation that short-period comets should show strong compositional variation with depth.

Diurnal and seasonal temperature variations of comet 67P’s nucleus are monitored by Rosetta’s imaging spectrometer VIRTIS during two months in 2014. The nucleus appears thermally homogeneous, with the temperature fluctuations mainly controlled by self-heating and heliocentric distance.

A refined analysis of infrared observations of comet 67P/Churyumov–Gerasimenko from the VIRTIS instrument on board the Rosetta spacecraft has revealed the presence of aliphatic organic molecules on the comet nucleus.

Interstellar comet Borisov has thus far looked very much like a Solar System comet in terms of its volatile content, but with new data from ALMA that show robust detections of CO and HCN, it is clear that Borisov is rich in CO, helping to pinpoint its origin.