The VIRTIS (Visible, Infrared and Thermal Imaging Spectrometer) instrument on board the Rosetta spacecraft has provided evidence of carbon-bearing compounds on the nucleus of the comet 67P/Churyumov-Gerasimenko. The very low reflectance of the nucleus (normal albedo of 0.060 ± 0.003 at 0.55 micrometers), the spectral slopes in visible and infrared ranges (5 to 25 and 1.5 to 5% kÅ ⁻¹ ), and the broad absorption feature in the 2.9-to-3.6–micrometer range present across the entire illuminated surface are compatible with opaque minerals associated with nonvolatile organic macromolecular materials: a complex mixture of various types of carbon-hydrogen and/or oxygen-hydrogen chemical groups, with little contribution of nitrogen-hydrogen groups. In active areas, the changes in spectral slope and absorption feature width may suggest small amounts of water-ice. However, no ice-rich patches are observed, indicating a generally dehydrated nature for the surface currently illuminated by the Sun.

Ammonium salts on comet 67P: The distribution of carbon and nitrogen in the Solar System is thought to reflect the stability of carbon- and nitrogen-bearing molecules when exposed to the heat of the forming Sun. Comets have a low nitrogen-to-carbon ratio, which is contrary to expectations because they originate in the outer Solar System where nitrogen species should be common. Poch et al. used laboratory experiments to simulate cometary surfaces and compared the resulting spectra with comet 67P/Churyumov-Gerasimenko. They assigned a previously unidentified infrared absorption band to nitrogen-containing ammonium salts. The salts could contain enough nitrogen to bring the comet's nitrogen-to-carbon ratio in line with the Sun's. Science , this issue p. eaaw7462

INTRODUCTION: The surface of the dwarf planet Ceres is known to host phyllosilicate minerals, but their distribution and origin have not previously been determined. Phyllosilicates are hydrated silicates, and their presence on the surface of Ceres is intriguing given that their structure evolves through an aqueous alteration process. In addition, some phyllosilicates are known to bear NH ₄ , which places a constraint on the pH and redox conditions during the evolution of Ceres. We studied the distribution of phyllosilicates across the planet’s surface to better understand the evolutionary pathway of Ceres. RATIONALE: Using the data acquired by the mapping spectrometer (VIR) onboard the Dawn spacecraft, we mapped the spatial distribution of different minerals on Ceres on the basis of their diagnostic absorption features in visible and infrared spectra. We studied the phyllosilicates through their OH-stretch fundamental absorption at about 2.7 µm and through the NH ₄ absorption at about 3.1 µm. From our composition maps, we infer the origin of the materials identified. RESULTS: We found that Mg- and NH ₄ -bearing phyllosilicates are ubiquitous across the surface of Ceres and that their chemical composition is fairly uniform. The widespread presence of these two types of minerals is a strong indication of a global and extensive aqueous alteration—i.e., the presence of water at some point in Ceres’ geological history. Although the detected phyllosilicates are compositionally homogeneous, we found variations in the intensity of their absorption features in the 3-µm region of the reflectance spectrum. Such variations are likely due to spatial variability in relative mineral abundance (see the figure). CONCLUSION: The large-scale regional variations evident in the figure suggest lateral heterogeneity in surficial phyllosilicate abundance on scales of several hundreds of kilometers. Terrains associated with the Kerwan crater (higher concentration of phyllosilicates) appear smooth, whereas the Yalode crater (lower concentration of phyllosilicates) is characterized by both smooth and rugged terrains. These distinct morphologies and phyllosilicate concentrations observed in two craters that are similar in size may reflect different compositions and/or rheological properties. On top of this large-scale lateral heterogeneity, small-scale variations associated with individual craters could result from different proportions of mixed materials in a stratified upper crustal layer that has been exposed by impacts. Variations associated with fresh craters, such as the 34-km-diameter Haulani, indicate the presence of crustal variations over a vertical scale of a few kilometers, whereas much larger craters, such as the 126-km-diameter Dantu, suggest that such stratification may extend for at least several tens of kilometers. Abundance maps.: Qualitative maps of the abundances of ( top ) phyllosilicates and ( bottom ) NH ₄ , based on the depth of their absorption features. The two maps have a similar global pattern, although they differ in some localized regions such as Urvara. The scale bar is valid at the equator.