No abstract

Recently published space-based observations of main-belt asteroids with the AKARI telescope provide a full description of the 3-µm band, related to the presence of OH bearing minerals. Here, we use laboratory spectra of carbonaceous chondrites obtained under controlled atmosphere (CI,CM,CO,CV,CR Tagish Lake) to derive spectral metrics related to the water content in the samples. After testing several spectral metrics, we use a combination of band depth at 2.75 µm and 2.80 µm that shows a correlation with [H2O] in the sample determined by TGA, though with a high uncertainty (4 wt. % H2O). This relation is used to determine water content at the surface of large C-complex mainbelt asteroids and discuss the origin of the variability found. On average C-complex Main-Belt Asteroids (MBA) have water contents of 4.5 wt.% (volume average, (1) Ceres excluded), significantly lower than average CM chondrites. The estimated water content for the most hydrated asteroids are lower than those of the most hydrated meteorites, a difference that could be attributed to space-weathering. An anti-correlation is also present between water content and overall spectral slope, which is opposite to expectation from laboratory simulations of space weathering on dark carbonaceous chondrites. This suggests that part of the variability in the surface hydration among the different C-complex asteroids is not due to space-weathering, but to the composition of surface material. When applied to Ceres, the hygrometer presented in this work enables us to estimate that at least 1.22 wt. % of the hydrogen is present in the form of organics. This richness in organics strengthens the connection between Ceres and cometary materials.

Miller Range (MIL) 07687 is a peculiar carbonaceous chondrite officially classified as a CO3. However, it has been found to display unique petrographic properties that are atypical of this group. Moreover, Raman spectra of its polyaromatic carbonaceous matter do not reflect a structural order consistent with the metamorphic history of a type 3 chondrite. As a result, it has been suggested to be an ungrouped C2 chondrite with CO affinities, although it has not been fully excluded as a CO chondrite. The ambiguity of the meteorite’s classification is the motivation behind the present study. We conclude that MIL 07687 is a unique carbonaceous chondrite with possible affinities to CO, CM, and/or some ungrouped carbonaceous chondrites. The difficulty in classifying this meteorite stems from (1) its heavily weathered nature, which interferes with the interpretation of our oxygen (O‐)isotopic measurements; (2) the overlap in the petrographic and O‐isotopic descriptions of various COs, CMs, and ungrouped meteorites in the Meteoritical Society Database. Optical and infrared spectra are consistent with the meteorite’s unequilibrated nature and indicate that it is probably mildly aqueously altered. Despite traces of aqueous alteration having previously been described in MIL 07687, this is the first time that the presence of hydrated amorphous silicates is reported. In fact, our results show that its present hydration is beyond that of most CO3s, less than most CM2s, and comparable to primitive CR2s. Consequently, we support the meteorite’s C2‐ung label, although a CO2 or CM2 classification cannot be fully excluded.