Laboratory Investigations Coupled to VIR/Dawn Observations to Quantify the Large Concentrations of Organic Matter on Ceres

Abstract

Organic matter directly observed at the surface of an inner planetary body is quite infrequent due to the usual low abundance of such matter and the limitation of the infrared technique. Fortuitously, the Dawn mission has revealed, thanks to the Visible and InfraRed mapping spectrometer (VIR), large areas rich in organic matter at the surface of Ceres, near Ernutet crater. The origin of the organic matter and its abundance in association with minerals, as indicated by the low altitude VIR data, remains unclear, but multiple lines of evidence support an endogenous origin. Here, we report an experimental investigation to determine the abundance of the aliphatic carbon signature observed on Ceres. We produced relevant analogues containing ammoniated-phyllosilicates, carbonates, aliphatic carbons (coals), and magnetite or amorphous carbon as darkening agents, and measured their reflectance by infrared spectroscopy. Measurements of these organic-rich analogues were directly compared to the VIR spectra taken from different locations around Ernutet crater. We found that the absolute reflectance of our analogues is at least two orders of magnitude higher than Ceres, but the depths of absorption bands match nicely the ones of the organic-rich Ceres spectra. The choices of the different components are discussed in comparison with VIR data. Relative abundances of the components are extrapolated from the spectra and mixture composition, considering that the differences in reflectance level is mainly due to optical effects. Absorption bands of Ceres’ organic-rich spectra are best reproduced by around 20 wt.% of carbon (a third being aliphatic carbons), in association with around 20 wt.% of carbonates, 15 wt.% of ammoniated-phyllosilicate, 20 wt.% of Mg-phyllosilicates, and 25 wt.% of darkening agent. Results also highlight the pertinence to use laboratory analogues in addition to models for planetary surface characterization. Such large quantities of organic materials near Ernutet crater, in addition to the amorphous carbon suspected on a global scale, requires a concentration mechanism whose nature is still unknown but that could potentially be relevant to other large volatile-rich bodies.

GAUSS - genesis of asteroids and evolution of the solar system

“…The shape of the 3.4-μm band and the lack of an associated 3.25-μm feature could exclude organics with a high content of aromatic carbon such as anthraxolites as main carriers of the features on Ceres, in favour of hydrocarbons rich in aliphatic carbon [25]. Laboratory studies have found organic-rich analogues that could reproduce absorption bands in the VIR spectra [117]. However, the exact nature and concentraton history of the organic matter is still unclear, making this a compelling scientific question for any future space mission to Ceres [117].…”

“…Laboratory studies have found organic-rich analogues that could reproduce absorption bands in the VIR spectra [117]. However, the exact nature and concentraton history of the organic matter is still unclear, making this a compelling scientific question for any future space mission to Ceres [117]. Furthermore, organic compounds possibly also exist on top of Cerealia Facula, the brightest spot located roughly in the middle of crater Occator [26].…”

Organic Matter and Associated Minerals on the Dwarf Planet Ceres

“…The comparison between the spectra of the organic-rich region of Ceres and that of Bennu shows different shape and depth. Ceres has a stronger band with clear minima at about 3.51 and 3.42 µm (see also [25]), while the minima of the organic band at Bennu are at 3.55 and 3.46 µm. The differences between the bands suggest that the mineralogy of the bodies is different and also the organics that characterize the surfaces of Bennu and Ceres have different composition.…”

“…A broad band at about 3.3-3.5 µm is also present in the spectrum, attributed to the contribution of different minerals, primarily carbonate and ammoniated phyllosilicates, but also organics. In particular, aliphatic organics contribute to that signature in the organic rich-regions (hundred km sized areas close to Ernutet crater), where the absorption at 3.3-3.5 dominates the spectrum [10,14,15,25].…”

“…Overall, the very low reflectance indicates the presence of dark and opaque phases that are difficult to identify unequivocally, given their featureless characteristics. Both carbon and magnetite can contribute to the dark material on Ceres and the retrieved abundances are not firmly estimated using only the VIS and IR spectra [17,19,25]. Even if ambiguity remains about the nature of the dark phase, the other minerals clearly identified in the Ceres average spectrum are aqueous alteration products, and imply a longstanding interaction with liquid water.…”