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IntroductionCeres, the largest object in the Solar System asteroid belt, has a complex geological and chemical history and experienced extensive water-related processes (1). Its surface is globally characterized by dark materials, phyllosilicates, ammonium-bearing minerals, carbonates, water ice, and salts (2-5) and the presence of carbon on a global scale, up to 20 wt.%, has also been inferred (6). Organic matter was unambiguously identified in the region of Ernutet crater (3) in the form of long-chain aliphatic organics (Aliphatic Organics). The origins and persistence of these organics are under debate due to the intense aliphatic signature and radiation levels in Ceres' orbit, which could lead to their destruction, hindering detection. To investigate this, we conducted laboratory experiments to replicate how the signature of the organic-rich regions would degrade by fast ions, ultraviolet radiation, and neutral atoms in conditions that simulate the environment of Ceres. Our experiments give hints on the lifetime of aliphatics on the Cerean surface, allowing us to constrain the age and mechanisms promoting the preservation of aliphatic material following exposure.Experiments- For simulants, we used a mixture of minerals, reproducing the average Ceres surface mineralogy and undecanoic acid (C10H21COOH) that shows a strong 3.4 m feature and that is thus spectrally representative of the Aliphatics compounds on the Ceres surface. The organic was then added to the mineral mixture. The experimental results were then scaled to the actual radiation environment estimated at the orbit of Ceres.UV irradiation experiments were performed at INAF-Osservatorio Astrofisico di Arcetri (Firenze, Italy). The Ceres mixture was exposed to a UV-enhanced Xe-lamp for about 7 hours, and the organic degradation process was monitored in real time. The 3.4 m band area decreased as the irradiation fluence increased, resulting in a very short lifetime. However, only the uppermost layers of the surface are affected by the destruction induced by UV photons.Exposure of Ceres mixture sample to hydrogen atoms was performed at INAF-Osservatorio Astronomico di Capodimonte (Napoli, Italy), resulting in a decrease of the organic band, suggesting that hydrogen atoms may also contribute to the degradation of organics on Ceres.Energetic ion irradiation experiments were performed at INAF Osservatorio Astrofisico di Catania (Italy). Three analogs in the form of compact pellets were irradiated under high vacuum at room temperature by 200 keV He+, N+, and H+ ions respectively. The results demonstrate a fast decrease in intensity of the 3.4 m feature in all irradiated samples.Results- Our experiments show that the radiation environment on the Cerean surface can destroy the 3.4 m band of Aliphatic Organics even if mixed with clays and carbonates, commonly assumed to preserve organic from degradation. Moreover, UV processing and H atom cause the alteration of Aliphatic Organics on very short timescales (from few days to 105 years), but the damage is limited to a few hundreds of nanometers to a few micrometers of the surface. Fast ions penetrate surfaces much deeper than UV photons and H atoms, and thus induce the destruction of the 3.4 m band to much larger depths. From our results, the presence of the strong aliphatic feature at Ernutet crater implies exposure on the surface 22% in the mixtures (7,8)).REFERENCES1 De Sanctis, et al., Nature 536, 5457 (2016). doi:10.1038/nature182902 Russell, et al. Science 353, 1008-1010 (2016). doi:10.1126/science.aaf42193 De Sanctis, et al., Science 355, 719-722 (2017). doi:10.1126/science.aaj23054 Ammannito, et al., Distribution of phyllosilicates on the surface of Ceres. Science 353, aaf4279 (2016). doi:10.1126/science.aaf4275 De Sanctis, et al., Characteristics of organic matter on Ceres from VIR/Dawn high spatial resolution spectra. Mon. Not. R. Astron. Soc. 482, 24072421 (2019). doi:10.1093/mnras/sty27726 Marchi, et al., Nat. Astron. 3, 140145 (2019). doi:10.1038/s41550-018-0656-07 . Vinogradoff, et al., Minerals 11, 7 (2021). doi:10.3390/min110707198 . Kaplan, R. E. et al., Geophys. Res. Let. 45, 5274-5282 (2018). doi:10.1029/2018GL077913
Sanctis et al. (Wed,) studied this question.