Bright material in Yalode-Urvara basin: in search of organic spectral signatures 

Abstract

Ceres is the largest object in the main asteroid belt, and target of NASA's Dawn mission, which orbited the dwarf planet between 2015 and 2018 with its scientific payload: the VIR spectrometer [1], the FC camera [2], and the gamma-ray and neutron detector (GRaND) [3]. Ceres is an intriguing target from an astrobiological perspective due to its diverse surface composition. It is primarily made up of dark material such as carbon, and features ubiquitous Ca-Mg-carbonates and phyllosilicates across its surface, indicating a past history as an oceanic world. Additionally, there are localized areas presenting Na-carbonates, salts, ammoniated species, water ice, and organic material. An intense spectral feature indicating the presence of aliphatic organics have been observed in the large area of the Ernutet crater [4]. The aim of this work is to identify new areas in which organic materials may be present, and to extract and compare the mineralogical composition of the areas. In particular, we focused on two adjacent craters, Yalode and Urvara, which have already been indicated to host organic material by Rizos et al. [5] and Nathues et al. [6], respectively. Using data collected from Dawn's FC, they identified spots of bright material in Yalode-Urvara basin with peculiar spectra, similar to those of the Ernutet area, therefore with a red-sloped spectrum at visible wavelengths (we called “bs1”, ”bs2” and “bs3” the three spots identified in Yalode [5], and “bsU” the spot identified in Urvara [6]). Then, they analyzed the VIR spectra corresponding to the areas identified with the FC data, finding absorption bands at 3.4 μm deeper than in the average spectrum of Ceres (fig.1). These absorption bands could indicate organic material. However, carbonates also absorb at the same spectral range, making challenging the identification of the components making up the surface. Here we analyse the full Yalode-Urvara area, included the bs, performing modelling on the basis of the Hapke theory [7]. The model is aimed in retrieving abundances and grain size of the endmembers, among which we include the organics indicated in Moroz et al. [8] and de Bergh et al. [9]. This analysis was carried out for all selected acquisitions to determine the mineralogical composition of the areas with peculiar spectra. We report in fig. 2, as an example, the result obtained for the spot bs1 [5]. The modelling show that the best fit involves a combination of medium anthraxolite [8] and semianthracite [9] as organic components. This combination led to a better match for the absorption at 3.4 μm. These organic materials differ from those previously used to explain the strong absorption in the Ernutet crater because of a lower degree of aliphaticity and the presence of an aromatic component. Figure 1. Left: comparison between some spectra in correspondence of the regions identified by Rizos et al. [5] and Nathues et al. [6] and a common spectrum of Ceres (in grey). Spectra are normalized to 2 μm. Right: comparison of the same spectra of the absorption band at 3.4 μm. The continuum calculated as a straight line passing through the extremes of the band was removed and the data were normalized to 3.2 μm. Figure 2. Measured (black) and modeled (red) spectrum of the spot BS1 [5] in terms of radiance factor (I/F). From the left to the right: 3.4 μm absorption band without organics in the model; 3.4 μm absorption band using Ernutet-like organic materials; 3.4 μm absorption band using organics from Moroz et al. [8] which return the best fit and are of a different nature to those of Ernutet. Acknowledgements: This work is supported by the INAF Large Grant "Nature and Evolution of the Organic Material on Ceres" (TERRAE).