MINERALOGY OF THE SURFACE OF CERES FROM 1 µm ABSORPTION

Introduction: Various minerals have been detected on Ceres and their abundance and spatial distribution has been mapped in the near infrared spectral domain [i.e. 1, 2]. In this work we study the 1 µm absorption to map Ceres mineralogy at a global scale. It is based on the determination of the whole 1 µm absorption from VIR spectra. VIR is the imaging spectrometer on board the NASA mission Dawn in orbit around Ceres and still operating. VIR is composed of two channels:  the VIS channel working in the visible wavelengths between 0.26-1.07 μm and the NIR channel operating in the near infrared between 1.02-5.1 µm. The 1 µm absorption is, indeed, in between the instrument channels.

Method:  To study the 1 micron spectral range, an automatic method to co-register the VIR and NIR channels, where they overlap, has been implemented, thus allowing the study of this band. After the spatial co-registration, a residual difference in the I/F at 1 µm between the two channels can still remain due to the difference in the PSF’s. To avoid this problem, each VIS spectrum is rescaled to the value of the I/F at ~1 µm of the corresponding NIR spectrum. The exact wavelength at which the two VIR channels are matched, inside the overlapping range between 0.92–1.8 µm, is selected for each spectrum on the basis of the best match. The difference between the VIS and NIR reflectance at ~1 µm (ΔR) is usually 1-2% and only about 1% of data show a ΔR>10%. The spectra with the largest mismatch between the two channels are located along the walls of the craters and valleys where the influence of PSF differences is expected to be larger. In this study the data with ΔR>10% are discarded. The maps have global longitudinal coverage, latitudinal coverage from 66°S to 66°N, and a spatial resolution of ~1.86 km/pixel at the equator. 

Each spectrum of the surface is the result of areal or intimate mixing of different minerals, and the resulting spectral reflectance properties are a complex combination of the spectra of each mineral end-member. Several studies have shown that it is possible to explore Ceres mineralogical diversity utilizing specific spectral parameters [1,2].

In order to extract compositional information from the spectra, we have introduced some spectral indices and studied the correlations between them all over the VIR data set. Global maps of the 1 µm signature on Ceres have been derived from various spectral indices, such as band centers, depths, integrated areas, etc.

Results:  A better understanding of the spatial distribution and the content of the different mineral phases can thus be obtained by investigating the spectral characteristics of the 1 µm band.

In general, the VIR spectrum of Ceres is dominated by a broad absorption at about 1.2 µm. It is mostly uniform across the surface, but some differences can be seen in the map of the band parametres, like the band depth. Variation in the band depth could be due to various factors, such as relative abundance and grain size of the present minerals.

However, the spectral properties of the 1 µm band have been related to other spectral parameters in the NIR domain where mineralogy is known. For example, the attribution of the 3.1 µm band to ammonia-bearing species [1] show that this mineral phase dominant where we see for low values of 1 micron band. Infact, the spatial variability in the 1.0 µm band intensity is anticorrelated respect to the 3.1 µm band.

Some terrains show a different behavior of the 1 micron band respect to 3.1 band. This suggests the presence of a distinctive mineralogy.

This work, together with the results of other authors [1,2], completes the global mapping of the Ceres mineralogy.

Acknowledgments: We thank the Italian Space Agency (ASI). The Visible and Infrared Mapping Spectrometer (VIR) was funded and coordinated by the Italian Space Agency, with the scientific leadership of the Institute for Space Astrophysics and Planetology, Italian National Institute for Astrophysics, Italy.

References: [1] E. Ammannito et al. (2016) Science, 353, issue 6303. [2] Carrozzo et al. (2018), Science Advances, vol. 4, 3, e1701645.