Dawn-vir at vesta and ceres: wrap-up of the corrections for the data of the visibile channel

Introduction: The Visible and InfraRed mapping spectrometer (VIR) was carried onboard the NASA Dawn mission [1] to study the spectral properties of the surface of the large asteroid Vesta and the dwarf planet Ceres in the visible to near infrared range. VIR is a mapping spectrometer made of two independent channels, the first operating in the visible (VIR-VIS, 0.25-1.07μm) and the second in the infrared (1.02-5.09μm). Here we focus on the visible data set. Examples are given for Ceres but are valid also for Vesta data. It has been observed that the data produced by VIR-VIS suffered of instrumental effects related to the temperature of the detectors: 1) spectral reddening with the increasing temperature of the visible detector (a charge coupled device CCD); 2) spectral distortion when the temperature of the infrared detector is high [2]. Here we report two corrections which have been implemented to improve the VIR-VIS data [2,3,4]: we first explain how a refinement of the correction dedicated to the global spectral shape – independent of the detectors’ temperatures – of the VIR-VIS spectra has been derived. Then, we detail the correction of the spectral dependence to the detectors’ temperatures, which improved the analysis of the VIR-VIS data. Refinement of the Spectral Correction: VIR data have been previously corrected, using groundbased observations, to remove a spurious positive slope affecting the spectra [5]. Recently, this correction has been refined [3] through a five-steps approach: 1) Ceres ground observations are collected on the basis of their consistency, considering the range where they overlap [6-13]; 2) those full-disk observations are converted in bidirectional reflectance at standard geometry (incidence angle = 30°, emergence angle = 0°, phase angle = 0°), using the Hapke theory [14] and the photometric parameters previously derived for Ceres [15); 3) a smooth average spectrum is computed using the spectra obtained in (2); 4) VIR data are converted at the same standard geometry and an average spectrum is calculated; 5) the last step consists in the calculation of the ratio between the spectrum derived from the ground-based observations (3) and the VIR average spectrum (4). The obtained ratio is a multiplicative factor that is used to correct every spectrum of VIR-VIS, acquired at Ceres but also at Vesta. Spectral Dependency to the CCD Temperature: VIR data are acquired in sequences of several hyperspectral cubes (typically 6 at a time). During a sequence, the CCD is used for several hours and its temperature increase all along, before to cooling down passively when it is not operating, i.e. in between two sequences. This is illustrated by Fig. 1 which displays a map of the hyperspectral cubes acquired during the Ceres Science HAMO (CSH) mission phase. The color scale shows the CCD temperature, the darker the colder. The black arrow shows a sequence within which the CCD temperature starts at 169K and ends at 184K after 2.5 h of operation.