Mars Express High Resolution Stereo Camera spectrophotometric data: Characteristics and science analysis

Abstract

The High Resolution Stereo Camera (HRSC) on the Mars Express (MEx) spacecraft in orbit about Mars is delivering images of the Mars surface and atmosphere from its high-inclination, elliptical orbit that are intended to cover most of the Mars surface by the end of the mission. These are acquired in four specific spectral passbands that sample the scene at different parts of the extended visual spectrum from near 0.4 μm to about 1μm and in five additional channels, four used for stereo and one nadir, that use the same fifth spectral passband. These channels view from different angles and can be used for photometric and spectral analysis. The data allow a variety of studies of the surface and atmosphere of Mars, but these require detailed understanding of the data characteristics, as there are several effects, including of the observation geometry and the atmosphere, that need to be treated. This article is a report to the science community on the current understanding of the HRSC color data after an initial analysis and also a demonstration of their capabilities for study of Mars, which, from this study, appear to be considerable. We first discuss the characteristics and calibration of the HRSC spectral data. The atmospheric influence on the photometric appearance of the surface materials is explored, and methods of compensating are considered. Reflectances are calculated for surface units by assuming the dark material is basalt and scaling the entire HRSC scene using a laboratory basalt spectrum for the selected dark material area; the results and the correction factors for different orbits are compared. We present several examples of what could become a large volume of future science results utilizing the HRSC spectrophotometric data in conjunction with photo-interpretation. For example, we find relatively few but very important distinct spectral components at the HRSC resolution, including what we interpret as iron oxide-rich material, unoxidized basalt, and polar ice, consistent with past works. The major spectral components are distributed in ways that suggest compositional mixing and the need to revise the current geologic understanding of major regions.