Patrick Pinet
Biography & lecture abstracts
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Lecture Mineralogy-1: Principles of spectroscopy
Reflectance spectroscopy in the 0.4-3 micron spectral domain is particularly adapted to remote-sensing planetary exploration given the number of mechanisms that cause absorptions in this wavelength region. These mechanisms involve electronic transitions within and between atoms (crystal field, metal-metal intervalence charge transfer, oxygen-metal charge transfer) as well as electronic transitions in molecular orbitals and vibrational transitions in molecules and crystals. Theoretical models from the solid state physics are used to interpret these spectra. However, laboratory measurements are absolutely needed for the controlled interpretation of remotely acquired spectra and very comprehensive laboratory databases have been generated along the years to do so. Beyond the detection of the presence of minerals, another aspect which is mandatory for approaching a quantitative characterisation of the mineralogy, addressing the composition and abundance of the minerals, especially in the case of complex mixtures, relies on progress recently made in the field of non-linear deconvolution of complex spectra. As an example, a powerful technique called Modified Gaussian Model (MGM) permits under some conditions to deconvolve overlapping absorptions of mafic mineral spectra into their fundamental absorption components, each Gaussian function used, directly accounting for an electronic transition.
Lecture Mineralogy-2: Mineralogy and Mars evolution
Relying on the fundamental principles and methodologies addressed earlier, it is now possible to model simple and complex mafic mineralogies including binary and ternary mixtures, for a large range of grain sizes. Accordingly, we will describe the progressive deciphering of the martian mineralogy as it is currently understood. Among many observations based on orbital hyperspectral coverages and in situ spectrophotometric measurements carried out at the Mars Exploration Rovers (Spirit and Opportunity) landing sites (Gusev crater and Meridiani), detailed analysis of reflectance spectra of mafic silicates, serpentines, sulfates, phyllosilicates (clay minerals) and carbonates appear extremely promising for studying the geological evolution of Mars. As an example, the occurrence of serpentine-bearing rocks on Mars restricted to the Noachian period is a strong indication in favour of the existence of early hydrothermal alteration of ultramafic rocks. This has obvious implications about the past and present environmental formation conditions and thus for search for traces of extinct life on Mars.
Lecture Mineralogy-3: Planetary regolith
This lecture will address the optical characterisation of the planetary regolith surface properties from theoretical, experimental and orbital spectrophotometry, with a special emphasis toward a multiscale understanding of the optical properties and their implications for the martian surface. Once the fundamental principles of radiative transfer modeling will have been introduced and reviewed, we will explore some applications to Mars. Given the huge amount of knowledge gained from the different spectroscopic surveys performed from orbit with the fleet of recent missions to Mars (e.g., Mars Global Surveyor, Mars Express, Mars Reconnaissance Orbiter, etc) and from the various in-situ measurements at the MER landing sites, we will focus on the Gusev crater to demonstrate how efficient the combination of remote sensing techniques can be for the purpose of characterising the physical surface properties of the martian regolith, with the propagation of the in-situ knowledge to large expanses of Mars only monitored from space. Aware of the intrinsic complexity associated with the spectrophotometric signal returned by the martian surface, which is influenced by both the surface and atmospheric contributions, we will then turn our attention towards the notion of surface changes and their possible detection.
Lecture Mineralogy-4: Optical properties and surface changes on Mars
Historically, the terminology 'variable features' on the surface of Mars was used to define albedo patterns on Mars that appeared, disappeared, or changed shape as a function of time, as seen on Mariner-9 images. They are attributed to the interaction of the atmosphere with the surface and are considered to represent erosion, deposition, and/or repositioning of sand and dust by the wind. Variable features associated with topographic landforms, such as craters, are called wind streaks and are thought to represent the prevailing wind direction at the time of their formation. However, the detection of albedo features is sensitive to many variables, including the imaging system on the spacecraft, the illumination and viewing geometry and the spectral range through which the image is acquired. As recently demonstrated, the optical instruments onboard Mars Express (ESA) and MRO (Mars Reconnaissance Orbiter from NASA) have allowed observations revealing the complexity associated with the colour observations produced under different geometry and illumination conditions, arising from the interplay between shade, shadow and the presence of a scattering atmosphere. This calls for a more advanced strategy than what has been undertaken so far for detecting and monitoring systematically martian variable features. This strategy would be based on the analysis of multi-angular observations repeated through time under close geometry and illumination conditions. This should be the "reference" case for disentangling surface changes through time (i.e., true variable features) from optical changes induced by the surface properties (e.g., subpixel roughness) and/or complex photometric effects related to the presence of a mixed granular rocky surface layer.