STAND-OFF RAMAN SPECTROSCOPY OF PLANETARY SURFACES

Spontaneous Raman spectroscopy is a powerful molecular spectroscopic technique for detecting both inorganic and organic materials in any state (solid, liquid or gas phase), and for identification of biogenic and abiogenic minerals. Raman spectra of minerals resulting from the Raman active lattice vibration and vibrational modes of a polyatomic species consist of well-defined sharp lines that can be used as fingerprints for the identification of various polymorphs and polyatomic ions and molecules.

In the Raman spectra of solid planetary surfaces, the spectral features are unique in the low-frequency lattice mode region even for various polymorphs of the same mineral composition, and also produce broad spectral features with no lattice modes in glasses of mineral compositions; hence Raman spectra can not only specify various polymorphs but can also differentiate between crystalline and mineral glasses formed from melt or by impact. For example, one can easily distinguish between the calcite, aragonite and vaterite polymorphs of CaCO₃, and various polymorphs of SiO₂ and SiO₂ glass from their respective Raman spectra. If the excitation wavelength approaches an electronically excited state of the molecule, the Raman signals show enhancement due to the resonance Raman Effect in addition to the (1/wavelength⁴) increase of the Raman scattering cross section. Phytopigments that are fundamental components of any surface-dwelling plant or organism on the Earth, excitation with 532 nm laser could result in the excitation of resonance Raman spectra (RRS) of these pigments, especially those containing carotene.

Both in situ Raman spectroscopy and remote Raman spectroscopy using a lander or rover, are currently being proposed for planetary exploration. At the University of Hawaii, the radial range up to 120 m for a field portable remote Raman spectroscopy system has been established. A remote Raman system with Raman imaging capability has potential for determining mineral distribution and composition on a rock surface by comparing a set of Raman images collected for various minerals. The present talk will be focused on current advances in the development of directly-coupled time-resolved remote-Raman (TR³) spectroscopy systems and their potential applications in the Earth and planetary science.