Analytical Chemistry at the Planetary Frontier

Planetary science depends on chemical measurements made under conditions very different from those in a terrestrial laboratory. This review treats the analytical methods of planetary science as one connected chain, from orbital remote sensing through in-situ measurements by landers and rovers and sample collection and curation, to the micro-analysis of returned material on Earth. Orbital methods, including visible and near-infrared reflectance, thermal emission and gamma-ray and neutron spectroscopy, are discussed first. In-situ techniques are then reviewed, including laser-induced breakdown spectroscopy (LIBS), micro-focus X-ray fluorescence (XRF), deep-ultraviolet Raman and fluorescence spectroscopy, evolved gas analysis with gas chromatography-mass spectrometry (GC-MS) and miniaturized mass spectrometry. Contamination control and curation practices are described, followed by laboratory techniques such as secondary ion mass spectrometry (SIMS and NanoSIMS), atom probe tomography (APT), thermal ionization mass spectrometry (TIMS), synchrotron micro-XRF and X-ray absorption near-edge structure (XANES) spectroscopy. Accuracy, calibration and reference materials are discussed as the link between in-situ and laboratory data, and the role of chemometrics and data fusion is examined. Flight and laboratory instruments are compared in terms of spatial resolution, detection limit, sample consumption and throughput. Active neutron-gamma spectroscopy planned for the Dragonfly mission to Titan is also discussed. This comparison shows that in-situ and laboratory techniques are complementary. In-situ instruments provide context and select samples, while laboratory techniques provide isotopic, molecular and speciation data that cannot yet be obtained in situ.