In the absence of human space exploration, we have learned a great deal about the compositions and mineralogy of planetary surfaces from a distance via orbital spacecraft and reflectance spectroscopy. For example, many rock-forming minerals exhibit diagnostic absorption and reflectance features at specific wavelengths of light. What’s more, slight variations in composition can shift these features in a systematic way. Thus, scientists use spectroscopy as a geochemical fingerprint of sorts. Most studies have focused on the visible to near-infrared (0.5–3 microns) and/or the mid-infrared (8–15 microns) wavelength ranges and have calibrated spectral features as a function of composition for major rock-forming minerals.

A new study from Brown University, led by Christopher Kremer, examined the spectral character of the mineral olivine [Mg,Fe]₂SiO₄ within the “cross-over” region of infrared light (4–8 microns). The authors measured the reflectance and thermal emission spectra of 47 natural and synthetic olivine samples that span the olivine solid solution from forsterite (Mg₂SiO₄) to fayalite (Fe₂SiO₄). The authors demonstrate that with increasing iron-content, the 5.6- and 6.0-micron absorption bands systematically shift to longer wavelengths. Using this newly calibrated “cross-over” region of the spectra, the magnesium and iron content of olivine observed from orbital spacecraft can be constrained, similar to previous work in the mid-infrared. The “cross-over” region is not observed from the ground because Earth’s atmosphere is not transparent at these wavelengths. However, this new method may become an additional, useful tool for future space missions. Because olivine is a primary mineral in mantle-derived rocks, this new tool can be used to better constrain the composition of planetary interiors. READ MORE