RADIATIVE TRANSFER MODELING OF NEAR-INFRARED REFLECTANCE DATA OF MERCURY

In this study we applied the radiative transfer model of Hapke [1-3] to predict the bidirectional reflectance spectra in a forward modeling approach [4,5] to produce spectral matches to MESSENGER data. To accurately interpret reflectance spectra for airless bodies, the effects of opaque phases (e.g., Fe, Ni, sulfides) must be considered properly. Opaque metals are present as native igneous minerals in meteorites and on planetary surfaces in the form of grains significantly larger than the wavelength of the incident light. However, submicroscopic metal grains are also a key by-product of space weathering [3,7-9] and introduce confounding effects on ultraviolet (UV), visible (VIS), and near-infrared (NIR) spectra as seen in laboratory and spacecraft data for airless bodies. Specifically, nanophase metal particles (<50 nm) decreases the overall reflectance and introduce a strong positive spectral slope across the visible to near-infrared (i.e., spectral “reddening”) [8, 9], whereas microphase metal "Britt-Pieters" particles (50-3000 nm) produce an overall decrease in reflectance only [9, 14, 15].

To address this issue, our model also includes optical constants for opaque metal and mineral phases. The model used for this study was adapted from the work of [17], which uses optical constants for metals (Fe, Ni) to introduce the darkening and/or reddening effects macroscopic and submicroscopic metal on reflectance spectra. We have two main goals of the current study. First, we seek to extend the opaque phases available for spectral mixing and space weathering, which were previously centered on the Fe-rich nature of the Moon (e.g., metallic Fe), including sufides (FeS, MgS, CaS) and graphite (C). Second, we aim to dissociate the microphase and the nanophase metal phases within the model. Results of these models for planetary spectra will be presented.

References: [1] Hapke, B. (1981). [2] Hapke, B. (1993). [3] Hapke, B. (2001. [4] Lucey (1998). [5] Wilcox et al. (2006). [7] Keller and McKay (1997). [8] Pieters et al. (2000). [9] Noble, et al. (2007). [10] Britt and Pieters (1994). [11] Pieters et al. (2000). [12] Noble and Pieters (2003). [13] Nobel et al. (2006). [14] Lucey and Noble (2008). [15] Lucey and Riner (2011). [16] Lawrence and Lucey (2007). [17] Cahill et al. (2015).