Figure 7. Representative Mercury Dual Imaging System (MDIS) reflectance spectra (on a logarithmic scale) for Mercury’s northern plains and low-reflectance material (LRM), as well as an average MDIS spectrum for the planet’s northern hemisphere. Laboratory spectra for three “darkening agent” candidates are also shown. Troilite and ilmenite were ruled out as major phases in Mercury’s surface (and as the phase responsible for Mercury’s low reflectance) because of the low Fe and Ti contents measured by MESSENGER’s geochemistry instruments. Graphite—with a dark, red-sloped, and featureless spectrum—is thought to be responsible for Mercury’s low reflectance. Modeling by Murchie et al. (2015) shows that it may be present at ~1 wt% globally, and up to ~5 wt% in Mercury’s darkest color unit (the LRM).