^Zhiyong Xiao, Robert G. Strom, David T. Blewett, Paul K. Byrne, Sean C. Solomon, Scott L. Murchie, Ann L. Sprague, Deborah L. Domingue, Jörn Helbert, 2013. Dark spots on Mercury: A distinctive low-reflectance material and its relation to hollows. Journal of Geophysical Research Planets. doi.org/10.1002/jgre.20115