130 Encyclopedia of the Solar System
FIGURE 15 Arecibo Observatory 2.4-Ghz radar images of radar features A, B, C, and the polar deposits (PD) in the north and south
hemispheres of Mercury (upper left and right). The resolution is 15 km (0.53◦). The lower left and right images are high-resolution
radar images of two impact craters seen in the top hemispheric rader images. Features A (85 km diameter) and B (95 km diameter) are
two of the brightest (freshest?) radar features on the planet. (Courtesy of John Harmon, Arecibo Observatory, Puerto Rico.)suggesting high opaque mineral areas. These have diffuse
boundaries that may be associated with fractures (Fig. 13).
These areas could be more mafic volcanic pyroclastic de-
posits. The bright-rayed craters on Mercury have a very low
opaque mineral index that may indicate the craters have
excavated into an anothositic crust. Color ratios of lunar
and Mercurian crater rays also suggest that the surface of
Mercury is low in Ti^4 +,Fe^2 +, and metallic iron compared
to the surface of the Moon. From spectroscopic measure-
ments, the FeO content of Mercury’s surface is less than
3%. This is consistent with Mercury’s lower ultraviolet re-
flectivity and smaller albedo contrast. The FeO content is