124 Encyclopedia of the Solar System
(a)FIGURE 6 (a) Mercury as viewed byMariner 10on its first
approach in March 1974. (b) Mercury’s opposite hemisphere
viewed byMariner 10as it left the planet on the first encounter,
and (c) the southern hemisphere viewed on the second
encounter in September 1974. (Courtesy of NASA.)
diameters between∼130 and 310 km Mercurian craters
have an interior concentric ring, and at diameters larger
than about 300 km they may have multiple inner rings. The
freshest craters have extensive ray systems, some of which
extend for distances over 1000 km. For a given rim diam-
eter, the radial extent of Mercurian continuous ejecta is
uniformly smaller by a factor of about 0.65 than that for the
Moon. Furthermore, the maximum density of secondary
impact craters occurs closer to the crater rim than for sim-
ilarly sized lunar craters: The maximum density occurs at
about 1.5 crater radii from the rim of Mercurian primaries,
whereas the maximum density occurs at about 2–2.5 crater
(b)(c)FIGURE 6 (Continued)