Mercury 129
FIGURE 13 Enhanced color mosaic of a portion of the
incoming side of Mercury as viewed byMariner 10.The area at
F has a sharp boundary that coincides with an intercrater plains
boundary and may have a different composition. The relatively
dark and blue unit at D is consistent with enhanced titanium
content. The bright orange unit at B may represent primitive
crustal material, and Kuiper crater at K shows a yellowish color
representing fresh material excavated from a subsurface unit that
may have an unusual composition. (Courtesy of Mark Robinson,
Northwestern Univ., Evanston, Illinois.)
of these craters could be the source of the polar water-ice
deposits.
6.2 Surface Composition
Little is known about the surface composition of Mercury.
If the plains units (intercrater and smooth) are lava flows,
then they must have been very fluid with viscosities similar
to fluid floodbasaltson the Moon, Mars, Venus, and Earth.
FIGURE 14 Photomosaic of the Borealis Basin showing
numerous craters (arrows) that have been flooded by smooth
plains. The largest crater is the Goethe Basin 340 km in
diameter. (Courtesy NASA.)The way in which light is reflected from the surface is very
similar to that of the Moon. However, at comparablephase
anglesand wavelengths in the visible part of the spectrum,
Mercury appears to have systematically higher albedos than
the Moon. Mercurian albedos range from 0.09 to 0.36 at
5 ◦phase angle. The higher albedos are usually associated
with rayed craters. However, the highest albedo (0.36) on
Mariner 10images is not associated with a bright-rayed
crater: It is a floor deposit in Tyagaraja Crater at 3◦N latitude
and 149◦longitude. The lunar highlands/mare albedo ratio
is almost a factor of 2 on the Moon, but it is only a factor
of 1.4 on Mercury. Furthermore, at ultraviolet wavelengths
(58–166 nm) Mercury’s albedo is about 65% lower than the
Moon’s at comparable wavelengths. These differences in
albedo suggest that there are systematic differences in the
surface composition between the two bodies.
A recalibration and color ratioing ofMariner 10im-
ages have been used to derive the FeO abundance, the
opaque mineral content, and the soil maturity over the re-
gion viewed byMariner 10.The probably volcanic smooth
plains have a FeO content of<6 weight percent that is
similar to the rest of the planet imaged byMariner 10.
The surface of Mercury, therefore, may have a more ho-
mogeneous distribution of elements affecting color (e.g.,
morealkali plagioclase) than does the Moon. At least
the smooth plains may be low iron or alkali basalts. Since
the iron content of lavas is thought to be representative of
their mantle source regions, it is estimated that Mercury’s
mantle has about the same FeO content (<6 weight per-
cent) as the crust, indicating Mercury is highly reduced
with most of the iron in the core. In contrast, the esti-
mated FeO contents of the mantle of the bulk Moon is
11.4 %, of Venus and the Earth 8%, and of Mars∼18%.
There are some low-albedo regions with spectral properties