Geology Reference
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Figure 5.13. A map showing
the Tolstoj multi-ring basin, the
distribution of its ejecta
(the Goya Formation, G), and
the suggested ring locations
determined by geologic
mapping (from Spudis and
Guest, 1988 ).
Figure 5.14. Average radial extent of continuous
ejecta deposits for fresh impact craters onMercury
and the Moon (after Gault et al., 1975 , copyright
American Geophysical Union).
370 cm/s 2 , or more than twice that of theMoon ' s 162 cm/s 2 ,
a block of ejecta might travel only half the distance from the
primary crater on Mercury.
On the other hand, the bright rays from some craters on
Mercury extend farther than those on the Moon
( Fig. 5.17 ). For example, one crater ray imaged by
MESSENGER is >4,500 km long, compared with the lon-
gest ray from the lunar crater Tycho, which is ~2,000 km
long. This suggests that the mercurian crater rays are not
as degraded as Tycho and might be younger, or that space
weathering effects (which obliterate albedo features) are
less ef cient on Mercury, or that there is some unknown
Analysis of the ejecta around mercurian impact craters
shows that both the continuous and the discontinuous
ejecta are deposited closer to the crater rim than is seen
on the Moon. The continuous ejecta typically occurs
within 0.5 crater diameter, whereas on the Moon the rule
of thumb is deposition within 0.7 - 1 crater diameter
( Fig. 5.14 ). Similarly, secondary craters and crater chains
occur closer to the primary crater rim ( Figs. 5.15 and
5.16 ), with some occurring within the continuous ejecta
deposits. These differences are at least partly the result of
the higher-gravity environment on Mercury ( Fig. 3.28 ).
For example, because the surface gravity on Mercury is
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