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 7 ]+ l# l/ |4 s' V6 _2 F3 E
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The most heavily cratered terrains on Mercury exhibit
" n6 O0 q& y* t2 L" C/ b3 I% La lower density of craters smaller than about 100 km in diameter than on the : M7 R8 x8 e! s& S( C
Moon, a deficit that has been attributed to resurfacing by formation of ancient 8 B+ ~1 b3 ^1 n+ f
intercrater plains. Simone Marchi et al. used a crater areal density map
! I6 k9 p$ }) Z* o1 y: _based on data from the MESSENGER spacecraft (the colour-coded foreground on
1 |% Z: Y g8 J$ G0 z: `! ?: ocover, with a global surface mosaic in the background) to locate the oldest }% v* F8 A: h0 H5 k
surfaces on Mercury and interpret the crater populations in the framework of a ! S& e! L# L/ {; L
recent lunar crater chronology. They conclude that the oldest surfaces were % m3 c$ \$ k4 G! Q1 G/ P7 t5 ~1 Y
emplaced just after the start of the Late Heavy Bombardment 4.0 to 4.1 billion ! m2 S5 t2 z4 p4 J, L% _. d2 ]
years ago. The large impact basins, not previously dated, yield a similar
& `1 |$ l1 i" i0 w. A7 S! q9 Rsurface age. This agreement implies that resurfacing was global and due to a8 C( M0 g; j9 Z M$ G: K
volcanism, perhaps aided by heavy bombardment as previously suggested. Cover:
2 Q9 H0 D1 B" d9 xSimone Marchi/MESSENGER data: NASA/JHUAPL/Carnegie Institution of Washington; p0 N/ c, j Q; N( D2 ^ u+ V
+ f2 G' {; P7 n4 M# P" c( x) n2 Q |
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发表于 2013-7-5 23:43
