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. e7 K. d0 m7 s5 r# p0 n; ]. O0 t' L; E3 ^
The most heavily cratered terrains on Mercury exhibit
8 H5 H9 |2 W: T# Q! E4 K3 Za lower density of craters smaller than about 100 km in diameter than on the # J( _' s3 e5 t: e* w/ o! m( R& U" o
Moon, a deficit that has been attributed to resurfacing by formation of ancient 8 }, c+ n$ y5 E: z
intercrater plains. Simone Marchi et al. used a crater areal density map
l# z) C. g& Y( d+ n' d2 pbased on data from the MESSENGER spacecraft (the colour-coded foreground on : `8 B: ~$ N- E1 A2 ~. P3 m9 h
cover, with a global surface mosaic in the background) to locate the oldest
+ }2 a {1 |$ ssurfaces on Mercury and interpret the crater populations in the framework of a 8 ?7 Z+ @+ i! w
recent lunar crater chronology. They conclude that the oldest surfaces were
( D8 o! u. t2 b5 q% uemplaced just after the start of the Late Heavy Bombardment 4.0 to 4.1 billion & q, U9 Y9 _$ _# V% C" ]
years ago. The large impact basins, not previously dated, yield a similar
* h9 w B8 u) h, ], R, s ]1 \surface age. This agreement implies that resurfacing was global and due to 4 h2 Q# D, E. J
volcanism, perhaps aided by heavy bombardment as previously suggested. Cover:
1 U0 u1 h, l; [5 x3 p* eSimone Marchi/MESSENGER data: NASA/JHUAPL/Carnegie Institution of Washington8 l: k% U: r" k1 V
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发表于 2013-7-5 23:43
发表于 2013-7-6 08:19
