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, q L6 l! u. |; }$ YThe most heavily cratered terrains on Mercury exhibit
3 a2 r. P5 B1 Y- k8 ?0 @/ [& ba lower density of craters smaller than about 100 km in diameter than on the
8 t% `0 u, k" p% L4 sMoon, a deficit that has been attributed to resurfacing by formation of ancient
& K; c6 F7 B; _' b: Eintercrater plains. Simone Marchi et al. used a crater areal density map ) T3 r- k# `( h1 H; j3 h# |$ {! ?2 @
based on data from the MESSENGER spacecraft (the colour-coded foreground on
+ w+ V! E3 E9 g, I, qcover, with a global surface mosaic in the background) to locate the oldest ' s6 k! ^; _3 B9 p# r, d: I
surfaces on Mercury and interpret the crater populations in the framework of a ' `1 b* P& O5 p1 Q B
recent lunar crater chronology. They conclude that the oldest surfaces were
- x9 r* g9 Y' r+ p' H5 \emplaced just after the start of the Late Heavy Bombardment 4.0 to 4.1 billion ; R, ?( {& u" P3 i7 S
years ago. The large impact basins, not previously dated, yield a similar
! B1 E4 U$ x: T( b8 B/ W+ |surface age. This agreement implies that resurfacing was global and due to
, y/ R3 d2 b4 U! T% Pvolcanism, perhaps aided by heavy bombardment as previously suggested. Cover: 6 m* Q9 Z B( F( {+ S b7 n# h
Simone Marchi/MESSENGER data: NASA/JHUAPL/Carnegie Institution of Washington
( q: W8 c9 `: K0 Z+ R; z3 K; K* O& ], z+ Q; A, d$ \
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发表于 2013-7-5 23:43
发表于 2013-7-6 08:19
