Mars has provoked much speculation on the possibili- (1): c9 N% g1 Q- h; h- J
ties of life beyond Earth than any planet in the Solar System. (2) __
9 q* Z% t4 p2 O/ S' c* i. @The presence of water is a prerequisite for the existing of life. (3) __4 H* g& ~# X) m/ J
Therefore, "follow the water" has been NASA' s chief guide-# I& W- I! ]9 S1 a
line for the exploration of a red planet. Although Mars experi- (4)5 x" |* `* Z8 p; b
ences seasons like on Earth and has polar caps which com- (5) __
; O8 x$ o. z& Q5 zposed of carbon dioxide and water ice, today it is bone-dry and+ D4 ]- K- e: B4 I: {" c0 _1 _
frigidly cold. But evidence is rapidly accumulating Mars was (6) __- Y, _/ |# r1 _6 b' c4 G# u
once much wetter, with a more clement climate. This evidence3 K2 ?8 I. G" O) c( V
comes from orbiting satellites and from data collected by rov-# ^& _) b5 Q: ~% A1 R* P% Q
ing landers.
+ G" W+ h* F3 v: p Since 1970s, space probes to Mars have revealed numer- (7) __8 a- W3 _3 I7 H6 W
ous features apparent carved by flowing water, such as wind-
# @" Z' }* b3 `- I& q/ nLng, branched valleys resembling dried-out streambeds and gi-
2 ^: ?. v4 p- H" e% D% ]5 oant outflow channels gouged by catastrophic floods. Recent) a/ P9 G8 M: x$ @. F0 N/ j+ H& {
high-resolution imagery from the Mars Global Surveyor Mars
: X! ^. \" U9 f# N8 \% M/ hOrbiter Camera and the Mars Odyssey THEMIS reveal nu- (8) __ u& X. G5 p+ P& I+ t, i0 j" N
merous examples of branched valleys that form tightly-packed,5 ]) C a' U$ f& y! @9 e
integrated drainage systems. These channels origins at topo- (9) __( Q g9 E; x7 F. B% T) u2 l$ p0 t
graphic high points; the valleys widen "downstream", some
: [6 z7 r0 \! h7 xeven displaying inner valleys. The valley networks exhibit
/ ]. _& p$ {" b9 h. G" Pmorphometric characteristics, including network densities,
/ d4 @% K$ P+ i' m7 G6 hcomparative to those of terrestrial drainage basins. These fea- (10) __
$ k# J4 _ d5 _+ c' ^0 Etures were most likely produced by rainfall, during wetter,) x, ~ T" f3 |: T6 q+ N; x6 N; B2 W
warmer periods in the past. |
发表于 2012-8-14 11:35:50
