Questions 10-19
' t: _2 ]: f! p( Z; x( b' e6 ]2 Z3 }No two comets ever look identical, but they have basic features in common, one of the
- ]# W/ e+ ]# g9 Imost obvious of which is a coma. A coma looks like a misty, patch of light with one or more) h6 x. F% t2 ^% i9 V9 {
tails often streaming from it in the direction away from the Sun.
& h# J+ Q7 v7 @9 p0 RLine At the heart of a comet's coma lies a nucleus of solid material, typically no more than
. a4 S l' ^# W5 D) [(5) 10 kilometers across. The visible coma is a huge cloud of gas and dust that has escaped+ R0 |% |& x9 c4 F7 W
from the nucleus, which it then surrounds like an extended atmosphere. The coma can extend as far as a million kilometers outward from the nucleus. Around the coma there is often an even larger invisible envelope of hydrogen gas.
+ L3 I' d" E, ^2 g* }The most graphic proof that the grand spectacle of a comet develops from a relatively; d1 j# ?- Z' y* d
(10) small and inconspicuous chunk of ice and dust was the close-up image obtained in 1986 by the European Giotto probe of the nucleus of Halley's Comet. It turned out to be a bit like a, \! ^6 q/ l8 ?/ l1 a9 k* o7 r
very dark asteroid, measuring 16 by 8 kilometers. Ices have evaporated from its outer layers
- n5 Q3 K8 |0 D6 }: I5 rto leave a crust of nearly black dust all over the surface. Bright jets of gas from evaporating0 p( S/ \, }0 u7 L5 F4 \
ice burst out on the side facing the Sun, where the surface gets heated up, carrying dust$ N7 `1 }0 E' O k% o U5 m
(15) with them. This is how the coma and the tails are created.; C. x6 W4 w# e9 d$ ]
Comets grow tails only when they get warm enough for ice and dust to boil off. As a( W3 ~+ p$ [( j+ v4 m
comet's orbit brings it closer to the Sun, first the coma grows, then two distinct tails usually% v4 e+ K7 R) I; d1 N! Z
form. One, the less common kind, contains electrically charged (i.e., ionized) atoms of gas,
5 c" I4 v( f6 S' s& f, y# Gwhich are blown off directly in the direction away from the Sun by the magnetic field of
" Q% h2 C6 c) F0 n& C: e5 u3 U(20) the solar wind. The other tail is made of neutral dust particles, which get gently pushed back by the pressure of the sunlight itself. Unlike the ion tail, which is straight, the dust tail8 {' i2 s L( u; H) }
becomes curved as the particles follow their own orbits around the Sun.
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I0. The passage focuses on comets primarily in terms of their
y' d1 o- v, i1 t0 _5 |(A) orbital patterns
7 n+ F$ g* M& a B" O(B) coma and tails
U$ T/ G% e( Q: m9 M7 B(C) brightness
# m1 l. {: i; Y/ ^( q(D) size
% H0 ^" N( G# T- m11. The word "identical" in line I is closest in meaning to
& ^8 P: j# x. \; K* j+ j3 W( {" }(A) equally fast5 W! c0 B5 p. g$ j6 y) \
(B) exactly alike+ d; Z; B0 H0 v
(C) near each other
) P& g. m8 z$ M% f: X(D) invisible |
发表于 2012-8-14 22:34:35
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