Questions 1-9:
3 g0 o, l; x5 w: w6 q3 M6 J: |Europa is the smallest of planet Jupiter’s four largest moons and the second moon , J- V6 X8 D u
out from Jupiter. Until 1979, it was just another astronomy textbook statistic. Then
- o% O: \, o/ g: @came the close-up images obtained by the exploratory spacecraft Voyager 2, and within & F# W. q; m( P* v2 s) N
days, Europa was transformed-in our perception, at least-into one of the solar system’s
% j% J `2 E( Y/ F(5)most intriguing worlds. The biggest initial surprise was the almost total lack of detail,
5 {0 ?, L# ]9 ?/ E- [especially from far away. Even at close range, the only visible features are thin, kinked
8 g% i0 u9 I6 u2 a9 F4 Zbrown lines resembling cracks in an eggshell. And this analogy is not far off the mark.
$ h8 e( {0 a" r( qThe surface of Europa is almost pure water ice, but a nearly complete absence of
, a8 y; W* X$ K3 [% e craters indicates that Europa’s surface ice resembles Earth’s Antarctic ice cap. The
1 X% h7 y1 j7 W(10) eggshell analogy may be quite accurate since the ice could be as little as a few kilometers
$ P0 U# b* } y( O Lthick –a true shell around what is likely a subsurface liquid ocean that , in turn, encases4 Z7 B. Q" B5 ~* ]8 p
a rocky core. The interior of Europa has been kept warm over the eons by tidal forces6 P8 p2 ~+ \( [& H: E/ ~
generated by the varying gravitational tugs of the other big moons as they wheel around! |8 t3 F% B$ z" i0 X6 z& X
Jupiter. The tides on Europa pull and relax in an endless cycle. The resulting internal heat
% D* ^3 R/ ?/ |6 m1 ^(15) keeps what would otherwise be ice melted almost to the surface. The cracklike marks on
3 `$ [# T1 W4 c% J Z5 | Europa’s icy face appear to be fractures where water or slush oozes from below.
- F$ C5 R0 [7 S, C# }# w. G5 H0 b Soon after Voyager 2’s encounter with Jupiter in 1979, when the best images of
+ T: n8 K' L7 F Europa were obtained, researchers advanced the startling idea that Europa’s subsurface, M! d' |5 C1 F8 \$ J3 U1 L9 }& G
ocean might harbor life. Life processes could have begun when Jupiter was releasing a- f+ T3 ]9 N4 m. @. O
(20 )vast store of internal heat. Jupiter’s early heat was produced by the compression of the: V6 y$ @5 ^% A0 X8 L- y8 B5 U
material forming the giant planet. Just as the Sun is far less radiant today than the primal4 ~ d$ X% ^7 j* r7 Q1 U
Sun, so the internal heat generated by Jupiter is minor compared to its former intensity.
0 m4 ^ b( f- V0 i During this warm phase, some 4.6 billion years ago, Europa’s ocean may have been liquid3 L8 F5 S7 k& C: F9 _8 W' A/ [7 ]
right to the surface, making it a crucible for life. % S' e8 \! L: o: ]
7 V9 n1 v4 Y: p4 Y8 H) v j3 t
What does the passage mainly discuss?
( ]( g6 m9 F6 a7 @, @/ hThe effect of the tides on Europa’s interior
6 E8 L: M4 }3 U7 h4 T4 GTemperature variations on Jupiter’s moons* n( Q0 b" K4 i; J4 j" w
Discoveries leading to a theory about one of Jupiter’s moons
3 z& v$ ^" L2 t9 D% N+ cTechniques used by Voyager 2 to obtain close-up images. |