Questions 1-9:
9 b+ M, q* J W9 mEuropa is the smallest of planet Jupiter’s four largest moons and the second moon
5 C. B n. j$ c& n; V: y. Nout from Jupiter. Until 1979, it was just another astronomy textbook statistic. Then
+ }1 z p& F2 [came the close-up images obtained by the exploratory spacecraft Voyager 2, and within
6 g' n' n. ?+ ?8 o Ldays, Europa was transformed-in our perception, at least-into one of the solar system’s
0 [0 v# S9 R( w, f* z3 X(5)most intriguing worlds. The biggest initial surprise was the almost total lack of detail,- F9 G" f" r, l. v# w
especially from far away. Even at close range, the only visible features are thin, kinked( b o3 R" D% L, `. @- j- x5 _
brown lines resembling cracks in an eggshell. And this analogy is not far off the mark.
* q0 E* S6 I/ OThe surface of Europa is almost pure water ice, but a nearly complete absence of* e- R: I: R7 ^+ b3 y
craters indicates that Europa’s surface ice resembles Earth’s Antarctic ice cap. The
- z& g) b2 y$ L1 I( }: M/ B(10) eggshell analogy may be quite accurate since the ice could be as little as a few kilometers ^! i8 B/ u& R8 Q- E6 U3 k
thick –a true shell around what is likely a subsurface liquid ocean that , in turn, encases
! W1 D, P2 r/ C$ f8 I/ Q a rocky core. The interior of Europa has been kept warm over the eons by tidal forces! l5 W7 T' m+ I' ?3 k9 V
generated by the varying gravitational tugs of the other big moons as they wheel around
7 v+ F" Z8 e4 J4 h0 ^; G Jupiter. The tides on Europa pull and relax in an endless cycle. The resulting internal heat : b4 r! C& c" V$ |# `
(15) keeps what would otherwise be ice melted almost to the surface. The cracklike marks on- E* y; K6 e! }: ]
Europa’s icy face appear to be fractures where water or slush oozes from below. 0 ?8 o H- M: i0 @# }0 u
Soon after Voyager 2’s encounter with Jupiter in 1979, when the best images of
) ?7 u2 q; d$ w; k0 h, _ Europa were obtained, researchers advanced the startling idea that Europa’s subsurface/ }: N. `) D* F$ g
ocean might harbor life. Life processes could have begun when Jupiter was releasing a% ?0 h; t+ b( L$ I/ f& @" Y' {6 a
(20 )vast store of internal heat. Jupiter’s early heat was produced by the compression of the- M+ g# i; u1 N( d- M' r
material forming the giant planet. Just as the Sun is far less radiant today than the primal
" K2 J/ u& l, K7 w, I# y Sun, so the internal heat generated by Jupiter is minor compared to its former intensity.+ s, Q% ?! V2 A0 f8 S+ ?/ B% }2 ~( P: b
During this warm phase, some 4.6 billion years ago, Europa’s ocean may have been liquid
9 ~1 i6 K- e; x! u4 | right to the surface, making it a crucible for life. % {' ^3 C2 _6 U+ B
+ x5 Z' g7 v( _- c6 `What does the passage mainly discuss?
+ F; e( N: w# ~, r2 QThe effect of the tides on Europa’s interior: p( `1 r# z8 e* P) U ~
Temperature variations on Jupiter’s moons _) [5 h% u! F% \. j+ S* n
Discoveries leading to a theory about one of Jupiter’s moons% t. y" l# u" }; H
Techniques used by Voyager 2 to obtain close-up images. |