Europa is the smallest of planet Jupiter’s four largest moons and the second moon out from

Europa is the smallest of planet Jupiter’s four largest moons and the second moon

out from Jupiter. Until 1979, it was just another astronomy textbook statistic. Then

came the close-up images obtained by the exploratory spacecraft Voyager 2, and within

days, Europa was transformed-in our perception, at least-into one of the solar system’s

(5) most intriguing worlds. The biggest initial surprise was the almost total lack of detail,

especially from far away. Even at close range, the only visible features are thin, kinked

brown lines resembling cracks in an eggshell. And this analogy is not far off the mark.

The surface of Europa is almost pure water ice, but a nearly complete absence of

craters indicates that Europa’s surface ice resembles Earth’s Antarctic ice cap. The

(10)eggshell analogy may be quite accurate since the ice could be as little as a few kilometers

thick –a true shell around what is likely a subsurface liquid ocean that , in turn, encases

a rocky core. The interior of Europa has been kept warm over the eons by tidal forces

generated by the varying gravitational tugs of the other big moons as they wheel around

Jupiter. The tides on Europa pull and relax in an endless cycle. The resulting internal heat

(15)keeps what would otherwise be ice melted almost to the surface. The cracklike marks on

Europa’s icy face appear to be fractures where water or slush oozes from below.

Soon after Voyager 2’s encounter with Jupiter in 1979, when the best images of

Europa were obtained, researchers advanced the startling idea that Europa’s subsurface

ocean might harbor life. Life processes could have begun when Jupiter was releasing a

(20)vast store of internal heat. Jupiter’s early heat was produced by the compression of the

material forming the giant planet. Just as the Sun is far less radiant today than the primal

Sun, so the internal heat generated by Jupiter is minor compared to its former intensity.

During this warm phase, some 4.6 billion years ago, Europa’s ocean may have been liquid

right to the surface, making it a crucible for life.

What does the passage mainly discuss？

A．The effect of the tides on Europa's interior

B．Temperature variations on Jupiter's moons

C．Discoveries leading to a theory about one of Jupiter's moons

D．Techniques used by Voyager 2 to obtain close-up images.