Question 41-50
) {6 j' @9 N% X* f) ^% K& \ The Moon, which has undergone a distinct and complex geological history, presents a
5 {2 Z3 c3 X9 {& y8 n, |) dstriking appearance. The moon may be divided into two major terrains: the maria (dark 7 |0 J' [1 X2 S
lowlands) and the terrace( bright highlands). The contrast in the reflectivity (the capability
3 M! F. R9 w5 jof reflecting light ) of these two terrains suggested to many early observers that the two
" e7 D4 S- J3 ^% A) p8 ^(5) terrains might have different compositions, and this supposition was confirmed by 1 w: u5 k# D3 Q
missions to the Moon such as Surveyor and Apollo. One of the most obvious differences
2 u% u6 ?8 P- Q3 x. J* |! \4 bbetween the terrains is the smoothness of the maria in contrast to the roughness of the
+ x7 R3 O9 j) s8 D8 bhighlands. This roughness is mostly caused by the abundance of craters; the highlands are 4 ]* O0 L+ m' C6 p5 u
completely covered by large craters( greater than 40-50 km in diameter), while the craters
, x5 ?1 f, M* h$ F(10) of the maria tend to be much smaller. It is now known that the vast majority of the Moon’s
7 n: |# X2 ?2 D+ S9 B- \! dcraters were formed by the impact of solid bodies with the lunar surface.0 ?& [+ c$ }. s" D
Most of the near side of the Moon was thoroughly mapped and studied from telescopic6 W/ p: y0 F1 H7 a# ~% X8 L
pictures years before the age of space exploration. Earth-based telescopes can resolve# ]: G8 Y0 M* ~. K3 P# \5 S: B
objects as small as a few hundred meters on the lunar surface. Close observation of 3 K8 G0 i- z2 n
(15) craters, combined with the way the Moon diffusely reflects sunlight, led to the & N' V U! o' \
understanding that the Moon is covered by a surface layer, or regolith, that overlies the
4 Y6 ^. e) m: c. h m7 c1 bsolid rock of the Moon. Telescopic images permitted the cataloging of a bewildering array) N: S Y2 O/ f+ e5 l/ ?
of land forms. Craters were studied for clues to their origin; the large circular maria were
9 Q C% ?1 B# P4 u8 Z( s5 }(20) seen. Strange, sinuous features were observed in the maria. Although various land forms: a( P& t6 ?+ s0 i) [' W
were catalogued, the majority of astronomers’ attention was fixed on craters and their , G- P) {. i. v0 [
origins., a8 y9 `) _) ^% _
Astronomers have known for a fairly long time that the shape of craters changes as7 x* U1 A0 E( l2 A
they increase in size. Small craters with diameters of less than 10-15 km have relatively
7 l, G: {5 X; }0 u(25) simple shapes. They have rim crests that are elevated above the surrounding terrain,
m- @# M3 ^# O8 S& {! |smooth, bowl-shaped interiors, and depths that are about one-fifth to one-sixth their diameters. The complexity of shape increases for larger craters. |