Question 41-50
5 ? r9 L h6 e# J9 S& h% S The Moon, which has undergone a distinct and complex geological history, presents a
! e v! ^; y+ X9 bstriking appearance. The moon may be divided into two major terrains: the maria (dark
% y" R+ r+ Y1 I3 q# mlowlands) and the terrace( bright highlands). The contrast in the reflectivity (the capability' P! n2 s: M# C9 H
of reflecting light ) of these two terrains suggested to many early observers that the two
1 _# o4 b' H- q+ f, ](5) terrains might have different compositions, and this supposition was confirmed by - ]& \/ u6 A$ a* ~1 [
missions to the Moon such as Surveyor and Apollo. One of the most obvious differences
$ a2 T- A- m& [9 ?between the terrains is the smoothness of the maria in contrast to the roughness of the
* a9 S, ~6 |: g) [2 r+ g6 n- F& Hhighlands. This roughness is mostly caused by the abundance of craters; the highlands are # Z" a" v9 T% @" O4 J+ v
completely covered by large craters( greater than 40-50 km in diameter), while the craters 2 \1 a( J; v9 Z2 n
(10) of the maria tend to be much smaller. It is now known that the vast majority of the Moon’s
* [- {3 d. t' ` r2 r% Ncraters were formed by the impact of solid bodies with the lunar surface.
- a7 @6 u" c* ?8 y- Q6 u F* I Most of the near side of the Moon was thoroughly mapped and studied from telescopic5 y0 _" O! S$ W2 Y$ ~0 Y) t6 [7 Q
pictures years before the age of space exploration. Earth-based telescopes can resolve# R# y ~) ~+ B/ r& k0 L" Y
objects as small as a few hundred meters on the lunar surface. Close observation of
& q. t# ]9 ^" d9 A" k6 I2 A) ~(15) craters, combined with the way the Moon diffusely reflects sunlight, led to the 9 u0 E2 F5 z8 ^! i- b3 K% N: j
understanding that the Moon is covered by a surface layer, or regolith, that overlies the
b7 b D% U, J& zsolid rock of the Moon. Telescopic images permitted the cataloging of a bewildering array
1 W7 H: |( O5 g/ }) Z% ~of land forms. Craters were studied for clues to their origin; the large circular maria were
) ]" I. O/ k- A9 H Y) B(20) seen. Strange, sinuous features were observed in the maria. Although various land forms
) q1 F! [9 J9 Dwere catalogued, the majority of astronomers’ attention was fixed on craters and their
$ n$ x2 y0 C1 \) iorigins.
, _1 N/ F5 K8 E) s" h7 {5 }6 m Astronomers have known for a fairly long time that the shape of craters changes as
6 u! O: |* |' F @# P$ O! y0 s& Nthey increase in size. Small craters with diameters of less than 10-15 km have relatively 5 w+ ?& d3 Q1 u6 ^
(25) simple shapes. They have rim crests that are elevated above the surrounding terrain,# o- d: X- ?4 H: P
smooth, bowl-shaped interiors, and depths that are about one-fifth to one-sixth their diameters. The complexity of shape increases for larger craters. |