For most of the 20th century, there has been life on Mars, or,
at least in the minds of the Earthlings, who live on its closest planetary
neighbor. But could life exist on Mars, given that it is a very (47)
world from what it was 3.8 billion years ago It has (48)
no atmosphere. It has a barren surface, which is bathed in
ultraviolet (49) from the sun, and there is no liquid water
on the surface of the planet (although there are ice caps at its poles). Mars
had moved from a warm, wet place -- a likely home for life -- to a dead world of
dust storms, (50) and vast canyons. It happened because its
carbon dioxide atmosphere could not remain (51) in a wet
environment. Carbon dioxide reacted with rocks, and it was rapidly absorbed by
water, where it became solid carbonates (碳酸盐) and (52) to
the bottom of seas. It is a greenhouse gas, which (53) heat
from the sun. As it disappeared from the Martian atmosphere, the planet began to
cool. Its atmosphere grew thinner and all the water on the surface (54)
This is a fate that the Earth escaped because of water vapor and
other greenhouse gases, which are (55) present in its
atmosphere. And this is not necessarily the end of the story.
Mars could one day be a warm, wet world again, if sufficient greenhouse gases
were (56) into the atmosphere -- a task our descendents may
one day set themselves when they begin to colonize the solar system. A) froze
I) radiation B)
different J) distinguished C) steady
K) virtually D) traps
L) earthquakes E) volcanoes
M) sank F) adequate
N) constantly G) stable
O) confines H) released