填空题
Why are there no (36) on the moon Great food. No atmosphere. The atmosphere is the layer of gases that surrounds a planet or other massive body. Although our (37) extends for tens of miles, almost ninety-nine percent of it is within eighteen miles of the earth’s (38) Our own atmosphere is mostly (39) , with about 20 percent being the oxygen we breathe. But, a planet doesn’t start out having an atmosphere like ours. Much of the carbon dioxide that envelops earth, for example, was (40) embedded inside rocks, and later released by heat from the sun. About a (41) years ago, simple green plants started generating oxygen, which soon became a major part of our atmosphere as well. But more than these kinds of (42) processes are needed to have an atmosphere, you also need (43) .
(44) . The earth loses some of its atmosphere this way, but in general it has enough mass to hold on. The moon, (45) isn’t big enough to do the job. Still, recent observations have shown that (46) . It’s so thin, however, that it only counts as one in technical terms, and certainly couldn’t be breathed, so much for that restaurant.