Why are there no restaurants on the moon? Greatfood. No atmosphere. The atmosphere is the layer ofgases that surrounds a planet or other massivebody. Although our atmosphere extends for tens ofmiles, almost ninety-nine percent of it is withineighteen miles of the earth's surface. Our ownatmosphere is mostly nitrogen, with about twenty-percent being the oxygen we breathe. But, a planetdoesn't start out having an atmosphere like ours.Much of the carbon dioxide that envelops earth, forexample, was originally embedded inside rocks, andlater released by heat from the sun. About a billion years ago, simple green plants startedgenerating oxygen, which soon became a major part of our atmosphere as well.

　　为何会月球上没有餐厅?很棒的食物。没有大气层。大气层是环绕星球或其他巨大天体的大气层。虽然我们的大气层延伸几十里，几乎99%的大气都在地球表面18英里范围内。我们自己的大气层主要成分是氮，约20%的成为我们呼吸的氧气。但是，一个星球最初时的大气层和我们的大气层并不一样。很多二氧化碳笼罩大地，比如，原本储存在岩石内的二氧化碳，经过太阳的加热而被释放出来。大约10亿年前，简单的绿色植物开始产生氧气，氧气很快成为了我们的大气层主要部分。