You might think, therefore, that modem land tortoises have probably stayed on land ever since those early terrestrialtimes, as most mammals did after a few of them went back to the sea.
The average density of interstellar material in the vicinity of our Sun is 1,000 to 10,000 times less than the best terrestrial laboratory vacuum.
在我们的太阳附近,星际物质的平均密度比最好的地球实验室真空还要低1000到10000倍。
3
The densities of the terrestrial planets average about 5 times the density of water, whereas the Jovian planets have densities that average only 1.5 times the density of water.