In 1969 when Neil Armstrong said “That’s one small step…”, he made a lasting impression. On the moon, that is. Marks on the surface of the moon don’t disappear the way marks on the Earth do; a great example is the impact craters we can see dotting the moon’s surface. Many of them are billions of years old!

　　But Why?

　　If you write your name in the sand at the beach, one day later there’s no sign you were there. What’s the difference?

　　Here’s an experiment you can do to find out. Take two pans and fill them with soil and sand mixed together. Now have some young person press her or his hand down into the mixture to make an imprint. One of these surfaces will represent the Earth, and one the moon.

　　Try This!

　　Leave your Earth pan outside. Take the moon pan inside and place a cardboard box over the top of it, weighted down with something heavy. Wait a couple days and check again. The moon pan will look the same. More than likely, though, the Earth pan won’t. Maybe the wind blew the sand around. Maybe it rained or snowed.

　　The point is that Earth has an atmosphere, which results in wind, rain, rivers and streams, lots of motion. Of course, Earth is also volcanically active: the ground shakes, lava pours out, continents rise and fall. By comparison, nothing much ever happens on the moon–except for the impacts that give rise to those craters–even across billions of years. So an impression on the moon can be a lasting impression indeed!

　　月球表面的记号为何不会消失？

　　1969年阿姆斯特朗的那句“人类一小步。”给人留下了难以磨灭的印象。月球表面的记号不会像地球那样轻易地消失。最典型的例子就是我们肉眼可见的月球表面散布的陨石坑。这些陨石坑中很多有数十亿年的历史。

　　但为什么呢？

　　如果你用沙子在沙滩上写下你的名字的话，一天过后，名字就会消失。差别在哪呢？

　　你可以做个实验来找出这种差别。拿出两个平底锅，里面放满土和沙子并将土和沙子掺在一起。然后找几个年轻人把手放在沙土中摁出手印。其中一个平底锅代表地球，另一个代表月球。

　　试试看！

　　把代表地球的平底锅放在户外，然后把代表月球的平底锅放在室内并在它的上面放上一个硬纸盒，再压上一些重物。几天过后再进行观察，你会发现代表月球的平底锅没有任何变化。但代表地球的平底锅却并非如此，也许是风把沙子吹得到处都是，也许是被雨雪冲刷过。

　　答案就在于地球有一层大气，大气可以产生风、雨、河流、小溪等引起运动的事物。当然，地球的火山运动是非常活跃的，会造成地震、岩浆喷发、大陆沉降。相比之下，月球在数十亿年里基本不会产生这些现象（形成火山坑的撞击除外）。因此，你在月球上留下的印记会一直留在那哦！