NARRATOR:Listen to part of a lecture in an astronomy class.

旁白：听一段天文学课上的讲座。

MALE PROFESSOR:Traditionally, astronomers worked out how old geologic features of planets and moons are by the number of marks on the surface.

教授：以前，天文学家通过行星和卫星表面上的痕迹数量来计算它们一些地质特征的年龄.

The more craters in one place—say, on a lava flow—the more asteroids and comets that place has encountered over time, so the older it must be.

一个地方的陨石坑越多，比如说，熔岩流上，随着时间的推移，它遇到的小行星和彗星就越多，所以它一定越老。

This seems to make sense for relative age. That is, a surface feature with fewer craters is younger than one with more craters.

这对于相对年龄来说似乎说得通，也就是说，陨石坑较少的地表特征相对陨石坑较多的地方要年轻一些。

But absolute age, actual age, is trickier.

但绝对年龄，实际年龄更棘手。

We have to know exactly how old one surface is—uh, for example, we do have a very clear idea of the ages of some surfaces of the moon from rocks we brought back—and then this information can allow us to extrapolate the age of another surface that has a similar concentration of craters.

我们必须确切地知道一个地表年龄，例如，从带回的岩石中，我们能够弄清楚月球某些地表部位的年龄。然后，这些信息可以让我们推断出另一个表面的年龄，因为它们有类似陨石坑密度。

That’s the traditional way to calculate it.

这是计算传统方法。

But two developments have brought this traditional way into question.

但是有两个发展使这种传统方式受到质疑。

For one, a recent study of the craters on one of Jupiter’s moons, Europa, suggests that at least 95 percent of its small craters were formed by secondary impacts.

一方面，对木星卫星木卫二上的陨石坑的一项最近研究表明，至少有 95% 的小陨石坑是由二次撞击形成的。

OK. Secondary impacts—they’re the impacts of the chunks of rock or ice that break off as a result of the primary impact.

二次撞击是由于首次撞击的破裂而产生的大块岩石或冰块引发的撞击。

The primary impact refers to the impactor itself—asteroid, comet—hitting the planet or moon.

首次撞击是指撞击体本身比如小行星、彗星等，直接撞击行星或月球。

And when that happens, pieces of rock or ice break off and go flying—and when those chunks come back down and smash into the planet, those are the secondary impacts.

当首次撞击发生时，岩石或冰块会破裂并飞散。而当这些大块落回来并撞击地球时，这些就是二次撞击。

So, using the old way we would have assumed that this surface of Europa is much older than it might actually be.

因此，使用旧方法，我们会假设木卫二的表面年龄比实际上要古老得多。

And it’s conceivable that a very large strike from an impactor might throw up some fairly large chunks, ones that’re larger than some of the smaller direct strikes.

并且可以想象，规模非常大的撞击可能会抛出一些相当大的碎片，比一些较小的直接撞击体还要大。

So we can’t use size to determine if a crater is the result of a primary impact or a secondary one.

因此，我们不能使用大小尺寸来确定陨石坑是首次撞击还是二次撞击的结果。

And of course impactors come in different sizes… though, actually, we think there are fewer small ones than there used to be.

当然，撞击体有不同的尺寸，但实际上，我们认为小撞击体的数量比以前估计的要少。

What really tells us more, though, is the arrangement, the way the craters are clustered together, or not.

然而，真正告诉我们更多信息的是陨石坑的排列方式，它们是否聚集在一起。

For example, on Venus, the craters are distributed randomly; they’re all over the place, which is what we’d expect.

例如，在金星上，陨石坑是随机分布的。它们到处都是，这是我们所预期的。

This suggests that there hasn’t been much geologic activity lately on Venus—lava or whatever.

这表明最近在金星没有太多的像熔岩之类的地质活动。

But on Europa, the craters are in clusters.

但在木卫二上，陨石坑成束分布。

And since asteroids come from all directions, if the craters are arranged in bunches, it’s a signal, especially if they’re arranged in long ray patterns from a center point—that there was a single primary impact that threw fragments outward from the impact site.

由于小行星来自四面八方，如果陨石坑成束排列，这是一种信号，而且特别是如果它们从中心点以长射线模式排列，那就是一次首次撞击，从撞击中心点向外抛出碎片。

Another thing… primary impactors hit a lot harder and usually more directly than secondary ones.

还有，首次撞击比二次撞击力度更大，撞击通常更直接。

So primary craters tend to be deeper—more bowl-shaped—and almost always circular… which isn’t the case with secondaries.

所以首次撞击陨石坑往往更深，呈碗状，并且几乎总是圆形，而二次陨石坑并非如此。

Anyway… now, let’s assume Europa is representative of the inner solar system.

无论如何，现在假设木卫二是内太阳系的代表。

That would mean there are a lot more secondaries on Mars or on Earth’s moon or other bodies than we had originally thought.

这意味着火星或月球或其他天体上的二次撞击比我们起先想象的要多得多。

And here’s some more proof: we got our hands on some nice photos of one particular crater on Mars—Zunil—and it turns out that this one impact caused many more secondary craters than we had thought, I mean, like 90 million more.

这里还有一些证据。我们得到了一个特别的陨石坑的照片，它位于火星的一颗卫星——祖尼尔上。事实证明，这一次撞击造成的二级陨石坑比想象的要多得多。我的意思是，多了九千万那么多。

So, if the impact causing each large primary crater—and Zunil isn’t even that big—results in this many secondaries, then most of the craters on Mars must be secondary.

因此，如果撞击造成了每个巨大的主陨石坑，而祖尼尔不算很大，就能导致这么多的二次陨石坑，那么火星上的大多数陨石坑肯定是次生的。

And that makes sense, actually, since if all of the craters, especially the small ones, if all of them are primary craters, well, there simply wouldn’t have been enough small objects out there in space to account for all of those craters.

实际上，这么讲是有道理的，因为所有的陨石坑，尤其是小的陨石坑的形成是没法解释的，如果它们都是首次撞击陨石坑，那么太空中根本就没有足够的小物体形成这些陨石坑。

And, unfortunately, this means most craters probably aren’t at all useful for dating surfaces on Mars.

不幸的是，这意味着大多数陨石坑可能对火星表面的年代测定毫无用处。

So, for example, some lava flows on Mars, which had been dated at about 5 million years old—very young—because of the relatively few craters there, well, it might only mean that this area was one of the random areas that wasn’t hit by a primary impactor.

例如，火星上的一些熔岩流，按照传统方法测定年代约为500万年，非常年轻，因为那里的陨石坑相对较少，但这可能只是意味着这个区域是没被首次撞击体击中的随机区域之一。

It just makes it less clear; this lava flow could be 100 million years old instead.

这反而让人更困惑了，因为这样的话，这种熔岩流可能有一亿年的历史。

In this case, we can’t predict the age with any accuracy unless we have actual samples from the planets.

在这种情况下，我们无法准确推测年龄，除非我们有来自行星的实际样本。

Y’know, we’re getting great information and photos from our space probes all the time, but they also remind us of just how much more we need to learn…

我们一直在从太空探测器中获得大量信息和照片，但它们也提醒我们还有多少需要学习。