[00:00.00]NARRATOR: Listen to part of a lecture in an astronomy class.
[00:04.35]MALE PROFESSOR: Traditionally, astronomers worked out how old geologic features of planets and moons are by the number of marks on the surface. [00:12.30]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.
[00:21.01]This seems to make sense for relative age. [00:24.23]That is, a surface feature with fewer craters is younger than one with more craters. [00:29.25]But absolute age, actual age, is trickier. [00:32.95]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. [00:50.22]That’s the traditional way to calculate it. [00:52.85]But two developments have brought this traditional way into question.
[00:57.20]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.
[01:08.25]OK. Secondary impacts—they’re the impacts of the chunks of rock or ice that break off as a result of the primary impact. [01:16.31]The primary impact refers to the impactor itself—asteroid, comet—hitting the planet or moon. [01:23.47]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. [01:33.60]So, using the old way we would have assumed that this surface of Europa is much older than it might actually be.
[01:39.80]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. [01:49.18]So we can’t use size to determine if a crater is the result of a primary impact or a secondary one. [01:55.86]And of course impactors come in different sizes… though, actually, we think there are fewer small ones than there used to be.
[02:02.43]What really tells us more, though, is the arrangement, the way the craters are clustered together, or not. [02:08.71]For example, on Venus, the craters are distributed randomly; they’re all over the place, which is what we’d expect. [02:15.39]This suggests that there hasn’t been much geologic activity lately on Venus—lava or whatever. [02:20.40]But on Europa, the craters are in clusters. [02:23.74]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.
[02:39.56]Another thing... primary impactors hit a lot harder and usually more directly than secondary ones. [02:46.18]So primary craters tend to be deeper-- more bowl-shaped-- and almost always circular… which isn’t the case with secondaries.
[02:54.59]Anyway… now, let’s assume Europa is representative of the inner solar system. [03:00.15]That would mean there are a lot more secondaries on Mars or on Earth’s moon or other bodies than we had originally thought. [03:06.78]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. [03:21.33]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. [03:33.40]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.
[03:45.99]And, unfortunately, this means most craters probably aren’t at all useful for dating surfaces on Mars. [03:52.86]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. [04:09.31]It just makes it less clear; this lava flow could be 100 million years old instead. [04:14.91]In this case, we can’t predict the age with any accuracy unless we have actual samples from the planets. [04:21.00]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…
托福14天冲刺微信战团