Listen to part of lecture in an astronomy class.
Traditionally, astronomers worked out how old geologic features of planets and moons are by the number of marks on the surface. The more
creator craters in one place ,say, on the a lava of flow, the more astrator ??? asteroids and comets that place s encountered over time, so the older ??? it must be. This things seems to make sense for relative age. That is, a surface feature with fewer creator craters ??? is younger than one with more creators craters.
But absolute age, actual age is
tricky year trickier. We have to know exactly how old one surface is. For example, we do have very a clear idea of the age of the sun some surfaces of the moon from the rocks we brought back and then this information can allow us to extrapolate the age of another surface that has a similar concerntration of craters. That the traditional way to calculate it.
??? developments have brought this traditional way into question.
For one, a recent study of creators
, on one of dupator Jupiter's moons, Europe Europa, suggests that at least 95% of its small craters sports creator were from formed by secondary impacts. OKay, Secondary impacts. ??? They're the impacts of the chunks of the chance aborker eye, of rock or ice that they break off as a result to of primary impact.
The Primary impact refers to the impact itself,
as ???, asteroid, comet, hitting the planet or moon, And when that happens. Pieces of rock eyes or ice break off often and go flying, and when those smells chunks come back down and under the smesh of into the planet, those are the secondary impacts.
use using the old way, we would have the soon assumed that the surface of Europe's Europa as is much already older than they it might actually be. And it's consivable conceivable that very large strike from an impactor might through up throw out some very fairly large chance chunks, once ??? ones are larger than some of the smaller direct strikes. So we can't use signs size to determine if the creator a crater is the result of the a primary impact or a secondary ways. And of course, impactor come from different sizes, though actually we think there are fewer small ones than there when they're used to be. We are What really tells us more ??? though is the arrangement, the way s the creators craters are classed clustered together or not.
unbeanous, on Venus the creator craters are distupated distributed brandondly randomly, there are all over the place, which is what we'd expect. This suggests that there hasn't been much geologic activity like undebenous lately on Venus, lava or whatever, but on Europer Europa, the creators craters are in classtures clusters. And since astro?? asteroids come from all directions if the creator crators are arranged in bunchies 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.
Another thing, primary
impacter has impactors hit a lot harder, and usually more directly than secondary ones. So primary crators craters tend to be deeper, more bowl's shaped bowl-shaped, and almost always circular, which isn't the case with secondaries.
let's ??? let's assume Europa is representative of the inner solar system to do the ??? system. That ??? mean ??? That would mean there are a lot more secondary secondaries ??? on Mars or Earth's or moon or other bodies that ????. than we had originally thought.
And here's more proof. We got our hands on some nice photos of one particular
crator crater on Mars, zoono, Zunil, and it turns out that this one impact caused many more secondary creators craters than we'd thought, I mean like 30 9 million more. So if the impact cause causing ?? each large primary crater and ??? that big ??? and Zunil isn't even that big, with many ??? result in this many secondaries, then most of the crator craters on Mars must be secondary. And that make sense actually, since if all the crators is , especially the smallest small ones, if all of them were primary crators craters, well there simply ???? wouldn't have been enough small objects out there in space to account for all of those craters.
And, unfortunately, this means most
crators craters probably ??? aren't at all useful for learning dating surface on Mars. So, for example, some lava flows on Mars which be dated at about 5 million years old, very young. Because of the relatively few crators craters there, well, it might only mean that this area was one of the random areas that wasn't be hit by a primary impactor. It just makes it less clear. These This lava flow could be a hundred million years old instead. In this case, we can't predict the age with any accuracy unless we have actual samples from ??? in the plantes. ??? grade You 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.
- Pieces of rock or ice
- craters 火山口
- asteroids 小行星
- comets 彗星
- trickier 棘手的
- extrapolate 推算
- concerntration 浓度
- Europa 欧罗巴（在这里是指一个planet的名字）
- Secondary impacts 次要影响
- chunks 大块
- impactor 撞击者
- fairly 相当
- arrangement 安排
- clustered 成簇
- clustered together 聚集在一起（真正告诉我们的是火山口的排列，聚集或不聚集的方式。）
- Venus 金星
- distributed randomly 随机分布
- clusters 集群
- asteroids 小行星
- outward 向外
- circular 圆
- Zunil 祖尼尔（这是一个火山口的名字）
- lava flow 熔岩流
- probes 探针
- space probes 太空探测器
- imply 暗示
- developments 事态发展
- assumed 假定
- conceivable 可以想到的；可想象的
- fairly large 相当 大
- let's assume 让我们假设（这两个单词连起来读的话，不容易反应）
- We got our hands
- distorted 扭曲的