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TPO 5 Lecture 2
Astronomy
Narrator
Listen to part of a lecture in an Astronomy Class
Professor:
Last week, we covered some arguments against going back to the Moon. But there are compelling reasons in favor of another Moon landing too, um… not the least of which is trying to pinpoint the moon’s age.
We could do this in theory by studying an enormous impact crater, known as the South Pole-Aitken Basin. Um…it’s located in the moon’s South Polar Region. But, since it’s on the far side of the moon, it can only be seen from space.
Here is an image of…we’ll call it the SPA Basin. This color-coated image of the SPA Basin, those aren’t its actual colors obviously, this image is from the mid 90s, from the American spacecraft called Clementine. Um… unlike earlier lunar missions, Clementine didn’t orbit only around the moon’s equator. Its orbits enabled it to send back data to create this topographical map of … well, the grey and white area towards the bottom is the South Pole, the purples and blues in the middle correspond to low elevations - the SPA Basin itself, the oranges and reds around it are higher elevations.
The basin measures an amazing 2,500 km in diameter, and its average depth is 12 km. That makes it the biggest known crater in our solar system and it may well be the oldest. You know planetary researchers love studying deep craters to learn about the impacts that created them, how they redistributed pieces of the planet’s crust. And in this case, we especially want to know if any of the mantle, the layer beneath the crust, was exposed by the impact. Not everyone agrees, but some experts are convinced that whatever created the SPA Basin did penetrate the Moon’s mantle. And we need to find out, because much more than the crust, the mantle contains information about a planet’s or Moon’s total composition. And that’s key to understanding planet formation. Um…Dian?
Dian:
So, the only way to know the basin’s age is to study its rocks directly?
Professor:
Well, from radio survey data, we know that the basin contains lots of smaller craters. So it must be really old, about 4 billion years, give or take a few hundred million years. But that’s not very precise. If we had rock samples to study, we’d know whether these small craters were formed by impacts during the final stages of planetary formation, or if they resulted from later meteor showers.
Dian:
But if we know around how old the Basin is, I’m not sure that’s reason enough to go to the Moon again.
Professor:
No…, but such crude estimates…um…we can do better than that.
Besides, there’s other things worth investigating, like is there water ice on the moon? Clementine’s data indicated that the wall of the south-polar crater was more reflective than expected. So some experts think there’s probably ice there. Also, data from a later mission indicates significant concentrations of hydrogen and by inference water less than a meter underground at both poles.
Student:
Well, if there’s water, how did it get there? Underground rivers?
Professor:
We think meteors that crashed into the moon or tails of passing comets may have introduced water molecules. Any water molecules that found their way to the floors of craters near the moon’s poles, that water would be perpetually frozen, because the floors of those craters are always in shadow. Um…furthermore, if the water ice was mixed in with rock and dust, it would be protected from evaporation.
Dian:
So are you saying there might be primitive life on the moon?
Professor:
That’s not my point at all. Um… o.k., say there is water ice on the moon. That would be of very practical value for a future moon base for astronauts. Water ice could be melted and purified for drinking. It could also be broken down into its component parts - oxygen and hydrogen. Oxygen could be used to breathe, and hydrogen could be turned into fuel, rocket fuel. So water ice could enable the creation of a self-sustaining moon base someday, a mining camp perhaps or a departure point for further space exploration.
Student:
But hauling tons of equipment to the moon to make fuel and build a life support system for a moon base, wouldn’t that be too expensive?
Professor:
Permanent base, maybe a ways off, but we shouldn’t have to wait for that. The dust at the bottom of the SPA Basin really does have a fascinating story to tell. I wouldn’t give for a few samples of it.
《天文学》
旁白：请听天文学的一篇演讲
教授：上周，我们谈到了反驳回到月球的一些点。而这节课我们要谈到的是支持登月的几个充分的原因。我们这样做绝不是试图给出月球准确的寿命。理论上，我们可以通过研究一个巨大的撞击坑，名为南极艾特肯盆地，来得出结论。嗯，这个坑位于月球的南极区。但是，由于它是位于离地球远的一边，我们只能在太空中才能看到它，我这里有一张图片。我们将之命名为 SPA 盆地。可以看到盆地上一层着色的图层，很明显这不是真实的月球的颜色。这张图拍摄于九十年代中期，来自名为克莱门的宇宙飞船。嗯，不像早些时候的月球探测一样，克莱门飞船并没有仅仅沿着月球的轨道飞行。这种特殊的飞行使得飞船能够发送数据回地球，从而得出这张地形图。延伸到底部的灰白区域是南极，中间紫色和蓝色的部分是海拔较低的地带，而 SPA 盆地，即橙红色地带，海拔较高。改盆地直径竟达 2500 千米，平均深度达到 12 千米。这使得它成为了太阳系中已知的最大的撞击坑，很可能也是年代最久远的。我们知道，在知道行星形成的原因之前，行星研究人员喜欢研究撞击深坑，它们是怎么重新分配行星的外壳物质分配。我们特别想知道在外壳下层中是否存在某些金属，因为撞击而受到影响。不是所有人都同意这样的说法，但是某些专家坚信不管什么东西使 SPA 盆地形成，它都会渗透过月球的金属层。而由于金属层包含信息比外壳要多得多，我们必须找出一个行星或是月球的整体构成要素。这是了解行星构成的关键所在。
Dian：那么知道盆地形成时间的唯一途径就是对其岩石进行直接的研究？
教授：嗯，辐射调查的数据显示，我们知道盆地里有很多更小的火山口。因此应该有一定历史了，大概 40 亿年，允许有一亿年左右的误差。但也不是很精确。如果我们研究岩石样本，我们就知道小火山口是不是在行星形成的最后阶段影响而形成的，或者是否由于最近的流星雨造成的。
Dian：但如果我们知道盆地的大概年龄，我不敢确定这是否足以成为我们再次登月的理由。
教授：不。。。这只是粗略的估计，嗯，我能得到更为准确的数字。此外，还有其他东西值得研究，比如月球上有水冰吗？关于克莱门的数据显示，火山南极墙比人们预期的要更具有反射效果，因此某些专家认为那里很可能有冰的存在。同样，最近一项月球探测任务的数据显示，那里氢气高度密集，据推测，两级的水就在地下不到一米处。
学生：嗯，如果有的话，水是怎么到达的呢？地下河？
教授：我们认为撞击月球的流星或经过的彗星尾巴会留下一些水分子。水分子会沿路到达月磁极旁的火山口地上，由于这些火山口的地面总是处在阴影中，水分子会永远冻结。嗯，还有，如果在水冰中混杂石头和灰尘，便不会容易蒸发掉。
Dian：那么你是不是说月球上可能有原始生物？
教授：我不是这个意思。嗯，好吧，假设月球上有水冰。这对宇航员未来在月球上的驻地将会有非常实际的价值。水冰可以融化过滤，供人饮用。同时也可以分解成氧和氢两种元素。氧气供人呼吸，氢气可以转化成燃料，特别是火箭燃料。因此，未来某天自给自足的月球基地将成为可能，也许是一个采矿营或者是成为未来太空探索的出发点。
学生：但是带着吨重的装备到月球去制造燃料，给月球研究基地提供生命支持系统，这代价不会太高了吗？
教授：永久基地，也许是一种解决方式，但我们不需要等那么久。SPA 盆地底部的灰尘蕴藏着非常有趣的故事。我不会错失一些样本的。
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