托福official68听力lecture2 Zircon原文解析+翻译音频

[00:00.00]Professor: OK, as we said in the last class, the general consensus among scientists is that earth is 4.5 billion years old. [00:14.47]So, what was earth like, when it was a new planet, 4.5 billion years ago? Kitty?
[00:21.73]Student: Um, it was pretty much covered in magma, with really fiery conditions, like volcanoes, plus there were like giant rocks, meteorites that collided with earth that would melt when they landed here.
[00:33.84]Professor: Great, melt or vaporize. This went on for hundreds of millions of years. [00:39.24]There was no life here then of course, just fire and molten rock. [00:44.16]And no liquid water either, no oceans. And, and when do you think start to change?
[00:49.90]Student: It lasts about five hundred million years, right? Until earth was like five hundred million years old?
[00:55.87]Professor: OK, researchers have deduced that since we don't have proof of any intact rocks from before then, that earth started cooling down after about five hundred million years, rocks earlier than that, would’ve been obliterated by the fire and heat. It makes sense, right?
[01:13.85]Student: Sure.
[01:14.88]Professor: OK, so a few weeks ago, we talked about Zircon. Anyone remembered anything about zircon.
[01:23.07]Student: Well, it's a mineral. Um, I know it has unusual properties, but...
[01:28.46]Professor: Great and know, that’s a good start. Zircon is a particularly sturdy mineral, it forms crystals. [01:36.38]And the thing about zircon is when it forms crystals, these crystals encapsulate or grow around other materials. [01:45.34]So, the zircon crystals retained evidence of the conditions that were present when they formed. [01:51.05]And even the other materials that were present at the time.
[01:56.12]Now, researchers in the 1980s discovered some zircon crystals in Australia. [02:02.14]At that time though, we lacked the technology to make any definite claims about the materials found in the zircon. [02:09.02]There seemed to be some evidence of material from way, way back before scientists thought earth had cooled enough for to exist, but researchers didn't have any conclusive evidence at the time.
[02:21.17]Student: So, they still thought earth started cooling, when it was five hundred million years old.
[02:26.50]Professor: Well, the discovery of those zircon crystals was pretty exciting, you know. [02:30.80]But really, most scientists had to stick with the theory, that had the clearest evidence. [02:36.90]And like I said, technology just wasn’t of the point yet where could really challenge accepted view.
[02:42.97]Student: But, how do they date the zircon in the material in it.
[02:46.09]Professor: Well, in the 1980s, they used a special kind of iron micro-probe. [02:52.82]So, with the special iron micro-probe, they removed a few atoms from the zircon’s surface, then they compare the mass of those items. [03:02.56]That’s how they measured the composition of the atoms. And this is what enabled the researchers to date the zircons. [03:09.53]In fact, using this technique, they could date, specific parts of a single crystal of zircon.
[03:21.08]And then years later, in 1999, they began to reinvestigate. [03:26.21]With an improved iron micro-probe, they analyzed another group of crystals and found that a few of them were more than four billion years old, which means, they were around, during those first five hundred million years when we thought earth was a very fiery place that destroyed any rocks of crystals they came to contact with it.
[03:47.82]Student: But for some reason, these zircon crystals weren’t destroyed?
[03:51.33]Professor: That’s right. I mean we know zircon can withstand a lot, but it definitely couldn't have survived constant fireball. [03:59.68]So, the question was, were these zircon crystals an indication that earth started cooling earlier than we thought. [04:07.68]Out of those crystals that we’re looking at, the oldest one, seemed to have been around when earth was only one hundred million years old. [04:16.92]If our view of earlier earth was correct, that shouldn't be possible. OK, now here’s where gets interesting. [04:24.45]They did some further testing on the zircon crystals. Remember how I say zircons contained evidence not just materials but also what the conditions were like when they formed, they found the rock that was crystallized inside the oldest zircons had high oxygen isotope ratios.
[04:44.32]Student: How do zircons in rocks get high oxygen isotope ratios.
[04:49.01]Professor: Hum, the required conditions are, and get this, liquid water and low temperatures. [04:57.44]This is only known way that high oxygen isotope ratios can occur in zircon in rocks. Yup, water and low temperatures, so.
[05:07.19]Student: So, and that's when earth was only one hundred million years old, that's really different from a...
[05:13.13]Professor: A very different, so we really had to rethink our ideas of the conditions on earth back then.