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Narrator:Listen to part of a lecture in an astronomy class.
獨白:聽下面一段天文學課堂的講解。
Today, I want to talk about a paradox the ties in with the topic we discuss last time.
今天，我想講一下與上次我們討論的話題相關的一個悖論。
We were discussing the geological evidence of water, liquid water on Earth and Mars three to four billion years ago.
上次我們討論了三四十億年前在地球和火星上有關水及液態水的地質證據。
So, what evidence of a liquid water environment did we find in rock samples taking from the oldest rocks on Earth?
那么我們在地球上最古老的巖石樣品中發現了什么液態水環境的證據呢？
Eh… Like pebbles, fossilized algae?
呃，比如鵝卵石，或者水藻化石?
Right. And on Mars?
對的。那么在火星上呢？
Dry channels?
干涸的水道？
Good. All evidence of water in liquid form, large quantities of it.
很好。有大量液態水存在的證據。
Now, remember when we talked about star formation, we said that as a star ages, it becomes brighter, right?
還記得我們講過恒星的形成嗎？就是說恒星越老，就變得越亮。
Hydrogen turns into Helium, which releases energy.
氫變成氦的時候會釋放能量。
So our standard model of star formation suggests that the Sun wasn't nearly as bright three to four billion years ago as it is today, which means the temperatures on Earth and Mars would have been lower, which in turn suggests…
因此恒星形成的標準模式就暗示出我們太陽的亮度在三十到四十億年前遠不如今天的太陽，這也就意味著當時地球和火星的溫度比現在更低，那就暗示著…
There would have been ice on Earth or Mars?
那地球和火星上就都是冰了？
Correct. If the young Sun was much fainter and cooler than the Sun today, liquid water couldn't have existed on either planet.
對。如果青年時期的太陽比現在更暗淡，溫度也沒有這么炙熱的話，那么液態水就不可能在這兩個星球上存在。
Now, this apparent contradiction between geologic evidence and the stellar evolution model became known as the faint young Sun paradox.
目前，地質證據和恒星進化模式之間的矛盾就是大家眾所周知的早期太陽悖論。
Now, there have been several attempts to solve this paradox.
現在,許多人試圖解開這個悖論。
First, there was the greenhouse-gas solution.
首先能解釋這個悖論的就是溫室氣體。
Well, you are probably familiar with the greenhouse gas effect, so I won't go into details now.
呃，你們可能已經很熟悉溫室氣體效應了，我就不多說了。
The idea was that trapped greenhouse gases in the atmospheres of Earth and Mars might have caused temperatures to raise enough to compensate for the low heat the young Sun provided.
這個理論就是說在地球和火星的大氣層里有足夠的溫室氣體來幫助提升溫度，從而彌補了年輕時期的太陽提供的較低的熱度。
And so it would have been warm enough on these planets for liquid water to exist.
這樣就有足夠的溫度讓這些星球上的水以液化形式存在了。
So, what gas do you think was the first suspect in causing the greenhouse effect?
那么，我們最先想到的是什么氣體能導致溫室效應呢？
Um…carbon dioxide, I guess. Like today?
呃，我想是 二氧化碳，和今天一樣？
In fact, studies indicate that four billion years ago, carbon dioxide levels in the atmosphere were much higher than today's levels.
事實上，研究顯示在四十億年前，二氧化碳在大氣層中的比重遠比今天的要高。
But the studies also indicate that they weren't high enough to do the job-make up for a faint Sun.
但是研究也顯示當時二氧化碳的量還不足以來彌補太陽沒能提供的熱度。
Then some astronomers came up with the idea that atmospheric ammonia might have acted as a greenhouse gas.
然后天文學家們就有另外一個想法，就是大氣氨可能扮演了溫室氣體的角色。
But ammonia would have been destroyed by the ultra-violet light coming from the Sun and it had to be ruled out too.
但是氨氣就會被來自太陽的紫外線所分解，所以氨氣一定會被排除。
Another solution, which is proposed much later, was that perhaps the young Sun wasn't faint at all, perhaps it was bright.
另外一種后來提出的解釋，就是年輕時期的太陽根本不暗淡，也許是明亮的。
So it is called the bright-young-Sun solution, according to which the Sun would have provided enough heat for the water on Earth and Mars to be liquid.
因此根據太陽能都為地球和火星上的液態水提供足夠的熱量，我們稱這種理論為“明亮的早期太陽解決方案”。
But how could the early Sun be brighter and hotter than predicted by the standard model?
但是早期太陽怎么會比按照標準模式預測的更明亮更炙熱呢？
Well, the answer is mass.
呃，答案就是質量。
You mean the Sun had more mass when it was young?
你的意思是說太陽年輕的時候質量更大。
Well, if the young Sun was more massive than today's, it would have been hotter and brighter than the model predicts.
呃，如果年輕時期的太陽比今天的太陽質量更大，那么它就會比預測的更加炙熱和明亮。
But this would mean that it had lost mass over the course of four billion years.
但是這也意味著在過去的四十億年中，它的質量也在大大降低。
Is that possible?
這個可能嗎？
Actually, the Sun is constantly losing mass through the solar wind, a stream of charged particles constantly blowing off the Sun.
事實上，在太陽風的影響下，太陽的質量一直在下降，太陽風就是一股從太陽表面放出的帶電粒子流。
We know the Sun's current rate of mass loss, but if we assume that this rate has been steady over the last four billion years, the young Sun wouldn't have been massive enough to have warmed Earth, let alone Mars, not enough to have caused liquid water.
我們知道目前太陽質量的減少率，但是假如這個率在過去四十億年間一直都沒有改變的話，年輕時期的太陽就不可能有足夠的質量來給地球提供溫度，就不用說火星了，不可能有足夠的熱量使液態水存在的。
Maybe the solar wind was stronger then?
也許那時候的太陽風更強烈呢？
There is evidence that the solar wind was more intense in the past.
的確有證據顯示太陽風在以前是更加強烈的。
But we don't know for sure how much mass our Sun's lost over the last four billion years.
但是我們不能確定的知道在過去這四十億年中太陽到底減少了多少質量。
Astronomers tried to estimate what solar mass could produce the required luminosity to explain liquid water on these planets.
天文學家試圖去推測是太陽質量是多少才能產生需要的發光度來解釋這兩個星球上的液體水。
They also took into account that with a more massive young Sun, the planets would be closer to the Sun than they are today.
他們還考慮到如果年輕時期的太陽質量更大，那么這兩個行星會比目前靠太陽更近。
And they found that about seven percent more mass would be required.
而且他們也發現太陽的質量需要大 7%才成。
So the young Sun had seven percent more mass than our Sun?
所以說年輕時期的太陽比現在的太陽質量大 7%？
Well, we don't know.
呃，這個我們還不知道。
According to observations of young Sun like stars, our Sun may have lost as much as six percent of its initial mass, which doesn't quite make it.
根據對其它年輕的恒星的觀察，推斷出我們的太陽可能比原始的質量小了 6%,這與 7%還是有很大的差距的。
On the other hand, this estimate is based on a small sample.
然而，這一推測僅僅是基于小部分的樣本觀察得出的。
And the bright-young-Sun solution is appealing.
而且“明亮的早期太陽解決方案”的理論也很有可能性。
We simply need more data to determine the mass loss rate of stars.
我們只是需要更多的數據來確定恒星的質量減少率。
So there's reason to believe that we will get an answer to that piece of the puzzle one day.
所以我們有理由相信總有一天我們能得到這個謎的答案。
