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旁白：听一段天文学课堂的讲解。

Narrator: Listen to part of a lecture in an astronomy class.

教授：今天，我想讲一下与上次我们讨论的话题相关的一个悖论。

Professor: Today, I want to talk about a paradox the ties in with the topic we discuss last time.

上次我们讨论了三四十亿年前在地球和火星上有关水及液态水的地质证据。

We were discussing the geological evidence of water, liquid water on Earth and Mars three to four billion years ago.

那么我们在地球上最古老的岩石样品中发现了什么液态水环境的证据呢？

So, what evidence of a liquid water environment did we find in rock samples taking from the oldest rocks on Earth?

学生：呃……比如鹅卵石，或者水藻化石?

Student: Eh… Like pebbles, fossilized algae?

教授：对的。那么在火星上呢？

教授：很好。有大量液态水存在的证据。

Professor: Good. All evidence of water in liquid form, large quantities of it.

还记得我们讲过恒星的形成吗？

Now, remember when we talked about star formation, we said that as a star ages, it becomes brighter, right?

就是说恒星越老，就变得越亮，氢变成氦的时候会释放能量。

Hydrogen turns into Helium, which releases energy.

因此恒星形成的标准模式就暗示出我们太阳的亮度在三十到四十亿年前远不如今天的太阳，这也就意味着当时地球和火星的温度比现在更低，那就暗示着……

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…

学生：那地球和火星上就都是冰了？

Student: There would have been ice on Earth or Mars?

教授：对。如果青年时期的太阳比现在更暗淡，温度也没有这么炙热的话，那么液态水就不可能在这两个星球上存在。

Professor: Correct. If the young Sun was much fainter and cooler than the Sun today, liquid water couldn't have existed on either planet.

目前，地质证据和恒星进化模式之间的矛盾就是大家众所周知的早期太阳悖论。

Now, this apparent contradiction between geologic evidence and the stellar evolution model became known as the faint young Sun paradox.

现在, 许多人试图解开这个悖论。

Now, there have been several attempts to solve this paradox.

首先能解释这个悖论的就是温室气体。

First, there was the greenhouse-gas solution.

呃，你们可能已经很熟悉温室气体效应了，我就不多说了。

Well, you are probably familiar with the greenhouse gas effect, so I won't go into details now.

这个理论就是说在地球和火星的大气层里有足够的温室气体来帮助提升温度，从而弥补了年轻时期的太阳提供的较低的热度。

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.

这样就有足够的温度让这些星球上的水以液化形式存在了。

And so it would have been warm enough on these planets for liquid water to exist.

那么，我们最先想到的是什么气体能导致温室效应呢？

So, what gas do you think was the first suspect in causing the greenhouse effect?

学生：呃……我想是二氧化碳，和今天一样？

Student: Um…carbon dioxide, I guess. Like today?

教授：事实上，研究显示在四十亿年前，二氧化碳在大气层中的比重远比今天的要高。

Professor: In fact, studies indicate that four billion years ago, carbon dioxide levels in the atmosphere were much higher than today's levels.

但是研究也显示当时二氧化碳的量还不足以来弥补太阳没能提供的热度。

But the studies also indicate that they weren't high enough to do the job—make up for a faint Sun.

然后天文学家们就有另外一个想法，就是大气氨可能扮演了温室气体的角色。

Then some astronomers came up with the idea that atmospheric ammonia might have acted as a greenhouse gas.

但是氨气就会被来自太阳的紫外线所分解，所以氨气一定会被排除。

But ammonia would have been destroyed by the ultra-violet light coming from the Sun and it had to be ruled out too.

另外一种后来提出的解释，就是年轻时期的太阳根本不暗淡，也许是明亮的。

Another solution, which is proposed much later, was that perhaps the young Sun wasn't faint at all, perhaps it was bright.

因此根据太阳能都为地球和火星上的液态水提供足够的热量，我们称这种理论为“明亮的早期太阳解决方案”。

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.

但是早期太阳怎么会比按照标准模式预测的更明亮更炙热呢？

But how could the early Sun be brighter and hotter than predicted by the standard model?

学生：你的意思是说太阳年轻的时候质量更大。

Student: You mean the Sun had more mass when it was young?

教授：呃，如果年轻时期的太阳比今天的太阳质量更大，那么它就会比预测的更加炙热和明亮。

Professor: Well, if the young Sun was more massive than today's, it would have been hotter and brighter than the model predicts.

但是这也意味着在过去的四十亿年中，它的质量也在大大降低。

But this would mean that it had lost mass over the course of four billion years.

教授：事实上，在太阳风的影响下，太阳的质量一直在下降，太阳风就是一股从太阳表面放出的带电粒子流。

Professor: Actually, the Sun is constantly losing mass through the solar wind, a stream of charged particles constantly blowing off the Sun.

我们知道目前太阳质量的减少率，但是假如这个率在过去四十亿年间一直都没有改变的话，年轻时期的太阳就不可能有足够的质量来给地球提供温度，就不用说火星了，不可能有足够的热量使液态水存在的。

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.

学生：也许那时候的太阳风更强烈呢？

Student: Maybe the solar wind was stronger then?

教授：的确有证据显示太阳风在以前是更加强烈的。

Professor: There is evidence that the solar wind was more intense in the past.

但是我们不能确定的知道在过去这四十亿年中太阳到底减少了多少质量。

But we don't know for sure how much mass our Sun's lost over the last four billion years.

天文学家试图去推测是太阳质量是多少才能产生需要的发光度来解释这两个星球上的液体水。

Astronomers tried to estimate what solar mass could produce the required luminosity to explain liquid water on these planets.

他们还考虑到如果年轻时期的太阳质量更大，那么这两个行星会比目前靠太阳更近。

They also took into account that with a more massive young Sun, the planets would be closer to the Sun than they are today.

而且他们也发现太阳的质量需要大7%才成。

And they found that about seven percent more mass would be required.

学生：所以说年轻时期的太阳比现在的太阳质量大7%？

Student: So the young Sun had seven percent more mass than our Sun?

教授：呃，这个我们还不知道。

根据对其它年轻的恒星的观察，推断出我们的太阳可能比原始的质量小了6%，这与7%还是有很大的差距的。

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.

然而，这一推测仅仅是基于小部分的样本观察得出的。

On the other hand, this estimate is based on a small sample.

而且“明亮的早期太阳解决方案”的理论也很有可能性。

And the bright-young-Sun solution is appealing.

我们只是需要更多的数据来确定恒星的质量减少率。

We simply need more data to determine the mass loss rate of stars.

所以我们有理由相信总有一天我们能得到这个谜的答案。

So there's reason to believe that we will get an answer to that piece of the puzzle one day.

Why is geological evidence of liquid water on Earth and Mars three to four billion years ago problematic?

Why did the greenhouse-gas solution fail to explain the early presence of liquid water on Earth and Mars?

Why does the professor mention the solar wind?

What factor did astronomers consider when calculating the required solar mass for liquid water to exist on Earth and Mars?

What is the professor's attitude about the bright-young-Sun solution?

Narrator: Listen to part of a lecture in an astronomy class. 
Professor: 
Today, I want to talk about a paradox the ties in with the topic we discuss last time. We were discussing the geological evidence of water, liquid water on Earth and Mars three to four billion years ago. So, what evidence of a liquid water environment did we find in rock samples taking from the oldest rocks on Earth? 
Student: 
Eh… Like pebbles, fossilized algae? 
Professor: 
Right. And on Mars? 
Student: 
Dry channels? 
Professor: 
Good. All evidence of water in liquid form, large quantities of it. Now, remember when we talked about star formation, we said that as a star ages, it becomes brighter, right? Hydrogen turns into Helium, which releases energy. 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… 
Student: 
There would have been ice on Earth or Mars? 
Professor: 
Correct. If the young Sun was much fainter and cooler than the Sun today, liquid water couldn’t have existed on either planet. Now, this apparent contradiction between geologic evidence and the stellar evolution model became known as the faint young Sun paradox. Now, there have been several attempts to solve this paradox. 
Professor: 
Good. All evidence of water in liquid form, large quantities of it. Now, remember when we talked about star formation, we said that as a star ages, it becomes brighter, right? Hydrogen turns into Helium, which releases energy. 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… 
Student: 
There would have been ice on Earth or Mars? 
Professor: 
Correct. If the young Sun was much fainter and cooler than the Sun today, liquid water couldn’t have existed on either planet. Now, this apparent contradiction between geologic evidence and the stellar evolution model became known as the faint young Sun paradox. Now, there have been several attempts to solve this paradox. 
First, there was the greenhouse-gas solution. Well, you are probably familiar with the greenhouse gas effect, so I won’t go into details now. 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. And so it would have been warm enough on these planets for liquid water to exist. So, what gas do you think was the first suspect in causing the greenhouse 
effect? 
Student: 
Um…carbon dioxide, I guess. Like today? 
Professor: 
In fact, studies indicate that four billion years ago, carbon dioxide levels in the atmosphere were much higher than today’s levels. But the studies also indicate that they weren’t high enough to do the job—make up for a faint Sun. Then some astronomers came up with the idea that atmospheric ammonia might have acted as a greenhouse gas. But ammonia would have been destroyed by the ultra-violet light coming from the Sun and it had to be ruled out too. 
Another solution, which is proposed much later, was that perhaps the young Sun wasn’t faint at all, perhaps it was bright. 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. But how could the early Sun be brighter and hotter than predicted by the standard model? Well, the answer is mass. 
Student: 
You mean the Sun had more mass when it was young? 
Professor: 
Well, if the young Sun was more massive than today’s, it would have been hotter and brighter than the model predicts. But this would mean that it had lost mass over the course of four billion years. 
Student: 
Is that possible? 
Professor: 
Actually, the Sun is constantly losing mass through the solar wind, a stream of charged particles constantly blowing off the Sun. 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. 
Student: 
Maybe the solar wind was stronger then? 
Professor: 
There is evidence that the solar wind was more intense in the past. But we don’t know for sure how much mass our Sun’s lost over the last four billion years. Astronomers tried to estimate what solar mass could produce the required luminosity to explain liquid water on these planets. They also took into account that with a more massive young Sun, the planets would be closer to the Sun than they are today. And they found that about seven percent more mass would be required. 
Student: 
So the young Sun had seven percent more mass than our Sun? 
Professor: 
Well, we don’t know. 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. On the other hand, this estimate is based on a small sample. And the bright-young-Sun solution is appealing. We simply need more data to determine the mass loss rate of stars. So there’s reason to believe that we will get an answer to that piece of the puzzle one day.