NARRATOR：

Listen to part of a lecture in an astronomy class.

MALE PROFESSOR：

Today I want to talk about a paradox that ties in with the topic we discussed 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 taken from the oldest rocks on Earth?

MALE STUDENT：

Uh, like pebbles... fossilized algae...

MALE PROFESSOR：

Right. And on Mars?

FEMALE STUDENT：

Dry channels...

MALE 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 that temperatures on Earth and Mars would have been lower... which, in turn, suggests...?

MALE STUDENT：

There would have been ice on Earth or Mars...?

MALE 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're 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 rise 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?

FEMALE STUDENT：

Um, carbon dioxide, I guess... like today?

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.

Well, then, some astronomers came up with the idea that atmospheric ammonia may have acted as a greenhouse gas, but ammonia would have been destroyed by the ultraviolet light coming from the Sun, and it had to be ruled out, too.

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

So it's 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, he answer is "mass."

MALE STUDENT：

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

MALE 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 has lost mass over the course of four billion years.

FEMALE STUDENT：

Is that possible?

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.

MALE STUDENT：

Maybe the solar wind was stronger then?

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.

FEMALE STUDENT：

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

MALE 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'll get an answer to that piece of the puzzle one day.