We have reasons to suspect, based on recent high resolution photography of the surface, that there may be a subsurface ocean there. And where we find water and where we find the right mix of organics, there is the possibility for life. So we have to entertain the idea then that Europa could provide in this subsurface ocean environment, a habitat that would be clement for life and where life could have originated.

"I don't know whether we'll find life beyond single cell. And when I talk about life, I want to be very, very clear. Single cell life would be an unbelievable find. And any higher level, I just don't know, I just don't know. But I doubt that we'll find a human species. "

When our spacecraft eventually penetrated Europa's icy sheets, they might find scenes like this, an undersea world where life prospers without sunlight or oxygen like our own deep ocean thermal beds. And beyond our own solar system, there is always the seductive possibility that an intelligent group of beings like ourselves exists somewhere. After all, our sun is one of billions of stars that shine in the universe, a universe that contains all the elements found in life on Earth. A distant star could be the sun of another civilization. Could aliens have paid us a visit already ? Maybe eons ago?

"It's always possible that there is something we haven't recognized that has been left here maybe a long time ago by alien visitation. But in the absence of presenting me with that evidence, I see no reason to, to believe this. I really think it's such an expensive exercise to physically travel from one star system to another. I can't imagine the motivation of any alien species that would do it. "

Humankind has explored the solar system, but we lack the technology to send men or robotic missions to another planetary system. But we've certainly left evidence of our own existance. The trail of debris from our space missions like Hansel and Gretel's breadcrumbs in the forest may one day mark the way for an alien group looking for us.

"Life probably does exist elsewhere in the universe, probably not sufficiently, frequently in the universe for us ever to be likely to meet it. So that would be my guess is that life is present elsewhere in the universe, but is probably so spaced-out that we may never know about it. "

Out there, way out there, in other galaxies, what is the likelihood? The president of the SETI Institute has developed a complicated equation to actually figure the odds.

It's called the Drake Equation. It's based upon things we do know about the universe, and other things we can make educated guesses about, things like how often new stars form, how many stars have planets, how many planets have life, the chance of that life developing intelligence, the chances that intelligence will lead to technology.

words in this passage

clement:describes weather which is pleasant or not severe:温和的，温暖的（气候等）
例：It's very clement for the time of year.

Hansel and Gretel：a fairy tale, for more information,

Drake Equation: