We've all seen movies about terrible insects from outer space or stories of abduction by little green men,
我们时常在电影里看到从外太空来的可怕的昆虫，或者地球人被小绿人绑架的故事，
but the study of life in the universe, including the possibility of extraterrestrial life, is also a serious, scientific pursuit.
但是有关宇宙生物的研究，包括外星生物存在的可能性，却是一门严肃的科学研究。
Astrobiology draws on diverse fields, such as physics, biology, astronomy, and geology,
天体生物学集合了物理学、生物学、天文学以及地质学的知识，
to study how life was formed on Earth, how it could form elsewhere, and how we might detect it.
来研究生命是怎样在地球上形成的，它如何能在地球以外的地方形成，以及我们怎样才能探知地外生命。
Many ancient religions described other worlds inhabited by known human beings,
很多古老的宗教都描述过人类居住的另外的世界，
but these are more like mythical realms or parallel universes than other planets existing in the same physical world.
但这些都属于神话或者是平行宇宙的范围，与其它存在于我们这个物理世界的行星无关。
It is only within the last century that scientists have been able to seriously undertake the search for extraterrestrial life.
从上个世纪开始，科学家们才有能力真正地着手有关地外生命的研究。
We know that at the most basic level organisms on Earth need three things:
我知道地球上最基本的生物需要三种物质：
liquid water, a source of energy, and organic, carbon-based material.
液态水、能量源以及有机碳化物。
We also know that the Earth is just the right distance from the Sun, so as not to be either frozen or molten.
我们也知道地球与太阳之间的距离刚刚好，以至于我们不会被冰封，也不会被熔化。
So, planets within such a habitable range from their own stars may be able to support life.
所以，与恒星保持适当距离的行星应该有孕育生命的可能性。
But while we used to think that life could only exist in such Earth-like environments,
我们曾经以为生命只能存在于类似地球的环境下，
one of the most amazing discoveries of astrobiology has been just how versatile life is.
但是，天体生物学的最惊人的发现之一，让我们知道了生命形式的多样性。
We now know that life can thrive in some of the most extreme environments that'd be fatal for most known organisms.
我们现在知道生命可以在一些极其严酷的环境中成长，而这些环境对大多数已知生物来说都是致命的。
Life is found everywhere, from black smoke of hydrothermal vents in the dark depths of Earth's oceans,
到处都可以找到生命的迹象，从地球深海热泉的黑烟，
to bubbling, hot, acidic springs on the flanks of volcanoes, to high up in the atmosphere.
到火山侧翼滚烫的酸性温泉，再到大气的高层。

Organisms that live in these challenging environments are called extremophiles,
生长于这些环境中的生物被叫做嗜极生物，
and they can survive at extremes of temperature, pressure, and radiation,
它们可以存活于极端的温度、压力、辐射、
as well as salinity, acidity, and limited availability of sunlight, water, or oxygen.
盐度、酸度以及有限的阳光、水分或氧气的条件下。
What is most remarkable about these extremophiles is that they are found thriving in environments that mimic those on alien worlds.
这些嗜极生物最引人注目的是，适合它们生长的环境往往与外星环境相似。
One of the most important of these worlds is our red and dusty neighbor, Mars.
最重要的地外行星之一，是我们布满尘土的邻居，火星。
Today, astrobiologists are exploring places where life might once have existed on Mars using NASA's Curiosity rover.
目前，天体生物学家们正在利用美国宇航局的好奇号火星探测器来勘探。
One of these is Gale Crater, an impact crater created when a meteor hit the surface of Mars nearly 3.8 billions years ago.
火星上其中一处可能存在过生命的地方便是盖尔撞击坑，它是于将近38亿年前彗星撞击火星表面造成的。
Evidence from orbit suggest past traces of water, which means the crater might once have supported life.
我们从彗星轨道上发现有少量水分的迹象，说明撞击坑可能曾经有过生命存在。
Planets are not the only places astrobiologists are looking at.
而天体生物学家们不仅仅关注地外行星。
For example, Europa, one of the moons of Jupiter, and Enceladus and Titan, two of Saturn's moons, are all exciting possibilities.
它们也关注其它有可能存在生命的地方，包括木星的卫星之一，木卫二，以及土星的两个卫星，土卫二和土卫六。
Although these moons are extremely cold and two are covered in thick ice, there is evidence of liquid oceans beneath the shell.
虽然这些卫星都极其寒冷，其中两个还被厚冰覆盖，但有证据表明它们的外壳下面存在液态海洋。
Could life be floating around in these oceans, or could it be living around black smoker vents at the bottom?
生命有可能浮游在这些海洋中，或有可能生存于深海热泉的黑烟中吗？
Titan is particularly promising as it has an atmosphere and Earth-like lakes, seas, and rivers flowing across the surface.
土卫六是很有希望的，因为它拥有大气层，并且其表面上分布着类似地球的湖泊，海洋，河流。
It is very cold, however, too cold for liquid water,
但是，它非常寒冷，以至没有液态水存在，
so these rivers may instead be flowing with liquid hydrocarbons such as methane and ethane.
所以土卫六的河流中流动的应该是液态的碳氢化合物，例如，甲烷和乙烷。
These are composed of hydrogen, and, more importantly, carbon, which is the basic building block of all life as we know it.
这些物质由氢和碳构成，而碳元素则是所有已知生命的基础组成部分。
So, could life be found in these lakes?
那么，我们有可能在这些湖泊中发现生命吗？
Although instruments are being designed to study these distant worlds,
虽然科学家们已经着手设计用于探测遥远星球的仪器，
it takes many years to build them and even longer to get them where they need to be.
但是制造这些仪器需要许多年，而把它们送到外星则需要更长时间。
In the meantime, astrobiologists work in our own natural laboratory, the Earth,
在此期间，天体生物学家们将利用我们的天然实验室，地球，
to learn about all the weird and wonderful forms of life that can exist
来了解可能存在的各种奇特的生命形式，
and to help us one day answer one of humanity's oldest questions: Are we alone?
从而帮助我们回答人类有史以来最古老的一个问题：我们是孤独的吗？