We were discussing the geological evidence of liquid water on Earth and Mars three to four billion years ago.
我们讨论了30到40亿年前地球和火星上液态水存在的地质证据。
In fact, studies indicate that four billion years ago, carbon dioxide levels in the atmosphere were much higher than today's levels.
事实上，研究表明40亿年前大气中的二氧化碳含量远远高于今天的水平。
Four billion years ago was a hellish time on planet Earth.
四十亿年前的地球如同地狱一般。
They say more than four billion years ago, Mars was covered with water 137 meters deep.
他们表示，在40多亿年前，火星覆盖着137米深的水。
The Caltech scientists say they have directly established the temperature of Mars four billion years ago.
加州理工学院的科学家们称他们已经确定了四十亿年前火星的温度。
He used computer simulations to explore what the solar system may have looked like some four billion years ago.
他利用电脑模拟来重现太阳系四十亿年前可能的样子。
Over 1,500 minerals are thought to have formed on Earth before the beginnings of life some four billion years ago.
大约四百万年前生命诞生前，地球上的矿物种类被认为已超过1500种。
But this result suggests quite strongly that even as long as four billion years ago, the surface of Mars would have been challenging for life.
但这一结果充分说明，即使在40亿年前，火星表面的生存环境仍然很恶劣。
Four and a half billion years ago the Earth was about sixty percent formed, though it did have a differentiated core, crust and mantle.
45亿年前，地球大约只有现在百分之六十，尽管它确有能够区分的地核，地壳和地幔。
Going back for another look at Tempel 1 will provide new insights on how comets work and how they were put together four-and-a-half billion years ago.
再次光顾彗星Tempel1，将会提供很多新的信息，以帮助我们认识彗星如何运行以及她们在45亿年前是如何组成的？
A modern PC typically has a processor running at 3.0ghz (3 billion clock ticks per second), little changed from a PC made three or four years ago.
现在一个电脑的处理器的频率通常是3.0GHz(每秒30亿次时针摆动)，这与三年或四前年制造的电脑没有太大的变化。
But four-and-a-half billion years ago, when our planet was born, it was just a molten inferno.
但四十五亿年前，在我们的星球诞生时，它只不过是一个炽热炼狱。
Instead, it is thought that lithium, composed of just three protons and four neutrons, was mainly produced just after the Big Bang, 13.7 billion years ago.
相反，仅由三个质子和四个中子构成的锂主要出现在137亿年之前的大爆炸之后。
Instead, it is thought that lithium, composed of just three protons and four neutrons, was mainly produced just after the Big Bang, 13.7 billion years ago.
相反，仅由三个质子和四个中子构成的锂主要出现在137亿年之前的大爆炸之后。
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