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 地球の最初の岩
When the Earth first formed four and a half billion years ago, it's too hot to form with water. But the planet has water now. If it wasn't delivered by comets, then has it been here all along? To find out where the water comes from, Professor Stephen Mojzsis is looking for clues to when it first appears. As a geologist, he uncovers the planet's past by looking at its rocks. But the past has a big hole. It's called the Hadean or Dark Period because nobody knows anything about it.The problem of the first 500 million years of Earth history for geologists has always been...that you have a nightmare scenario where you don't have any rocks or minerals to investigate. It's thought early Earth is a fireball of molten rock. But with no evidence from this time, it's just a theory. Then in 2002, Mojzsis makes an earthshaking discovery - the oldest mineral sample ever found on our planet - a crystal of zircon. Locked inside these tiny grains is a vital clue. Chemical analysis of zircon crystals reveals whether they formed in the presence of water. Over seven years, Mojzsis finds and dates nearly 100,000 crystals of zircon. One stands alone. The oldest one that we've found so far is 4.38 billion years old. Now, I'd like that to sink in. When this crystal was made, the Earth was just 180 million years old. If this line represents the age of the earth, then the dinosaurs died out here. Rewind about 4 billion years to the end of the dark period. The zircon crystal was formed here. We're talking about time spans that are beyond comprehension. For instance, in this rock here, there are zircon crystals that are one-third of the age of the universe. So these things have been around for almost as long as the Earth has been a planet. Talk about time capsules! Analysis shows this 4.4 billion-year-old zircon crystal forms in liquid water rather than a dry planet...the crystal reveals that early Earth already has water on its surface.