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When Earth was a Snowball | |
admin | |
2018-01-22 | |
发布年 | 2018 |
语种 | 英语 |
国家 | 法国 |
领域 | 地球科学 |
正文(英文) |
Artist's impression of Snowball Earth 720 million years ago.
The Earth hasn't always been the blue, hospitable planet it is today. On at least three occasions our planet was completely covered with ice. How did the Earth get into this state and, above all, how did it manage to get out of it?
In the course of its history Earth has experienced periods so cold that its surface was entirely covered with ice. Long questioned, these so-called "Snowball Earth" episodes are today attested by considerable geological and geochemical evidence. Since the 2000s, climatologist Gilles Ramstein1 has been investigating these events, in which the Earth seen from space would have resembled the icy moons Europa and Enceladus. He tells CNRS News about these icy spells that impacted our planet. The Earth is located in the habitable zone of its star, which normally means that water can exist in a liquid state there. In that case, how could the planet have completely iced over? From the very beginning, Earth's atmosphere has behaved like a heating blanket.
The climate is thus a balance between sources of greenhouse gases and sinks that remove them from the atmosphere. On our planet, the main natural source of CO2 is volcanic activity, while its principal sink depends on the weathering of continental silicate rocks caused by water runoff. When CO2 reacts with the rock it is removed from the atmosphere and sequestered as carbonate, which accumulates as underwater sediment. Over 3 billion years ago, volcanism was already in full swing, although the total area of the continents was still limited and erosion was negligible. As a result, the atmospheric concentration of CO2 was very high. In addition, the same period saw the emergence, 3.5 billion years ago, of methanogenic archaea. The metabolism of these unicellular microorganisms produces methane, whose greenhouse effect is thirty times greater than CO2. The build-up of these two gases—carbon dioxide and methane—in Earth's early atmosphere explains why our planet was so warm in its youth. 750 million years ago, the atmospheric concentration of greenhouse gases fell to such an extent that the Earth's mean temperature dropped by around 50 °C.
And yet, 700 million years ago solar luminosity was 94% of its current value, so surely the Earth should have been spared this! In fact, this time it was the CO2 engine that failed. The Rodinia supercontinent—which had formed 500 million years earlier—began to break up along the tropical belt (see map) 800 million years ago. These tectonic processes were accompanied by massive volcanic eruptions that covered vast regions with basalt. This major event was associated with the opening of oceans and seaways, generating new sources of atmospheric moisture near the continents. This led to increased precipitation and runoff on the continents and, therefore, to greater erosion of continental rocks. Since recently formed basalt rocks weather six to eight times faster than granite, much of the CO2 disappeared into this sink. 750 million years ago, the atmospheric concentration of greenhouse gases fell to such an extent that the Earth's mean temperature dropped by around 50 °C. As a result, the polar ice sheets extended as far as the equator.
How did the Earth manage to escape from such Snowball Earth episodes of runaway global glaciation? The last few billion years of Earth's history may well be lifeless, due to lack of water and photosynthesis.
At the time, nobody believed him, as the albedo of an ice-covered Earth is so high that in order for it to emerge from a global glaciation, the Sun's luminosity would have needed to be 1.5 times greater. Then, in the 1990s, scientists realized that it was not the Sun that put an end to Snowball Earth episodes, but CO2. Although covered in ice, the Earth continued to experience high volcanic activity. So what happened is that, for millions of years, the CO2 emitted by volcanoes built up in the atmosphere. This eventually caused a super-greenhouse effect that outdid that of the ice- and snow-albedo, leading to a sudden meltdown. G. R.: If the Earth had completely frozen, any life surviving in the oceans should have suffocated. At the LSCE, we were able to show that our planet was not completely ice-covered, thanks in particular to hot vents and volcanoes. Life may have been able to take refuge in water-filled holes in the ice known as cryoconite holes. This was sufficient to allow exchanges between the atmosphere and the ocean and enable life to survive until deglaciation. Although the Earth has known mass extinctions, it has remained highly conducive to life for the last 4 billion years. All in all, it has experienced remarkable climate stability throughout its history. Global glaciation episodes represent only a fraction of it. However, climate conditions will not always be favorable to life: as the Sun becomes warmer and warmer, it will eventually evolve into a red giant. The last few billion years of Earth's history may well be lifeless, due to lack of water and photosynthesis. Just as on Venus, life will have been unable to adapt—but it will have been present on Earth for a very long time. And that's good news for all those who are looking for it elsewhere. In the Solar System, we are today exploring worlds such as Europa and Enceladus, icy moons that are reminiscent of the distant past of our own planet. Footnotes
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来源平台 | Centre national de la recherche scientifique |
文献类型 | 新闻 |
条目标识符 | http://119.78.100.173/C666/handle/2XK7JSWQ/129656 |
专题 | 地球科学 |
推荐引用方式 GB/T 7714 | admin. When Earth was a Snowball. 2018. |
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