Asteroids and comets that repeatedly smashed into the early Earth covered the planet ’s surfacewith molten rock during its earliest days, but still may have left oases of water that could havesupported the evolution of life, scientists say. The new study reveals that during the planet ’sinfancy, the surface of the Earth was a hellish environment, but perhaps not as hellish as oftenthought, scientists added.
Earth formed about 4.5 billion years ago. The first 500 million years of its life are known as theHadean Eon. Although this time amounts to more than 10 percent of Earth’s history, little isknown about it, since few rocks are known that are older than 3.8 billion years old.
For much of the Hadean, Earth and its sister worlds in the inner solar system were pummeled withan extraordinary number of cosmic impacts. “It was thought that because of these asteroids andcomets flying around colliding with Earth, conditions on early Earth may have been hellish, ” saidlead study author Simone Marchi, a planetary scientist at the Southwest Research Institute inBoulder, Colorado. This imagined hellishness gave the eon its name —Hadean comes fromHades, the lord of the underworld in Greek mythology.
However, in the past dozen years or so, a radically different picture of the Hadean began toemerge. Analysis of minerals trapped within microscopiczircon crystals dating from this econ“suggested that there was liquid water on the surface of the Earth back then, clashing with theprevious picture that the Hadean was hellish,” Marchi said. This could explain why the evidenceof the earliest life on Earth appears during the Hadean —maybe the planet was less inhospitableduring that eon than previously thought.
The exact timing and magnitude of the impacts that smashed Earth during the Hadean areunknown. To get an idea of the effects of this bombardment, Machi and his colleagues looked atthe moon, whose heavily cratered surface helped model the battering that its close neighbor Earthmust have experienced back then.
“We also looked at highly siderophile elements (elements that bind tightly to iron), such as gold,delivered to Earth as a result of these early collisions, and the amounts of these elements tells usthe total mass accreted by Earth as the results of these collisions,”Marchi said. Prior researchsuggests these impacts probably contributed less than 0.5 percent of the Earth’s present-day mass.The researchers discovered that “the surface of the Earth during the Hadean was heavily affectedby very large collisions, by impactors [ ?m'p?kt?] larger than 100 kilometers (60 miles) or so —really, really big impactors, ’ Marci said. “When Earth has a collision with an object that big, thatmelts a large volume of the Earth’s crust and mantle, covering a large fraction of the surface,”Marchi added. These findings suggest that Earth ’s surface was buried over and over again by largevolumes of molten rock —enough to cover the surface of the Earth several times. This helpsexplain why so few rock survive from the Hadean, the researchers said.
Why is little known about the Earth ’s first 500 million years?
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