Asteroids and comets that repeatedly smashed into the early Earth covered the planet ’s su

rface with molten rock during its earliest days, but still may have left oases of water that could have supported the evolution of life, scientists say. The new study reveals that during the planet ’s infancy, the surface of the Earth was a hellish environment, but perhaps not as hellish as often thought, scientists added.

Earth formed about 4.5 billion years ago. The first 500 million years of its life are known as the Hadean Eon. Although this time amounts to more than 10 percent of Earth’s history, little is known 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 with an extraordinary number of cosmic impacts. “It was thought that because of these asteroids and comets flying around colliding with Earth, conditions on early Earth may have been hellish, ” said lead study author Simone Marchi, a planetary scientist at the Southwest Research Institute in Boulder, Colorado. This imagined hellishness gave the eon its name —Hadean comes from Hades, the lord of the underworld in Greek mythology.

However, in the past dozen years or so, a radically different picture of the Hadean began to emerge. 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 the previous picture that the Hadean was hellish,” Marchi said. This could explain why the evidence of the earliest life on Earth appears during the Hadean —maybe the planet was less inhospitable during that eon than previously thought.

The exact timing and magnitude of the impacts that smashed Earth during the Hadean are unknown. To get an idea of the effects of this bombardment, Machi and his colleagues looked at the moon, whose heavily cratered surface helped model the battering that its close neighbor Earth must 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 us the total mass accreted by Earth as the results of these collisions,”Marchi said. Prior research suggests 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 affected by very large collisions, by impactors [ ？m&39;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, that melts 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 large volumes of molten rock —enough to cover the surface of the Earth several times. This helps explain why so few rock survive from the Hadean, the researchers said.

Why is little known about the Earth ’s first 500 million years？

A.Because it is an imagined period of time.

B.Because this period is of little significance.

C.Because it is impossible to know about this period.

D.Because no rocks are available as research evidence.

Why is the early Earth imagined to be hellish？A.Because it was often smashed by asteroids and comets.

B.Because back then Hades, the lord of Hell, resigned.

C.Because it was so according to Greek mythology.

D.Because back then there was no life.

Why was the early Earth in fact less inhospitable than often thought？A.Because minerals of the Hadean have been found suggesting the existence of life.

B.Because the clashing brought by asteroids and comets was not completely damaging.

C.Because during the Hadean there already existed the evidence of life.

D.Because there had already been liquid water on the Earth back then.

How can the moon help with the understanding of the impacts that smashed the Earth？A.The moon once smashed into the Earth too.

B.The moon was battered earlier than the Earth.

C.The moon, as a close neighbor, is easier to observe.

D.The moon’s surface is heavily cratered as the Earth ’s.

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