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Comets, not asteroids, scarred Moon's face about 4 billion years ago.

Byline: ANI

London, July 28 (ANI): A new study of ancient rocks in Greenland has suggested that icy comets - not rocky asteroids This is a list of numbered minor planets, nearly all of them asteroids, in sequential order.

As of late September 2007 there are 164,612 numbered minor planets, and many more not yet numbered. Most asteroids are ordinary and not particularly noteworthy.
 - launched a dramatic assault on the Earth and moon around 3.85 billion years ago, thus causing the lunar surface to become scarred.

"We can see craters on the moon's surface with the naked eye, but nobody actually knew what caused them - was it rocks, was it iron, was it ice?" Uffe Grae Jorgensen, an astronomer at the Niels Bohr Institute Coordinates:

The Niels Bohr Institute is part of the Niels Bohr Institute for Astronomy, Physics and Geophysics of the University of Copenhagen.
 in Copenhagen, Denmark, told New Scientist.

"It's exciting to find signs that it was actually ice," he said.

Evidence suggests that the Earth and moon had both formed around 4.5 billion years ago.

But, almost all the craters on the moon This is a list of craters on the Moon. The large majority of these features are impact craters. The crater nomenclature is governed by the International Astronomical Union, and this listing only includes features that are officially recognized by that scientific society.  date to a later period, the "Late Heavy Bombardment The Late Heavy Bombardment (commonly referred to as the lunar cataclysm, or LHB) is a period of time approximately 3800 to 4100 million years ago (Mya) during which a large number of impact craters are believed to have formed on the Moon, and by inference on Earth, " 3.8 to 3.9 billion years ago, when around 100 million billion tonnes of rock or ice crashed onto the lunar surface.

To find out whether asteroids or comets were the main culprits for the bombardment, Jorgensen decided to measure levels of the element iridium iridium (ĭrĭd`ēəm), metallic chemical element; symbol Ir; at. no. 77; at. wt. 192.22; m.p. about 2,410°C;; b.p. about 4,130°C;; sp. gr. 22.55 at 20°C;; valence +3 or +4.  in ancient terrestrial rocks.

Iridium is rare on the Earth's surface because almost all of it bound to iron and sank into the Earth's core soon after the planet had formed. But iridium is relatively common in comets and meteorites Meteorites
See also astronomy.

aerolithology

the science of aerolites, whether meteoric stones or meteorites. Also called aerolitics.

astrolithology

the study of meteorites. Also called meteoritics.
.

His team calculated the amount of iridium that asteroids would leave on the Earth and moon compared to comets.

Because comets have more volatile elements and higher impact speeds due to their more elongated e·lon·gate  
tr. & intr.v. e·lon·gat·ed, e·lon·gat·ing, e·lon·gates
To make or grow longer.

adj. or elongated
1. Made longer; extended.

2. Having more length than width; slender.
 orbits around the sun, they would create giant plumes on impact, allowing more iridium to escape into space than during asteroid impacts.

The team predicted that asteroid bombardment would leave iridium levels of 18,000 and 10,000 parts per trillion in rocks on the Earth and moon respectively, while the same figures for comet bombardment would be about 130 and 10.

Ancient moon rocks returned by NASA's Apollo missions have already confirmed that the lunar iridium levels are 10 parts per trillion or less.

To find out the terrestrial value, Jorgensen's team sampled some of the world's oldest rocks from Greenland, aged 3.8 billion years, and asked a Japanese laboratory to assess their iridium levels more accurately than ever before.

They contained iridium levels of 150 parts per trillion, which strongly suggests comets, rather than asteroids, caused the violent bombardment. (ANI)

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Publication:Asian News International
Date:Jul 28, 2009
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