Comets (Not Asteroids) Heavily Battered the Moon

Author’s Note: 3.8 billion years ago, at the beginning of the solar system’s formation, an "earth-shattering" impact event occurred. This created the craters on the moon and the pitted surfaces of many celestial bodies. However, it may also have been the source of life. Research indicates it might have brought water to Earth.

This translation was not very difficult, though some technical terms were troublesome. For instance, "Giant plumes"—I was initially unclear on its meaning. After consulting others, I learned it refers to "mantle plumes" (or impact plumes). I hope fellow enthusiasts will not jump to conclusions when encountering unfamiliar terms in the future; try searching on Google more often.

Comets (Not Asteroids) Heavily Battered the Moon
by Hazel Muir

A new study of ancient rocks in Greenland suggests that the "perpetrators" of the violent impacts on the Earth and Moon 3.85 billion years ago were icy comets—rather than rocky asteroids. This work indicates that most of the water currently on Earth may have been brought by comets.

"We can observe the craters on the lunar surface with the naked eye, but no one knew how they were formed—whether by rock, iron, or ice," says Uffe Grae Jorgensen, an astronomer at the Niels Bohr Institute in Copenhagen, Denmark. "It is exciting that the discovered signs indicate this was caused by ice."

Research has found that both the Earth and the Moon formed 4.5 billion years ago. However, almost all the craters on the Moon formed later, originating from a "Late Heavy Bombardment" (LHB, see: Wikipedia: Late Heavy Bombardment) approximately 3.9 to 3.8 billion years ago. It is estimated that 100 million billion tonnes (10^{17} tonnes) of meteoritic ice or rock struck the lunar surface. At the same time, Earth was also being struck by debris—though our restless planet has erased these scars through "plate tectonics."

To determine whether asteroids or comets were the true "culprits" of the impact, Jorgensen decided to measure the concentration of iridium in ancient geological rocks. Since almost all iridium transitioned into iron and sank into the Earth’s core shortly after the Earth’s formation, iridium is extremely rare on the Earth’s surface. However, iridium is a relatively common element in comets and meteorites.

Ice? Rock?

His team calculated and compared the iridium content that asteroid and comet impacts would leave on the Earth and Moon. Because comets have more volatile components and higher impact velocities due to their elongated orbits, their impacts would create giant plumes. Compared to asteroid impacts, this would allow more iridium to escape into space.

The team estimated that if it were an asteroid impact, the iridium content in Earth and Moon rocks would be 18,000 ppt and 10,000 ppt (parts per trillion), respectively. If it were a comet impact, these two figures would be only 130 and 10.

Ancient lunar rocks brought back by NASA’s Apollo missions have confirmed that the iridium content in lunar rocks is only 10 ppt or even less. To find relevant information on Earth, Jorgensen’s team collected a batch of the world’s oldest rocks (3.8 billion years old) from Greenland and sent them to a Japanese laboratory to measure the iridium content with unprecedented precision. The results showed an iridium content of 150 ppt.

This provides strong evidence that comets—not asteroids—caused the major bombardment.

Giant Plumes

If this is the case, according to the Jorgensen team’s calculations, comets brought 3,400 tonnes of ice to every square meter of the Earth. It is estimated that half of the cometary material returned to space through giant plumes, leaving approximately 1 billion cubic kilometers of water on Earth.

This figure is roughly equal to the total volume of Earth’s current oceans, though it remains unclear whether water was produced during the chemical changes of the early Earth. (i.e., it is unknown whether Earth’s water is "home-grown" or "imported"; see this English reference or this Chinese reference).

Michael Mumma, a comet expert at NASA’s Goddard Space Flight Center in Maryland who was not involved in the study, stated that the new results are very interesting: "The paper is certain to stimulate lively debate."

Original Article: http://www.newscientist.com/article/dn17520-comets-not-asteroids-to-blame-for-moons-scarred-face.html

Comets, not asteroids, to blame for moon’s scarred face
by Hazel Muir

Icy comets – not rocky asteroids – launched a dramatic assault on the Earth and moon around 3.85 billion years ago, a new study of ancient rocks in Greenland suggests. The work suggests much of Earth’s water could have been brought to the planet by comets.

"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?" says Uffe Græ Jørgensen, an astronomer at the Niels Bohr Institute in Copenhagen, Denmark. "It’s exciting to find signs that it was actually ice."

Evidence suggests that the Earth and moon had both formed around 4.5 billion years ago. But almost all the craters on the moon date to a later period, the "Late Heavy Bombardment" 3.8 to 3.9 billion years ago, when around 100 million billion tonnes of rock or ice crashed onto the lunar surface. The Earth would have been pummelled by debris at the same time, although plate tectonics on our restless planet have since erased the scars.

To find out whether asteroids or comets were the main culprits for the bombardment, Jørgensen decided to measure levels of the element iridium 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.

Rock or ice
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 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, Jørgensen’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.

That strongly suggests comets, rather than asteroids, caused the violent bombardment.

Giant plumes
If so, Jørgensen’s team calculates that around 3400 tonnes of icy comet material fell on each square metre of the Earth. About half the comet material would ricochet back into space in giant plumes, leaving behind roughly a billion cubic kilometres of cometary water in total.

That is a similar amount to that in the Earth’s oceans today, although it is not clear whether there was already water on the planet due to chemical reactions on the early Earth (see Earth’s water brewed at home, not in space).

Michael Mumma, a comet expert at NASA’s Goddard Space Flight Center in Maryland who was not involved in the research, says the new report is interesting: "The paper is certain to stimulate lively debate."

When reposting, please include the original address: https://kexue.fm/archives/65

For more details on reposting, please refer to: Scientific Space FAQ