The Earth-Moon system during the Late Heavy Bombardment period

arXiv ^ | Jul 23, 2009 | un

[decimon]

The Late Heavy Bombardment (LHB) period is the narrow time interval between 3.8 and 3.9 Gyr ago, where the bulk of the craters we see on the Moon formed. Even more craters formed on the Earth. During a field expedition to the 3.8 Gyr old Isua greenstone belt in Greenland, we sampled three types of metasedimentary rocks, that contain direct traces of the LHB impactors by a seven times enrichment (150 ppt) in iridium compared to present day ocean crust (20 ppt). We show that this enrichment is in agreement with the lunar cratering rate, providing the impactors were comets, but not if they were asteroids. Our study is a first direct indication of the nature of the LHB impactors, and the first to find an agreement between the LHB lunar cratering rate and the Earth's early geochemical record (and the corresponding lunar record). The LHB comets that delivered the iridium we see at Isua will at the same time have delivered the equivalent of a km deep ocean, and we explain why one should expect a cometary ocean to become roughly the size of the Earth's present-day ocean, not only in terms of depth but also in terms of the surface area it covers.

[decimon]

Judging crater.

[allmost]

Let’s go check it out.

[Seruzawa]

I wish they would stop presenting wild guesses as fact.

[commish]

interesting, but at the moment I am quietly contemplating the metaphysical properties of a boal of Corn Flakes.

[JimRed]

Gyr? I’m assuming that means billions of years, but I’ve never seen it stated that way before (B4).

[martin_fierro]

Gyr?

That's Gazillions, silly.

[SunkenCiv]

I always thought it meant “gigayears” but now that I’ve seen martin’s idea...

[decimon]

...“gigayears”...

What do the Rolling Stones have to do with this?

[SunkenCiv]

Scientists Confirm Age Of The Oldest Meteorite Collision On Earth
Science Daily
Stanford University
8/23/2002

...Scientists have yet to locate any trace of the extraterrestrial object itself or the gigantic crater it produced, but other geological evidence collected on two continents suggests that the meteorite was approximately 12 miles (20 kilometers) wide -- roughly twice as big as the one that contributed to the demise of the dinosaurs some 65 million years ago... To pinpoint when the huge meteorite collided with Earth, Lowe and his colleagues performed highly sensitive geochemical analyses of rock samples collected from two ancient formations well known to geologists: South Africa's Barberton greenstone belt and Australia's Pilbara block... Although thousands of miles apart, both sites contain 3.5-billion-year-old layers of rock embedded with "spherules" -- tiny spherical particles that are a frequent byproduct of meteorite collisions... The meteorite that led to the dinosaur extinction produced spherule deposits around the world that are less than 2 centimeters deep. But the spherule beds in South Africa and Australia are much bigger -- some 20 to 30 centimeters thick. A chemical analysis of the rocks also has revealed high concentrations of rare metals such as iridium -- rare in terrestrial rocks but common in meteorites... isotopic studies confirmed that much of the chromium buried in the rock samples came from an extraterrestrial source... In addition to the 3.47-billion-year-old impact, Lowe and Byerly have found evidence of meteorite collisions in three younger rock layers in the South African formation. According to Lowe, the force of those collisions may have been powerful enough to cause the cracks -- or tectonic plates -- that riddle the Earth's crust today.

[SunkenCiv]

Meteor bombardment 4 bln yrs ago may have made Earth more habitable
ANI
June 2nd, 2009

When a meteorite enters a planet's atmosphere, extreme heat causes some of the minerals and organic matter on its outer crust to be released as water and carbon dioxide (CO2) before it breaks up and hits the ground.

Researchers suggest the delivery of this water could have made Earth's and Mars' atmospheres wetter.

The release of the greenhouse gas CO2 could have trapped more energy from sunlight to make Earth and Mars warm enough to sustain liquid oceans.

Yeah, right.

[SunkenCiv]

Physics News Update Number 262
by Phillip F. Schewe and Ben Stein
March 14, 1996

IN THE LATE HEAVY BOMBARDMENT (LHB) EPOCH , a span of about 200 million years some 4 billion years ago, the Moon sustained many large impacts. Some astronomers believe that the projectiles responsible may have pestered Mercury, Venus, Earth, and Mars as well. Others assert that the LHB phenomenon was unique to the Earth-Moon system or that it did not happen at all, at least not so suddenly. Now, a group of scientists at the University of Manchester (UK) has dated a rock found here on Earth but which is believed to have been a meteorite originating at Mars. The 4-billion-year age of the object, determined by isotope dating, is much older than previously studied Martian meteorites. The antiquity of the rock, say the researchers, provides evidence for a widespread LHB effect. (R.D. Ash et al, Nature, 7 March 1996.)

[SunkenCiv]

Asteroids Resurfaced The Earth and Inner Solar System 3.9 Billion Years Ago
by Julieta Gonzalez
28-Feb-02

The bombardment that resurfaced the Earth 3.9 billion years ago was produced by asteroids, not comets, according to David Kring of the University of Arizona Lunar & Planetary Laboratory and Barbara Cohen, formerly at the UA and now with the University of Hawaii... The significance of this conclusion is that the bombardment was so severe that it destroyed older rocks on Earth... the reason why the oldest rocks found are less than 3.9 billion years old... This same bombardment according to Kring and Cohen, affected the entire inner solar system, producing thousands of impact craters on Mercury, Venus, the Moon and Mars. Most of the craters in the southern hemisphere of Mars were produced during this event. On Earth, at least 22,000 impact craters with diameters greater than 20 kilometers were produced, including about 40 impact basins with diameters of about 1,000 kilometers in diameter.

[SunkenCiv]

Thanks decimon.

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[SunkenCiv]

“Glad to be here, glad to be anywhere.” — Keith Richards

[rdl6989]

Thanks. I was gonna ask what that meant.

[Quix]

THANKS.

And the implications are???

[SunkenCiv]

Somewhere (I read it yesterday, apparently not in one of the files around here) I've seen an older article about a Canadian study which uses various techniques and images taken from orbit to turn up hundreds of "lost" impact craters on the Earth's surface. Hope it turns up again. :')

Theory of Periodic Mass Extinctions
Frank R. Ettensohn

Why has interest in this hypothesis subsided? There are two reasons. First, there are statistical questions about extinction rates that just cannot be answered. It is not clear whether other hypotheses can fit the empirical data as well as does the hypothesis of periodicity. The distribution of extinction events in time is certainly not random, but there is more than one way to analyze the data. Does the fact that the data appear to fit well to a model of periodicity mean that the events truly are periodic? This question was debated throughout the late 1980s without reaching a resolution.

The second reason for the loss of interest is one not uncommon to science: Here is an intriguing observation that no one knows how to explain. Researchers formulated a number of very interesting astronomical hypotheses to account for the 26-million-year periodicity of extinction. The most famous of these was the Nemesis, or 'death star,' hypothesis, which stated that the sun has a distant companion whose highly elliptical orbit brings it into the Oort Cloud (a swarm of frozen comets orbiting far from the sun) once every 26 million years. During each pass through the Oort Cloud, the companion's gravity would scatter huge numbers of comets, some of which would crash into Earth. The environmental damage caused by these impacts would lead to an elevated rate of extinctions.

[SunkenCiv]

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[null and void]

interesting, but at the moment I am quietly contemplating the metaphysical properties of a boal of Corn Flakes.

Did you find any thing in your bolide?

[null and void]

Gyr

Reporting for duty.