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Blobrana


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Meteorite storms brought Earth alive
Massive bombardments of meteorites from space may have turned Earth into a haven for life, scientists said today. And the same could have happened on Mars. The storms of stones from the sky, around four billion years ago, made the planets more habitable by producing the oceans, according to the new research. As each meteor entered the atmosphere, extreme heat caused minerals and organic material on its outer crust to be released.

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Earth's Bombardment by Asteroids 3.9 Billion Years Ago May Have Enhanced Early Life, Says University of Colorado Study
The bombardment of Earth nearly 4 billion years ago by asteroids as large as Kansas would not have had the firepower to extinguish potential early life on the planet and may even have given it a boost, says a new University of Colorado at Boulder study.
Impact evidence from lunar samples, meteorites and the pockmarked surfaces of the inner planets paints a picture of a violent environment in the solar system during the Hadean Eon 4.5 to 3.8 billion years ago, particularly through a cataclysmic event known as the Late Heavy Bombardment about 3.9 million years ago. Although many believe the bombardment would have sterilised Earth, the new study shows it would have melted only a fraction of Earth's crust, and that microbes could well have survived in subsurface habitats, insulated from the destruction.

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"These new results push back the possible beginnings of life on Earth to well before the bombardment period 3.9 billion years ago. It opens up the possibility that life emerged as far back as 4.4 billion years ago, about the time the first oceans are thought to have formed"- CU-Boulder Research Associate Oleg Abramov.

A paper on the subject by Abramov and CU-Boulder geological sciences Professor Stephen Mojzsis appears in the May 21 issue of Nature.

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Blobrana


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About four billion years ago, during a time called the late heavy bombardment, Earth, Mars and the moon were flinging rocks into space at a tremendous rate. Now astrobiologists are keen to track down terrestrial meteorites that may have survived on the moon, in the hope of finding within them biomarkers to reveal information about the origins of life on Earth.
Recent research by University of NSW astronomer and associate professor Jeremy Bailey, Ian Crawford of London's Birbeck College and others has cleared up a crucial question. Some of the rocks may well have survived the journey and the crash landing.

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It seems we got off lightly in the cosmic lottery. Deadly comet impacts may be much rarer in our solar system than in others nearby.
We can't directly measure the rate of comet collisions in other solar systems but we can detect signs of the dust that such smashes kick up because the dust gets warmed by the star and so gives off infrared radiation. That radiation shows up as extra infrared in the spectrum of light coming from the star. Because such dust should dissipate quickly, it is thought to provide a good snapshot of the recent collision rate.
Jane Greaves of the University of St Andrews, UK, analysed observations by the Spitzer Space Telescope and found that the vast majority of sun-like stars near us have more dust than our solar system does and therefore have had more collisions in their vicinity. Our solar system may be one of the few that have been safe for life. Greaves presented her results at the Cosmic Cataclysms and Life symposium in Frascati, Italy, this month.

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Blobrana


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The fiery demise of a fifth rocky planet in our Solar System might have led to a flurry of asteroid impacts that pockmarked the Moon and Earth billions of years ago.
The Late Heavy Bombardment (LHB) is a relatively brief period, about 3.9 billion years ago, when wayward space projectiles heavily pelted the Moon and inner planets. Craters from that chaotic time are still visible on the Moon, but have been erased from Earth, where the crust is continually recycled.
Try as they might, astronomers have not yet been able to pin down a cause for the bombardment. Some experts have postulated that a shuffling up of the arrangement of the planets in their youth may have been responsible. One popular theory is that the outward migration of a young Neptune perturbed rocky bodies in the distant Kuiper Belt, causing some to veer into the inner Solar System.

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Blobrana


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There may have been a dramatic event early in the history of the Solar System--the intense bombardment of the inner planets and the Moon by planetesimals during a narrow interval between 3.92 and 3.85 billion years ago, called the late heavy bombardment, but also nicknamed the lunar cataclysm. The evidence for this event comes from Apollo lunar samples and lunar meteorites. While not proven, it makes for an interesting working hypothesis. If correct, what caused it to happen?

A group of physicists from the Observatoire de la Côte d'Azur (Nice, France), GEA/OV/Universidade Federal do Rio de Janeiro and Observatório Nacional/MTC (Rio de Janeiro, Brazil), and the Southwest Research Institute (Boulder, Colorado) conducted a series of studies of the dynamics of the early Solar System. Alessandro Morbidelli, Kleomenis Tsiganis, Rodney Gomes, and Harold Levison simulated the migration of Saturn and Jupiter. When the orbits of these giant planets reached the special condition of Saturn making one trip around the Sun for every two trips by Jupiter (called the 1:2 resonance), violent gravitational shoves made the orbits of Neptune and Uranus unstable, causing them to migrate rapidly and scatter countless planetesimals throughout the Solar System. This dramatic event could have happened in a short interval, anywhere from 200 million years to a billion years after planet formation, causing the lunar cataclysm, which would have affected all the inner planets.

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New age measurements of lunar rocks returned by the Apollo space missions have revealed that a surprising number of the rocks show signs of melting about 3.9 billion years ago, suggesting that the moon – and its nearby neighbour Earth – were bombarded by a series of large meteorites at that time.

The idea that meteorites have hammered the moon's surface isn't news to scientists. The lunar surface is pockmarked with large craters carved out by the impact of crashing asteroids and meteorites.
But the narrow range of the impact dates suggests to researchers that a large spike in meteorite activity took place during a 100-million year interval – possibly the result of collisions in the asteroid belt with comets coming from just beyond our solar system.

Results of the study are being published in Geochimica et Cosmochimica Acta, the journal of the international Meteoritical Society. Co-authors with Robert Duncan, a professor and associate dean in the College of Oceanic and Atmospheric Sciences at Oregon State University, are Marc Norman of the Australian National University and John Huard, also an oceanographer at Oregon State University.
The study was funded by NASA.

Tiny melted fragments from the lunar rocks were dated at the noble gas geochronology laboratory at Oregon State. Duncan and Huard were able to use radiometric dating techniques to determine when the rocks had melted after being struck by meteorites. What is particularly intriguing is that this apparent spike in meteorite activity took place about 3.8 to 4 billion years ago – an era that roughly coincides with when scientists believe life first began on Earth, as evidenced by the fossil record of primitive one-cell bacteria.

It is possible that life was introduced to Earth from one of these meteorites. Or it could have developed spontaneously once the bombardment subsided, or developed beneath the ocean near life-nurturing hydrothermal vents. The lack of evidence on Earth makes the analysis of moon rocks much more compelling. The meteorite activity that bombarded the moon likely struck our planet as well.

"Unfortunately, we haven't found many very old rocks on Earth because of our planet's surface is constantly renewed by plate tectonics, coupled with erosion. By comparison, the moon is dead, has no atmosphere and provides a record of meteorite bombardment that we can only assume is similar to that on Earth." - Robert Duncan.

When the solar system was formed, scientists say, it spun away from the sun like a huge, hot disk that subsequently condensed into planets. At least nine planets survived, sucking in loose space matter from around them. Those planets closer to the sun were more solid, while those farther away were comprised primarily of gases.

Over time, the space debris has lessened, either being gravitationally collected into the planets, or smashed into cosmic dust through collisions with other objects. The discovery of this apparent spike in meteorite activity suggests to the authors that a major event took place.

"We may have had a 10th and 11th planet that collided, or it's possible that the outward migration of Neptune may have scattered comets and small planet bodies, inducing collisions in the asteroid belt. The close passing of a neighbouring star could have had a similar effect." - Robert Duncan.

Duncan and his colleagues examined about 50 different rock samples scooped up by astronauts on the Apollo missions. All but a few of them produced ages close to 3.9 billion years and they exhibited different chemical "fingerprints," indicating that they had melted from different meteorites and lunar surface rocks.

"The evidence is clear that there was repeated bombardment by meteorites" - Robert Duncan.

When meteorites collide with the moon, the surface rock and the meteorites partially melt, and then turn to glass. After the glasses quenched, they slowly began to accumulate argon gas that scientists can measure and calculate from the known isotopic decay rate (from potassium) to determine age.

"The formation of glass from the melting is like starting a clock. It resets the time for us to determine billions of years later." - Robert Duncan

Duncan and his colleagues say the intense bombardment ended about 3.85 billion years ago, and there has been a slowly declining pattern of meteorite activity since. Many of the prominent craters found on the moon date back to that era, including Imbrium, at 3.84 billion years; Serenitatis, 3.89 billion years; and Nectaris, 3.92 billion years.
Many of the moon's craters are 10 to 100 kilometres across and scientists say that meteorites of that size or larger may have struck the Earth in the past. Such meteorites impacts may have been responsible for the extinction of dinosaurs 65 million years ago, and a mass extinction that wiped out an estimated 75 percent of the Earth's plant and animal species 250 million years ago.

However, these mass extinctions could also be linked to climate, disease and volcanism – or a combination of such factors.

"It is clear that there was a spike of meteorite activity on the moon about 3.9 billion years ago, and that it lasted for roughly 100 million years. The moon provides important information about the early history of our solar system that is missing from the Earth's geologic record." - Robert Duncan.

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Our solar system may have had a fifth terrestrial planet, one that was swallowed up by the Sun. But before it was destroyed, the now missing-in-action world made a mess of things.

The fifth terrestrial planet may once have orbited between Mars and Jupiter. Although gravitational disturbances would have sent the planet hurtling into the sun or out into space long ago, traces of this long-gone world may still be visible in part of the asteroid belt today.
Recent simulations have suggested that the gas giants of our solar system formed with circular orbits but moved into their more elongated paths about 4 billion years ago - 700 million years after the solar system formed. While the gas giants were in circular orbits, rocky planets should have formed in stable orbits out to a distance of 2.2 astronomical units (1 AU = 1 Earth-Sun distance).

However, there are no planets between Mars, which lies at 1.5 AU from the sun, and Jupiter at 5.2 AU. That puzzled Sean Raymond of the University of Colorado in Boulder and John Chambers of the Carnegie institute.

Space scientists John Chambers and Jack Lissauer of NASA's Ames Research Centre in 2002 hypothesised that along with Mercury, Venus, Earth, and Mars -- the terrestrial, rocky planets -- there was a fifth terrestrial world, likely just outside of Mars's orbit and before the inner asteroid belt.

The computer modelling findings of Chambers and Lissauer were presented during the 33rd Lunar and Planetary Science Conference, held in March 11-15, 2002 and sponsored by NASA and the Lunar and Planetary Institute.
The basic idea is that among the "planetary embryos" that formed in the early solar system, an additional Mars massed object formed and persisted at about 2 AU. It orbited there in a near circular for about 700 million years before being ejected by Jupiter.

It is commonly believed that during the formative years of our solar system, between 3.8 billion and 4 billion years ago, the Moon and Earth took a pounding from space debris. However, there is an on-going debate as to whether or not the bruising impacts tailed off 3.8 billion year ago or if there was a sudden increase - a "spike" -- in the impact rate around 3.9 billion years ago, with quiet periods before and afterwards?
This epoch of time is tagged as the "lunar cataclysm" - also a wakeup call on the cosmological clock when the first evidence of life is believed to have appeared on Earth.
Having a swarm of objects clobbering the Moon in a narrow point of time would have resurfaced most of our celestial next door neighbour, covering up its early history. Being that the Moon is so small, Earth would have been on the receiving end of any destructive deluge too.
Moon walking astronauts brought back a cache of lunar material. Later analysis showed that virtually all impact rocks in the "Apollo collection" sported nearly the same age, 3.9 billion years, and none were older. But some scientists claim that these samples were "biased", as they came from a small area of the Moon, and are the result of a localised pummelling, not some lunar big bang.
There is a problem in having a "spike" in the lunar cratering rate.

That scenario is tough to devise. Things should have been settling down, according to solar system creation experts. Having chunks of stuff come zipping along some hundreds of millions of years later out of nowhere and create a lunar late heavy bombardment is a puzzler.
If real, what were these bodies, and where were they before they scuffed up the Moon big time? The answer, according to Chambers and Lissauer, might be tied to the Planet V hypothesis.

"The extra planet formed on a low-eccentricity orbit that was long-lived, but unstable" - John Chambers.

About 3.9 billion years ago, Planet V was perturbed by gravitational interactions, and was tossed onto a highly eccentric orbit that crossed the inner asteroid belt, a reservoir of material much larger than it is today.
Planet V's close encounters with the inner belt of asteroids stirred up a large fraction of those bodies, scattering them about. The perturbed asteroids evolved into Mars crossing orbits, and temporarily enhanced the population of bodies on Earth-crossing orbits, and also increased the lunar impact rate.
After doing its destabilising deeds, Planet V was lost too, most likely spinning into the Sun.
The temporary existence of more than 4 planet-sized bodies in the inner Solar System is consistent with the currently favoured model for the formation of the Moon.
Work by Chambers and Lissauer also supports the view that our Moon is a leftover of a massive collision between Earth and a Mars-sized body 50 million to 100 million years after the formation of the Solar System.

"This idea and others within the last few years show that the Solar System is filled with all sorts of gravitational resonances...that a lot of potential orbits in the Solar System are chaotic and unstable. My sense is that this is a new idea. It's another thing to throw into the pot that's not totally crazy" - Wendell Mendell, a planetary scientist at the Johnson Space Centre.

The work suggests there's a match up in timing, with asteroids striking the Moon and causing the effects that are seen in the dating of Apollo lunar rocks.

"By thinking that the Solar System was really quite different in a major way with an extra inner planet, we might be able to develop some sort of self-consistent scenario that explains a lot of things. But all this is at the very early stages now. We're moving into a really new regime, where the Solar System is not a static dynamic place from day one to now. It really might have had some nuances and synchronicities associated with it that we have not really tried to exploit before" - Wendell Mendell.

It takes a drill hole setting the early Solar System and lunar history record straight means going back to the Moon.
Places on the Moon where older, large basins have deposited ejecta are ideal research zones. Digging into such sites could yield impact glass formed by basins perhaps dating older than 3.9 billion years old.
Just taking spot samples -- say from the Moon's South Pole Aitken basin -- could be risky, in terms of uncovering the Moon's rocky history. Such a huge area would take multiple robotic or human exploration missions, each with significant roving abilities.
Aitken, also known as the "Big Backside Basin," is the largest impact crater on the Moon, and one of the biggest in the Solar System.
For the near term, sets of low-cost, mini-robotic landers carrying specialised gear would be ideal in opening up the Moon to further exploration.

Getting back to the Moon with a settlement for resource exploitation is another step forward. From such a site, human explorers can survey various lunar locales - even the Moon's side that we Earthlings never see.

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Blobrana


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Ancient Lunar Impacts
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Ohio State University planetary scientists have found the remains of ancient lunar impacts that may have helped create the surface feature commonly called the "man in the moon."
Their study suggests that a large object hit the far side of the moon and sent a shock wave through the moon's core and all the way to the Earth-facing side. The crust recoiled -- and the moon bears the scars from that encounter even today.

The finding holds implications for lunar prospecting, and may solve a mystery about how past impacts on Earth affect it's geology today.
The early Apollo missions revealed that the moon isn't perfectly spherical. Its surface is warped in two spots; an earth-facing bulge on the near side is complemented by a large depression on the Moon's far side. Scientists have long wondered whether these surface features were caused by Earth's gravity tugging on the moon early in its existence, when its surface was still molten and malleable.
According to Laramie Potts and Ralph von Frese, a postdoctoral researcher and professor of geological sciences respectively at Ohio State, these features are instead remnants from ancient impacts.
Potts and von Frese came to this conclusion after they used gravity fluctuations measured by the Clementine and Lunar Prospector satellites to map the moon's interior. They reported the results in a recent issue of the journal Physics of the Earth and Planetary Interiors.

They expected to see defects beneath the moon's crust that corresponded to craters on the surface. Old impacts, they thought, would have left marks only down to the mantle, the thick rocky layer between the moon's metallic core and its thin outer crust. And that's exactly what they saw, at first.


Ohio State University planetary scientists have found the remains of ancient lunar impacts. This diagram shows a depression on the far side of the moon, a bulge on the near (Earth-facing) side of the moon, and the interior features that connect them -- all signs of a giant impact.
Image courtesy of Laramie Potts, Ohio State University.

Potts pointed to a cross-sectional image of the moon that the scientists created using the Clementine data. On the far side of the moon, the crust looks as though it was depressed and then recoiled from a giant impact, he said. Beneath the depression, the mantle dips down as he and von Frese would expect it to do if it had absorbed a shock.
Evidence of the ancient catastrophe should have ended there. But some 700 miles directly below the point of impact, a piece of the mantle still juts into the moon's core today.
That was surprising enough.

"People don't think of impacts as things that reach all the way to the planet's core" - Ralph von Frese.

But what they saw from the core all the way to the surface on the near side of the moon was even more surprising. The core bulges, as if core material was pushed in on the far side and pulled out into the mantle on the near side. Above that, an outward-facing bulge in the mantle, and above that -- on the Earth-facing side of the moon -- sits a bulge on the surface.
To the Ohio State scientists, the way these features line up suggests that a large object such as an asteroid hit the far side of the moon and sent a shock wave through the core that emerged on the near side.

The scientists believe that a similar, but earlier impact occurred on the near side.
Potts and von Frese suspect that these events happened about four billion years ago, during a period when the moon was geologically active -- with its core and mantle still molten and magma flowing.
Back then, the moon was much closer to the Earth than it is today, so the gravitational interactions between the two were stronger. When magma was freed from the Moon's deep interior by the impacts, Earth's gravity took hold of it and wouldn't let go.
So the warped surfaces on the near and far sides of the moon and the interior features that connect them are all essentially signs of injuries that never healed.

"This research shows that even after the collisions happened, the Earth had a profound effect on the moon" - Laramie Potts.

The impacts may have created conditions that led to a prominent lunar feature.
The "man in the moon" is a collection of dark plains on the Earth-facing side of the moon, where magma from the moon's mantle once flowed out onto the surface and flooded lunar craters. The moon has long since cooled, but the dark plains are a remnant of that early active time -- "a frozen magma ocean."

How that magma made it to the surface is a mystery, but if he and Potts are right, giant impacts could have created a geologic "hot spot" on the moon – a site where magma bubbles to the surface. Some time between when the impacts occurred and when the moon solidified, some magma escaped the mantle through cracks in the crust and flooded the nearside surface and formed a lunar “hot spot”.
A hot spot on Earth forms the volcanoes that make the Hawaiian island chain. The Ohio State scientists wondered: could similar ancient impacts have penetrated the Earth, and caused the hot spots that exist here today? von Frese thinks that it's possible.

"Surely Earth was peppered with impacts, too. Evidence of impacts here is obscured, but there are hot spots like Hawaii . Some hot spots have corresponding hot spots on the opposite side of the Earth. That could be a consequence of this effect" - Ralph von Frese.

He and Potts are exploring the idea, by studying gravitational anomalies under the Chicxulub Crater on Mexico 's Yucatan Peninsula . A giant asteroid struck the spot some 65 million years ago, and is believed to have set off an environmental chain reaction that killed the dinosaurs.
NASA funded this research. The space agency has been charged with returning astronauts to the moon to prospect for valuable gases and minerals.

But even today, scientists don't entirely know what the moon is made of – not down to the core, anyway. They can calculate where certain minerals should be, given the conditions they believe existed when the moon formed. But impacts like the one Potts and von Frese discovered have since shuffled materials around. Gravity measurements, they said, will play a key role as scientists figure out what materials lie within the moon, and where.

"We don't fully understand the way these minerals settle out under temperature and pressure, so the exact composition of the moon is difficult to determine. We have to use gravity measurements to calculate the density of materials, and then use that information to extrapolate the likely composition" - Laramie Potts.

von Frese said a lunar base would be needed before scientists can more completely answer these questions.

"Once we have more rock samples and soil samples, we will have a lot more to go on. Nothing is better than having a person on the ground" - Laramie Potts.

Source

-- Edited by Blobrana at 19:23, 2006-02-09

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Blobrana


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The Earth and the inner planets of our solar system have been pelted by two different populations of asteroids, a new study suggests. It also gives insights into when Jupiter and Saturn settled into their respective orbits.
It is known that something unusual happened in the solar system about 3.9 billion years ago. Either there was an abrupt end to the constant pelting of the Earth by the leftover debris of solar system construction, or there was a sudden, short-lived episode of heavy bombardment long after the early battering had died down.
Now, a new analysis of asteroid and crater populations shows that it was almost certainly the latter. And the so-called Late Heavy Bombardment (LHB) was caused by a sudden influx of asteroids pulled out of their usual place in the main asteroid belt between Mars and Jupiter.
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