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Earth's Water
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New Class of Comets May Be the Source of Earth's Water.

Three icy comets orbiting among the rocky asteroids in the main asteroid belt between Mars and Jupiter may hold clues to the origin of Earth's oceans.

The newly discovered group of comets, dubbed "main-belt comets" by University of Hawaii graduate student Henry Hsieh and Professor David Jewitt, has asteroid-like orbits and, unlike other comets, appears to have formed in the warm inner solar system inside the orbit of Jupiter rather than in the cold outer solar system beyond Neptune.

The existence of these main-belt comets suggests that asteroids and comets are much more closely related than previously thought and supports the idea that icy objects from the main asteroid belt could be a major source of Earth's present-day water. This work appears in the March 23 edition of Science Express (pdf) and will also appear in an April print edition of Science.



The crucial observations were made on November 26, 2005, using the 8-meter Gemini North Telescope on Mauna Kea. Hsieh and Jewitt found that an object designated as Asteroid 118401 was ejecting dust like a comet. Together with a mysterious comet (designated 133P/Elst-Pizarro) known for almost a decade but still poorly understood, and another comet (designated P/2005 U1) discovered by the Spacewatch project in Arizona just a month earlier, "Asteroid" 118401 forms an entirely new class of comets.

"The main-belt comets are unique in that they have flat, circular, asteroid-like orbits, and not the elongated, often tilted orbits characteristic of all other comets. At the same time, their cometary appearance makes them unlike all other previously observed asteroids. They do not fit neatly in either category" - Henry Hsieh.

In both 1996 and 2002, the "original" main-belt comet, 133P/Elst-Pizarro (named after its two discoverers), was seen to exhibit a long dust tail typical of icy comets, despite having the flat, circular orbit typical of presumably dry, rocky asteroids. As the only main-belt object ever observed to take on a cometary appearance, however, 133P/Elst-Pizarro's true nature remained controversial. Until now.

"The discovery of the other main-belt comets shows that 133P/Elst-Pizarro is not alone in the asteroid belt. Therefore, it is probably an ordinary (although icy) asteroid, and not a comet from the outer solar system that has somehow had its comet-like orbit transformed into an asteroid-like one. This means that other asteroids could have ice as well" - Professor David Jewitt.

The Earth is believed to have formed hot and dry, meaning that its current water content must have been delivered after the planet cooled. Possible candidates for supplying this water are colliding comets and asteroids. Because of their large ice content, comets were leading candidates for many years, but recent analysis of comet water has shown that comet water is significantly different from typical ocean water on Earth.

Asteroidal ice may give a better match to Earth's water, but until now, any ice that the asteroids may have once contained was thought to either be long gone or so deeply buried inside large asteroids as to be inaccessible for further analysis. The discovery of main-belt comets means that this ice is not gone and is still accessible (right on the surfaces of at least some objects in the main belt, and at times, even venting into space). Spacecraft missions to the main-belt comets could provide new, more detailed information on their ice content and in turn give us new insight into the origin of the water, and ultimately life, on Earth.

As conventionally defined, comets and asteroids are very different. Both are objects a few to a few hundred miles across that orbit throughout our solar system. Comets, however, are thought to originate in the cold outer solar system and consequently contain much more ice than the asteroids, most of which are thought to have formed much closer to the Sun in the asteroid belt between Mars and Jupiter.

Comets also have large, elongated orbits and thus experience wide temperature variations. When a comet approaches the Sun, its ice heats up and sublimates (changes directly from ice to gas), venting gas and dust into space, giving rise to a tail and a distinctive fuzzy appearance. Far from the Sun, sublimation stops, and any remaining ice stays frozen until the comet's next pass close to the Sun. In contrast, objects in the asteroid belt have essentially circular orbits and are expected to be mostly baked dry of ice by their confinement to the inner solar system. Essentially, they should be just rocks. With the discovery of the main-belt comets, we now know this is not the case, and that, in general, the conventional definitions of comets and asteroids are in need of refinement.

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Iron meteorites
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Using computer simulations, A team of researchers, from Southwest Research Institute (SwRI) and Observatoire de la Cote d'Azur in Nice, France, have simulated how the early solar system evolved. By tracking the evolution of how a disk of asteroids interact with protoplanetary objects in the terrestrial planet region, they showed that some of these asteroids were scattered to form the asteroid belt.

Iron and nickel meteorites found in the asteroid belt are probably the surviving fragments of asteroid-like bodies that once formed the Earth and other nearby rocky planets.
The iron-meteorite parent bodies probably emerged from the same disk of planetary debris that produced the Earth and other inner solar system planets.

"Small bodies that form quickly in the inner solar system end up melting and differentiating from the decay of short-lived radioactive elements. Iron meteorites came from the molten material that sinks to the centre of these objects, cools and solidifies" - Dr. William Bottke, SwRI research scientist.

The computer simulations showed that the asteroids could have remained in the asteroid belt located between the orbits of Mars and Jupiter for billions of years.

"This means that certain iron meteorites may tell us what the precursor material for the primordial Earth was like, while also helping us unlock several fundamental questions about the Earth's origins. There's also the possibility that larger versions of this material may still be hiding among the asteroids. The hunt for them is on" - Dr. William Bottke.

A potential problem in using meteorites to understand the formation of Earth and other terrestrial planets is that most members of the asteroid belt are assumed to have formed there, rather than from the formation of the terrestrial planets.
Meteorite compositions are very varied which leads further support that they did not all form in that region of space.

"While tens of thousands of stony meteorites have been collected, most can be traced back to perhaps a few tens of parent asteroids. What is strange is that the iron meteorites, despite their smaller numbers, represent almost two-thirds of all of the unique parent asteroids sampled to date" - Dr. Alessandro Morbidelli, Observatorie de la Cote d'Azur.

By tracking the origin and evolution of iron-meteorite parent bodies using computer simulations, they showed that the precursors of most iron meteorites formed in the terrestrial planet region.

"It is hard to produce small differentiated bodies in the asteroid belt without also melting lots of large asteroids. These events would produce a number of telltale signs that would be easily detected by observers" - Dr. Robert Grimm, assistant director of the SwRI Space Studies Department.

"While the amount of material reaching the asteroid belt was limited, much of it was placed in regions likely to produce meteorites" - Dr. David Nesvorny, SwRI Research Scientist

NASA's Origins of Solar Systems and Planetary Geology and Geophysics programs funded the research of the SwRI investigators.
Their paper "Iron Meteorites as Remnants of Planetesimals from the Terrestrial Planet Region," by Bottke, Nesvorny, Grimm, Morbidelli, and O'Brien is described in the February 16 issue of Nature.

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RE: Earths Formation
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Collisions between embryonic planets during a critical period in the early history of the Solar System may account for some previously unexplained properties of planets, asteroids, and meteorites, according to researchers at the University of California, Santa Cruz, who describe their findings in a paper to appear in the January 12 issue of the journal Nature.

The four "terrestrial" or rocky planets (Earth, Mars, Venus, and Mercury) are the products of an initial period, lasting tens of millions of years, of violent collisions between planetary bodies of various sizes. Scientists have mostly considered these events in terms of the accretion of new material and other effects on the impacted planet, while little attention has been given to the impactor. (By definition, the impactor is the smaller of the two colliding bodies.)
But when planets collide, they don't always stick together. About half the time, a planet-sized impactor hitting another planet-sized body will bounce off, and these hit-and-run collisions have drastic consequences for the impactor.

"You end up with planets that leave the scene of the crime looking very different from when they came in--they can lose their atmosphere, crust, even the mantle, or they can be ripped apart into a family of smaller objects" - Erik Asphaug, associate professor of Earth sciences at UCSC and first author of the Nature paper.

The remnants of these disrupted impactors can be found throughout the asteroid belt and among meteorites, which are fragments of other planetary bodies that have landed on Earth, he said. Even the planet Mercury may have been a hit-and-run impactor that had much of its outer layers stripped away, leaving it with a relatively large core and thin crust and mantle. That scenario remains speculative and requires additional study.
Asphaug and postdoctoral researcher Craig Agnor used powerful computers to run simulations of a range of scenarios, from grazing encounters to direct hits between planets of comparable sizes. Co-author Quentin Williams, professor of Earth sciences at UCSC, analysed the outcomes of these simulations in terms of their effects on the composition and final state of the remnant objects.
The researchers found that even close encounters in which the two objects do not actually collide can severely affect the smaller object.

"As two massive objects pass near each other, gravitational forces induce dramatic physical changes--decompressing, melting, stripping material away, and even annihilating the smaller object. You can do a lot of physics and chemistry on objects in the Solar System without even touching them" - Quentin Williams.

A planet exerts enormous pressure on itself through self-gravity, but the gravitational pull of a larger object passing close by can cause that pressure to drop precipitously. The effects of this depressurisation can be explosive.

"It's like uncorking the world's most carbonated beverage. What happens when a planet gets decompressed by 50 percent is something we don't understand very well at this stage, but it can shift the chemistry and physics all over the place, producing a complexity of materials that could very well account for the heterogeneity we see in meteorites" - Quentin Williams.

The formation of the terrestrial planets is thought to have begun with a phase of gentle accretion within a disk of gas and dust around the Sun. Embryonic planets gobbled up much of the material around them until the inner Solar System hosted around 100 Moon-sized to Mars-sized planets. Gravitational interactions with each other and with Jupiter then tossed these protoplanets out of their circular orbits, setting off an era of giant impacts that probably lasted 30 to 50 million years.
Scientists have used computers to simulate the formation of the terrestrial planets from hundreds of smaller bodies, but most of those simulations have assumed that when planets collide they stick.

"We've always known that's an approximation, but it's actually not easy for planets to merge. Our calculations show that they have to be moving fairly slowly and hit almost head-on in order to accrete" - Erik Asphaug

It is easy for a planet to attract and accrete a much smaller object than itself. In giant impacts between planet-sized bodies, however, the impactor is comparable in size to the target. In the case of a Mars-size impactor hitting an Earth-size target, the impactor would be one-tenth the mass but fully one-half the diameter of the Earth.

"Imagine two planets colliding, one half as big as the other, at a typical impact angle of 45 degrees. About half of the smaller planet doesn't really intersect the larger planet, while the other half is stopped dead in its tracks. So there is enormous shearing going on, and then you've got incredibly powerful tidal forces acting at close distances. The combination works to pull the smaller planet apart even as it's leaving, so in the most severe cases the impactor loses a large fraction of its mantle, not to mention its atmosphere and crust" - Erik Asphaug.

According to Agnor, the whole problem of planet formation is highly complex, and unravelling the role played by hit-and-run fragmenting collisions will require further study. By examining planetary collisions from the perspective of the impactor, however, the UCSC researchers have identified physical mechanisms that can explain many puzzling features of asteroids.
Hit-and-run collisions can produce a wide array of different kinds of asteroids.

"Some asteroids look like small planets, not very disturbed, and at the other end of the spectrum are ones that look like iron-rich dog bones in space. This is a mechanism that can strip off different amounts of the rocky material that composes the crust and mantle. What's left behind can range from just the iron-rich core through a whole suite of mixtures with different amounts of silicates" - Quentin Williams.

One of the puzzles of the asteroid belt is the evidence of widespread global melting of asteroids. Impact heating is inefficient because it deposits heat locally. It is not clear what could turn an asteroid into a big molten blob, but depressurisation in a hit-and-run collision might do the trick.

"If the pressure drops by a factor of two, you can go from something that is merely hot to something molten" - Erik Asphaug.

Depressurisation can also boil off water and release gases, which would explain why many differentiated meteorites tend to be free of water and other volatile substances. These and other processes involved in hit-and-run collisions should be studied in more detail.

"It's a new mechanism for planetary evolution and asteroid formation, and it suggests a lot of interesting scenarios that warrant further study" - Erik Asphaug.

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Moons Formation
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The collision that spawned the Earth's Moon was relatively mild, reveals the longest and most detailed computer simulation ever done of the impact. The research puts limits on the size and velocity of space rocks that can lead to the formation of satellites in cosmic smash-ups.
Computer models suggest the Moon formed after an object the size of Mars (just over half the diameter of Earth) crashed into Earth about 4.5 billion years ago. Debris from the impact formed a disc around Earth that eventually coalesced to become the Moon.
But modelling the process realistically is extremely difficult, and researchers have tried a variety of approaches. Most have used single particles in the models to represent some larger number of real particles, a method called Smoothed Particle Hydrodynamics (SPH).
But the best of these models use just a few thousand particles in the debris disc, and therefore can not reveal detailed disc structures. As a result, the models can only recreate conditions for less than a day after the impact.
Now, researchers led by Keiichi Wada at the National Astronomical Observatory of Japan in Tokyo have used another approach to model the disc for about four days. They divided the disc into a three-dimensional grid of boxes – each with its own properties, such as temperature and density – and evolved the boxes over time. They ran two "extreme" simulations – one in which the disc was made mostly of hot gas, and another where it was mostly liquid and solid.



Both simulations behaved similarly for the first 10 hours after the initial impact, with the damaged impactor circling back and hitting Earth a second time, when it is destroyed. This accords with SPH models as well, suggesting gravity is the dominant force in the early formation of the disc.
But the two models begin to diverge after that. If the impactor vaporises when it is destroyed, spiral shock waves are created that slow down the disc's rotation. This allows the disc material to fall onto the Earth and prevents the formation of a moon.
In contrast, if the impactor produces mostly liquid or solid debris, the shocks cannot slow the disc down enough to make it fall to Earth, and the Moon is formed. The researchers suggest that any impact powerful enough to vaporise the impactor would not form a satellite.
In the case of the Earth, they estimate the Mars-sized object must have been travelling at less than 15 kilometres per second. In more general terms, they conclude that if an impactor is more than a few times the mass of Earth, then "the giant impact never results in forming a large satellite".

"The gaseous disc would most likely collapse faster than a solid or liquid disc" - Scott Kenyon, astronomer at the Smithsonian Astrophysical Observatory in Cambridge, Massachusetts, US

But he points out that astronomers have long struggled with modelling the viscosity of gas in rotating discs. He says all models have this problem, but that the 3D-grid approach may be more vulnerable to it because the viscosity must be chosen by the researchers, and the value selected could affect the timescale over which the disc falls to Earth.
Journal reference: Upcoming issue of the Astrophysical Journal.

-- Edited by Blobrana at 04:31, 2006-01-12

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RE: Earths Formation
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Earth's future was determined at birth. Using refined techniques to study rocks, researchers at the Carnegie Institution’s Department of Terrestrial Magnetism (DTM) found that Earth’s mantle--the layer between the core and the crust--separated into chemically distinct layers faster and earlier than previously believed. The layering happened within 30 million years of the solar system’s formation, instead of occurring gradually over more than 4 billion years, as the standard model suggests.
The new work was recognized by Science magazine, in its December 23 issue, as one of the science breakthroughs for 2005.

Carnegie scientists Maud Boyet and Richard Carlson analysed isotopes--atoms of an element with the same number of protons, but a different number of neutrons--of elements in rock samples for their work.

"Isotopes exist naturally in different proportions and are used to determine conditions under which rock forms. Radioactive isotopes are particularly handy because they decay at a predictable rate and can reveal a sample’s age and when its chemical composition was established" - Richard Carlson.

In the standard model of the geochemical evolution of the Earth, the Earth’s mantle has been evolving gradually over Earth’s 4.567-billion-year history primarily through the formation of the chemically distinct continental crust. Shortly after solid material began condensing from the hot gas of the cooling early solar system, the object that would become Earth grew by the collision and accretion of smaller rocky bodies. The chemical composition of these building blocks is preserved today in primitive meteorites called chondrites.
In the 1980s, scientists analysed the ratio of isotopes of the rare earth element neodymium in chondrites and various terrestrial rocks collected at or near the Earth’s surface and found that the samples shared a common composition. Researchers believed that this ratio remained constant from the beginning of Earth formation. Using new-generation equipment, Boyet and Carlson found, surprisingly, that the terrestrial samples did not have the same ratio as the meteorites. Compared to chondrites, all terrestrial rocks measured have an excess of the mass 142 isotope of neodymium (142Nd), which is the decay product of a now-extinct radioactive isotope of samarium of mass 146 (146Sm) that was present at the birth of the solar system but decayed away shortly thereafter. The excess in 142Nd allowed the researchers to determine when the composition of the Earth diverged from that of the meteorites--within the first 30 million years after solar system formation, which is less than 1% of the age of our planet.

To explain the excess of 142Nd found in the terrestrial samples, the Carnegie scientists believe that the Earth was largely molten during its formation and that rapid crystallization of Earth’s early magma ocean caused the mantle to separate into chemically distinct layers, one containing a high ratio of Sm to Nd similar to that observed today in the mantle source of the volcanism along ocean ridges. The complementary reservoir, with low 142Nd abundance, has never been sampled at the surface and hence could now be deeply buried in the so-called D" layer at the very base of the mantle, above the core.
This “missing” layer should be rich in the elements uranium, thorium, and potassium, whose long-lived radioactive decay heats Earth's interior and causes our planet to remain geologically active. This hot layer above the core could help to keep the outer core molten so that circulation of liquid iron can produce Earth's magnetic field, and it could instigate the hot plumes of upwelling mantle material that give rise to volcanically active islands, such as Hawaii.

Measurements by Boyet and Carlson also show that lunar rocks have the same abundance of 142Nd as the terrestrial samples, a finding that adds to the evidence that the Moon formed from the Earth. Since Mars also experienced early melting, as indicated by the chemical and isotopic composition of Martian meteorites, the new results now link the early evolution of Earth, Moon, and Mars and highlights the importance of early events in determining the chemical characteristics of the terrestrial planets.

"The work of Boyet and Carlson, when added to what has already been determined for the Moon and Mars, shows that the earliest days of the inner planets were violent times in solar system history. Theoretical work by Carnegie scientist George Wetherill had pointed to this result, but now we have a clear chemical signature of this episode of Earth history" - Sean Solomon, DTM director.


During Earth formation, decay of short-lived radioactive isotopes and surface bombardment from large bodies heated Earth’s mantle and created a deep magma ocean. Crystallization of the magma ocean created compositionally distinct layers (blues), while leftover liquid (red) remained just under the primordial crust.


In the conventional model of Earth history, the mixing caused by mantle convection has erased this early chemical differentiation. The only chemical variation in the mantle is that caused by the formation of the continental crust, leaving the upper mantle (light blue) deficient in those elements concentrated in the crust (black), while most of the mantle is still similar in composition to the chondritic meteorites from which Earth accumulated.


The Boyet and Carlson result requires the Earth to have differentiated early, within 30 million years, leaving most of Earth's mantle (light blue) depleted in those elements that prefer melts over crystallizing solids. The chemical complement to the depleted mantle could be small and quite enriched in radioactive elements, such as uranium and thorium; this complementary material may coincide with the seismically observed D" layer, located between the core and the mantle some 2700 km deep.
Credit Maud Boyet



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A new study by an international team of researchers has concluded Earth's continents most likely were in place soon after the planet was formed, overturning a long-held theory that the early planet was either moon-like or dominated by oceans.

The team, led by Professor Mark Harrison of the Australian National University, analysed the rare metal element known as hafnium in ancient minerals from the Jack Hills in Western Australia, thought to be among the oldest rocks on Earth. Hafnium is found in association with zircon crystals in the Jack Hills rocks, which date to almost 4.4 billion years ago.

"These results support the view that the continental crust had formed by 4.4-4.5 billion years ago and was rapidly recycled into the mantle".

The researchers used hafnium as a "tracer" element, using isotopes to infer the existence of early continental formation on Earth dating to Hadeon Eon, which took place during the first 500 million years of Earth's history

"The evidence indicates that there was substantial continental crust on Earth within its first 100 million years of existence. It looks like the Earth started off with a bang"- Stephen Mojzsis, University of Colorado Assistant Professor.

A 2001 study led by Mojzsis published in the journal Nature showed evidence for the presence of water on Earth's surface roughly 4.3 billion years ago.

"The view we are taking now is that Earth's crust, oceans and atmosphere were in place very early on, and that a habitable planet was established rapidly" - Stephen Mojzsis.

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Oceans of molten rock, or magma, covered some asteroids in the early solar system, reveals a new study of meteorites. But researchers are still puzzled over why other asteroids apparently did not melt at all.
In the solar system's first few tens of millions of years, collisions between rocky objects and the decay of radioactive isotopes melted the interiors of large objects. Magma oceans - perhaps hundreds of kilometres deep - lapped over the Moon, the Earth, and other planets, allowing dense material to settle towards their centres in a process called differentiation.
But the extent of asteroid melting had remained unclear.

Now, Richard Greenwood at the Open University in Milton Keynes, UK, and colleagues have analysed groups of meteorites thought to have come from the 530-kilometre-wide asteroid Vesta and from a second, still-unknown, asteroid.
They found all of the meteorites from each source shared the same ratios of oxygen isotopes, suggesting both asteroids must have melted almost completely.

"It's an exquisite piece of work" - Michael Drake, geochemist at the University of Arizona in Tucson, US.

But the research fails to explain why other asteroids do not show any evidence of melting. Ceres, the largest known asteroid - 930 kilometres wide - appears to be totally undifferentiated.

The difference may be down to timing.
Previous research has suggested asteroids were heated by the decay of radioactive aluminium-26 in the dusty disc from which the solar system condensed. That isotope has a half-life of only 700,000 years.
So if it was the main heat source for the first asteroids, too little may have remained to warm those that formed later.



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Melting asteroids and the building blocks of early Earth
Important new research documenting how the Earth formed from melted asteroids 4.5 billion years ago is published in the 16 June issue of Nature. The paper was written by Dr Richard Greenwood and Dr Ian Franchi of the Open University’s Planetary and Space Sciences Research Institute (PSSRI).

"This research is important, because it demonstrates that events and processes on asteroids during the birth of the Solar System determined the present-day composition of our Earth" - Dr Richard Greenwood

Immediately following the formation of our Solar System 4.5 billion years ago, small planetary bodies formed, with some melting to produce volcanic and related rocks. The OU researchers analysed meteorites to see how processes on asteroids may have contributed to the formation of Earth.

In their paper “Widespread magma oceans on asteroidal bodies in the early Solar System” Drs Greenwood and Franchi show that some asteroids experienced large-scale melting, with the formation of deep magma oceans. Such melted asteroids would have become layered with lighter rock forming near the surface, while denser rocks were deeper in the interior. Since large bodies, such as Earth, grew by incorporation of many such smaller bodies these important results shed new light on the processes involved in building planets.

The researchers suggest that in the chaotic, impact-rich environment of the early Solar System, significant amounts of the outer layers of these melted asteroids would have been removed prior to becoming part of the growing Earth. This process is a better explanation for the composition of the Earth than earlier theories which called for large amounts of light elements in the Earth’s dense core, or unknown precursor materials.

The Open University researchers point to recent astronomical observations which show that these processes are also important in other planetary systems, such as that around the star Beta Pictoris.

Open University Press Release (.Doc)

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