* Astronomy

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.

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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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.