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TOPIC: Earths Formation


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The history of planet Earth is a fascinating story, involving catastrophic collisions with other small planets and a veritable plethora of asteroid impacts. The prevailing theory about the formation of the moon is called the giant impact hypothesis: the theory goes that a Mars-sized object, known as Theia, crashed in to the young Earth. What left was Earth, and its moon.
A new computer model suggests, however, that the Moon may not have been the only reminder of that big collision. Jack J. Lissauera of the Space Science and Astrobiology Division, NASA Ames Research Centre, and John E. Chambers of the Department of Terrestrial Magnetism, Carnegie Institution of Washington, have suggested that moonlets called Trojans may have been left behind in the collision.


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Theia
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NASA's twin STEREO probes are entering a mysterious region of space to look for remains of an ancient planet which once orbited the Sun not far from Earth. If they find anything, it could solve a major puzzle--the origin of the Moon.

"The name of the planet is Theia. It's a hypothetical world. We've never actually seen it, but some researchers believe it existed 4.5 billion years ago - and that it collided with Earth to form the Moon" - Mike Kaiser, STEREO project scientist at the Goddard Space Flight Centre.

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Les isotopes du fer, nouveaux traceurs de la genèse de la Terre
Les isotopes du fer peuvent-ils servir de traceur pour élucider certains aspects de la genèse de la Terre, de la Lune et d'autres planètes telluriques, dont Mars, notamment, qui présentent toutes les trois un fer de caractéristiques différentes?
Pour répondre à cette question, des chercheurs du Laboratoire d'étude des Mécanismes de Transfert en Géologie (CNRS-INSU, Université de Toulouse), du Laboratoire de Structure et Propriétés de l'Etat Solide, (CNRS, Université de Lille), de l'Institution Carnegie à Washington et de l'Université Macquarie à Sydney ont analysé les compositions isotopiques du fer de phases métalliques et silicatées synthétisées à l'équilibre dans des conditions de haute pression et haute température, reproduisant celles de l'océan de magma qui aurait précédé l'apparition du noyau terrestre.
Dans les conditions de leurs expériences, on n'observe pas de fractionnement isotopique lié à l'apparition de deux phases métal et silicate. Ceci implique que les différences observées entre les planètes résultent plus de la manière dont les planètes se sont formées, que de leur différenciation.

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The history of planet Earth is a fascinating story, involving catastrophic collisions with other small planets and a veritable plethora of asteroid impacts. The prevailing theory about the formation of the moon is called the giant impact hypothesis: the theory goes that a Mars-sized object, known as Theia, crashed in to the young Earth. What was left was Earth, and its moon.
A new computer model suggests, however, that the Moon may not have been the only reminder of that big collision. Jack J. Lissauera of the Space Science and Astrobiology Division, NASA Ames Research Centre, and John E. Chambers of the Department of Terrestrial Magnetism, Carnegie Institution of Washington, have suggested that moonlets called Trojans may have been left behind in the collision.


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Earth's crust
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The continuous overturning, melting and re-casting of Earth's crust over the eons may have started with a massive asteroid impact in Earth's infancy, suggests one geologist.
The unusual and iconoclastic hypothesis, if true, could help point the way to how and why plate tectonics did or did not get started on other worlds in our solar system and beyond. That's important because one of the critical ingredients of life on Earth is a constantly recycling crust.

"Everyone argues about when (plate tectonics) starts, but never asks about outside processes" - geologist Vicki Hansen of the University of Minnesota at Duluth.

Hansen had spent years studying Venus when it occurred to her there was a blind spot in the thinking of Earth-focused geological research: The role of impacts in a once asteroid-thick early solar system.

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A team of scientists from NASA's Johnson Space Center (JSC) and the Lunar and Planetary Institute (LPI), both in Houston, and the University of California, Davis (UCD) has found that terrestrial planets such as the Earth and Mars may have remained molten in their early histories for tens of millions of years. The findings indicate that the two planets cooled slower than scientists thought and a mechanism to keep the planet interiors warm is required.
These new data reveal that the early histories of the inner planets in the solar system are complex and involve processes no longer observed. Evidence of these processes has been preserved in Mars, while it has been erased in Earth. So Mars is probably the best opportunity to understand how Earth formed.
Vinciane Debaille (LPI), Alan Brandon (JSC), Qing-zhu Yin and Ben Jacobsen (UCD) present these new findings in a paper published in the Nov. 22 issue of Nature.
Scientists think that early crust formation alone cannot account for the slow cooling magma ocean seen in large planets. This new evidence instead implies that Mars, at one time, had a primitive atmosphere that acted as the insulator.

The primitive atmosphere was composed mostly of hydrogen left over from accretion into a rocky planet, but was removed, probably by impacts, about 100 million years after the planet formed - Vinciane Debaille.

Debaille and her colleagues performed precise measurements of neodymium isotope compositions of nine rare Martian meteorites called shergottites using mass spectrometers at JSC and UCD. Shergottites, named after the first-identified meteorite specimen that fell at Shergotty, India, in 1865, are a group of related meteorites from Mars composed primarily of pyroxene and feldspar.
The scientists examined shergottites because their large range in chemical compositions is thought to be a fingerprint of the formation of their deep sources very early in the history of Mars.

These rocks were lavas that were made by melting deep in Mars and then erupted on the surface. They were delivered to Earth as meteorites following impacts on Mars that exhumed them and launched them into space"  - Alan Brandon.

Mars meteorites are a treasure chest of information about that planet and have been the focus of extensive research by scientists.
The metallic element samarium has two radioactive isotopes that decay at a known rate to two daughter neodymium isotopes. By precisely measuring the quantities of neodymium isotopes, Debaille was able to use these two radiometric clocks to derive the times of formation of the different shergottite sources in the Martian interior.

We expected to find that their sources all formed at the same time. But what we found instead was that the shergottite sources formed at two different times. The oldest formed at 35 million years after the solar system began to condense from ice and dust into large planets about 4,567 million years ago. The youngest formed about 110 million years after the solar system began to condense - Vinciane Debaille.

Debaille and her colleagues found that the scenario that best fits the data is one where a global-scale magma ocean formed from melting in Mars during the final stages of accretion and then slowly solidified over this time period.

The most recent physical models for magma oceans suggest they solidify on timescales of a few million years or less, so this result is surprising. Some type of insulating blanket, either as a rocky crust or a thick atmosphere, is needed as an insulator to have kept the Martian interior hot - Alan Brandon.

Source NASA

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Geochemists at Rensselaer Polytechnic Institute are challenging commonly held ideas about how gases are expelled from the Earth. Their theory, which is described in the Sept. 20 issue of the journal Nature, could change the way scientists view the formation of Earths atmosphere and those of our distant neighbours, Mars and Venus. Their data throw into doubt the timing and mechanism of atmospheric formation on terrestrial plants.
Lead by E. Bruce Watson, Institute Professor of Science at Rensselaer, the team has found strong evidence that argon atoms are tenaciously bound in the minerals of Earths mantle and move through these minerals at a much slower rate than previously thought. In fact, they found that even volcanic activity is unlikely to dislodge argon atoms from their resting places within the mantle. This is in direct contrast to widely held theories on how gases moved through early Earth to form our atmosphere and oceans, according to Watson.

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Title: Origin of the Ocean on the Earth: Early Evolution of Water D/H in a Hydrogen-rich Atmosphere
Authors: Hidenori Genda, Masahiro Ikoma

The origin of the Earth's ocean has been discussed on the basis of deuterium/hydrogen ratios (D/H) of several sources of water in the solar system. The average D/H of carbonaceous chondrites (CC's) is known to be close to the current D/H of the Earth's ocean, while those of comets and the solar nebula are larger by about a factor of two and smaller by about a factor of seven, respectively, than that of the Earth's ocean. Thus, the main source of the Earth's ocean has been thought to be CC's or adequate mixing of comets and the solar nebula. However, those conclusions are correct only if D/H of water on the Earth has remained unchanged for the past 4.5 Gyr. In this paper, we investigate evolution of D/H in the ocean in the case that the early Earth had a hydrogen-rich atmosphere, the existence of which is predicted by recent theories of planet formation no matter whether the nebula remains or not. Then we show that D/H in the ocean increases by a factor of 2-9, which is caused by the mass fractionation during atmospheric hydrogen loss, followed by deuterium exchange between hydrogen gas and water vapour during ocean formation. This result suggests that the apparent similarity in D/H of water between CC's and the current Earth's ocean does not necessarily support the CC's origin of water and that the apparent discrepancy in D/H is not a good reason for excluding the nebular origin of water.

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Scientists have compared silicon samples from Earth, meteorites and planetary materials to provide new evidence that the Earths core formed under very different conditions from those that existed on Mars.
The research also shows that the Earth and the Moon have the same silicon isotopic composition, supporting the theory that atoms from the two mixed in the early stages of their development.
 Carried out by scientists from Oxford University along with colleagues from University of California, Los Angeles (UCLA) and the Swiss Federal Institute of Technology in Zurich (ETH), the study compared silicon isotopes from rocks on Earth with samples from meteorites and other solar system materials.
It is the first time that isotopes have been used in this way and it has opened up a new line of scientific investigation into how the Earths core formed.

The research also shows that the Moon has the same silicon isotopic composition as the Earth. This cannot be caused by high pressure core formation on the Moon which is only about one percent of the mass of the Earth.
 However, it is consistent with the recent proposal that the material that made the Moon during the giant impact between the proto-Earth and another planet, usually called 'Theia', was sufficiently energetic that the atoms of the disk from which the Moon formed mixed with those from the silicate Earth.
This means the silicon in the silicate Earth must have already had a heavy isotopic composition before the Moon formed about 40 million years after the start of the Solar System.
The research was published in the 28 June edition of Nature and supported from grants provided by the UKs Science and Technology Facilities Council, and the USAs and Switzerlands National Science Foundation.

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Asteroids, Comets, and Planets
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Could all of the asteroids, comets, and planets in our Milky Way galaxy be made of a similar mix of dusty components?
After analysing the dust particles of a variety of comets with NASA's Spitzer Space Telescope, the Deep Impact spacecraft, and the internationally funded Infrared Space Observatory, Dr. Carey Lisse, of the Johns Hopkins University Applied Physics Laboratory in Laurel, Md., suspects that the answer is yes.

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