Thursday, 18 July 2013

Hidden mantle material may help explain Earth’s origins Posted on July 17, 2013 by Physics Today

Science Daily: Scientists have been puzzled by the fact that Earth’s mantle appears to have less lead than predicted by standard theories of planetary evolution. It has long been assumed that the planet formed from meteoritic material ejected from asteroids that smashed into each other, and thus the amount of lead in Earth’s mantle should be comparable to that found in meteorites. Yet until now, such a reservoir has gone undetected. To look for that hidden cache, researchers at MIT have been collecting rock samples from a region of northern Pakistan called the Kohistan arc; a collision of two massive tectonic plates there some 40 million years ago exposed some of Earth’s mantle. An analysis of those rocks revealed that some were much denser than the mantle and contained more lead. Based on that finding, the researchers calculated that roughly 70% of the magma that rises from the mantle during subduction events is so heavy with lead that it crystallizes into dense rock and drops back down into the mantle, where it collects and remains undisturbed. Their results could help further the study of how Earth has evolved.

The Earth's Gold --"A Neutron Star Collision Was the Source" -The Daily Galaxy via CfA, July 18, 2013

A odd glow from a short gamma-ray burst (GRB) in a galaxy 3.9 billion light years away in theconstellation Leo on June 3 by NASA’s Swift space telescope hints that all of Earth's gold is the product of collisions between dead neutron stars. The gamma-ray explosion resulting from dead stars crashing together 24 sextillion miles away created an initial burst that lasted only only two-tenths of a second. But the infrared glow that lingered around the area afterward suggests that gold may have been among the elements thrown out in the collision. After comparing their observations using the powerful ESO telescope in Chile and the Hubble Space Telescope with theoretical models, the astronomers concluded that they were seeing the afterglow from a huge quantity of heavy metals formed in the collision.
The image above is a first direct look, in visible light, at a lone neutron star (RX J185635-3754). Produced with the Wide-Field Planetary Camera 2, Hubble Space Telescope.Current observation and theories say that the heavy elements of the periodic table, such as gold, platinum, lead and  uranium, had their origin in supernova explosions, but failed to explain the volume of gold in our solar system. About a decade ago, researchers in Europe used supercomputers to test their theory that heavier metals like gold and platinum could be formed from the massive explosion that occurs when two ultra-dense dead neutron stars collide.
It has long been understood that Earth’s elements are of cosmic origin. Carbon and oxygen atoms in our bodies, for  example, come from the interior of stars, where they were formed under high pressure and heat. They were later spewed into  the universe in supernova explosions.
A single neutron star might be roughly the size of Manhattan, but contain as much mass as our sun, or more, with all of it crammed together by the force of gravity until even the atoms have collapsed, leaving the object with the density of an atomic nucleus. A teaspoon of neutron-star mass would weigh, on Earth, about 5 billion tons.
“We are all star stuff, and our jewelry is colliding-star stuff,” said Edo Berger, an astronomer who led the research at  the Harvard-Smithsonian Center for Astrophysics in Cambridge, Mass. In the Milky Way a neutron-star collision is likely to happen about once every 100,000 years, Berger said. But the universe is big, containing many billions of galaxies, and so astronomers doing an all-sky survey will occasionally see one of these rare GRBs.

The Daily Galaxy via CfA