http://www.abc.net.au/science/news/stories/s822757.htm
Hot asteroids make Earth-like planets more likely Mark Horstman ABC Science Online (Australia) April 3, 2003 Young stars beyond our Sun can form rocky planets like Earth from nearby gas and dust, suggests a new study of meteorites which found terrestrial planets formed quicker than previously thought. In a report in today's issue of the journal Nature, German and French geologists have reconstructed the temperature histories of several meteorites that split from a single ancient asteroid dating back to the birth of our Solar System. Known as 'thermochronometry', the technique also indicates the asteroid was heated from the inside out. Led by Dr Mario Trieloff of Mineralogisches Institut der Universit�t Heidelberg in Germany, the team used an ingenious array of 'radiometric clocks' - natural radioactive isotopes, such as lead (Pb), potassium-argon (K-Ar), and plutonium (Pu) that decay at known rates - in order to measure the asteroid's rate of cooling. Asteroids have a layered structure rather like an onion. One type was heated to incredibly high temperatures, melted, and formed metallic cores of iron and nickel, with an outer mantle of silicate minerals - similar to the terrestrial planets Mercury, Venus, Earth and Mars. Another class of 'undifferentiated' asteroids were not heated enough to melt or form a metallic core. They contain millimetre-sized droplets of rock that are the melted dust grains of early solar nebula, before larger bodies were formed more than 4 billion years ago. These tiny time capsules are called 'chondrules', and the meteorites in which they are found, 'chondrites'. Meteorites are samples from different depths - and therefore, temperatures - of an asteroid, reflecting the history of its formation. The researchers used 'H-chondrites' (H denoting 'high metallic iron'), that they knew were derived from the same parent because they shared the same composition of oxygen isotopes, oxidation, and rock-forming minerals. "Part of our problem was that we had meteorites, or small fragments from this asteroid, but we didn't know exactly from which depths they were excavated, Trieloff told ABC Science Online. "Our study checked the models against our experiments to reconstruct the thermal histories of the rocks coming from different depths." Radioactive decay "Our dating methods use decay of natural radioactive elements such as 40K (decaying to 40Ar), and 244Pu that was only alive in the early Solar System, due to its relatively short half-life of 80 million years. The 'ages' determined in this way do not mean the rocks came into existence at that time, but are 'cooling ages' when the rock fell below a specific temperature." For example, if the K-Ar 'age' of the feldspar in the sample was 4.4 billion years, this would indicate when the mineral was first heated to 280�C. At less than this temperature, the natural radioactive decay product of 40K would be retained in the mineral as 40Ar. The energy source to heat up small asteroids on the inside - enough to melt and form iron cores - has long been a mystery. Theories included heat from the harsh glow of an early proto-Sun, induction heating by 'wind' blasts of ion particles, or collisions with larger bodies. However, none of these could account for the necessary high energy levels required. "Such energy sources would have yielded asteroids where the exposed outer layers were heated most strongly and cooled most rapidly," Trieloff said. "The insulated inner regions would have been heated most weakly, and cooled most slowly. Our results did not confirm such a cooling behaviour." Instead, the researchers found a progression of different exposures to heat, from the coolest on the outside to hottest in the centre. This implied that the asteroid had been heated from within. "We used several meteorites in which the radiometric clocks had not been disturbed by collisions. The cooling behaviour we observed agrees with a mathematical model in which an asteroid is heated by the 'decay energy' of 26Al." [A short-lived isotope of aluminium with a half-life of 700,000 years that was present in the early Solar System] "This is the first time that the decay energy of 26Al has been demonstrated as the energy source that heated one specific small body in the early Solar System," said Trieloff. "These results also imply that accretion of bodies of 100 km size occurred with a few million years, otherwise 26Al would have been mostly decayed." Combining all their information from the meteorites, the team describes the parent asteroid: born in the first few million years - the dawn of our Solar System - with a radius of about 100 km and an 'onion shell' structure, internally heated by 26Al to a peak temperature of about 850�C, cooling over the next 160 million years to about 120�C, and more iron-rich than any rock on Earth. "If accretion was this fast in our early Solar System, then formation of planets around other stars may occur similarly fast, and the existence of other terrestrial planets is likely," Trieloff concluded. ______________________________________________ Meteorite-list mailing list [EMAIL PROTECTED] http://www.pairlist.net/mailman/listinfo/meteorite-list
