N°30 I quarterly I july 2013 Live from the Labs | 11 w BY Tom Ri dgway Paleoclimatology w Since May 2011, the Alpha Magnetic Spectrometer (AMS) installed on the International Space Station has recorded 25 billion particles in the flux of near-Earth cosmic rays. Its first published results1 have surprised researchers: amounts of antimatter with energy levels between 0.5 and 350 GeV—as expressed in the “positron fraction” or the ratio of positrons to the total number of electrons and positrons—are far higher than expected. “If these positrons were simply the product of secondary interactions between hydrogen or helium nuclei and the interstellar medium, then the positron fraction should drop off with increasing energy levels,” says Sylvie Rosier-Lees from the LAPP,2 one of the CNRS labs involved in the project. “Instead, AMS has observed a rise with higher energy levels, pointing to the existence of primary sources of positrons.” Since positrons and electrons quickly lose their energy, and the antimatter produced in the Big Bang was annihilated long ago, these positrons must come from “new” sources relatively close to Earth. These “electron-positron pairs can be produced by pulsar nebulae or by the annihilation of dark matter”—an exciting prospect for scientists. Dark matter is thought to make up 26.8% of the universe’s total mass, but its exact nature remains a mystery. Part of it could be solved if the excess positrons prove to be the result of dark matter annihilation. “Further AMS work is necessary,” says Rosier- Lees. “We need to measure the flux of positrons and electrons separately, and above all, extend our research into higher energy levels. Dark matter annihilation could also produce anti-protons and gamma photons, so investigating them could help lift some of the ambiguity.” 01. M. Aguilar et al. (AMS Collaboration), “First Result from the Alpha Magnetic Spectrometer on the International Space Station: Precision Measurement of the Positron Fraction in Primary Cosmic Rays of 0.5–350 GeV,” Phys. Rev. Lett., 2013. 110: 141102. 02. Laboratoire d’Annecy-le-Vieux de physique des particules (CNRS / Université de Savoie). Astrophysics proportionally to the depth at which it was trapped,” explains Frédéric Parrenin of the LGGE. Analyzing the isotope’s concentration makes it possible to accurately ascertain when the original air bubble was trapped. Not only did the researchers review gas age scales using these measurements, but they also synchronized temperature readings from the five ice cores extracted to fine-tune their data. As Parrenin observes, one thing is now clear: the massive impact of CO2 on past and present climatic variations. 01. F. Parrenin et al., “Synchronous change of atmospheric CO2 and Antarctic temperature during the last deglacial warming.” Science, 2013. 339: 1060-3. 02. Laboratoire de glaciologie et de géophysique de l’environnement (CNRS / Université Joseph Fourier). 03. Laboratoire des sciences du climat et de l’environnement (IPSL / CNRS / CEA / Université de Versailles Saint-Quentin-en-Yvelines). Contact information: LA PP, Annecy. Sylvie Rosier-Lees > sylvie.lees-rosier@lapp.in2p3.fr BY Fui Lee Luk w Climate skeptics, think again. A study of five ice cores drilled in Antarctica has contradicted former findings that temperatures increased up to 800 years prior to atmospheric CO2 surges at the end of the last Ice Age.1 Using the nitrogen isotope δ15N to date gas, the team from the LGGE2 and the LSCE3 showed that CO2 spikes coincided with warming during the last deglaciation period (20,000-10,000 years ago), thus contradicting the idea that climate change was unrelated to CO2 emissions. Some 800,000 years of climatic data frozen into ice cores have fuelled a longstanding debate: which came first, higher temperatures or excess CO2? Data on temperature and on CO2 at a given period are trapped at different levels of the ice core, creating confusion. While snow water molecules infer on temperature, gas from the same period keeps circulating throughout the ice core’s uncompacted layers and is only locked in air bubbles 50-120 meters deeper, where the snow becomes ice. Ice is thus older than the air it traps. The team used a method based on δ15N levels to determine the age of the gas. “This heavy isotope, found in air bubbles in the ice core, is enriched by gravity Contact information: LGGE , Grenoble. Frédéric Parrenin > frederic.parrenin@lgge.obs.ujf-grenoble.fr q Frédéric Parrenin cutting an ice core at Talos Dome (Antarctica) in 2005-2006. q T he AMS spectrometer placed on the International Space Station by Atlantis in 2011. Syncing Ice Core Data AMS Results: New Clue to Dark Matter © LGGE/CNRS/UJF © nasa Grenoble Annecy
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