Features Compilers & Researchers: Rachel Courtney & Sarah Williams Elizabeth Gibney & Nature Magazine | Scientific American staff September 26, 2014 Earth has water older than the sun Not all water in the solar system today could have formed in our solar system As much as half of the water in earth’s oceans could be older than the Sun, a study has found. By reconstructing conditions in the disk of gas and dust in which the Solar System formed, scientists have concluded that the Earth and other planets must have inherited much of their water from the cloud of gas from which the Sun was born 4.6 billion years ago, instead of forming later. The authors say that such interstellar water would also be included in the formation of most other stellar systems, and perhaps of other Earth-like planets. The dense interstellar clouds of gas and dust where stars form contain abundant water, in the form of ice. When a star first lights up, it heats up the cloud around it and floods it with radiation, vapourising the ice and breaking up some of the water molecules into oxygen and hydrogen. Until now, researchers were unsure how much of the old water would be spared in this process. If most of the original water molecules were broken up, water would have had to reform in the early Solar System. But the conditions that made this possible could be specific to the Solar System, in which case many stellar systems could be left dry, says Ilsedore Cleeves, an astrochemist at the University of Michigan in Ann Arbor, who led the new study. But if some of the water could survive the star-forming process, and if the Solar System’s case is typical, it means that water “is available as a universal ingredient during planet formation”, she says. To find out, Cleeves and her colleagues modelled the conditions soon after the Sun lit up. They calculated the amount of radiation that would have hit the Solar System, both from the young star and from outer space, and how far that radiation would have travelled through the cloud. Those conditions determine how new water molecules form from hydrogen and oxygen, and in particular the odds that the molecules include deuterium, an isotope of hydrogen whose nucleus contains a neutron, in addition to the usual single proton. The model predicted an abundance of deuterium-containing water, also known as heavy water, that was lower than that in the Solar System’s water today. But the interstellar clouds where Sun-like stars are currently forming — and thus, presumably, the material from which the Sun formed — have a higher proportion of heavy water compared to the current Solar System. This is because these clouds are subject to the continuous bombardment of cosmic rays, which tend to favour the inclusion of deuterium. Therefore, the authors concluded, the young Sun’s radiation was insufficient to account for the amount of heavy water seen in the Solar System today, and some must have existed before. They estimate that somewhere between 30% and 50% of the water in Earth’s oceans must be older than the Sun. “If the disk can’t do it, that means we must have inherited some level of these very deuterium-enriched interstellar ices from the birth environment of the Sun,” says Cleeves. The study was published in Science on 25 September. Ewine van Dishoeck, an astrochemist at the Leiden Observatory in the Netherlands, says that the study’s conclusions are based on good arguments but are still only theoretical. But confirmation could come next year, she adds, when the Atacama Large Millimeter Array, a radio telescope in Chile’s Atacama Desert, begins to study the chemical processes underlying the proportion of heavy water in protoplanetary disks. Even if the formation of typical stellar systems does not destroy all of the pre-existing water, it does not mean that water-drenched planets need to be the norm throughout the Universe. Venus and Mercury have no water, and Mars seems to have lost most of the water it once had — and it is still unclear what determines whether a planet gets to become wet and to stay that way, says Cecilia Ceccarelli, an astronomer at the Institute of Planetology and Astrophysics in Grenoble, France. See Diagram 1 Clara Moskowitz | Scientific American staff October 02, 2014 New particle is both matter and antimatter Researchers see signature of “Majorana particles” inside superconducting iron Since the 1930s scientists have been searching for particles that are simultaneously matter and antimatter. Now physicists have found strong evidence for one such entity inside a superconducting material. The discovery could represent the first so-called Majorana particle, and may help researchers encode information for quantum computers. Physicists think that every particle of matter has an antimatter counterpart with equal mass but opposite charge. When matter meets its antimatter equivalent, the two annihilate one another. But some particles might be their own antimatter partners, according to a 1937 prediction by Italian physicist Ettore Majorana. For the first time researchers say they have imaged one of these Majorana particles, and report their findings in the October 3 Science. The new Majorana particle showed up inside a superconductor, a material in which the free movement of electrons allows electricity to flow without resistance. The research team, led by Ali Yazdani of Princeton University, placed a long chain of iron atoms, which are magnetic, on top of a superconductor made of lead. Normally, magnetism disrupts superconductors, which depend on a lack of magnetic fields for their electrons to flow unimpeded. But in this case the magnetic chain turned into a special type of superconductor in which electrons next to one another in the chain coordinated their spins to simultaneously satisfy the requirements of magnetism and superconductivity. Each of these pairs can be thought of as an electron and an antielectron, with a negative and a positive charge, respectively. That arrangement, however, leaves one electron at each end of the chain without a neighbor to pair with, causing them to take on the properties of both electrons and antielectrons—in other words, Majorana particles. As opposed to particles found in a vacuum, unattached to other matter, these Majoranas are what’s called “emergent particles.” They emerge from the collective properties of the surrounding matter and could not exist outside the superconductor. The new study shows a convincing signature of Majorana particles, says Leo Kouwenhoven of the Delft University of Technology in the Netherlands who was not involved in the research but previously found signs of Majorana particles in a different superconductor arrangement. “But to really speak of full proof, unambiguous evidence, I think you have to do a DNA test.” Such a test, he says, must show the particles do not obey the normal laws of the two known classes of particles in nature—fermions (protons, electrons and most other particles we are familiar with) and bosons (photons and other force-carrying particles, including the Higgs boson). “The great thing about Majoranas is that they are potentially a new class of particle,” Kouwenhoven adds. “If you find a new class of particles, that really would add a new chapter to physics.” Physicist Jason Alicea of California Institute of Technology, who also did not participate in the research, said the study offers “compelling evidence” for Majorana particles but that “we should keep in mind possible alternative explanations—even if there are no immediately obvious candidates.” He praised the experimental setup for its apparent ability to easily produce the elusive Majoranas. “One of the great virtues of their platform relative to earlier works is that it allowed the researchers to apply a new type of microscope to probe the detailed anatomy of the physics.” The discovery could have implications for searches for free Majorana particles outside of superconducting materials. Many physicists suspect neutrinos—very lightweight particles with the strange ability to alter their identities, or flavors—are Majorana particles, and experiments are ongoing to investigate whether this is the case. Now that we know Majorana particles can exist inside superconductors, it might not be surprising to find them in nature, Yazdani says. “Once you find the concept to be correct, it’s very likely that it shows up in another layer of physics. That’s what’s exciting.” The finding could also be useful for constructing quantum computers that harness the laws of quantum mechanics to make calculations many times faster than conventional computers. One of the main issues in building a quantum computer is the susceptibility of quantum properties such as entanglement (a connection between two particles such that an action on one affects the other) to collapse due to outside interference. A particle chain with Majoranas capping each end would be somewhat immune to this danger, because damage would have to be done to both ends simultaneously to destroy any information encoded there. “You could build a quantum bit based on these Majoranas,” Yazdani says. ”The idea is that such a bit would be much more robust to the environment than the types of bits people have tried to make so far.” Physicists used a scanning-tunneling microscope to image a thin chain of iron atoms atop a superconductor made of lead (yellow bar). The colours here represent the quantum probability that any given spot contains a so-called Majorana particle, which is both matter and antimatter. The zoomed-in portion shows that the propability of finding a Majorana particle increases greatly at the ends of the wire, as theory predicts it should. See Diagram 2
Posted on: Sat, 04 Oct 2014 10:34:46 +0000
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