Thursday, November 19, 2009


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Dust samples collected by high-flying aircraft in the upper atmosphere have yielded an unexpectedly rich trove of relicts from the ancient cosmos, report scientists from the Carnegie Institution. The stratospheric dust includes minute grains that likely formed inside stars that lived and died long before the birth of our sun, as well as material from molecular clouds in interstellar space. This “ultra-primitive” material likely wafted into the atmosphere after the Earth passed through the trail of an Earth-crossing comet in 2003, giving scientists a rare opportunity to study cometary dust in the laboratory.

At high altitudes, most dust in the atmosphere comes from space, rather than the Earth’s surface. Thousands of tons of interplanetary dust particles (IDPs) enter the atmosphere each year. “We’ve known that many IDPs come from comets, but we’ve never been able to definitively tie a single IDP to a particular comet,” says study coauthor Larry Nittler, of Carnegie’s Department of Terrestrial Magnetism. “The only known cometary samples we’ve studied in the laboratory are those that were returned from comet 81P/Wild 2 by the Stardust mission.” The Stardust mission used a NASA-launched spacecraft to collect samples of comet dust, returning to Earth in 2006.

Comets are thought to be repositories of primitive, unaltered matter left over from the formation of the solar system. Material held for eons in cometary ice has largely escaped the heating and chemical processing that has affected other bodies, such as the planets. However, the Wild 2 dust returned by the Stardust mission included more altered material than expected, indicating that not all cometary material is highly primitive.

The IDPs used in the current study were collected by NASA aircraft in April 2003, after the Earth passed through the dust trail of comet Grigg-Skjellerup. The research team, which included Carnegie scientists Nittler, Henner Busemann (now at the University of Manchester, U.K.), Ann Nguyen, George Cody, and seven other colleagues, analyzed a sub-sample of the dust to determine the chemical, isotopic and microstructural composition of its grains. The results are reported on-line in Earth and Planetary Science Letters.

“What we found is that they are very different from typical IDPs” says Nittler. “They are more primitive, with higher abundances of material whose origin predates the formation of the solar system.” The distinctiveness of the particles, plus the timing of their collection after the Earth’s passing through the comet trail, point to their source being the Grigg-Skjellerup comet.

“This is exciting because it allows us to compare on a microscopic scale in the laboratory dust particles from different comets,” says Nittler. “We can use them as tracers for different processes that occurred in the solar system four-and-a-half billion years ago.”

The biggest surprise for the researchers was the abundance of so-called presolar grains in the dust sample. Presolar grains are tiny dust particles that formed in previous generations of stars and in supernova explosions before the formation of the solar system. Afterwards, they were trapped in our solar system as it was forming and are found today in meteorites and in IDPs. Presolar grains are identified by having extremely unusual isotopic compositions compared to anything else in the solar system. But presolar grains are generally extremely rare, with abundances of just a few parts per million in even the most primitive meteorites, and a few hundred parts per million in IDPs. “In the IDPs associated with comet Grigg-Skjellerup they are up to the percent level,” says Nittler. “This is tens of times higher abundances than we see in other primitive materials.”

Also surprising is the comparison with the samples from Wild 2 collected by the Stardust mission. “Our samples seem to be much more primitive, much less processed, than the samples from Wild 2,” says Nittler, “which might indicate that there is a huge diversity in the degree of processing of materials in different comets.”

(Photo: Carnegie I.)

Carnegie Institution


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Remains of one of the oldest known marsupials have been recovered in Charente-Maritime by a palaeontologist team from the Muséum national d'Histoire naturelle (CNRS) and the University of Rennes 1. This discovery raises a new hypothesis about the dispersal route of the earliest marsupial mammals. Results are published in the journal PNAS.

In the history of the first modern mammals (i.e., marsupials and placentals), during the Cretaceous, Europe is almost a Terra incognita. No fossils are known between 125 and 84 million years (my), and very few up to the Cretaceous-Tertiary boundary (65 my). In the Cenomanian (99 my) of Charente-Maritime, the discovery of the scientist team from the Muséum1 (CNRS) and the University of Rennes 12 thus provides important information on the early history of these mammals in Europe. The discovery consists of a few teeth, collected after screenwashing of 5 tons of sediment. They belong to a new tiny mammal, named Arcantiodelphys marchandi, which is one of the oldest and most primitive marsupial known in the world. It is also the oldest known representative of the modern therians in Europe.

This discovery is the result of a research program of the University of Rennes 1 on the vertebrates from the Cretaceous of Charentes, in collaboration with the MNHN.

Arcantiodelphys marchandi improves our knowledge of the earliest stages of the marsupial history, so far known mostly from North American fossils. Its main significance is that the beginning of the marsupial history also involved Europe. Furthermore, it confirms faunal links between North America and Europe during the mid-Cretaceous.

It is from these primitive marsupials from the “Euramerican” Cretaceous that the modern marsupials colonized the southern landmasses, South America and mainly Australia where they are nowadays well diversified. Opossums and kangaroos are extant representatives of this very old northern origin of the marsupials.

(Photo: © MNHN/ CNRS – Rennes 1)

Centre National de la Reserche Scientifique


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Cornell researchers have developed an ingenious method to time-compress optical signals. The process could enable optical communication systems to carry many more bits per second or could also be used to generate short bursts of light with complex waveforms needed to control chemistry and physics experiments where changes are induced by light.

In tests, the researchers compressed a series of optical pulses carrying information at 10 gigabits per second (Gps) into a much shorter burst carrying the same information at 270 Gps.

The research is reported in the online edition of the journal Nature Photonics and in a forthcoming print edition.

Alexander Gaeta, professor of applied and engineering physics, calls the compression device a "temporal telescope." The "lenses" are two tiny optical waveguides on a silicon chip designed by Michal Lipson, associate professor of electrical and computer engineering, in which signals can be manipulated by a process called "four-wave mixing." A signal and a "pump" are combined inside a waveguide only 300x550 nanometers in cross section, smaller than the wavelength of the infrared light traveling through it. (A nanometer is a billionth of a meter, about the length of three atoms in a row.) In the confined space, the two light beams mix together to create a new, combined signal.

In the first waveguide a signal that varies in intensity over time -- in the demonstration case, a series of on and off pulses representing ones and zeros -- is combined with a pump pulse containing a broad range of wavelengths of light. The output is a spectrum of wavelengths in which the intensity, or brightness, at each wavelength corresponds to the amplitude of the original signal at a particular moment in time.

The second waveguide combines this spectrum with another pump pulse that is much shorter than the original signal and reverses the process, creating a signal that varies in amplitude corresponding to variations in intensity across the spectrum. The resulting output signal mirrors the original input, but is compressed to the length of the second pump pulse.

As Gaeta puts it, the "focal length" of the temporal lens is determined by the length of the pump pulse. As with conventional glass lenses, putting two temporal lenses together creates a telescope. In this case the system looks through the wrong end of the telescope, making things look smaller.

The demonstration was also done with more complex waveforms, including amplitude- and frequency-modulated signals.

Modulating light into complex waveforms over very short time scales is difficult and expensive, Gaeta said. This process, he said, makes it possible to generate a signal on a long time scale using off-the-shelf methods, and then compress the result to the desired time scale. The process also offers a new way to connect the relatively slow outputs of silicon electronics to photonic systems, he added.

The research was supported by the Defense Advanced Projects Agency and the Cornell Center for Nanoscale Systems, which is funded by the National Science Foundation and the New York State Office of Science, Technology and Academic Research.

(Photo: Cornell U.)

Cornell University




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