The origin of comet material formed at high temperatures

ScienceDaily (July 22, 2011) — Comets are icy bodies, yet they are made of materials formed at very high temperatures. Where do these materials come from? Researchers from the Institut UTINAM(1)(CNRS/Université de Besançon) have now provided the physical explanation behind this phenomenon. They have demonstrated how these materials migrated from the hottest parts of the solar system to its outer regions before entering the composition of comets.See Also:Space & TimeSunAsteroids, Comets and MeteorsAstronomySolar FlareSolar SystemStarsReferenceComet Hale-BoppComet Shoemaker-Levy 9Orion NebulaNear-Earth asteroid

Their results are published in the July 2011 issue of the journal Astronomy & Astrophysics.

On 15 January 2006, after an eight-year voyage, NASA's Stardust Mission (Discovery program) brought dust from Comet Wild 2 back to Earth. Comets are formed at very low temperatures (around 50 Kelvin, i.e. -223°C). However, analyses have revealed that Comet Wild 2 is made of crystalline silicates and CAIs (Calcium-Aluminium-rich Inclusions). Considering that the synthesis of these minerals requires very high temperatures (above 1 000 Kelvin or 727°C), how can this composition be explained?

A team from the Institut UTINAM1 (CNRS/Université de Besançon), in collaboration with researchers from the Institut de Physique de Rennes (CNRS/Université de Rennes), the University of Duisburg-Essen (Germany) and the Laboratoire Astrophysique, Instrumentation et Modélisation (CNRS/CEA/Université Paris Diderot), have provided the answer on the basis of a physical phenomenon called photophoresis. This force depends on two parameters: the intensity of solar radiation and gas pressur e. At the birth of the solar system, the comets were formed from the protoplanetary disk(2). Inside this disk, a mixture of solid grains ranging in size from a few microns to several centimeters was bathed in a dilute gas that let sunlight through.

According to the researchers, photophoresis drove the particles towards the outer regions of the disk. Under the effect of solar radiation, one face of the grains was "hotter" than the other and the behavior of gas molecules on the surface of these grains was modified: on the "sunny" side, the gas molecules were more unstable and moved about more rapidly than on the "cold" side. By causing a pressure difference, this imbalance moved the grains away from the Sun. Through digital simulations, the researchers have borne out this photophoresis phenomenon. They demonstrated that the grains of crystalline silicates formed in the inner, hot region of the protoplanetary disk near to the Sun migrated to its outer, cold region before playing a part in the formation of the comets! . This novel physical explanation could account for the position of certain dust rings observed in protoplanetary disks and thus shed light on the conditions of planet formation.

(1)Institut "Univers, Transport, Interfaces Nanostructures, Atmosphère et Environnement, Molécules" (CNRS/Université de Besançon). (2)The protoplanetary disk of a young star (for example the Sun) is the disk of gas and dust that surrounds it, and in which planets are likely to form.

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Shining a light on the elusive 'blackbody' of energy research: Designer material has potential applications for thermophotovoltaics

ScienceDaily (July 23, 2011) — A designer metamaterial has shown it can engineer emitted "blackbody" radiation with an efficiency beyond the natural limits imposed by the material's temperature, a team of researchers led by Boston College physicist Willie Padilla report in the current edition of Physical Review Letters.See Also:Matter & EnergyMaterials ScienceThermodynamicsPhysicsPetroleumEnergy TechnologyEngineeringReferenceBlack bodyRadiant energyElectromagnetic spectrumHeat

A "blackbody" object represents a theorized ideal of performance for a material that perfectly absorbs all radiation to strike it and also emits energy based on the material's temperature. According to this blackbody law, the energy absorbed is equal to the energy emitted in equilibrium.

The breakthrough reported by Padilla and colleagues from Duke University and SensorMetrix, Inc., could lead to innovative technologies used to cull energy from waste heat produced by numerous industrial processes. Furthermore, the human-made metamaterial offers the ability to control emissivity, which could further enhance energy conversion efficiency.

"For the first time, metamaterials are shown to be able to engineer blackbody radiation and that opens the door for a number of energy harvesting applications," said Padilla. "The energy a natural surface emits is based on its temperature and nothing more. You don't have a lot of choice. Metamaterials, on the other hand, allow you to tailor that radiation coming off in any desirable manner, so you have great control over the emitted energy."

Researchers have long sought to find the ideal "blackbody" material for use in solar or thermoelectric energy generation. So far, the hunt for such a class of thermal emitters has proved elusive. Certain rare earth oxides are in limited supply and expensive, in addition to being almost impossible to control. Photonic crystals proved to be inferior emitters that failed to yield significant efficiencies.

Constructed from artificial composites, metamaterials are designed to give them new properties that exceed the performance limits of their actual physical components and allow them to produce "tailored" responses to radiation. Metamaterials have exhibited effects such as a negative index of refraction and researchers have combined metamaterials with artificial optical devices to demonstrate the "invisibility cloak" effect, essentially directing light around a space and masking its existence.

Three years ago, the team developed a "perfect" metamaterial absorber capable of absorbing all of the light that strikes it thanks to its nano-scale geometric surface features. Knowing that, the researches sought to exploit Kirchoffs's law of thermal radiation, which holds that the ability of a material to emit radiation equals its ability to absorb radiation.

Working in the mid-infrared range, the thermal emitter achieved experimental emissivity of 98 percent. A dual-band emitter delivered emission peaks of 85 percent and 89 percent. The results confirmed achieving performance consistent with Kirchoff's law, the researchers report.

"We also show by performing both emissivity and absorptivity measurements that emissivity and absorptivity agree very well," said Padilla. "Even though the agreement is predicted by Kirchoff's law, this is the first time that Kirchoff's law has been demonstrated for metamaterials."

The researchers said altering the composition of the metamaterial can results in single-, dual-band and broadband metamaterials, which could allow greater control of emitted photons in order to improve energy conversion efficiency.

"Potential applications could lie in energy harvesting area such as using this metamaterial as the selective thermal emitter for thermophotovoltaic (TPV) cells," said Padilla. "Since this metamaterial has the ability to engineer the thermal radiation so that the emitted photons match the band gap of the semiconductor -- part of the TPV cell -- the converting efficiency could be greatly enhanced.

In addition to Padilla, the research team included BC graduate student Xianliang Liu, Duke University's Nan Marie Jokerst and Talmage Tyler and SensorMetrix, Inc., researchers Tatiana Starr and Anthony F. Starr.

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