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Spirit struggles with soft soil

...NASA's Mars Exploration Rover Spirit is facing one of its biggest challenges yet with a patch of soft soil that is currently holding the rover hostage...

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Rogue black holes skulk Milky Way perimeter

...Hundreds of rogue black holes left over from the galaxy building days of the early Universe could be wandering loose in the Milky Way...

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Postcards from MESSENGER

...A previously unknown 690 kilometre wide impact basin and evidence that Mercury's atmosphere and the interaction of its magnetic field with the solar wind are more active than previously thought are the latest offerings from NASA's MESSENGER mission...

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Video archive

STS-120 day 2 highlights

Flight Day 2 of Discovery's mission focused on heat shield inspections. This movie shows the day's highlights.


STS-120 day 1 highlights

The highlights from shuttle Discovery's launch day are packaged into this movie.


STS-118: Highlights

The STS-118 crew, including Barbara Morgan, narrates its mission highlights film and answers questions in this post-flight presentation.

 Full presentation
 Mission film

STS-120: Rollout to pad

Space shuttle Discovery rolls out of the Vehicle Assembly Building and travels to launch pad 39A for its STS-120 mission.


Dawn leaves Earth

NASA's Dawn space probe launches aboard a Delta 2-Heavy rocket from Cape Canaveral to explore two worlds in the asteroid belt.

 Full coverage

Dawn: Launch preview

These briefings preview the launch and science objectives of NASA's Dawn asteroid orbiter.

 Launch | Science

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Comet crystals feel the heat



Posted: 14 May, 2009

Since comets formed out in the cold depths of the Solar System, the existence of materials in them that must have been created in high temperatures has been a real puzzle, until now. NASA’s Spitzer Space Telescope has observed the infrared signature of tiny silicate crystals, of the type found in comets, being created in the planet-forming disc around a young star called EX Lupi, in the constellation of Lupus.

Back in 2005 a team of European astronomers studied EX Lupi, which is believed to be a lot like our Sun was 4.5 billion years ago, they saw amorphous grains of silicate dust in the disc. When they came back to EX Lupi three years later, after a major outburst on the star, they found those dust grains had been crystallised. The question is, how?

An artist’s impression of the planet-forming dust disc around EX Lupi, where silicate crystals have been discovered. Image: NASA/JPL-Caltech.

The stellar outbursts occur when the growing young star

accumulates a large amount of mass from the dusty, gaseous disc that is spinning around it. Each outburst sends a flash of heat permeating through the disc. At the distance the crystals were seen at, the temperature reached 725 degrees Celsius (about 1,000 kelvin), enough to thermally ‘anneal’ the silicate dust. Annealing describes the process of heating a material to the point that its chemical bonds break and then re-form to fundamentally change the material’s properties. In this case, the dust was annealed into crystals of a material called forsterite, which can form part of olivine. Turbulent convection currents running through the planet-forming disc then mixes up the forsterite so some of it ends up in the outer reaches of the disc where icy comets form. Consequently, forsterite and olivine are commonly found in comets and meteorites.

The two lines on this graph indicate the change in the spectra of EX Lupi’s disc between 2005 and 2008, revealing the crystallising event. Image: NASA/JPL–Caltech/P Abraham (Konkoly Observatory, Hungarian Academy of Sciences).

“This is a completely new scenario about how this material

could be created,” says Attila Juhasz of the Max Planck Institute for Astronomy in Heidelberg, Germany, who participated in the research that is published in the 14 May edition of Nature. Two previous explanations had been put forward: one, that exposure to heat from a young star could, over a long period, anneal silicate dust; and two, shockwaves produced by a growing planet moving through the disc could suddenly heat the grains sufficiently to anneal them.

“We concluded that this is a third way… one not considered before,” says lead researcher Peter Abraham of the Hungarian Academy of Sciences’ Konkoly Observatory in Budapest. The implication is that silicate crystals in comets, such as those returned to Earth in sample–return expeditions like NASA’s Stardust mission, formed when our own Sun was undergoing frequent outbursts during its dim and distant formative years, thus providing new insight into the early times of our Solar System.

Readers may be interested in an animation of the outburst and the formation of the crystals on NASA’s Spitzer website.