SD: Hot Water in Cold Comets

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SD: Hot Water in Cold Comets

Post by bystander » Thu Sep 16, 2010 9:45 pm

Hot Water in Cold Comets: Water Around Comets Produced With Unusual Properties
Science Daily | 16 Sept 2010
Comets, sometimes called "dirty snowballs," are largely composed of water. An international research team led by Andreas Wolf of the Max-Planck Institute for Nuclear Physics in Heidelberg, Germany, recently succeeded in deciphering an important aspect of the way in which water molecules often form in space. As a surprise, the water molecules produced under cold, dilute conditions turned out to be produced as particles as hot as 60,000 Kelvin. In their research the physicists, though, did not use a telescope, but a particle accelerator.

The research appears in the journal Physical Review Letters.

In comets as well as in interstellar clouds, the precursor molecule of water is the positively charged hydronium ion H3O+. This molecular ion can be detected from earth by telescopes. In the cosmic clouds negatively charged electrons are also present, causing frequent collisions. In those the hydronium ion converts to the neutral instable radical H3O, which rapidly decays. "For this break-up reaction, nature offers three choices," describes Andreas Wolf, forming either H2O plus H, or OH plus H2, or OH plus two H atoms. Present research tries to determine the yields of these production channels, including that of water.

Wolf and his colleagues investigated this question by reproducing the electron attachment in the laboratory. They used the Heidelberg Test Storage Ring, a racetrack of sorts, with a 55-meter circumference, on which charged particles race around, guided by magnets.
Figuring out how to get hot water from cold ice
ars technica | Science | 31 Aug 2010
New laboratory research has attempted to figure out how hot water molecules have ended up near the icy regions of comets. Recent spectroscopic observations of the gaseous cloud surrounding comets—which are essentially big balls of flying, dirty ice—have found hot water vapor molecules within the comet's coma. No thermodynamic phase change process should lead to hot water being released by the <100K ice, so how it got there has been quite a mystery.

In cold, ionized media, an important source of water is the dissociative recombination of the hydronium ion, H3O+. When a slow-moving electron hits this, one of the possible reaction pathways leads to neutral water and atomic hydrogen, along with a release of energy. Using a new type of detection apparatus, researchers from Germany, Israel, and the US examined the relative frequency and associated energies of the various reaction pathways that occur when D3O+ interacts with an electron (where D is deuterium, a heavy isotope of hydrogen).

In addition to understanding the reaction energies, the team suggested a mechanism that explained how D3O+ becomes D2O and D. They hypothesized that an electron attaches to the hydronium ion, forming an unstable intermediate that decays into the final products. The team found that the pathway that leads to D2O and D released an amount of energy far below the predicted reaction energy, suggesting that the remainder remains trapped in the resulting D2O molecule, held in the form of internal excitation.

The heavy water molecules generated in the laboratory reaction had temperatures in excess of 60,000K, a finding that explains the signature of hot water found in the cold icy environment of a comet.
Hot Water Molecules from Dissociative Recombination of D3O+ with Cold Electrons - H Buhr et al

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