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, H 3 O+. 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 D 3 O+ 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 D 3 O+ becomes D 2 O 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 D 2 O and D released an amount of energy far below the predicted reaction energy, suggesting that the remainder remains trapped in the resulting D 2 O 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.

Physical Review Letters, 2010. DOI: Upcoming