Refractory elements in the gas phase for comet 67P/Churyumov-Gerasimenko

TL;DR

This study reports the detection of refractory elements like silicon, sodium, and iron in the gas phase of comet 67P/Churyumov-Gerasimenko, suggesting direct release from small grains rather than gaseous parent molecules.

Contribution

It provides new evidence for the presence of refractory elements in the gas phase and proposes a novel explanation for their release mechanism on comet 67P.

Findings

Gas-phase silicon was detected throughout the mission.

Sodium and iron atoms were observed near perihelion.

Release of refractory elements likely from small grains, not gaseous molecules.

Abstract

Results. We found that gas-phase silicon was present throughout the Rosetta mission. Furthermore, the presence of sodium and iron atoms near the comet's perihelion confirms that sputtering cannot be the sole release process for refractory elements into the gas phase. Nickel was found to be below the detection limit. The search for parent species of any of the identified gas phase refractories has not been successful. Upper limits for a suite of possible fragment species (SiH, SiC, NaH, etc.) of larger parent and daughter species have been obtained. Furthermore, Si did not exhibit the same drop in signal as do common cometary gases when the spacecraft is pointed away from the nucleus. The combined results suggest that a direct release of elemental species from small grains on the surface of the nucleus or from small grains in the surrounding coma is a more likely explanation than the previous assumption of release via the dissociation of gaseous parent molecules.