Refractory elements in the gas phase for comet 67P/Churyumov-Gerasimenko. Possible release of atomic Na, Si, and Fe from nanograins

Context. Gas-phase sodium, silicon, potassium, and calcium were previously identified in mass spectra recorded in the coma of comet 67P/Churyumov-Gerasimenko, the target of the European Space Agency’s Rosetta mission. The major release process for these atoms was identified as sputtering by the solar wind. More recently, remote observations of numerous comets over a range in heliocentric distances revealed the presence of metal atoms of iron and nickel that had been released either from the nucleus or from a distributed source with a short scale length. Sputtering, however, has been dismissed as a major release process due to the attenuation of the solar wind in the comae of some of the observed targets. Aims. We investigated the presence of refractory species in the gas phase of the coma of 67P/Churyumov-Gerasimenko. This investi- gation includes a period close to perihelion when the solar wind was likely absent from the near-nucleus region due to the increased cometary activity. Additionally, we extended our search to iron and nickel. Methods. We analyzed in situ data from the Rosetta/ROSINA Double Focusing Mass Spectrometer DFMS. 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.