Evolution of the ion environment of comet 67P/Churyumov-Gerasimenko

Abstract

Context. The Rosetta spacecraft is escorting comet 67P/Churyumov-Gerasimenko from a heliocentric distance of >3.6 AU, where the comet activity was low, until perihelion at 1.24 AU. Initially, the solar wind permeates the thin comet atmosphere formed from sublimation. Aims. Using the Rosetta Plasma Consortium Ion Composition Analyzer (RPC-ICA), we study the gradual evolution of the comet ion environment, from the first detectable traces of water ions to the stage where cometary water ions accelerated to about 1 keV energy are abundant. We compare ion fluxes of solar wind and cometary origin. Methods. RPC-ICA is an ion mass spectrometer measuring ions of solar wind and cometary origins in the 10 eV-40 keV energy range. Results. We show how the flux of accelerated water ions with energies above 120 eV increases between 3.6 and 2.0 AU. The 24 h average increases by 4 orders of magnitude, mainly because high-flux periods become more common. The water ion energy spectra also become broader with time. This may indicate a larger and more uniform source region. At 2.0 AU the accelerated water ion flux is frequently of the same order as the solar wind proton flux. Water ions of 120 eV-few keV energy may thus constitute a significant part of the ions sputtering the nucleus surface. The ion density and mass in the comet vicinity is dominated by ions of cometary origin. The solar wind is deflected and the energy spectra broadened compared to an undisturbed solar wind. Conclusions. The flux of accelerated water ions moving from the upstream direction back toward the nucleus is a strongly nonlinear function of the heliocentric distance.

Dynamic Deuterium Enrichment in Cometary Water via Eley–rideal Reactions

“…"Cold" water ions matter most, since their flux is typically 100 times the "accelerated" water ion flux and they possess kinetic energies between 10 and 50 eV, where the D atom abstraction reaction is favored. Nilsson et al (2015b) provide flux estimates with the caveat that "the cold water ion flux can be severely underestimated due to the limited field of view at low energy. " Fuselier et al (2015) further state that "much less than 1 percent of the total ion flux" is detected.…”

“…Energetic H 2 O + ions have been discovered in the coma of comet 67P at heliocentric distances between 3.6 and 2.0 au, produced by the photoionization of cometary H 2 O (Nilsson et al 2015a(Nilsson et al , 2015b. Two populations have been found: (i) The first is comprised of "accelerated" water ions, formed over an extended source region far from the nucleus and then picked up by the solar wind electric field and accelerated along the Suncomet direction to energies between 120 and 800 eV (Nilsson et al 2015b).…”

“…Two populations have been found: (i) The first is comprised of "accelerated" water ions, formed over an extended source region far from the nucleus and then picked up by the solar wind electric field and accelerated along the Suncomet direction to energies between 120 and 800 eV (Nilsson et al 2015b). These ions bombard the nucleus surface with fluxes up to the solar wind flux until a diamagnetic cavity forms (Goetz et al 2016).…”

“…(ii) The second is comprised of "cold" water ions, formed near the nucleus and affected by the outgassing velocity of the parent neutrals. These ions move away from the nucleus and can reach energies between 10 and 50 eV; they are more persistent and have a flux density about two orders of magnitude higher than that of "accelerated" water ions (Nilsson et al 2015b). Both ion populations undergo charge exchange with background neutrals as they traverse the coma-which implies that fast neutral H 2 O molecules must also be present in both directions, increasing the flux of energetic H 2 O in the extended coma.…”

“…Water ions in the extended coma are accelerated toward the nucleus, where they collide with the outer nucleus crust of 67P (Nilsson et al 2015a(Nilsson et al , 2015b). This crust is dehydrated (Filacchione et al 2016) and includes typical cometary minerals, such as olivine and pyroxene silicates (Krüger et al 2015;Hilchenbach et al 2016), and carbonaceous refractory materials (chondrites; Capaccioni et al 2015;Fray et al 2016).…”