Implantation of multiply charged carbon ions in water ice

Context: Several objects in the solar system like Europa, Ganymede, and Callisto have frozen surface (main component: H2O). The associated thickness is bigger than the penetration depth of the relevant projectile ions. Additionally, other species such as H2O2, SO2, and CO2 have been detected on these surface. The formation mechanisms of these molecules are still under discussion.

Aims: We present new experimental results on the implantation of 13Cq+ (q = 2, 3) ions at an energy of 30 keV in water ice at low temperatures (15 and 80 K). Experiments with multiply-charged ions at energies of tens of keV are particularly relevant to simulating the complexity of the irradiation environment to which the surfaces of the icy moons in the outer solar system are exposed.

Methods: The experiments were performed at the low-energy ion beam facility ARIBE of GANIL in Caen (France). 30 keV 13Cq+ (q = 2, 3) ions have been used to bombard solid H2O surface, which were frozen at 15 K and 80 K. Fourier transform infrared spectrometer (FTIR) was used to analyze the sample in the 5000–600 cm-1 (2–16.7 μm) region with a spectral resolution of 1 cm-1.

Results: The results of our experiments indicate that implantation produces 13CO2 with yields in the range of 0.32–0.57 molecules ion-1. This yield seems to be independent of the temperature of the ices in the range studied. We have estimated the time scale necessary to accumulate by implantation of magnetospheric carbon ions the observed quantity of carbon dioxide on the surface of Europa, a Jovian moon. This time scale is of the order of 1.0–1.3  ×  104 yrs, which is higher than that evaluated for carbon dioxide production by other relevant processes.

Conclusions: We conclude that although a relevant quantity of CO2 can be formed by carbon ion implantation, this is not the dominant formation mechanism at Europa.