Production of ammonia-depleted surface layers on the saturnian satellites by ion sputtering

Abstract

It is generally believed that some type of plains-forming resurfacing has continued into the recent geological past on the surfaces of the saturnian satellites, particularly Enceladus1–3. In conditions of tidally-driven heating, ‘magma’ could emerge and produce the plains especially evident on this satellite, as well as on Tethys, Dione, and Rhea. The magma favoured at present, considering the temperatures of the satellites and their likely formation, is the peritectic melt 2H2O·NH3 (refs 1–4). Surprisingly, however, visible and near-IR spectra have not shown any evidence of ammonia on the optical surface of any of the satellites, although absorption bands indicative of water ice are abundantly in evidence. On the basis of laboratory-derived data on the ion erosion of water and ammonia ices, we suggest that the satellite surfaces could have become enriched with water ice over a reasonable geological time period (106–109 yr) owing to preferential erosion of the ammonia by magnetosphere ion bombardment. This mechanism would be an alternative explanation to other processes invoked for the absence of ammonia spectral features. The concept is similar to that of Half5 who has proposed that a crust of non-volatiles could eventually coat the Galilean satellites J3 and J4 (Ganymede and Callisto) through preferential water ice erosion (an ‘armouring’ process).