Enceladus: A source of nitrogen and an explanation for the water vapor plume observed by Cassini

Recently, the Cassini spacecraft observed an unexpected emission of plumes of water vapor, nitrogen, and icy particles from the southern polar region of Saturn's icy moon Enceladus. While these findings support previous ideas of geological activity in this icy moon, there is no experimental evidence explaining how these plumes could be produced at the low (∼130-160 K) surface temperatures. Here we show that similar behavior appears when heating water-ammonia ices that have been irradiated with protons that simulate Saturn's energetic ion environment. In our experiments, the behavior results from the eruption of high-pressure bubbles of hydrogen and nitrogen molecules that accumulate in the ice due to the radiolytic decomposition of ammonia. The thermal release of nitrogen can explain the intriguing finding of N⁺ in the inner magnetosphere. Thus, our laboratory simulations indicate that radiation processing of the surface of Enceladus may explain much of the extraordinary phenomena that have been observed by Cassini.