Surveying exogenous species in Saturn with ALMA. II. Detecting and Mapping HCN

Abstract

Several external sources (the rings, the satellites, cometary impacts, interplanetary dust particles) supply material to the upper atmosphere of Saturn. The signature of this external influx has been documented on the ionospheric electronic and ionic densities, and in the stratosphere on oxygenated molecules (CO, CO2, H2O). We search for the chemical signature of the external sources in nitrogen-bearing species in Saturn's stratosphere. We have targeted the HCN(4-3) rotational line and mapped its emission line at the planet limb with ALMA in 2018 with configuration C43-3, providing a synthetic beam of 0.61"x0.50". HCN was detected at all observable latitudes from 20°S to 60°N. The HCN abundance was measured at two independent pressure levels, 0.1 and 1 hPa. The measured volume mixing ratios are latitudinally uniform within uncertainties at both pressure levels. The mean vmr is $(7\pm 2)\times 10^{-11}$ and $(4\pm 1)\times 10^{-12}$ at 0.1 and 1 hPa respectively. This corresponds to a mean column-density of $(4\pm 1)\times 10^{12}$ cm${}^{-2}$, and a total nitrogen mass of $(3.7\pm 0.9)\times 10^{10}$ g. The steep vertical profile suggests that another loss mechanism, possibly sticking or heterogeneous chemistry on aerosols, is active besides photodissociation. The huge influx of material detected in the ring plane by Cassini left no detectable chemical signature on HCN. The detected HCN mass seems compatible with several sources: the ring rain at mid-latitudes, Enceladus, or an old cometary impact. The detected HCN could also be a chemical product of internal N2 transported from the deep atmosphere up to the stratosphere.

arXiv commentto be submitted to A&A