Pluto's atmosphere observations with ALMA: spatially-resolved maps of CO and HCN emission and first detection of HNC

TL;DR

This study uses ALMA to produce spatially-resolved maps of CO and HCN in Pluto's atmosphere, detects HNC for the first time, and reveals temperature and compositional variations with altitude and latitude.

Contribution

It provides the first spatially-resolved observations of Pluto's atmospheric molecules and reports the first detection of HNC, offering new insights into atmospheric composition and structure.

Findings

Tentative evidence for non-uniform temperature in Pluto's lower atmosphere.

Extended HCN emission peaking beyond Pluto's limb, indicating high-altitude presence.

First detection of HNC with a ratio similar to Titan's atmosphere.

Abstract

Following the detection of CO and HCN in Pluto's atmosphere, we report on new ALMA observations of Pluto with two main goals: (i) obtaining spatially-resolved measurements (~0.06'' on the ~0.15'' disk subtended by Pluto and its atmosphere) of CO(3-2) and HCN(4-3) (ii) targetting new chemical compounds, primarily hydrogen isocyanide (HNC). The CO line shows an absorption core at beam positions within Pluto's disk, a direct signature of Pluto's cold mesosphere. Analysis provides tentative evidence for a non-uniform temperature field in the lower atmosphere (near 30 km), with summer pole latitudes being 7$\pm$3.5 K warmer than low latitudes. This unexpected result may point to shorter radiative timescales in the atmosphere than previously thought. The HCN emission is considerably more extended than CO, peaking at radial distances beyond Pluto limb, and providing a new method to determine Pluto's HCN vertical profile in 2017. The mean (column-averaged) location of HCN is at 690+/-75 km altitude, with an upper atmosphere (> 800 km) mixing ratio of ~ 1.8 x 10$^{-4}$. Little or no HCN (<5 x 10$^{-9}$ at 65 km) is present in the lower atmosphere, implying undersaturation of HCN there. The HCN emission appears enhanced above the low-latitude limb, but interpretation, in terms of an enhanced HCN abundance or a warmer upper atmosphere there, is uncertain. The first detection of HNC is reported, with a (7.0$\pm$2.1) x 10$^{12}$ cm$^{-2}$ column density, referred to Pluto surface, and a HNC / HCN ratio of 0.095+/-0.026, very similar to their values in Titan's atmosphere. We also obtain upper limits on CH3CN (< 2.6 x 10$^{13}$ cm$^{-2}$) and CH$_3$CCH (< 8.5 x 10$^{14}$ cm$^{-2}$); the latter value is inconsistent with the reported detection of CH$_3$CCH from New Horizons. These upper limits also point to incomplete resublimation of ice-coated aerosols in the lower atmosphere.