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

Abstract

Pluto exhibits a tenuous, predominantly N2-CH4 atmosphere, with Titan-like chemistry. Previous observations with ALMA have permitted the detection of CO and HCN at 345.796 and 354.505 GHz in this atmosphere, yielding vertically resolved chemical and thermal information. We report on new observations of Pluto’s atmosphere with ALMA, performed in April and July/August 2017 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) . These observations are modeled with radiative transfer codes coupled with inversion methods. The CO line shows an absorption core at beam positions within Pluto’s disk, a direct signature of Pluto’s cold mesosphere. Analysis of the CO line map provides tentative evidence for a non-uniform temperature field in the lower atmosphere (near 30 km), with summer pole latitudes being 7 ± 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 ∼1.8×10−4. Little or no HCN (<5 × 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 ± 2.1)×1012 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 (<2.6×1013 cm−2) and CH3CCH (<8.5×1014 cm−2); the latter value is inconsistent with the reported detection of CH3CCH from New Horizons. These upper limits also point to incomplete resublimation of ice-coated aerosols in the lower atmosphere.