Detection of CO and HCN in Pluto’s atmosphere with ALMA

Abstract

Observations of the Pluto-Charon system, acquired with the ALMA interferometer on June 12-13, 2015, have led to the detection of the CO(3-2) and HCN(4-3) rotational transitions from Pluto (including the hyperfine structure of HCN), providing a strong confirmation of the presence of CO, and the first observation of HCN in Pluto's atmosphere. The CO and HCN lines probe Pluto's atmosphere up to ∼450 km and ∼900 km altitude, respectively, with a large contribution due to limb emission. The CO detection yields (i) a much improved determination of the CO mole fraction, as 515±40 ppm for a 12 µbar surface pressure (ii) strong constraints on Pluto's mean atmospheric dayside temperature profile over ∼50-400 km, with clear evidence for a well-marked temperature decrease (i.e., mesosphere) above the 30-50 km stratopause and a best-determined temperature of 70±2 K at 300 km, somewhat lower than previously estimated from stellar occultations (81±6 K), and in agreement with recent inferences from New Horizons / Alice solar occultation data. The HCN line shape implies a high abundance of this species in the upper atmosphere, with a mole fraction > 1.5×10 −5 above 450 km and a value of 4×10 −5 near 800 km. Assuming HCN at saturation, this would require a warm (>92 K) upper atmosphere layer; while this is not ruled out by the CO emission, it is inconsistent with the Alicemeasured CH 4 and N 2 line-of-sight column densities. Taken together, the large HCN abundance and the cold upper atmosphere imply supersaturation of HCN to a degree (7-8 orders of magnitude) hitherto unseen in planetary atmospheres, probably due to a lack of condensation nuclei above the haze region and the slow kinetics of condensation at the low pressure and temperature conditions of Pluto's upper atmosphere. HCN is also present in the bottom ∼100 km of the atmosphere, with a 10 −8 -10 −7 mole fraction; this implies either HCN saturation or undersaturation there, depending on the precise stratopause temperature. The HCN column is (1.6±0.4)×10 14 cm −2 , suggesting a surface-referred vertically-integrated net production rate of ∼2×10 7 cm −2 s −1 . Although HCN rotational line cooling affects Pluto's atmosphere heat budget, the amounts determined in this study are insufficient to explain the well-marked mesosphere and upper atmosphere's ∼70 K temperature, which if controlled by HCN cooling would require HCN mole fractions of (3-7)×10 −4 over 400-800 km. We finally report an upper limit on the HC 3 N column density (< 2×10 13 cm −2 ) and on the HC 15 N / HC 14 N ratio (< 1/125).

An intense thermospheric jet on Titan

“…Spectral calculations were based on a home-made radiative transfer model in full spherical geometry, inherited from [4,34,35]. Line opacities are calculated using spectroscopic parameters from the Cologne Database for Molecular Spectroscopy (CDMS, https://www.astro.uni-koeln.de/cdms).…”

A new spectroscopically-determined potential energy surface and ab initio dipole moment surface for high accuracy HCN intensity calculations

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The thermal emission of Centaurs and trans-Neptunian objects at millimeter wavelengths from ALMA observations

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