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Register a new userDetection of CO and HCN in Plutos atmosphere with ALMA
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Observations of the Pluto-Charon system, acquired with the ALMA interferometer on June 12-13, 2015, have yielded a detection of the CO(3-2) and HCN(4-3) rotational transitions from Pluto, providing a strong confirmation of the presence of CO, and the first observation of HCN, in Plutos atmosphere. The CO and HCN lines probe Plutos atmosphere up to ~450 km and ~900 km altitude, respectively. The CO detection yields (i) a much improved determination of the CO mole fraction, as 515+/-40 ppm for a 12 ubar surface pressure (ii) 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, 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 >1.5x10-5 above 450 km and a value of 4x10-5 near 800 km. 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 the slow kinetics of condensation at the low pressure and temperature conditions of Plutos 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)x10^14 cm-2, suggesting a surface-referred net production rate of ~2x10^7 cm-2s-1. Although HCN rotational line cooling affects Plutos atmosphere heat budget, the amounts determined in this study are insufficient to explain the well-marked mesosphere and upper atmospheres ~70 K temperature. We finally report an upper limit on the HC3N column density (< 2x10^13 cm-2) and on the HC15N / HC14N ratio (< 1/125).
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Combining stellar occultation observations probing Plutos atmosphere from 1988 to 2013 and models of energy balance between Plutos surface and atmosphere, we conclude that Plutos atmosphere does not collapse at any point in its 248-year orbit. The occultation results show an increasing atmospheric pressure with time in the current epoch, a trend present only in models with a high thermal inertia and a permanent N2 ice cap at Plutos north rotational pole.
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We analyze two multi-chord stellar occultations by Pluto observed on July 18th, 2012 and May 4th, 2013, and monitored respectively from five and six sites. They provide a total of fifteen light-curves, twelve of them being used for a simultaneous fit that uses a unique temperature profile, assuming a clear (no-haze) and pure N_2 atmosphere, but allowing for a possible pressure variation between the two dates. We find a solution that fits satisfactorily (i.e. within the noise level) all the twelve light-curves, providing atmospheric constraints between ~1,190 km (pressure ~ 11 mubar) and ~ 1,450 km (pressure ~0.1 mubar) from Plutos center. Our main results are: (1) the best-fitting temperature profile shows a stratosphere with strong positive gradient between 1,190 km (at 36 K, 11 mubar) and r = 1,215 km (6.0 mubar), where a temperature maximum of 110 K is reached; above it is a mesosphere with negative thermal gradient of -0.2 K/km up to ~ 1,390 km (0.25 mubar), where, the mesosphere connects itself to a more isothermal upper branch around 81 K; (2) the pressure shows a small (6 %) but significant increase (6-sigma level) between the two dates; (3) without troposphere, Plutos radius is found to be R_P = 1,190 +/- 5km. Allowing for a troposphere, R_P is constrained to lie between 1,168 and 1,195 km; (4) the currently measured CO abundance is too small to explain the mesospheric negative thermal gradient. Cooling by HCN is possible, but only if this species is largely saturated; Alternative explanations like zonal winds or vertical compositional variations of the atmosphere are unable to explain the observed mesospheric trend.
The origin of the atmosphere of the largest moon of Saturn, Titan, is poorly understood and its chemistry is rather complicated. Ground-based millimeter/sub-millimeter heterodyne spectroscopy resolves line shapes sufficiently to determine information in Titans atmospheric composition (on vertical profiles and isotopic ratios). We test the capabilities of the Swedish Heterodyne Facility Instrument (SHFI), Receiver APEX-1, together with the Atacama Pathfinder EXperiment APEX 12-m telescope for Titans atmospheric observations. In particular we present sub-millimeter observations of the CO(2-1) and HCN(3-2) lines of the Titan stratosphere with APEX, and with SHFI taken during the Science Verification (SV) instrument phase on March and June 2008. With the help of appropriate radiative transfer calculations we investigate the possibility to constrain the chemical concentrations and optimize the performance of the APEX-1 instrument for inferring vertical profiles of molecular components of the atmosphere of Titan.
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