PLUTO’s ATMOSPHERE FROM STELLAR OCCULTATIONS IN 2012 AND 2013

Abstract

We analyze two multi-chord stellar occultations by Pluto observed on July\n18th, 2012 and May 4th, 2013, and monitored respectively from five and six\nsites. They provide a total of fifteen light-curves, twelve of them being used\nfor a simultaneous fit that uses a unique temperature profile, assuming a clear\n(no-haze) and pure N_2 atmosphere, but allowing for a possible pressure\nvariation between the two dates. We find a solution that fits satisfactorily\n(i.e. within the noise level) all the twelve light-curves, providing\natmospheric constraints between ~1,190 km (pressure ~ 11 \\mubar) and ~ 1,450 km\n(pressure ~0.1 \\mubar) from Pluto's center. Our main results are: (1) the\nbest-fitting temperature profile shows a stratosphere with strong positive\ngradient between 1,190 km (at 36 K, 11 \\mubar) and r = 1,215 km (6.0 \\mubar),\nwhere a temperature maximum of 110 K is reached; above it is a mesosphere with\nnegative thermal gradient of -0.2 K/km up to ~ 1,390 km (0.25 \\mubar), where,\nthe mesosphere connects itself to a more isothermal upper branch around 81 K;\n(2) the pressure shows a small (6 %) but significant increase (6-\\sigma level)\nbetween the two dates; (3) without troposphere, Pluto's radius is found to be\nR_P = 1,190 +/- 5km. Allowing for a troposphere, R_P is constrained to lie\nbetween 1,168 and 1,195 km; (4) the currently measured CO abundance is too\nsmall to explain the mesospheric negative thermal gradient. Cooling by HCN is\npossible, but only if this species is largely saturated; Alternative\nexplanations like zonal winds or vertical compositional variations of the\natmosphere are unable to explain the observed mesospheric trend.\n