Pluto's Atmosphere from Stellar Occultations in 2012 and 2013

A. DIAS-OLIVEIRA, B. SICARDY, E. LELLOUCH, R. VIEIRA-MARTINS, M. ASSAFIN, J. I. B. CAMARGO, ..., R. GIL-HUTTON Y 39 AUTORES MÁS

ASTROPHYSICAL JOURNAL

IOP PUBLISHING LTD

Lugar: Londres; Año: 2015 vol. 811 p. 1 - 20

0004-637X

We analyze two multi-chord stellar occultations by Pluto that were observed on 2012 July 18th and 2013 May 4th,and respectively monitored from five and six sites. They provide a total of fifteen light curves, 12 of which wereused for a simultaneous fit that uses a unique temperature profile, assuming a clear (no haze) and pure N 2atmosphere, but allowing for a possible pressure variation between the two dates. We find a solution thatsatisfactorily fits (i.e., within the noise level) all of the 12 light curves, providing atmospheric constraints between∼1190 km (pressure ∼11 micro bar) and ∼1450 km (pressure ∼0.1 micro bar) from Pluto?s center. Our main results are: (1)the best-fitting temperature profile shows a stratosphere with a strong positive gradient between 1190 km (at 36 K,11 micro bar) and r = 1215 km (6.0 micro bar), where a temperature maximum of 110 K is reached; above it is a mesospherewith a negative thermal gradient of −0.2 K km −1 up to ∼1390 km (0.25 micro bar), where the mesosphere connectsitself 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 a troposphere, Pluto?s radius is found to be R P = 1190 +- 5 km.Allowing for a troposphere, R P is constrained to lie between 1168 and 1195 km; and (4) the currently measured COabundance is too small to explain the mesospheric negative thermal gradient. Cooling by HCN is possible, but onlyif this species is largely saturated. Alternative explanations like zonal winds or vertical compositional variations ofthe atmosphere are unable to explain the observed mesospheric negative thermal gradient.