Pluto's atmosphere from stellar occultations in 2012 and 2013

DIAS-OLIVEIRA, ALEX; SICARDY, BRUNO; LELLOUCH, EMMANUEL; VIEIRA-MARTINS, ROBERTO;...; GIL-HUTTON, RICARDO Y OTROS 41 AUTORES

Washington

Congreso; 47th Annual Meeting of the Division of Planetary Sciences; 2015

We present results from two Pluto stellar occultations observed on 18July 2012 and 04 May 2013, and monitored respectively from five and sixsites in South America. Both campaigns involved large telescopes(including the 8.2-m VLT at ESO/Paranal). The high SNR ratios andmulti-chord coverage provide amoung the best Pluto atmospheric profilesever obtained from the ground.We show that a spherically symmetric,clear (no-haze) and pure N2 atmosphere with a unique temperature profilesatisfactorily fits the twelve lightcurves provided by the two events.We find, however, a small but significant increase of pressure of 6%(6-sigma level) between the two dates, with values of 2.16 ± 0.2and 2.30 ± 0.01 μbar at the reference radius 1275 km,respectively.We provide atmospheric constrains between 1190 km and 1450km from Pluto´s center, and we determine the temperature profile withaccuracy of a few km in vertical scale. Our model shows a stratospherewith strong positive gradient between 1190 km (at 36 K, 11 μbar) andr =1215 km (6.0 μbar), where a temperature maximum of 110 K isreached. Above it is a mesosphere with negative thermal gradient of -0.2K/km up to 1,390 km (0.25 μbar), at which point, the mesosphereconnects itself to a more isothermal upper branch at 81 K. This profileprovides (assuming no troposphere) a Pluto surface radius of 1190± 5 km, consistent with preliminary values obtained by NewHorizons. Currently measured CO abundances are too low to explain thenegative mesospheric thermal gradient. We explore the possibility of anHCN (recently detected by ALMA) cooling. This model, however, requireslargely supersaturated HCN. Zonal winds and vertical compositionalvariations of the atmosphere are also unable to explain the observedmesospheric trend.These events are the last useful ground-basedoccultations recorded before the 29 June 2015 occultation observed fromAustralia and New Zealand, and before the NASA´s New Horizons flyby ofJuly 2015. This work can serve as a benchmark in the New Horizonscontext, enabling comparisons between ground-based and space resultsconcerning Pluto´s atmospheric structure and temporal evolution.