By Kassandra Bell, Arielle Moullet, and Joan Schmelz
Paper: Haze in Pluto’s atmosphere: Results from SOFIA and Ground-based Observations of the 2015 June 29 Pluto Occultation
M. J. Person et al., 2020, Icarus, in press.
SOFIA observed the occultation by Pluto of a bright star on 29 June 2015, enabling scientists to measure pressure, density, and temperature profiles of the atmosphere of the dwarf planet. Pre-event astrometry allowed the SOFIA team to position the aircraft deep within the central flash zone, just 22 km from the center of the occultation path.
During the occultation, Pluto’s shadow traveled at 53,000 mph across the Pacific Ocean. SOFIA flew from its Southern Hemisphere base in New Zealand to observe the occultation event for 120 seconds. It was the only observatory able to position itself in the predicted center of the shadow’s path, and therefore able to observe a strong, distinct brightening near the middle of the occultation, called the central flash. This allowed scientists to probe Pluto’s atmosphere at low altitudes.
About two hours before the occultation, scientists at MIT contacted the SOFIA in-flight team with the news that the center of the shadow would cross more than 200 miles north of the position on which the airborne observatory’s flight plan had been based. After recalculating and filing a revised flight plan, SOFIA’s flight crew and science team had to wait an anxious 20 minutes before receiving permission from air traffic control to alter the flight path accordingly.
Observations were taken simultaneously with the First Light Test Camera (FLITECAM), an infrared camera with grism spectroscopy, the High-Speed Imaging Photometer for Occultations (HIPO), an extremely fast and accurate electronic multi-colored imager, and the Focal Plane Imager Plus (FPI+), a fast frame-rate imaging photometer. Together these instruments provided a multi-wavelength view over four photometric bands, from 0.57 to 1.8 µm. These instruments and their ability to be co-mounted on the telescope were designed with this precise observation in mind. Only this sort of simultaneous multi-colored observation can detect the differences in the occultation signatures of clear and hazy atmospheres.
In order to compare atmospheric characteristics with information retrieved from earlier occultations, the light curve region near half-light was first modeled with previously-used clear atmospheric models. No significant pressure variation was identified between 2011 and 2015, providing information on how the atmosphere is effectively resupplied by nitrogen ice sublimation.
Neither a pure isothermal nor temperature-gradient atmospheric model resulted in good fits to the full 2015 SOFIA data. However, a thick, hazy region within the atmosphere produces an excellent fit to the entire dataset, including in and around the central flash, which is sensitive to lower altitudes of the atmosphere (where haze is presumed to be thicker).
In addition, a distinctive variation of residual flux across the different observed wavelengths was detected, which is strongly indicative of wavelength-dependent scattering by a haze component. Modeling of the residual flux gradient, assuming the haze composition, allowed scientists to constrain the characteristic haze particle size to ~0.06–0.10 µm. This finding suggests that the haze must be replenished on short timescales, an important implication for understanding the photochemistry of Pluto’s atmosphere.
The observations occurred just two weeks before the New Horizons spacecraft flew past the dwarf planet, providing researchers with multiple, nearly co-temporal datasets. Ultraviolet data from the spacecraft confirmed the existence of haze in the upper layers of Pluto’s atmosphere while infrared and visible light data from SOFIA confirmed the haze extending to atmospheric layers much closer to the surface.
This was SOFIA’s second Pluto occultation. The first set of observations, in 2011, confirmed that Pluto’s atmosphere was not collapsing as its eccentric orbit took it farther from the Sun. These observations predicted that the atmosphere would remain stable for the 2015 New Horizons flyby. Continued monitoring is necessary as the dwarf planet gets even further from the Sun, as numerous models predict Pluto’s atmosphere will at least decline if not vanish over the coming decades. The agreement between the flyby and SOFIA data where they overlap indicate that occultations continue to be an accurate method to remotely study Pluto’s atmosphere.