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Pluto’s Atmosphere Just Prior to New Horizon’s Arrival: Consistent Pressure and Hazy


Image of Pluto from New Horizons, looking back toward the Sun 15 minutes after close approach.  The backlighting highlights multiple layers of haze in Pluto’s tenuous atmosphere.  (More information is at, Credits: NASA/JHUAPL/SwRI.)

The study of an occultation of Pluto by a bright star showed that the planet’s atmospheric pressure has remained consistent even as it has moved away from the Sun in its orbit, and found a haze component in Pluto’s atmosphere, according to a new paper on which PSI Senior Scientist Amanda Sickafoose is an author. 

“For the past 30 years, Pluto has been moving away from the Sun in its orbit. The resulting decrease in solar insolation has been predicted to cause a decrease in Pluto’s atmospheric pressure, or even a complete collapse. There was some question as to whether or not Pluto would have a detectable atmosphere by the time the New Horizons spacecraft arrived,” Sickafoose said. “These observations showed that 15 days before the New Horizons flyby in 2015, Pluto’s atmosphere was at essentially the same size and pressure as it had been in previous observations in 2013 and 2011 — there was no significant expansion or contraction since that time. Furthermore, the multi-wavelength occultation data suggested that Pluto’s atmosphere contained a low-altitude haze composed of submicron-sized particles. 

The occultation was observed from the Stratospheric Observatory for Infrared Astronomy (SOFIA), an airborne observatory, and several ground based stations in New Zealand and Australia. Sickafoose traveled to New Zealand to take data from the Stardome Observatory in Auckland, where she worked with local astronomers to use the 0.5-meter Edith Winstone Blackwell Telescope and a 0.4-meter Meade telescope.  Sickafoose generated light curves from multiple telescope datasets and led the work on the flux versus wavelength dependence, which indicated the presence of haze particles less than 0.1 micron in radius. 

Stellar occultations, when a star is hidden by a foreground object that passes between it and the observer, are the best way to study Pluto’s tenuous atmosphere.  Recording the starlight as it passes through the atmosphere, gets blocked by Pluto’s surface, and then reappears through the atmosphere on the other side provides an extremely sensitive method of measuring atmospheric density, temperature, and pressure. This work allows comparison between nearly coincident spacecraft and ground-based occultation data – the results of which are in general agreement – providing an important connection and verification between the two methods. 

Sickafoose is a co-author on “Haze in Pluto’s atmosphere: Results from SOFIA and ground-based observations of the 2015 June 29 Pluto occultation.” Michael J. Person of the MIT Department of Earth, Atmospheric, and Planetary Sciences is lead author.

stardome observatory 

Amanda Sickafoose worked at the Stardome Observatory in Auckland, New Zealand, where she and her colleagues used a 0.5-meter Edith Winstone Blackwell Telescope and a 0.4-meter Meade telescope.

Credit: Stardome Observatory 


Jan. 24, 2021
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