Stellar Occultation Observed from Devasthal Observatory Shows Atmospheric Pressure on Pluto’s is More Than 80,000 Times Less Than Earth's
Owing to its high obliquity or axial tilt (122.5°) and high orbital eccentricity (0.25), Pluto suffers intense seasonal episodes. Its poles remain, for decades, in permanent sunlight or darkness over its 248-year long orbital period. This leads to strong effects on its Nitrogen (N2) atmosphere that is mainly controlled by vapour pressure equilibrium with the surface N2 ice. The NASA New Horizons flyby in July 2015 revealed a large depression, Sputnik Planitia, filled by N2 ice, which appeared to be the main engine that controls the seasonal variation of atmospheric pressure during one seasonal cycle.
In astronomy, an occultation happens when a celestial object gets hidden from the view of the observer due to another celestial object passing in between them. A compilation of twelve stellar occultations by Pluto observed between 1988 and 2016 shows a three-fold monotonic increase of atmospheric pressure during this period, that can be explained by the progression of summer over the northern hemisphere, exposing Sputnik Planitia to the solar radiation. This increase can be explained consistently by a Pluto volatile transport model, which predicts that the pressure should peak around 2020. A gradual decline should then last for two centuries under the combined effects of Pluto’s recession from the Sun and the prevalence of the winter season over Sputnik Planitia.
An international team of scientists including members from Aryabhatta Research Institute of Observational Sciences (ARIES), an autonomous institute under the Department of Science & Technology, used ARIES’s 3.6-m DOT (India’s largest optical telescope) and 1.3-m DFOT telescopes located at Devasthal, Nainital, India to observe a stellar occultation by Pluto on 6 June 2020 using imaging systems in the I (ANDOR CCD) and H (TIFR Infrared Camera-II) bands, respectively. The high signal-to-noise ratio light curves obtained with these sophisticated instruments allowed the team to derive an accurate value of Pluto’s atmospheric pressure at its surface to be 12.23 [+0.65 −0.38] μbar, which is more than 80,000 times smaller than the atmospheric pressure at mean sea level on Earth. The results showed that since mid-2015, Pluto’s atmosphere is in a plateau phase close to peak and is in excellent agreement with the model values given by the Pluto volatile transport model of Meza et al. (2019). The study is now published in the Astrophysical Journal Letters (ApJL).
This occultation was particularly timely as it can test the validity of the current models of Pluto’s atmosphere evolution. Moreover, as Pluto is now moving away from the Galactic plane as seen from Earth, stellar occultations by the dwarf planet are becoming increasingly rare, making this event a decisive one.
Figure: The blue curves are a simultaneous fit to the 6 June 2020 stellar occultation by Pluto light curves (black squares) obtained with the 3.6-m and 1.3-m telescopes of ARIES at Desvasthal, over a 320-s interval bracketing the event. The residuals (observation-minus-model) are plotted in green below each light curve. The value of χ2d of, the χ2 per degree of freedom for each fit, is displayed at the lower right corner of each light curve. The lower and upper horizontal lines are the normalized total flux (star+Pluto+Charon) and the zero flux levels, respectively. The 3.6-m light curve has been shifted vertically by +1.2 for better viewing.
Contributors: Bruno Sicardy, Nagarhalli M. Ashok, Anandmayee Tej, Ganesh Pawar, Shishir Deshmukh,Ameya Deshpande, Saurabh Sharma, Josselin Desmars, Marcelo Assafin, Jose Luis Ortiz, Gustavo Benedetti-Rossi, Felipe Braga-Ribas, Roberto Vieira-Martins, Pablo Santos-Sanz, Krishan Chand, and Bhuwan C. Bhatt
Publication: Pluto’s atmosphere in plateau phase since 2015 from a stellar occultation at Devasthal
Publication link: https://iopscience.iop.org/article/10.3847/2041-8213/ac4249
For more details, Dr. Saurabh (ARIES) (saurabh[at]aries.res.in), Prof. N M Ashok (PRL) (ashoknagarhalli[at]gmail.com), Prof. Anandmayee Tej (IIST) (tej[at]iist.ac.in) can be contacted.
A version of this story is also available on DST website.