The dynamic nature of Pluto’s atmosphere prompts speculation of cryovolcanism

3 March 2015
The dynamic nature of Pluto’s atmosphere prompts speculation of cryovolcanism

Pluto, an intriguing dwarf planet located at the outer edge of our solar system, has planetary astronomers including S.A. Astronomical Observatory’s Dr. Amanda Gulbis, investigating its atmospheric properties and evolution. The first detailed stellar occultation observed in 1988 detected a tenuous atmosphere on Pluto. Observations in 2002 surprisingly showed a noticeable change in the light curve that was attributed to a rise in the isothermal atmospheric pressure and atmospheric size. Pluto’s eccentric orbit results in significant changes over the course of its 248-year revolution around the sun. At first, the atmospheric increase was attributed to the 1989 closest approach of Pluto to the sun and was expected to drop as Pluto receded, but additional occultation results between 2006 and 2013 are enough for astronomers to think otherwise.

A team of astronomers observed stellar occultations by Pluto using multiple telescopes in the United States (2012) and Chile (2013). They compared those data to observations taken in previous years. The results revealed that Pluto’s atmosphere is currently holding stable and is not showing any signs of a global collapse. The occultation light curves show very little change in Pluto’s upper atmosphere between 1988 and 2013. However, the portions of the light curves corresponding to the lower atmosphere have changed significantly, exhibiting a distinct change in slope in 1988 and 2011, and a “bowl-like” shape in 2002, 2006, and 2013.

Dr. Gulbis explains, “We use stellar occultations to explore in detail the faintest objects in our distant solar system. Comparing results obtained from the first definitive Pluto occultation in 1988 till recently, we use our data to determine whether Pluto’s atmospheric pressure will continue to rise or wholly dissipate before the arrival of the New Horizons spacecraft in July 2015. Thus far, the data convincingly show that the atmosphere will hold through July”.

A separate paper by the same team contains a detailed analysis of an observation of a stellar occultation by Pluto in 2011. Those unique observations, consisting of simultaneous visible-wavelength images and low-resolution near-infrared spectra with NASA’s 3-m Infrared Telescope Facility, showed a full occultation of the star by Charon (one of Pluto’s moon) followed by an atmospheric graze by Pluto. The double occultation allows accurate measurement of the distance between Pluto and Charon, as well as accurate calibration of the light curve. The multiwavelength light curve data match that predicted by models containing micron-sized particles, suggesting that haze was present in Pluto’s lower atmosphere at that time.

“Pluto is a very distant, very cold object. The fact that it has an atmosphere, much less an active one, is a bit surprising. In 2007, stellar occultation data detected atmospheric waves. Now, the evolution of the light curves through 2013 and the multiwavelength data from 2011 suggest that there is intermittent haze in the lower atmosphere. Pluto is much more interesting than we expected,” states Dr. Gulbis.

The dynamic nature of Pluto’s atmosphere prompts speculation of cryovolcanism, essentially an underground force that causes melted volatiles to shoot up from the surface. The New Horizons spacecraft arrival in July 2015 will provide more clues to this mystery.

These results appeared in January in the journal Icarus, under Bosh et al. (2015) and Gulbis et al. (2015). This special issue was dedicated to the current understanding of the Pluto System on the eve of the arrival of New Horizons.

Illustrations

(1) Computer-generated images of the dwarf planet Pluto, based on data from the Hubble Space Telescope. Pluto is approximately 2300 km in diameter and is currently 4.8 billion km from the Sun.
(2) Comparison between Pluto stellar occultation light curves between 1998 and 2013. The upper atmosphere (about ~ 0.5 flux level) is constant, while the shape of the lower atmosphere has evolved. Figure from Bosh et al. (2015).

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