Scientists use occultation event to confirm Pluto’s decrease in atmospheric density
When Pluto passed a star on the night of August 15, 2018, a team of astronomers led by the Southwest Research Institute had deployed telescopes to numerous sites in the United States and Mexico to observe Pluto’s atmosphere while it was briefly backlit by the well-placed star. Scientists used this occultation event to measure the overall abundance of Pluto’s thin atmosphere and found compelling evidence that it is starting to disappear, refreezing on its surface as it moves away from the Sun. .
The occultation lasted for about two minutes, during which time the star disappeared from view as the atmosphere and solid body of Pluto passed in front of it. The speed at which the star disappeared and reappeared determined the density profile of Pluto’s atmosphere.
“Scientists have used occultations to monitor changes in Pluto’s atmosphere since 1988,” said Dr Eliot Young, senior program director in the Division of Space Science and Engineering at SwRI. “The New Horizons mission achieved an excellent density profile from its 2015 flyby, consistent with Pluto’s global atmosphere doubling every decade, but our 2018 observations do not show this trend continuing from 2015. “
Several telescopes deployed near the middle of the shadow’s path observed a phenomenon called “central flash,” caused by the refraction of light from Pluto’s atmosphere in a region in the very center of the shadow. When measuring an occultation around an object with an atmosphere, the light decreases as it passes through the atmosphere and then gradually returns. This produces a moderate slope at each end of the U-shaped light curve. In 2018, the refraction of Pluto’s atmosphere created a central flash near the center of its shadow, turning it into a W-shaped curve. .
“The center flash seen in 2018 was by far the most powerful anyone has ever seen in a Pluto occultation,” Young said. “The central flash gives us very precise knowledge of the path of Pluto’s shadow on Earth.”
Like Earth, Pluto’s atmosphere is mainly made up of nitrogen. Unlike Earth, Pluto’s atmosphere is supported by the vapor pressure of its surface ice, which means that small changes in surface ice temperatures would cause large changes in the bulk density of its atmosphere. Pluto takes 248 Earth years to complete a full orbit around the Sun, and its distance varies from its closest point, about 30 astronomical units from the Sun (1 AU is the distance from Earth to the Sun), to 50 AU from the Sun.
Over the past quarter of a century, Pluto has received less and less sunlight as it moved away from the Sun, but, until 2018, its surface pressure and atmospheric density continued to increase. Scientists attributed this to a phenomenon known as thermal inertia.
“An analogy to this is how the Sun heats sand on a beach,” said Dr. Leslie Young, a scientist at SwRI, who specializes in modeling the interaction between surfaces and atmospheres of icy bodies in the outer solar system. “The sunlight is most intense at midday, but the sand then continues to absorb heat in the afternoon, so it gets hotter in the late afternoon. The continued persistence of Pluto’s atmosphere suggests that the nitrogen ice reservoirs on Pluto’s surface were kept warm by the heat stored below the surface. The new data suggests they are starting to cool down.
The largest known reservoir of nitrogen is Sputnik Planitia, a brilliant glacier that forms the western lobe of the heart-shaped Tombaugh Regio. The data will help atmospheric modelers improve their understanding of Pluto’s subterranean layers, particularly with respect to compositions compatible with the observed limits on heat transfer.