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Hazy Blue Yonder

by Perry Pezzolanella

A major nor’easter blasts the area with over a foot of snow. It all ends as fast as it began and the once bleak countryside is now a winter wonderland. The Sun rises after a calm, clear, starlit night and temperatures hover near 0ºF. Streaks of fine haze and fog are visible in the distance, high and low above the frigid snowscape. It is beginning to look a lot like Christmas, but perhaps it is looking more like Pluto!

When New Horizons flew past Pluto on July 14, 2015, one of the most dramatic images ever taken of any world was the departing shot of Pluto’s distant mountains next to a smooth glacial plain with streaks of haze above. The image conjured a sense of distance and depth and proved that Pluto is a wonderfully diverse and active world complete with weather. Observations of Pluto by NASA’s airborne telescope, the Stratospheric Observatory for Infrared Astronomy, or SOFIA, shows that the thin haze surrounding Pluto is made of very small particles that remain in the atmosphere for an extended time, instead of immediately falling to the surface. The haze particles must be continually replenished to maintain it, which may require a radical rethinking of the status of the atmosphere. It was once thought that the atmosphere would eventually freeze out onto the surface of Pluto as it traveled to its most distant point, about 4.6 billion miles from the Sun, in its highly elongated orbit, but with a more resilient atmosphere, it may persist throughout its 248-year journey around the Sun.

SOFIA studied Pluto about two weeks before New Horizons flew by pointing its 9-foot telescope at Pluto while it was occulting a star. Haze or clouds would naturally cause a fluctuation in starlight as Pluto passed in front of it. SOFIA observed the middle layers of Pluto’s atmosphere in infrared and visible light. New Horizons probed the upper, and lower, layers using radio waves and ultraviolet light. The combined observations give a complete and accurate picture of Pluto’s atmosphere. The atmosphere is created when surface ice vaporizes under the distant light of the Sun and is composed primarily of nitrogen, along with small amounts of methane and carbon monoxide. Haze particles form high up in the atmosphere, about 20 miles above the surface, where methane and other gases react with sunlight, before slowly settling down onto the icy surface. New Horizons found evidence of these particles when it sent back images showing a blue-tinted haze, especially when it flew behind Pluto witnessing a beautiful backlit blue ring. These particles are 1000 times smaller than the width of a human hair and are so tiny that they can scatter blue light more than other colors creating the blue tint as they drift slowly towards the surface.

The haze thickens and fades in a cycle lasting only a few years, which indicates that the particles are being created relatively quickly. Pluto is tipped over on its side even more than Uranus, which causes some areas to be exposed to more sunlight during different points in its orbit. When ice-rich regions, such as the vast glacial plain of Sputnik Planitia, which is part of the heart-shaped feature known as Tombaugh Regio, are exposed to sunlight, the atmosphere may expand and create more haze particles. As areas receive less sunlight, it may shrink and become clearer. This cycle also creates winds that transport particles to the night side, and cold pockets such as Sputnik Planitia where the temperature plunges to a mind-numbing -420ºF, hardly 40ºF above absolute zero.

Pluto is a fascinating world finally brought into focus by New Horizons. We can study it from Earth now with sophisticated, sensitive equipment such as SOFIA. The next phase in the exploration of Pluto is a dedicated orbiter mission known as Persephone that will orbit Pluto for several years perhaps as early as October 2058. If approved for funding, it could launch in February 2031 and fly to Pluto’s hazy blue yonder.