Scientists Use Starlight to Probe Pluto’s Blue Sky

Pluto 7-15-15
Pluto’s atmosphere imaged by New Horizons on July 15, 2015 after it passed behind the dwarf planet and saw it silhouetted in front of the Sun.

From a May 12, 2020 NASA article:

When the New Horizons spacecraft passed by Pluto in 2015, one of the many fascinating features its images revealed was that this small, frigid world in the distant solar system has a hazy atmosphere. Now, new data helps explain how Pluto’s haze is formed from the faint light of the Sun 3.7 billion miles away as it moves through an unusual orbit.

Remote observations of Pluto by NASA’s telescope on an airplane, the Stratospheric Observatory for Infrared Astronomy, or SOFIA, show that the thin haze enshrouding Pluto is made of very small particles that remain in the atmosphere for prolonged periods of time rather than immediately falling to the surface. SOFIA’s data clarify that these haze particles are actively being replenished – a discovery that is revising predictions on the fate of Pluto’s atmosphere as it moves into even colder areas of space on its 248-Earth-year orbit around the Sun. The results are published in the scientific journal Icarus.

Created as surface ice vaporizes under the distant light of the Sun, Pluto’s atmosphere is predominantly nitrogen gas, along with small amounts of methane and carbon monoxide. Haze particles form high up in the atmosphere, more than 20 miles above the surface, as methane and other gases react to sunlight, before slowly raining down to the icy surface.

Pluto haze NH 7-14-15
Detail of Pluto’s layered blue haze

New Horizons found evidence of these particles when it sent back images showing a blue-tinted haze to Pluto’s atmosphere. Now, SOFIA’s data fills in even more details by discovering that the particles are extremely small, just 0.06–0.10 microns thick, or about 1,000 times smaller than the width of a human hair. Because of their small size, they scatter blue light more than other colors as they drift toward the surface, creating the blue tint.

How could such detailed measurements be taken from Earth? The airborne SOFIA team aimed for the stars—literally! SOFIA was uniquely positioned to study Pluto from afar by taking advantage of a rare moment when Pluto passed in front of a distant star, casting a faint shadow across the Earth’s surface. Momentarily backlit by the star, Pluto’s atmosphere could be analyzed.

With these new insights, scientists are reevaluating their predictions on the fate of Pluto’s atmosphere. Many forecasts indicated that as the dwarf planets moved away from the Sun, less surface ice would be vaporized — creating fewer atmospheric gases while losses to space continued — eventually leading to atmospheric collapse. But rather than collapsing, the atmosphere appears to change on a shorter cyclical pattern.

“There’s still a lot we don’t understand, but we’re forced now to reconsider earlier predictions. Pluto’s atmosphere may collapse more slowly than previously predicted, or perhaps not at all. We have to keep monitoring it to find out.”
— Michael Person, director of Massachusetts Institute of Technology’s Wallace Astrophysical Observatory

Read the full article here, and learn more about SOFIA here.