The conference “Progress inUnderstanding the Pluto System: Ten Years Since Flyby” covered a wide range of topics regarding Pluto, Charon, other KBOs, ocean worlds, and more. With apologies for the delay in this entry, I will focus now on two of these topics: Pluto’s atmospheric hazes and Charon’s geology and history.
Launched in late 2021, the James Webb Space Telescope (JWST) observed Pluto’s atmosphere in infrared wavelengths. Other than New Horizons, no instrument has been able to study Pluto’s atmosphere because it is so cold. These hazes, which likely have a high ice content, likely control Pluto’s climate and keep its upper atmosphere cool.
While the spectrum of Pluto’s hazes somewhat resembles the spectrum of the atmosphere of Titan, Saturn’s large moon, Titan’s atmosphere has much less ice than Pluto’s. The sublimation and condensation of ices plays a significant role in Pluto’s atmosphere but not in Titan’s.
New Horizons observed layered hazes covering all of Pluto. While both Pluto and Titan have nitrogen in their atmospheres, Pluto’s also contains carbon monoxide. Given that Pluto is farther from the Sun than Titan, it is understandable that the former’s atmosphere is colder than the latter’s.
Several of New Horizons’ instruments observed Pluto’s atmospheric hazes, which are believed to form through photochemistry of methane and nitrogen. These hazes vary with Pluto’s seasons and during its elliptical solar orbit, as well as with the Sun’s 11-year cycle. When Pluto gets closer to the Sun, Pluto’s volatile ices sublimate and are lifted into its atmosphere.
For Charon, New Horizons’ LORRI, MVIC, and LEISA instruments were used to image the planet and create a mosaic making up 80 percent of its surface. Topographic features were best seen when the Sun was low in Charon’s sky while reflectivity features were best seen when the Sun was high.
Though Charon’s non-encounter hemisphere was imaged in low resolution, topographic features such as massive canyon systems, moated mountains, mottled terrain, and alternating areas of smooth and rocky terrain could still be seen. There are many impact craters but not enough to saturate the surface. Patterns of bright and dark ejecta not seen anywhere else in the solar system are visible on Charon’s surface.
These features suggest Charon has a volatile crust as well as strong layering beneath its surface. All of them indicate a long ago freezing of the planet’s ice shell. This freezing of what was once a subsurface ocean happened very early in the system’s history, before Pluto and Charon became tidally locked to one another.
Charon has about 200 scarps or steep cliffs that are taller than the Grand Canyon on Earth. These might be the largest canyons relative to planet size in the entire solar system.
While Pluto has both young and old surface terrain, Charon’s surface is approximately two billion years old. Large craters on both objects were likely created by ancient impacts of KBOs approximately the size of Arrokoth. Charon’s craters contain a record of impactor populations, which created its large craters. Few small craters have been seen on Charon’s surface. Impactors that hit Charon excavated material from its subsurface.
New Horizons’ instruments showed that water ice is ubiquitous on Charon’s surface. Observations by JWST revealed the presence of carbon dioxide, hydrogen peroxide, and ammonia diluted in water on Charon’s northern hemisphere. Significantly, Jupiter’s moon Europa also has carbon dioxide in its spectrum. Impactors that hit ancient Charon appear to have exposed what had been subsurface carbon dioxide.
What is especially significant is that Charon’s surface appears to have accurately preserved its formation and impact history.
The red spot visible on Charon’s north pole is comprised of tholins, complex organic molecules found in space created by the interaction of ultraviolet sunlight with methane, nitrogen, and water on an object’s surface or in its atmosphere. These tholins originate from methane that escapes Pluto’s atmosphere that is then gravitationally captured by Charon. Only Charon’s nighttime temperatures are cold enough for methane to condense on its surface. Most of this methane is converted to ethane; only ten percent is processed into sticky surface materials.
Like Pluto as well as Titan, Europa, and other solar system bodies, Charon may have once been an ocean world. The freezing of that ocean likely caused expansion of surface features once the planet’s internal heating shut down. Features such as canyons, rifts, and fractures, all seen on Charon, have also been observed on other icy moons that may have subsurface oceans.
There is some evidence Charon may once have experienced cryovolcanism, such as shallow fractures and landslides, which are seen on elsewhere in the solar system on Ceres, Europa, and Uranus’s moon Ariel. Charon’s north appears to be heavily cratered while other areas are composed of smoother cryovolcanic planes.
These factors reveal Charon to have once been one of a growing number of ocean worlds in the solar system. While Charon’s ocean has since frozen, Pluto’s, like those of Europa, Titan, Ceres, Saturn’s moon Enceladus, Jupiter’s moon Ganymede, and Neptune’s moon Triton could still exist in liquid form. While no signs of microbial life have yet been found on any of these worlds, the presence of liquid water and organic compounds raises the possibility that these oceans could harbor such life. This is one of many reasons we need further exploration, especially by instruments that can drill through the ice layers and study the liquid below.
