As part of its extensive slate of observations, the James Webb Space Telescope (JWST) recently took a closer look at Pluto’s moon Charon, where it helped scientists to make a surprising discovery.
Charon, the largest and most well-studied of Pluto’s five moons, was first visited in 2015 as part of the groundbreaking New Horizons mission, which characterized it as a stunningly beautiful world with unexpectedly complex surface features. However, this new discovery makes it plain that there is chemistry ongoing on the surface of the distant moon, making it a surprisingly active place in the far reaches of the solar system.
As presented in a study published in Nature in October 2024, scientists from the Southwest Research Institute (SwRI) and a number of cohort organizations have detected both carbon dioxide (CO2) and hydrogen peroxide (H2O2) on the surface of Pluto’s largest moon. The team drew upon new information gathered by JWST during a recent period of observation that has followed on the heels of a sort of virtual “grand tour” of the outer system and its many worlds by the well-known space telescope.
Tiny team
Charon was discovered by James W. Christy in 1978, almost 50 years after Clyde Tombaugh discovered Pluto. In the years since its detection, more and more has been found out about this primordial world, including the exciting detail that Charon is actually so large compared to Pluto that both of them orbit a shared center of gravity that is outside of Pluto itself. This has led some to characterize the Pluto-Charon system as a sort of “binary planet,” which defies the traditional characteristics of worlds and their moons.
The Pluto-Charon system is located an average of 3.5 billion miles (5.7 billion kilometers) from the Sun, and at that distance Charon’s surface gets down to a frigid –364° F (–220° C). Although a thin atmosphere was detected on Pluto by the New Horizons probe during its 2015 flyby, scientists believed at the time that Charon was simply too small to maintain an atmosphere. But none of the tools brought to bear on Charon were capable of returning spectral data with wavelengths longer than 2.5 µm, leaving plenty of uncovered ground for future work.
JWST has that power, specializing in infrared observing and bringing several spectral instruments to the table. Silvia Protopapa of SwRI and her team compiled and examined data from one of JWST’s recent observations of of the Pluto-Charon system, using this new, longer-wavelength spectral data.
The data show both carbon dioxide (CO2) and hydrogen peroxide (H2O2) on the surface, as well as ammonia and crystalline water ice. Hydrogen peroxide is often formed when light and radiation interact with water ice, indicating active surface chemistry on the moon’s surface. The carbon dioxide, the team contends, probably originated underground, and was exposed to the surface during impact events. Both discoveries shed more light on the frigid happenings of a small, distant world like Charon, and hints at the compounds that might comprise other objects in the vast and poorly understood Kuiper Belt.
Hints at history
The team’s discovery might also reveal clues to the mystery of how Charon formed. One popular theory contends a method similar to what is believed to have formed Earth’s moon, namely an impact by a massive third object which shattered proto-Pluto and allowed Charon to coalesce from the ensuing debris. If the CO2 and H2O2 on Charon were delivered by external impactors, that could indicate the validity of this theory.
This discovery comes on the heels of the 2023 detection of CO2 on Jupiter’s moon Europa, and adds a new chapter in the study of how Charon typifies the formation and evolution of surface environments on Trans-Neptunian Objects (TNOs) like Pluto and Charon. The capability of JWST to observe these outer system bodies at longer wavelengths than previously possible means that scientists like Protopapa and her colleagues will be able to make better observations of these exciting worlds and explain the ways their stories fit into the larger narrative of the solar system we all call home.
