Artist’s illustration of the dwarf planet Sedna, located in the distant Kuiper Belt. | Credit: NASA/JPL-Caltech
New Chemical Findings Differentiate Pluto and Sedna
A remarkable chemical disparity between Pluto and Sedna, two dwarf planets residing in the far-off Kuiper Belt, is instrumental in helping researchers pinpoint their masses, according to a recent study.
Understanding the Kuiper Belt
The Kuiper Belt represents a zone in space extending beyond Neptune’s orbit. It is populated by Pluto and the majority of recognized dwarf planets, alongside comets considered remnants from the solar system’s planetary genesis.
Amelia Bettati, the study’s lead author and a scientist at Elon University, stated, “Kuiper Belt objects are frigid celestial bodies that can reveal conditions from billions of years ago. Investigating them aids scientists in comprehending planetary formation and evolution.”
Methane and Ethane Discrepancy
Recent investigations utilizing near-infrared spectroscopy by the James Webb Space Telescope (JWST) have unveiled the presence of both methane and ethane on Pluto’s surface. These volatile compounds are frequently detected in the outer solar system and are believed to be vestiges from the era of planet formation. In contrast, Sedna, which is less than half the size of Pluto, was observed to possess only methane.
Volatile Retention and Planetary Mass
“We posited that the observed variation stems from Sedna’s smaller size and consequently weaker gravitational pull compared to Pluto,” Bettati informed Space.com. “This diminished gravity permits methane to escape into space over eons, whereas ethane, being a heavier molecule, is retained.”
While prior research established a general threshold differentiating objects capable of retaining volatiles from those that cannot, the distinction between Pluto and Sedna offers novel insight into how specific escape mechanisms might shape the surface compositions of these remote objects. Sedna’s proximity to the mass threshold for volatile loss underscores the significance of deciphering how particular chemicals are either preserved or dissipated, especially when contrasting different Kuiper Belt objects.
Refining Sedna’s Mass Estimate
“By examining methane and ethane escape from Sedna, we calculated Sedna’s minimum mass required to account for its present surface composition,” Bettati explained. “To elucidate the absence of methane but presence of ethane on Sedna, we must elevate the minimum mass estimation for Sedna. This refinement is crucial for enhancing our understanding of Sedna’s internal structure and evolutionary history.”
Comparative Modeling with Comets and Moons
In their research, Bettati and Jonathan Lunine, her co-author from NASA’s Jet Propulsion Laboratory and the California Institute of Technology, constructed models for methane and ethane levels on Sedna. They validated their model’s precision using two analogous bodies: Comet 67P/Churyumov-Gerasimenko and Saturn’s moon Enceladus.
The Rosetta probe from Europe conducted in-depth studies of Comet 67P, while NASA’s Cassini spacecraft amassed extensive data on Enceladus during its mission within the Saturnian system.
Bettati clarified, “Both bodies possess well-defined measurements and are outer solar system entities, justifying their consideration as analogues for our model.”
Evaluating Volatile Escape Scenarios
To ascertain if sufficient methane and ethane had dissipated from these objects to become undetectable in their surface spectra, the scientists needed to approximate the initial quantities of these chemicals trapped within.
They employed two distinct scenarios for this estimation. One scenario presumed the ratio of methane and ethane to water ice mirrored measurements from Enceladus, whereas the second considered the ratio observed on Comet 67P during winter. These comparisons aided in comprehending the potential loss of these compounds over time.
Escape Models: Jeans and Hydrodynamic
Bettati elaborated, “[We utilized] the Jeans escape model, a thermal escape mechanism driven by atmospheric temperature. In this model, the fastest-moving molecules exceed the escape velocity, but the majority of molecules do not.”
They also implemented another model, hydrodynamic escape, which occurs when the bulk of molecules, not solely the high-velocity ones, are capable of escaping. “A significant portion of the atmosphere is in motion, escaping into space,” Bettati noted.
Model Findings and Gonggong
The models indicated methane’s stability on Pluto but its escape from Sedna due to its reduced mass. Conversely, ethane remained stable on both, even with varying outgassing rates—100% (complete volatile release) and 10% (minimal release).
This outcome aligns with observed surface spectra and yields a more precise mass estimate for Sedna. The model also elucidates the absence of methane on another Kuiper Belt object, Gonggong.
Implications for Future Missions
“Similar to Sedna, Gonggong also lacks surface methane,” Bettati pointed out. “Given Gonggong’s size similarity to Sedna, we infer methane likely escaped from it in a comparable manner. This suggests smaller Kuiper Belt objects experience methane loss over time, while larger ones, such as Pluto, retain it.”
“Knowing the probable gases present on diverse Kuiper Belt objects, their depletion rates, and their historical compositions allows scientists to better strategize forthcoming missions.”
JWST’s Revolutionary Insights
These discoveries, coupled with JWST observations, will assist researchers in deciphering atmospheric and surface composition changes within and beyond the Kuiper Belt.
“It underscores how JWST is transforming our comprehension of the solar system’s most distant bodies,” Bettati concluded.
Study Publication
The study was featured in the February issue of the journal Icarus.