Atmospheric Escape Explains Diverse Surface Compositions of Pluto vs Sedna

Kuiper Belt Objects are thought to be remnants of early solar system materials, which have retained volatiles such as 〖CH〗_4 and C_2 H_6. Spectroscopic data from various sources reveals that while Pluto retains 〖CH〗_4and C_2 H_6, Sedna shows a lack of 〖CH〗_4 but an abundance of C_2 H_6, (Emery et. al 2024). We hypothesized that 〖CH〗_4, which is stable on Pluto, is able to escape from less massive Sedna. Sufficiently rapidly to be depleted over the age of the solar system while C_2 H_6 has been retained due to its larger molecular mass. Utilizing models for Jeans escape and hydrodynamic escape, as appropriate for a given mass of object and of escaping species, we analyzed the stability and escape rates of 〖CH〗_4 and C_2 H_6 on both Pluto and Sedna. We explored a wide range of masses of Sedna, ranging from ½ to ⅒ the mass of Pluto. We also tested three different abundances of 〖CH〗_4 and C_2 H_6 using measured values from Comet 67-P, and Enceladus, and an upper limit assuming clathrate hydrate. We tested these abundances for two different outgassing rates from the interior, 10% and 100%. We find that 〖CH〗_4 remains stable on Pluto, but escapes from Sedna due to its lower mass, whereas C_2 H_6 remains stable when using both 100% and 10% outgassing rates. This result is in agreement with the observed spectra and leads to tighter mass constraints for Sedna. Our model also explains the observed absence of methane on another KBO, Gonggong.

柯伊伯带天体（Kuiper Belt Objects，KBO）被认为是太阳系早期物质的残留物，保留了甲烷（CH₄）与乙烷（C₂H₆）等挥发性物质。多项来源的光谱数据显示，尽管冥王星仍保有CH₄与C₂H₆，但塞德娜却缺乏CH₄且富含C₂H₆（Emery et al., 2024）。我们提出假说：在冥王星上保持稳定的CH₄，可从质量更小的塞德娜逃逸，其逃逸速率足够快，足以在太阳系的演化时长内被完全耗尽，而C₂H₆则因分子质量更大得以保留。我们针对天体质量与逃逸组分的实际适配场景，采用金斯逃逸（Jeans escape）与流体动力逃逸（hydrodynamic escape）模型，分析了冥王星与塞德娜表面CH₄和C₂H₆的稳定性及逃逸速率。我们对塞德娜的质量范围开展了广泛探索，取值覆盖冥王星质量的1/2至1/10。同时，我们基于67P彗星与土卫二（Enceladus）的实测丰度，以及笼形水合物（clathrate hydrate）的丰度上限假设，共测试了三种不同的CH₄与C₂H₆丰度方案，并针对两种不同的内部脱气速率（10%与100%）验证了上述丰度方案的合理性。研究结果显示，CH₄在冥王星上保持稳定，但因塞德娜质量更低而发生逃逸；而无论采用100%还是10%的脱气速率，C₂H₆均能保持稳定。该结果与观测光谱一致，并为塞德娜的质量提供了更为严格的约束条件。我们的模型同时解释了另一颗柯伊伯带天体共工星（Gonggong）上未观测到甲烷的现象。