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Deuterium decoded: How the D/H ratio can clarify the origin of methane in the Kuiper belt

Presentation #301.01 in the session TNOs and Centaurs as Viewed by JWST (Oral Presentation)

Published onOct 23, 2023
Deuterium decoded: How the D/H ratio can clarify the origin of methane in the Kuiper belt

Methane is abundant on several large icy worlds. This makes the origin of methane an important issue in the outer solar system. So far, though, almost all of the attention on this topic has been focused on Titan, and we still lack a comprehensive framework to determine the origin of methane. Moreover, opportunities are emerging on more distant bodies, which are now being characterized in stunning detail by the James Webb Space Telescope (JWST). We are thus motivated to develop new geochemical models for the origin of methane on solid bodies in the outer solar system. Here, we describe a framework to constrain the origin of methane based on its D/H ratio. We consider three types of methane that may exist on icy worlds: primordial, abiotically synthesized, and thermogenic. From a detailed review, we were able to set limits on the D/H ratio for each type of methane using data from comet 67P, Earth hydrothermal systems, and carbonaceous chondrites. We have also considered how escape and photochemical processes could have led to deuterium enrichment at the surfaces of icy worlds. We find that primordial methane should be isotopically distinct from abiotic and thermogenic methane in the outer solar system. Primordial methane is expected to have a high D/H ratio (>5×10-4). Abiotic methane should have a lower D/H than thermogenic methane. Hydrogen atoms in abiotic methane would be derived from water, while thermogenic methane could acquire its hydrogen atoms from both organic matter and water. Using our geochemical models, we can predict the D/H ranges for different origin-of-methane scenarios on Kuiper Belt Objects. We will present our predictions and show how they can be used to interpret JWST data from Eris and Makemake, providing our first isotopic clues to the formation conditions and interior evolution of these enigmatic worlds.

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