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Nitrogen Loss from Pluto’s Birth to the Present Day via Atmospheric Escape, Photochemical Destruction, and Impact Delivery and Erosion

Presentation #308.01 in the session Pluto System (Oral Presentation)

Published onOct 23, 2023
Nitrogen Loss from Pluto’s Birth to the Present Day via Atmospheric Escape, Photochemical Destruction, and Impact Delivery and Erosion

Pluto’s early years were tumultuous, as it and the neighboring plutinos were tossed throughout the solar system in response to Neptune’s outward migration 4.5 Gy ago (Nesvorný, 2015). We have previously shown (Johnson, 2023) that Pluto could have spent as much as 10 My at heliocentric distances less than 30 AU, but it also could have reached 100+ AU. While accounting for the Faint Young Sun, the higher solar UV flux, and Pluto’s higher radiogenic heat flux during this time period from 4.5 to 4.4 Ga (termed the “Wild Years”), we estimate the effect of this orbital migration on Pluto’s early climate and its primordial nitrogen inventory. We find that photochemical destruction of nitrogen in the upper atmosphere is the largest loss mechanism, accounting for nearly 13 m GEL (global equivalent layer) of loss over the age of the solar system. Atmospheric Jeans escape represents an additional loss of 1 m GEL. We also investigate the effect of impacts on the nitrogen inventory. Impactors deliver nitrogen, but they also erode nitrogen from the atmosphere, and we find that for early Pluto, impacts represent a net delivery of nitrogen on the order of 1 m GEL, rather than a loss. Considering these three mechanisms (photochemical destruction, atmospheric loss, and impact delivery/erosion), the net change to Pluto’s nitrogen inventory is around 13 m GEL. The present-day known nitrogen inventory can be approximated as the volume of ice in Sputnik Planitia, which is roughly 40 - 440 m GEL (the atmospheric nitrogen contributes a negligible 0.2 cm GEL). Thus, we predict that Pluto’s primordial nitrogen inventory was not significantly larger than the present-day inventory. However, significant fractions of these total values of photochemical destruction, escape, and impact delivery occurred during the Wild Years, so loss estimates made using present-day rates will be underestimates of the total amount of nitrogen lost via each mechanism.

Johnson, P. E. 2023. PhD Thesis, Univ. of Colorado

Nesvorný, D. 2015. Astrophysical Journal, 150, 73

Funded by NASA Solar System Workings 80NSSC20K0819

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