Atmospheric implications of the lack of H 3 + detection at Neptune

L. Moore*, J. I. Moses, H. Melin, T. S. Stallard, J. O’Donoghue

*Corresponding author for this work

Research output: Contribution to journalArticlepeer-review

11 Citations (Scopus)

Abstract

H3+ has been detected at all of the solar system giant planets aside from Neptune. Current observational upper limits imply that there is far less H3+ emission at Neptune than rudimentary modelling would suggest. Here, we explore via modelling a range of atmospheric conditions in order to find some that could be consistent with observational constraints. In particular, we consider that the upper atmosphere might be much cooler than it was during the 1989 Voyager 2 encounter, and we examine the impact of an enhanced influx of external material that could act to reduce H3+ density. Resulting ionosphere models that are consistent with existing H3+ observational constraints have an exospheric temperature of 450 K or less, 300 K lower than the Voyager 2 value. Alternatively, if a topside CO influx of 2 × 108 cm−2 s−1 is imposed, the upper atmospheric temperature can be higher, up to 550 K. The potential cooling of Neptune’s atmosphere is relevant for poorly understood giant planet thermospheric energetics, and would also impact aerobreaking manoeuvers for any future spacecraft. Such a large CO influx, if present, could imply Triton is a very active moon with prominent atmospheric escape, and/or that Neptune’s rings significantly modify its upper atmosphere, and the introduction of so much exogenic material would complicate interpretation of the origin of species observed in Neptune’s lower atmosphere.
Original languageEnglish
Article number20200100
Pages (from-to)1-13
Number of pages13
JournalPhilosophical Transactions of the Royal Society A: Mathematical, Physical and Engineering Sciences
Volume378
Issue number2187
Early online date9 Nov 2020
DOIs
Publication statusPublished - 25 Dec 2020
Externally publishedYes

Cite this