Neptune’s HCl upper limit from Herschel/HIFI

Journal article

Here we search for hydrogen chloride (HCl) in Neptune’s stratosphere using observations of the 1876.22 GHz J=3–2 transition from the Heterodyne Instrument for the Far-Infrared (HIFI) on Herschel. Observations comprise a 7.2 hr disc-averaged integration, originally designed to investigate stratospheric methane. Significant HCl emission was not detected. Instead, we determine upper limits using step-type abundance profiles, defined by zero deep abundance and uniform volume mixing ratio for pressures less than a transition pressure (assumed to be 0.1 or 1 mbar). These profiles are a reasonable first-order approximation for an externally sourced species; at higher pressures HCl is expected to be removed by aerosol scavenging and reactions with ammonia. The 3 upper limits are 0.70 parts per billion (ppb) for a 0.1 mbar transition pressure and 0.076 ppb for a 1 mbar transition pressure. These upper limits are the most stringent to date and are consistent with current estimates of interplanetary dust particle flux and the hypothesis that Neptune experienced a large comet impact in the past 1000 years.

Authors: Teanby, NA; Gould, B; Irwin, PGJ

• Publication status: Published
• Peer review status: Peer reviewed
• Publisher: Elsevier
• Journal: Icarus
• Volume: 354
• Article number: 114045
• Publication date: 2020-08-14
• Acceptance date: 2020-08-06
• DOI: 10.1016/j.icarus.2020.114045
• ISSN: 0019-1035
• Language: English
• Keywords: sub-millimetre; Herschel; Neptune; FFR; atmosphere; composition