Constraints on Jupiter׳s stratospheric HCl abundance and chlorine cycle from Herschel/HIFI

Abstract

Detection of HCl on Jupiter would provide insight into the chlorine cycle and external elemental fluxes on giant planets, yet so far has not been possible. Here we present the most sensitive search for Jupiter's stratospheric HCl to date using observations of the 625.907 and 1876.221 GHz spectral lines with Herschel's HIFI instrument. HCl was not detected, but we determined the most stringent upper limits so far, improving on previous studies by two orders of magnitude. If HCl is assumed to be uniformly mixed, with a constant volume mixing ratio above the 1 mbar pressure level and has zero abundance below, we obtain a 3-sigma upper limit of 0.056 ppb; in contrast, if we assume uniform mixing above the 1~mbar level and allow a non-zero but downward-decreasing abundance from 1~mbar to the troposphere based on eddy diffusion, we obtain a 3-sigma upper limit of 0.024 ppb. This is below the abundance expected for a solar composition source, such as comets, and implies that upper atmosphere HCl loss processes are important. We investigated aerosol scavenging using a simple diffusion model and conclude that it could be a very effective mechanism for HCl removal. Transient scavenging by stratospheric NH3 from impacts is another potentially important loss mechanism. This suggests that it is extremely unlikely that HCl is present in sufficient quantities to be detectable in the near future. We summarise the implications for Jupiter's chlorine cycle and conclude that based on a combination of our observations and previous studies of external oxygen supply, a solar composition external source for Jupiter's chlorine combined with stratospheric scavenging by aerosols and NH3 appears the most plausible. 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 AbstractDetection of HCl on Jupiter would provide insight into the chlorine cycle and external elemental fluxes on giant planets, yet so far has not been possible. Here we present the most sensitive search for Jupiter's stratosphericHCl to date using observations of the 625.907 and 1876.221 GHz spectral lines with Herschel's HIFI instrument. HCl was not detected, but we determined the most stringent upper limits so far, improving on previous studies by two orders of magnitude. If HCl is assumed to be uniformly mixed, with a constant volume mixing ratio above the 1 mbar pressure level and has zero abundance below, we obtain a 3-σ upper limit of 0.056 ppb; in contrast, if we assume uniform mixing above the 1 mbar level and allow a non-zero but downward-decreasing abundance from 1 mbar to the troposphere based on eddy diffusion, we obtain a 3-σ upper limit of 0.024 ppb. This is below the abundance expected for a solar composition source, such as comets, and 6 Herschel is an ESA space observatory with science instruments provided by Europeanled Principal Investigator consortia and wit...

Uranus’ Stratospheric HCl Upper Limit from Herschel/SPIRE*

“…This is consistent with Cl/O=3.3×10 −4 for a solar composition source (Lodders 2010). Additional loss processes such as aerosol scavenging (Showman 2001;Teanby et al 2014) or reactions with hydrocarbons (Teanby et al 2021) may be occurring in Uranus' stratosphere, but they are not required to explain the HCl non-detection.…”

“…Stringent HCl searches have been undertaken on Jupiter (Teanby et al 2014), Saturn (Fletcher et al 2012) and Neptune (Teanby et al 2021), but no detections have been made. Lack of HCl may require loss mechanisms in addition to downward mixing, particularly for large cometary injections (Teanby et al 2021).…”

“…Lack of HCl may require loss mechanisms in addition to downward mixing, particularly for large cometary injections (Teanby et al 2021). Possible loss mechanisms include aerosol scavenging (Showman 2001;Teanby et al 2014) or reactions with hydrocarbons (Teanby et al 2021). It appears unlikely that HCl is detectable on Uranus, given its non-detection on Jupiter, Saturn, and Neptune.…”

Neptune’s HCl upper limit from Herschel/HIFI

“…Stratospheric HCl should be stable to loss via ammonia salt creation as any residual NH 3 advected from the deep troposphere will be effectively removed by condensation, reactions with excess H 2 S to form NH 4 SH (Irwin et al, 2019b), and photolysis (Kaye and Strobel, 1983;Moses, 2000). However, other HCl loss processes such as aerosol scavenging provide alternative potentially efficient removal processes, but are not well constrained (Showman, 2001;Teanby et al, 2014). Therefore, observations of HCl provide important constraints on external sources, loss processes, and photochemical pathways in Neptune's stratosphere.…”

“…Previous studies have searched for HCl on Jupiter (Fouchet et al, 2004;Teanby et al, 2014) and Saturn (Teanby et al, 2006;Fletcher et al, 2012), but have been unsuccessful. The current best 3σ upper limit for Jupiter is 0.061 ppb (parts per billion) (Teanby et al, 2014) and for Saturn is 130 ppb (Fletcher et al, 2012), assuming for both planets that HCl is confined to pressures less than 1 mbar.…”