Herschel map of Saturn’s stratospheric water, delivered by the plumes of Enceladus

Abstract

Context. The origin of water in the stratospheres of Giant Planets has been an outstanding question ever since its first detection by ISO some 20 years ago. Water can originate from interplanetary dust particles, icy rings and satellites and large comet impacts. Analysis of Herschel Space Observatory observations have proven that the bulk of Jupiter's stratospheric water was delivered by the Shoemaker-Levy 9 impacts in 1994. In 2006, the Cassini mission detected water plumes at the South Pole of Enceladus, placing the moon as a serious candidate for Saturn's stratospheric water. Further evidence was found in 2011, when Herschel demonstrated the presence of a water torus at the orbital distance of Enceladus, fed by the moon's plumes. Finally, water falling from the rings onto Saturn's uppermost atmospheric layers at low latitudes was detected during the final orbits of Cassini's end-of-mission plunge into the atmosphere. Aims. In this paper, we use Herschel mapping observations of water in Saturn's stratosphere to identify its source. Methods. Several empirical models are tested against the Herschel-HIFI and -PACS observations, which were collected on December 30, 2010, and January 2 nd , 2011 (respectively). Results. We demonstrate that Saturn's stratospheric water is not uniformly mixed as a function of latitude, but peaking at the equator and decreasing poleward with a Gaussian distribution. We obtain our best fit with an equatorial mole fraction 1.1 ppb and a half-width at half-maximum of 25˝, when accounting for a temperature increase in the two warm stratospheric vortices produced by Saturn's Great Storm of 2010-2011. Conclusions. This work demonstrates that Enceladus is the main source of Saturn's stratospheric water.

First direct measurement of auroral and equatorial jets in the stratosphere of Jupiter

“…We analyzed the vertical distributions of CO and HCN as a function of latitude from the line widths. Using empirical vertical profiles of CO and HCN with a cutoff pressure p 0 , below which the species have a constant mole fraction, and the radiative transfer model of Cavalié et al (2019), we found that CO is present at p 0 < 5 mbar at all latitudes, whereas HCN is found at the same pressure levels only at the low-to-mid latitudes (60 • S-50 • N). At higher latitudes, HCN is restricted to p 0 < 0.1 mbar (see Fig.…”

“…2 (top), we simulated the Doppler shifts induced by constant winds within the auroral ovals on the spectral lines. We took the radiative transfer model from Cavalié et al (2019) and the HCN vertical profiles derived from the line shape analysis: from 60 • S to 50 • , HCN was set constant to 1 ppb for p 0 < 5 mbar, and to zero at higher pressures. For latitudes lower than 60 • S and higher than 50 • N, HCN was set constant to 1 ppb for p 0 < 0.1 mbar, and to zero at higher pressures.…”

Orbiting Astronomical Satellite for Investigating Stellar Systems (OASIS): following the water trail from the interstellar medium to oceans

“…Interplanetary dust particles (IDP) should impact the Giant Planets from all directions, while local sources such as rings and icy moons would preferentially deliver water to the Equator. For instance, Herschel/PACS measurements of an H2O enhancement between 25°N and 25°S on Saturn favor Enceladus and its H2O torus as the dominant source [192]. Cassini/CIRS measurements of H2O at the poles of Saturn indicate that there is a smaller component due to IDP [193].…”

Monitoring of the evolution of H2O vapor in the stratosphere of Jupiter over an 18-yr period with the Odin space telescope

“…We applied the radiative transfer model described in Cavalié et al (2008bCavalié et al ( , 2019 and used the temperature profile as well as the output mole fraction profiles of the photochemical model. Both are therefore applied uniformly in latitude and longitude over the Jovian disk.…”

“…Indeed, H 2 O cannot be transported from the tropospheres to the stratospheres due to a cold trap at the tropopause of all these planets. Regarding the nature of the external sources, it has been shown that Enceladus plays a major role in delivering H 2 O to Saturn's stratosphere (Waite et al 2006;Hansen et al 2006;Porco et al 2006;Hartogh et al 2011;Cavalié et al 2019), while an ancient comet impact is the favored hypothesis in the case of Neptune for carbon monoxide (CO), hydrogen cyanide (HCN), and carbon monosulfide (CS) (Lellouch et al 2005(Lellouch et al , 2010Hesman et al 2007;Luszcz-Cook & de Pater 2013;Moreno et al 2017). For Uranus, the situation remains unclear (Cavalié et al 2014;Moses & Poppe 2017).…”