First Observations of CH4 and H3+ ${mathrm{H}}_{3}^{+}$ Spatially Resolved Emission Layers at Jupiter Equator, as Seen by JIRAM/Juno

Abstract

In this work, we present the detection of CH4 and H3+ ${mathrm{H}}_{3}^{+}$ emissions in the equatorial atmosphere of Jupiter as two well-separated layers located, respectively, at tangent altitudes of about 200 and 500–600 km above the 1-bar level using the observations of the Jovian InfraRed Auroral Mapper (JIRAM), on board Juno. This provides details of the vertical distribution of H3+ ${mathrm{H}}_{3}^{+}$ retrieving its Volume Mixing Ratio (VMR), concentration, and temperature. The thermal profile obtained from H3+ ${mathrm{H}}_{3}^{+}$ shows a peak of 600–800 K at about 550 km, with lower values than the ones reported in Seiff et al. (1998), https://doi.org/10.1029/98JE01766 above 500 km using VMR and temperature as free parameters and above 650 km when VMR is kept fixed with that model in the retrieval procedure. The observed deviations from the Galileo’s profile could potentially point to significant variability in the exospheric temperature with time. We suggest that vertically propagating waves are the most likely explanation for the observed VMR and temperature variations in the JIRAM data. Other possible phenomena could explain the observed evidence, for example, dynamic activity driving chemical species from lower layers toward the upper atmosphere, like the advection-diffusion processes, or precipitation by soft electrons, although better modeling is required to test these hypothesis. The characterization of CH4 and H3+ ${mathrm{H}}_{3}^{+}$ species, simultaneously observed by JIRAM, offers the opportunity for better constraining atmospheric models of Jupiter at equatorial latitudes.Plain Language SummaryThe Jovian Infrared Auroral Mapper (JIRAM) is the infrared imager and spectrometer on board the Juno mission, designed to investigate Jupiter’s atmosphere. A key objective of JIRAM is the investigation of the minor species, such as CH4 and H3+ ${mathrm{H}}_{3}^{+}$ that are very important to understanding the energy balance of the middle and upper atmosphere of Jupiter. These species have strong signatures in the 3.3–3.8 μm spectral region, well within the nominal wavelength range of the instrument. We present the analysis of recent images and spectra obtained by JIRAM, in the period December 2018–September 2020, plus additional measurements in March 2017, to study methane and H3+ ${mathrm{H}}_{3}^{+}$ vertical distribution at equatorial latitudes. We find that CH4 is localized around 200 km above the 1-bar level, while a distinct layer of H3+ ${mathrm{H}}_{3}^{+}$ is observed around 500–600 km (0.04–0.016 μbar). The observed vertical distribution and intensity variation of H3+ ${mathrm{H}}_{3}^{+}$ is likely to be the result of vertically propagating waves. However, other possible phenomena can be invoked to explain these findings, like for example, an uplifting of chemical species from lower layers toward the upper atmosphere, or soft electrons precipitation, although a rigorous modeling is needed to confirm the latter hypothesis.Key PointsDetection of CH4 and H3+ ${mathrm{H}}_{3}^{+}$ emissions over Jupiter’s disc as two well separated layers in the equatorial region at 200 and 600 kmThe H3+ ${mathrm{H}}_{3}^{+}$ temperature profile shows a peak of 600–800 K at about 600 km with some differences with respect to the Galileo’s profileThe observed features point out the presence of localized variability with altitude, perhaps indicative of wave activities