Presentation #306.08 in the session New Chemicals, New Clouds, New Toys for Giants.
James Webb Space Telescope commissioning observations of Jupiter using near infrared filters at 2.12 μm and 3.23 μm revealed what seemed to be a surprising detached haze layer far above the east limb of Jupiter. It was soon recognized, however, that the seeming detachment was caused by the geometry of the observation: due to the 11.8° phase angle, the terminator of Jupiter is visible on the eastern limb, with Jupiter’s shadow rising to an altitude of 1500 km above the 1-bar level at the tangent point where hazes reach their maximum brightness. If the “detached haze” reached that altitude, as seems necessary to be illuminated by sunlight, why was there no high-altitude haze seen above the opposite sunlit limb? To better understand this situation, we made a quantitative analysis of the 3.23-μm image, which has the larger pixel scale (222 km/pixel) but is the more specifically sensitive to stratospheric hazes. At that wavelength, due to strong methane absorption, the planet’s reflectivity over the sunlit portion of the disk is dominated by stratospheric haze scattering. We used a limb-to-terminator equatorial scan from that image to constrain a simple single-scattering model with just two parameters: the scattering optical depth of the haze (found to be about 7x10-4) and the absorption optical depth of the overlying methane (found to be about 0.13). The absorption optical depth at 3.23-μm corresponds to a pressure of approximately 3 mbar and an altitude of roughly 125 km above the 1-bar level. This result also helped to identify a small but important image navigation error. Once the image navigation was adjusted to put the peak brightness of the sunlit haze just outside the 1-bar sunlit limb, we found that the “detached haze” feature on the opposite (shaded) terminator limb fell within the shadow of Jupiter and only about 200 km above the 1 bar level. This suggests either a remarkable coincident alignment of two unrelated phenomena or that the “detached haze” may in fact be the same haze inferred from modeling the illuminated disk, with an altitude closer to 160 km. The problem with the latter interpretation is that the haze should not be visible in the dark! All the indirect illumination options we have investigated so far fall far short of producing the observed brightness of the shaded feature, which is about an order of magnitude dimmer than the peak of the directly illuminated haze (on the sunlit limb). Perhaps what appears to be a shadowed haze is actually a molecular emission of some sort, but the feature’s altitude seems to be much too low for H3+ emissions. This mystery has so far defied explanation.