Long Term Evolution of Neptune's Atmosphere

Abstract

We present an analysis of the temporal evolution of hazes on Neptune between 1994 and 2018, using HST archival data at visible wavelengths from theWFPC2 and WFC3 cameras. We assess the changes over time of the global reflectivity in the F850LP/F845M (methane) and F467M (blue) filters, adding more than 10 years of data to previous analyses. We see an increase in brightness in the methane filter from at least 1996 until 2009, followed by a decrease until today, as seen from cloud-free regions only. A similar behavior is observed in the blue filter, which reaches a maximum disk-averaged I/F value in 2002 and stays nearly constant until 2015, when the planet starts to darken. We also evaluate variations in individual latitudinal bands between 90°S and 50°N. We note the largest increases for southern mid-latitudes (65°S-30°S) for both filters, and the northernmost band (20°N-50°N) for the methane filter. The largest decreases are seen in the northernmost band for the blue filter and for southern near-polar (90°S-65°S) and mid- to low latitudes (50°S-5°N) in the methane filter. We perform radiative transfer modeling to assess the latitudinal variability in the hazes altitudes and to identify changes in the atmospheric vertical structure that may give rise to the observed trends. Adopting the SUNBEAR code and assuming a two-haze-layer model, we find an optically thin (τ α Æ 0.03) upper haze layer placed around 0.6 bar and an optically thicker (τ ß 2.2) lower cloud based at 4.1 bar. This model describes the observed I/F values of the atmosphere in 2009 as well as for 2015 for latitudes north of 30°S, but does not agree with southern latitudes, where the best fitting model demands a shallower pressure base (3.4 bar) and methane depletion (methane mixing ratio lowered from 0.05 to 0.03) for the lower haze. The parameters retrieved for 2002 hint at a higher altitude (2.6-3.0 bar) lower haze layer compared to both 2009 and 1996, with the largest difference occurring in relatively cloud-free latitudes (65°S-50°S and 30°S-5°N). The upheaval of the lower haze might be related to the strong cloud activity seen in 2002. Today, southern latitudes’ lower hazes have descended to deeper pressure levels (3.6-4.1 bar) with respect to 2009, and different scattering properties are seen for them at southern mid-latitudes and northern regions.