Evolution of Neptune at Near-Infrared Wavelengths from 1994 through 2022

TL;DR

This study analyzes nearly three decades of near-infrared observations to track Neptune's cloud activity evolution, revealing periodic maxima and minima linked to solar UV-driven photochemical processes.

Contribution

It provides the first long-term near-infrared cloud activity record for Neptune, linking cloud variability to solar UV irradiance.

Findings

Cloud activity peaks in 2002 and 2015, minima in 2007 and 2020.

2020 showed near-total cloud loss at mid-latitudes.

Cloud brightness variations correlate with solar Lyman-Alpha irradiance.

Abstract

Using archival near-infrared observations from the Keck and Lick Observatories and the Hubble Space Telescope, we document the evolution of Neptune's cloud activity from 1994 to 2022. We calculate the fraction of Neptune's disk that contained clouds, as well as the average brightness of both cloud features and cloud-free background over the planet's disk. We observe cloud activity and brightness maxima during 2002 and 2015, and minima during 2007 and 2020, the latter of which is particularly deep. Neptune's lack of cloud activity in 2020 is characterized by a near-total loss of clouds at mid-latitudes and continued activity at the South Pole. We find that the periodic variations in Neptune's disk-averaged brightness in the near-infrared H (1.6 $\mu$m), K (2.1 $\mu$m), FWCH4P15 (893 nm), F953N (955 nm), FWCH4P15 (965 nm), and F845M (845 nm) bands are dominated by discrete cloud activity, rather than changes in the background haze. The clear positive correlation we find between cloud activity and Solar Lyman-Alpha (121.56 nm) irradiance lends support to the theory that the periodicity in Neptune's cloud activity results from photochemical cloud/haze production triggered by Solar ultraviolet emissions.