The Secular and Rotational Brightness Variations of Neptune

TL;DR

This study analyzes decades of photometric and spectroscopic data to understand Neptune's brightness variations, revealing that high-altitude clouds significantly influence its albedo and flux over time.

Contribution

It provides a comprehensive analysis combining long-term photometry and spectroscopy to explain Neptune's secular brightness changes and cloud-related effects.

Findings

Neptune's brightness increased by over 10% in recent decades.

Cloud cover variations correlate with changes in methane absorption and albedo.

Spectrophotometry and photometry results are consistent and validate each other.

Abstract

Neptune has brightened by more than 10% during the past several decades. We report on the analysis of published Johnson-Cousins B and V magnitudes dating back to 1954 along with new U, B, V, R, Rc, I and Ic photometry that we recorded during the past 24 years. Characteristic magnitudes, colors and albedos in all seven band-passes are derived from the ensemble of data. Additionally, 25 spectra spanning 26 hours of observation on 5 nights are analyzed. The spectrophotometry demonstrates that planetary flux and albedo is inversely related to the equivalent widths of methane bands. We attribute the changes in band strength, flux and albedo to the high altitude clouds which rotate across the planet's visible disk. Bright clouds increase albedo and flux while reducing methane absorption. Synthetic V magnitudes derived from the spectroscopy also agree closely with the photometric quantities, which cross-validates the two techniques. The spectroscopic and photometric results are discussed within the framework of the secular brightness variations of Neptune.