The Mid-IR Albedo of Neptune Derived from Spitzer Observations

TL;DR

This study estimates Neptune's mid-infrared albedo using Spitzer data, revealing very low albedo values around 1% or less, with variability over short and long timescales, impacting atmospheric models.

Contribution

The paper provides the first detailed derivation of Neptune's mid-IR albedo from Spitzer observations and discusses its variability and implications for atmospheric modeling.

Findings

Mid-IR albedo of Neptune is about 1% or less in 2016.

Short-term variability shows albedo at 3.6 microns ranges from 0.2% to 0.6%.

Historical brightness suggests lower albedo in earlier decades.

Abstract

Mid-IR albedo values of Neptune are derived from Spitzer Space Telescope measurements reported by Stauffer et al. (2016). The method of this derivation is described and the results indicate that the geometric albedo was about 1% or less at the time of the observations in 2016. Short-term mid-IR variability of Neptune, evidenced by the Spitzer observations themselves, indicates an albedo at 3.6 microns ranging from 0.2% to 0.6% with a mean of 0.4%. The corresponding albedos at 4.5 microns are 0.7%, 1.3% and 0.9%. Furthermore, the 60-year history of visible-light brightness variations, which show that Neptune was significantly fainter a few decades ago, suggests that the mid-IR albedo during that earlier period of time may have been much less than 1%. The albedo values reported here can have implications for models of Neptune's atmosphere. However, the mid-IR brightness of Neptune cannot contribute very strongly to its total albedo because the Sun emits only about 2% of its flux long-ward of 3 micron. By contrast, the Sun emits 42% of its flux at visible and near-UV wavelengths where the planet's albedo is in the tens of percents.