Constraining the surface properties of Helene

TL;DR

This study uses Cassini's infrared data to analyze Helene's surface temperature and albedo, constraining its surface properties despite uncertainties in thermal inertia and comparisons to Dione.

Contribution

It provides new estimates of Helene's surface albedo and temperature using infrared observations, and assesses the constraints on its thermal inertia.

Findings

Surface albedo between 0.25 and 0.70, mean 0.43.

Surface temperatures around 83-89 K from FP3 data.

Helene's albedo may be darker than Dione, but uncertainties remain.

Abstract

We analyze two sets of observations of Dione's co-orbital satellite Helene taken by Cassini's Composite Infrared Spectrometer (CIRS). The first observation was a CIRS FP3 (600 to 1100cm-1, 9.1 to 16.7 ${\mu}$m) stare of Helene's trailing hemisphere, where two of the ten FP3 pixels were filled. The daytime surface temperatures derived from these observations were 83.3${\pm}$0.9 K and 88.8${\pm}$0.8 K at local times 223${\deg}$ to 288${\deg}$ and 180${\deg}$ to 238${\deg}$ respectively. When these temperatures were compared to a 1-D thermophysical model only albedos between 0.25 and 0.70 were able to fit the data, with a mean and standard deviation of 0.43${\pm}$0.12. All thermal inertias tested between 1 and 2000 J m$^{-2}$ K$^{-1}$ s$^{-1/2}$ could fit the data (i.e. thermal inertia was not constrained). The second observation analyzed was a FP3 and FP4 (1100 to 1400cm-1, 7.1 to 9.1 ${\mu}m)$ scan of Helene's leading hemisphere. Temperatures between 77 and 89 K were observed with FP3, with a typical error between 5 and 10 K. The surface temperatures derived from FP4 were higher, between 98 and 106 K, but with much larger errors (between 10 and 30 K) and thus the FP3- and FP4-derived temperature largely agree within their uncertainty. Dione's disk-integrated bolometric Bond albedos have been found to be between 0.63${\pm}$0.15 (Howett et al., 2010) and 0.44${\pm}$0.13 (Howett et al., 2014). Thus Helene may be darker than Dione, which is the opposite of the trend found at shorter wavelengths (c.f. Hedman et al., 2020; Royer et al., 2020). However few conclusions can be drawn since the albedos of Dione and Helene agree within their uncertainty.