Letter to the Editor

Thermal clustering of solid ethane with potential relevance to Charon’s Mordor region

¹ University of Texas at San Antonio, Department of Physics and Astronomy, San Antonio, TX, 78249, USA
² Center for Laboratory Astrophysics and Space Science Experiments (CLASSE), Southwest Research Institute, Space Science Division, San Antonio, TX, 78238, USA
³ Southwest Research Institute, Space Science Division, San Antonio, TX, 78238, USA
⁴ Lowell Observatory, Flagstaff, AZ, 86001, USA
⁵ Northern Arizona University, Department of Astronomy and Planetary Science, Flagstaff, AZ, 86011, USA

^★ Corresponding author: This email address is being protected from spambots. You need JavaScript enabled to view it.

Received: 29 December 2025
Accepted: 4 March 2026

Abstract

The methane condensed in Charon’s winter hemisphere undergoes photolysis by interplanetary medium Lyman-α photons, which yields solid ethane as the dominant photoproduct. With the onset of spring, the photolyzed surface experiences increased surface temperatures and solar wind exposure, conditions that promote further structural and chemical modification. We report laboratory measurements that show solid ethane undergoes a marked spectral reddening when thermally processed at Charon’s subsolar maximum temperature of ∼60 K. The reddening follows an Arrhenius temperature dependence, proceeding on hour timescales at 60 K but requiring on the order of 100 years at 45 K. This behavior is consistent with thermally driven diffusion and aggregation of ethane molecules into nanoclusters that preferentially scatter shorter wavelengths. On Charon, simultaneous solar wind radiolysis may convert ethane nanoclusters into higher-order nonvolatile organic nanoclusters that can plausibly account for the observed reddish polar hue of the object.