Radiolysis by-products on the surface of Kuiper Belt Object (20000) Varuna

We present the results of an investigation into the presence of radiolysis by-products on the surface of the intermediate-sized Kuiper Belt Object (KBO) (20000) Varuna. Interaction of extreme-UV photons and cosmic rays with volatile methane (CH4) ice results in the formation of other hydrocarbons such as ethane (C2H6), acetylene (C2H2), and ethylene (C2H4). Ethane is the most common by-product, and all by-products are non-volatile at Kuiper Belt temperatures. Near-infrared spectra of Varuna were obtained with the SpeX instrument in Prism mode (0.7-2.52 microns, R=100) at NASA’s Infrared Telescope Facility (IRTF) on the nights of February 20-23, 2017. A handful of absorption features in the spectrum of Varuna between 2.2 and 2.5 μm are not consistent with absorption from the non-volatile species H2O and CH3OH (methanol). The features are also inconsistent with absorption due to CH4 ice, which was presented as a possible component of Varuna’s surface by Lorenzi et al. (2014). Preliminary analysis suggests these features are consistent with absorption from ethane and ethylene (Hudson et al., 2014). Volatile retention theories (e.g., Schaller and Brown, 2007) favor the retention of ethane and ethylene and the loss of methane on Varuna due its diameter (∼700 km; Lellouch et al., 2013) and estimated equilibrium temperature (∼41 K). Portions of this work were funded by NASA Solar Systems Observations grant NNX17AG16G.