Title: Colour variations of Comet C/2013 UQ4 (Catalina) Author: Oleksandra Ivanova, Evgenij Zubko, Gorden Videen, Michael Mommert, Joseph L. Hora, Zuzana Seman Kriandová, Ján Svoren, Artyom Novichonok, Serhii Borysenko, Olena Shubina
We report observations of colour in the inner coma of Comet C/2013 UQ4 (Catalina) with the broadband B and R filters. We find significant temporal variations of the colour slope, ranging from -12.67 ± 8.16 % per 0.1~m up to 35.09±11.7% per 0.1~m. It is significant that the comet changes colour from red to blue over only a two-day period. Such dispersion cannot be characterized with an average colour slope. We also observe Comet C/2013 UQ4 (Catalina) in infrared using Spitzer and find no significant CO/CO2 gaseous species in its coma. Therefore, we classify Comet C/2013 UQ4 (Catalina) as a dust-rich comet and attribute the measured colour slope to its dust. We analyse the colour slope using the model of agglomerated debris particles and conclude that the C/2013 UQ4 coma was chemically heterogeneous, consisting of at least two components. The first component producing the bluest colour is consistent with Mg-rich silicates. There are three different options for the second component producing the reddest colour. This colour is consistent with either Mg-Fe silicates, kerogen type II, or organic matter processed with a low dose of UV radiation.