Detection of volatiles undergoing sublimation from 67P/Churyumov-Gerasimenko coma particles using ROSINA/COPS

Context. The ESA Rosetta mission has allowed for an extensive in situ study of the comet 67P/Churyumov-Gerasimenko. In measurements performed by the ram gauge of the COmet Pressure Sensor (COPS), observed features are seen to deviate from the nominal ram gauge signal. This effect is attributable to the sublimation of the volatile fraction of cometary icy particles containing volatiles and refractories. Aims. The objective of this work is to investigate the volatile content of icy particles that enter the COPS ram gauge. Methods. We inspected the ram gauge measurements to search for features associated with the sublimation of the volatile component of cometary particles impacting the instrument. All the sublimation features with a high-enough signal-to-noise ratio were modelled by fitting one or more exponential decay functions. The parameters of these fits were used to categorise different compositions of the sublimating component. Results. Based on features that are attributable to ice sublimation, we infer the detection of 73 icy particles containing volatiles. Of these, 25 detections have enough volatile content for an in-depth study. From the values of the exponential decay constants, we classified the 25 inferred icy particles into three types, interpreted as different volatile compositions, which are possibly further complicated by their differing morphologies. The available data do not give any indication as to which molecules compose the different types. Nevertheless, we can estimate the total volume of volatiles, which is expressed as the diameter of an equivalent sphere of water (density of 1 g cm−3 ). This result was found to be on the order of hundreds of nanometres.