# # Program information file # PROGRAM_ID 2022A072 PROGRAM_TITLE Temporal variability of the physical properties of water-ice grains in cometary comae. PROGRAM_INV1 Silvia Protopapa PROGRAM_INV2 Michael S.P. Kelley PROGRAM_INV3 Bin Yang PROGRAM_INV4 PROGRAM_INV5 PROGRAM_SCICAT comets PROGRAM_ABSTRACT_BEG Cometary comae are the only means to remotely investigate the primitive interior of a large sample of cometary nuclei. The compositional taxonomy of comets has been based primarily on coma abundances of gas species as well as chemical and physical properties of dust in the coma. These observations have been critical to constrain formation and evolution models of comets. However, there is still a missing piece of the puzzle, which is water ice, a major constituent of comets. While water-ice grains have been detected in cometary comae, it is unclear whether their characteristics mirror the comet nucleus interior or are the results of sublimation processes in the coma. We propose to monitor the properties of the icy grains as insolation varies with heliocentric distance on the nuclei of nine comets. The objective of this proposal is to study the long-term behavior, rather than snapshots, of individual comets and to ascertain which comets have detectable water-ice grains and how the properties of the ice [abundance, purity, size, phase] change over a long period of time. The proposed set of measurements will be unique and will be used to infer, in a systematic way, the physical evolution of the ice and, potentially, the nucleus composition. Our comet sample has been chosen such that we can examine the development of water ice throughout a very long orbital arc. Among our targets is the long-period comet C/2017 K2, which is extraordinary for showing activity since at least 24 au from the Sun. Using the NASA/IRTF we intend to acquire, in the next four years, near-IR spectroscopic observations of each of the proposed targets using the prism mode, and when possible, the LXD mode as well. These data will be analyzed by means of spectroscopic and sublimation lifetime models. PROGRAM_ABSTRACT_END