# # Program information file # PROGRAM_ID 2023B075 PROGRAM_TITLE Comet 2P/Encke: Investigating Possible Causes for the Unusual Compositional Variability in a Short Period Comet. PROGRAM_INV1 Chemeda Ejeta PROGRAM_INV2 Erika Gibb PROGRAM_INV3 Younas Khan PROGRAM_INV4 Nathan Roth PROGRAM_INV5 Michael DiSanti PROGRAM_SCICAT comets PROGRAM_ABSTRACT_BEG We propose to use iSHELL at NASA-IRTF to study the volatile composition of short-period comet 2P/Encke. Comets are generally assumed to be relatively pristine and thus can reveal information about the early solar system. However, short-period comets may [and do] exhibit changes in volatile composition when observed over time. For example, significant differences were noted when 2P/Encke's post-perihelion primary volatile abundance ratios in 2017 were compared to pre-perihelion values from 2003, particularly for C2H6, CH3OH, and H2CO. Differences in the heliocentric distance, seasonal or evolutionary effects, asymmetry in mixing ratios about perihelion, and rotational/diurnal effects were proposed to explain the observed compositional differences. The 2023B semester provides an excellent opportunity to test two scenarios: [1] evolutionary effects on mixing ratios due to multiple perihelion passages and [2] seasonal and/or diurnal effects on the potentially heterogeneous nucleus of 2P/Encke. In 2023, we propose observing 2P/Encke at the same heliocentric distance range [0.40-0.55 AU] as observations performed in 2017 but in different seasonal phases [pre-perihelion]. We will measure abundance ratios [relative to H2O] of six primary volatiles using three iSHELL settings: Lp1 will sample CH4, C2H6, CH3OH, and H2CO, M2 will sample H2O, and CO, and a custom L setting [covering roughly 2.80-3.10 micro m] will sample H2O, and HCN. We will use an observing cadence that will permit us to sample an entire rotation of the nucleus with Lp1 to test for possible rotational effects on mixing ratios. Owing to its cross-dispersed capability, excellent sensitivity, superior guiding while obtaining spectra in M-band, and daytime observing capability, iSHELL at NASA-IRTF is the only suitable instrument/facility combination for this study. PROGRAM_ABSTRACT_END