# # Program information file # PROGRAM_ID 2021B105 PROGRAM_TITLE Comprehensive compositional study of exceptionally bright comet C/2021 A1 [Leonard] PROGRAM_INV1 Younas Khan PROGRAM_INV2 Erika Gibb PROGRAM_INV3 Chemeda Ejeta PROGRAM_INV4 Boncho Bonev PROGRAM_INV5 Michael DiSanti PROGRAM_SCICAT solar system PROGRAM_ABSTRACT_BEG We propose using iSHELL to measure the composition of parent volatiles in the long-period comet C/2021 A1 Leonard [hereafter A1] over a range of heliocentric distances [Rh]. A1's favorable apparition presents a unique opportunity to measure the abundances of a large suit of molecules: H2O, C2H6, CH3OH, CH4, HCN, HC3N, H2CO, C2H2, C4H2, NH3, OCS, CO, OH, CH3D, and HDO. We will carry out a deep search for the deuterated species HDO and CH3D, which is important for understanding the formation conditions in the protosolar nebula. We will search for HC3N and C4H2, which can be useful for understanding the organic complexity in comets. Measurements of the hypervolatiles CH4 and CO are important for understanding natal vs. post-formation processing effects in comets. Although, this requires sufficiently large geocentric velocity [D-dot] to Doppler-shift emission lines of these molecules away from their telluric counterparts, the D-dot of A1 is quite large. A1's exceptional apparition will allow us to measure the underrepresented hypervolatiles CH4 and CO at multiple epochs, which is uncommon. Enhanced abundances of H2CO, NH3, and C2H2 have been reported in many comets measured at Rh < 0.8 AU, possibly due to activation of extended sources in the coma. We will sample A1 in the critical [and underrepresented] range Rh ~1.0 - 0.6 AU, to test for the possible dependence of its chemistry on distance from the Sun. This exceptional opportunity [owing to the brightness, favorable geocentric velocity, and favorable heliocentric and geocentric distances of A1] will enable us to obtain a rich data set that will figure prominently in the PhD theses of the PI and one of the Co-Is. Our observations of A1 with iSHELL will deepen our understanding of comets in significant ways. PROGRAM_ABSTRACT_END