A comparative study of the blue comets C/1908 R1 (Morehouse) and C/2016 R2 (Pan-STARRS)

Context. The long-period comet C/1908 R1 (Morehouse) is distinguished by its early spectroscopic tail photography, which uncovered notably intense emission bands of N2+ and CO+, similar to the unusual characteristics of the atypical blue comet C/2016 R2 (Pan-STARRS). Aims. To probe potential parallels with C/2016 R2 further, we revisited the historical spectroscopic plates of C/1908 R1 while leveraging the New Astrometric Reduction of Old Observations (NAROO) project’s advanced sub-micrometric scanner. Methods. We first reviewed the intensity ratio method, followed by a comprehensive spectroscopic analysis of the original historical plates to determine the comet’s composition. Our analysis also encompassed an evaluation of C/1908 R1’s dynamic trajectory using an N-body integrator and a detailed examination of tail morphology records. Results. Our findings suggest that C/1908 R1 experienced no significant close encounters as it crossed the inner Solar System, anchoring its origins directly in the Oort Cloud and allowing us to ascertain that this was its inaugural voyage near the Sun. We determined a N2+/CO+ ratio of ~7% along with a dust-poor composition, particularities it shares with C/2016 R2. Moreover, by synthesizing observations of the tail’s structure over the three-month period of visibility, we uncovered a link between tail dislocation events and aurora borealis sightings on Earth. This association underscores the comet tail’s heightened sensitivity to solar wind fluctuations due to its volatile makeup. Conclusions. The comet C/1908 R1 (Morehouse) emerges as one of the most unaltered relics of our Solar System’s formation, offering another instance of a C/2016 R2-analogous comet. This underscores the importance of preserving and reexamining historical astronomical datasets, not only for historical significance but as a critical resource for contemporary scientific advancement.