Century-old comet C/1908 R1 (Morehouse) finally reveals its secrets
Cracking the Comet Cold Case: the analysis with modern techniques of old archived photographic plates taken in 1908 of one of the brightest comets of the past, uncovered a unique composition and explain previously unexplained behaviors.
Acollaboration of French and University of Liège astronomers has solved a century-old mystery : the unusual nature of comet C/1908 R1 (Morehouse), observed only once during its passage in 1908. By leveraging historical archives from the Meudon Observatory and cutting-edge technology from the NAROO project, researchers have rediscovered and analyzed forgotten data, revealing an exceptional composition.
Comet Morehouse had already fascinated astronomers of the time with its blue hue and hyperbolic trajectory, which doomed it to a single visit to Earth.
“This passage coincided with a major astronomical advancement: it was only the second time scientists managed to capture the spectrum of a comet’s ionic tail, a technique then under development at the Meudon Observatory,” explains Dr. Sarah Anderson, from the Laboratoire d’Astrophysique de Marseille and lead researcher of the study.
While these early spectroscopic observations revealed unusual behavior, the technology of the time was too limited to fully understand the comet's composition.
Left: The New Astrometric Reduction of Old Observations (NAROO) scanner. Right top. Scan of a plate obtained over two combined nights on 1908 October 31 and November 1 (de La Baume Pluvinel & Baldet 1908,1911). It was obtained at the Juvisy observatory using a 24 cm refractor telescope equipped with an objective prism. The comet is at number 2. As a prism objective spectrograph has no slit the spatial extension of the comet ion tail is visible for each of the bright CO+ bands. Bottom: Scan of a photographic plate from 1908 October 30 (Deslandres H. 1908). This spectrum (2-, indicating the comet nucleus region) was obtained at the Meudon observatory using also an objective prism.
In 2018, comet C/2016 R2 captivated the scientific community with its unique composition, poor in water but rich in ionized molecular nitrogen (N2+) and ionized carbon monoxide (CO+), the origin of its distinctive blue color. This discovery prompted researchers to revisit the historical data of comet Morehouse, seeking potential similarities. Thanks to the NAROO project, which specializes in digitizing astronomical photographic plates, the research team was able to reexamine the original 1908 observations using modern spectroscopic analysis techniques, namely the most up-to-date fluorescence models of N2+ and CO+ that were recently developed to analyze the unusual spectrum of comet comet C/2016 R2.
“It was really interesting to work on these old and pioneering data, at a time when N2+ was not yet known, and to realize that and to realize that we could calibrate them precisely and get useful measurements using the same modern tools that we used already in 2019 to analyze the Very Large Telescope spectra of comet C/2016 R2” says Dr. Emmanuel Jehin of the STAR Institute at the University of Liège.
The results confirm that comet Morehouse shared similar characteristics with C/2016 R2, including a high concentration of N2+ and CO+, while being dust-poor.
“This result is extremely rare among observed comets and gives us another point of comparison for these unique comets, which are water-poor but rich in hyper-volatile ices like CO and N2. This makes Morehouse a first in this category, more than a century before the discovery of C/2016 R2,” adds Prof. Philippe Rousselot of the UTINAM institute at Besançon, who also contributed to the study.
Another long-unexplained phenomenon has finally been clarified: why did the tail of comet Morehouse rip off about every 15 days? Researchers have linked this behavior to the solar cycle. In 1908, the Sun was experiencing a period of high activity (a solar maximum), producing intense solar winds that caused the repeated disconnection events of the comet’s tail. At the time, the existence of the solar wind was not yet known.
This study highlights the importance of preserving and analyzing historical scientific archives. Often regarded as heritage pieces, these old datasets still hold significant scientific value. “These findings show that historical data can still reveal groundbreaking discoveries,” concludes Emmanuel Jehin.
This rediscovery of Comet Morehouse and the understanding of its enigmatic behaviors open new perspectives for the study of comets and underline the importance of revisiting the archives in the light of modern technologies.
Scientific reference
A comparative study of the blue comets C/1908 R1 (Morehouse) and C/2016 R2 (Pan-STARRS)
Anderson, Rousselot, Jehin et al. "A comparative study of the blue comets C/1908 R1 (Morehouse) and C/2016 R2 (Pan-STARRS)"
