The ratio of ordinary water to deuterated water in the mysterious interstellar visitor that passed through our Solar System a few months ago is a chemical fingerprint from a distant planetary system, which points to an origin in an environment that is exceptionally cold and chemically distinct from our own.
New observations carried out by ALMA (Atacama Large Millimeter / submillimeter Array) have allowed the first measurement of deuterated water – also known as semi-heavy water – in an interstellar object.
The discovery reveals that the interstellar comet contains at least 30 times the proportion of semi-heavy water found in comets in our own Solar System, providing a direct chemical window to the frigid conditions in which their home star system formed.
The investigation was led by Luis E. Salazar Manzanoby University of Michigan doctoral student, in collaboration with assistant professor Teresa Paneque-Carreñowho served as Principal Investigator for the ALMA Director’s Discretionary Time program, which made these observations possible.
The data was obtained with ALMA’s Atacama Compact Array (ACA) just six days after 3I/ATLAS reached its closest point to the Sun – a narrow observation window made possible by ALMA’s unique ability to point toward the Sun, unlike most optical telescopes.
“Our new observations show that the conditions that led to the formation of our Solar System are very different of the way planetary systems evolved in different parts of our Galaxy”, said Salazar Manzano.
Comets are often nicknamed “dirty snowballs“, in part due to its high water content – water that contains frozen chemical records of the environment in which they formed.
In addition to the ordinary water (H₂O)comets contain a molecular variant called deuterated water (HDO), in which a hydrogen atom is replaced by deuterium, a hydrogen atom with an additional neutron.
In comets in the Solar System, there is approximately one semi-heavy water molecule for every ten thousand molecules of ordinary water. In 3I/ATLAS, this proportion is at least 30 times greater – and more than 40 times greater than that found in Earth’s oceans.
Notably, ordinary water (H₂O) itself fell below ALMA’s detection threshold during these observations.
The team determined the D/H ratio indirectlydetecting HDO directly and inferring water production rate through excitation of methanol lines – a sophisticated modeling approach that demonstrates ALMA’s unique analytical capabilities.
This proportion points to a origin in an exceptionally cold environment and chemically distinct.
“The chemical processes that lead to the increase in deuterated water are very temperature sensitive and typically require environments colder than approximately 30 Kelvin,” explained Salazar Manzano.
The ratio was established when the comet’s home system formed and was preserved, intact, throughout its interstellar journey. ALMA played a key role in this discovery.
“A Most instruments cannot point to the Sunbut radio telescopes like ALMA can. We were able to observe the comet a few days after perihelion, precisely when it emerged from its transit behind the Sun. This allowed us to constrain these molecules, which would not have been possible with other instruments”, highlights Paneque-Carreño.
In addition to being a chemical fingerprint of a distant planetary systemthe HDO/H₂O relationship has a special cosmological significance: the abundances of deuterium and hydrogen were defined during the Big Bang itselfmaking this measurement a fundamental and unique probe of the conditions under which other worlds are born.
“Each interstellar comet brings with it a bit of its history, fossils from other places. We don’t know exactly wherebut with instruments like ALMA we can begin to understand the conditions there and compare them to our own,” said Paneque-Carreño.
