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Astronomers Trace Journey of Phosphorus from Stellar Nurseries to Solar System Comets

To understand how molecules containing phosphorus — one of life’s building blocks — are formed in star-forming regions, a team of astronomers has analyzed ALMA (Atacama Large Millimeter/Submillimeter Array) observations of the massive star-forming region AFGL 5142, combined with an analysis of the data of the comet 67P/Churyumov-Gerasimenko taken with the ROSINA (Rosetta Orbiter Spectrometer for Ion and Neutral Analysis) instrument onboard ESA’s Rosetta spacecraft. Their research shows, for the first time, where phosphorus-bearing molecules form, how this element is carried in comets, and how a particular molecule may have played a crucial role in starting life on Earth.

Astronomers Trace Journey of Phosphorus from Stellar Nurseries to Solar System Comets

Phosphorus, present in our DNA and cell membranes, is an essential element for life as we know it. But how it arrived on the early Earth is something of a mystery.

“Life appeared on Earth about 4 billion years ago, but we still do not know the processes that made it possible,” said Dr. Víctor Rivilla, an astronomer at the INAF-Osservatorio Astrofisico di Arcetri.

With the power of ALMA, which allowed a detailed look into the star-forming region AFGL 5142, astronomers could pinpoint where phosphorus-bearing molecules, like phosphorus monoxide, form.

The observations showed that phosphorus-bearing molecules are created as massive stars are formed.

Flows of gas from young massive stars open up cavities in interstellar clouds. Molecules containing phosphorus form on the cavity walls, through the combined action of shocks and radiation from the infant star.

Phosphorus monoxide is the most abundant phosphorus-bearing molecule in the cavity walls, according to the study.

After searching for this molecule in star-forming regions with ALMA, Dr. Rivilla and colleagues moved on to 67P/Churyumov-Gerasimenko. The idea was to follow the trail of these phosphorus-bearing compounds.

If the cavity walls collapse to form a star, particularly a less-massive one like the Sun, phosphorus monoxide can freeze out and get trapped in the icy dust grains that remain around the new star.

Even before the star is fully formed, those dust grains come together to form pebbles, rocks and ultimately comets, which become transporters of phosphorus monoxide.

The ROSINA instrument on Rosetta, which collected data at 67P/Churyumov-Gerasimenko for over two years, had already found hints of phosphorus at the comet in 2016, but it was not clear what molecule had carried it there.

After the ALMA observations suggested that phosphorus monoxide would be a very likely candidate, the scientists went back to their data and eventually found evidence of this molecule at the comet.

“Phosphorus monoxide would be a very likely candidate, so I went back to our data and there it was,” said University of Bern’s Dr. Kathrin Altwegg, the principal investigator for ROSINA.

This first sighting of phosphorus monoxide on a comet helps astronomers draw a connection between star-forming regions, where the molecule is created, all the way to Earth.

“The combination of the ALMA and ROSINA data have revealed a sort of chemical thread during the whole process of star formation, in which phosphorus monoxide plays the dominant role,” Dr. Rivilla said.

“Phosphorus is essential for life as we know it. As comets most probably delivered large amounts of organic compounds to the Earth, the phosphorus monoxide found in 67P/Churyumov-Gerasimenko may strengthen the link between comets and life on Earth,” Dr. Altwegg said.

This intriguing journey could be documented because of the collaborative efforts between astronomers.

“The detection of phosphorus monoxide was clearly thanks to an interdisciplinary exchange between telescopes on Earth and instruments in space,” Dr. Altwegg said.

“Understanding our cosmic origins, including how common the chemical conditions favorable for the emergence of life are, is a major topic of modern astrophysics,” said Dr. Leonardo Testi, ESO astronomer and ALMA European Operations Manager.

The research is published in the Monthly Notices of the Royal Astronomical Society.

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V.M. Rivilla et al. 2020. ALMA and ROSINA detections of phosphorus-bearing molecules: the interstellar thread between star-forming regions and comets. MNRAS 492 (1): 1180-1198; doi: 10.1093/mnras/stz3336

This article is based on text provided by the European Southern Observatory.

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