Tracing back one of the ingredients of life

Read time: 4 minutes

If you bake a cake, you need all kinds of ingredients, like flour, butter, sugar and eggs. You can’t make
something from nothing! Likewise, when life first emerged on our planet, it needed its own
ingredients. Some of those, like carbon and oxygen, are readily available in the universe. But for
other ingredients of life, it’s not so clear where they come from. One of those ingredients is
phosphorous.
Phosphorous is an important part of DNA – the fundamental molecule of all living organisms. Every
single cell in your body contains countless DNA molecules, and they all contain phosphorous. But
where did the phosphorous come from, and how did it arrive here on Earth?
Using observations of ALMA and of the European space probe Rosetta, astronomers are now
beginning to find the answer to that question. In a sense, they have traced back the history of the
phosphorous in our bodies.
Phosphorous is produced by nuclear reactions in the interiors of stars. At the end of their lives, stars
shed their outer layers, or they explode altogether. As a result, the phosphorous atoms are blown
into space. ALMA has now discovered that they can team up with oxygen atoms to form molecules of
phosphorous monoxide.
According to the observations of ALMA, these molecules form under the influence of energetic
radiation and shock waves from young, massive stars. These stars blow out empty regions in the
surrounding clouds of gas and dust. The phosphorous monoxide molecules form preferentially on the
inner walls of these empty regions.
ALMA also revealed that phosphorous monoxide is the most abundant phosphorous-bearing
molecule in the universe. So how did it finally arrive on Earth? That’s where the observations of
Rosetta came in. The European spacecraft carried out close-up studies of a comet in our own solar
system.
Comets are the icy building blocks of planets. Rosetta discovered that the nucleus of comet 67P
contains molecules of phosphorous monoxide. Apparently, when the Sun was born, these molecules
became trapped in the first, frozen clumps of matter that formed in the outer parts of the
surrounding disk.
So here’s the story. Phosphorous monoxide molecules form in interstellar space, in the neighborhood
of massive stars. They become part of the clouds of gas and dust that give birth to stars like our own
Sun. These stars are surrounded by flat, rotating disks from which planets are eventually born. The
phosphorous monoxide molecules get trapped in icy comets that clump together in the outer parts
of these disks.
Finally, when comets crash into newborn planets, the phosphorous ends up on those planets,
including on our own Earth. There, it is available as an ingredient for living organisms like you and
me!

What?

ALMA studied the distribution of phosphorous-bearing molecules in a nebula known as AFGL5142. In
this huge cloud of gas and dust, new massive stars are forming. ALMA found two phosphorous-
bearing molecules: phosphorous monoxide (abbreviated as PO) and phosphorous nitride (PN). PO
molecules turned out to be the most abundant. The European Space Agency’s Rosetta spacecraft
visited a comet known as 67P/Churyumov-Gerasimenko (named after its two discoverers) and used
its ROSINA instrument to study the composition of the comet. 67P also turned out to contain
phosphorous monoxide molecules. If the same is true for comets in general, cometary impacts may
have seeded the Earth with phosphorous, one of the building blocks of life.

Who?

The study of phosphorous-bearing molecules in space and in comets was carried out by a large
international team of astronomers and space scientists, led by Victor Rivilla of the Arcetri
Asstrophysical Observatory in Italy and Kathrin Altwegg of the University of Bern in Switzerland.
Victor and Kathrin worked together with eleven colleagues from many European countries, and with
the science team that operated the ROSINA instrument on the Rosetta spacecraft. The team
published their results in the Monthly Notices of the Royal Astronomical Society.

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