The heart of the Milky Way is apparently a hotspot for molecules that combine to form RNA.
A new survey of the thick, molecular clouds that shroud the galactic center has revealed the presence of a wide range of nitriles – organic molecules that are often toxic in isolation, but also constitute the building blocks of molecules essential for life.
The increase in prebiotic molecules (molecules involved in the emergence of life) identified in the galactic center, particularly those associated with RNA, has implications for our understanding of how life emerges in the Universe – and how it did so here on Earth.
"Here we show that the chemistry that takes place in the interstellar medium is able to efficiently form multiple nitriles, which are key molecular precursors of the 'RNA World' scenario," explained astrobiologist VÃctor Rivilla of the Spanish National Research Council and the National Institute of Aerospace Technology in Spain.
Precisely how life emerged on Earth is a mystery whose bottom scientists are extremely keen to reach. That information will yield important clues to discovering exoplanets likely to harbor living organisms.
One version is that RNA emerged first from the metaphorical ooze, self-replicating and diversifying all on its own; this is what's called the RNA World Hypothesis.
We're not likely to ever get direct evidence from Earth, but we can put together more and more clues to figure out how plausible and likely this scenario is. One of the questions raised by this hypothesis is about the source of RNA prebiotic molecules such as nitriles. Were they here on Earth from the start, or could they have been carried in from space on meteorites and asteroids?
We know the inner Solar System, including Earth, was subject to a period of intense asteroid bombardment very early in its history. We've also found prebiotic molecules on meteors, comets, and asteroids hanging around the Solar System today. And where do meteors, comets and asteroids get them?
Well, probably the clouds they were born in: cold molecular clouds that give birth to stars. Once a star finishes forming from a section of cloud, the cloud leftovers go on to form everything else in a planetary system – planets, comets, asteroids, dwarf planets, and whatever else might be lurking about.
The Solar System's birth cloud is long gone, but the center of the galaxy is thick with molecular clouds. It's called the Central Molecular Zone, and scientists have found a bunch of prebiotic molecules hanging around there.
One particular cloud, named G+0.693-0.027, is especially interesting. There's no evidence of star formation there yet, but scientists believe that a star or stars will form there in the future.
"The chemical content of G+0.693-0.027 is similar to those of other star-forming regions in our galaxy, and also to that of Solar System objects like comets," Rivilla said.
"This means that its study can give us important insights about the chemical ingredients that were available in the nebula that give rise to our planetary system."
The researchers used two telescopes to study the spectrum of light coming from the cloud. When certain elements or molecules absorb and re-emit light, this can be seen on the spectrum as a darker or lighter line. Interpreting these absorption and emission lines can be tricky, but it can also be used to identify which molecules are present: each one has its own spectral signature.
By carefully studying and analyzing emission features from G+0.693-0.027, Rivilla and his colleagues identified a range of nitriles, including cyanic acid, cyanoallene, propargyl cyanide, and cyanopropyne. They also made tentative detections of cyanoformaldehyde, and glycolonitrile.
Previous observations of G+0.693-0.027 revealed the presence of cyanoformaldehyde, and glycolonitrile. This suggests that nitriles are among the most abundant chemical families in the Milky Way, and that the most basic building blocks for RNA can be found in the clouds that give birth to stars and planets.
But there is – of course, as there always is – more work to be done.
"We have detected so far several simple precursors of ribonucleotides, the building blocks of RNA," explained astrobiologist Izaskun Jiménez-Serra, also of the Spanish National Research Council and the National Institute of Aerospace Technology.
"But there are still key missing molecules that are hard to detect. For example, we know that the origin of life on Earth probably also required other molecules such as lipids, responsible for the formation of the first cells. Therefore we should also focus on understanding how lipids could be formed from simpler precursors available in the interstellar medium."
The research has been published in Frontiers in Astronomy and Space Sciences.
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