Key molecular building blocks may have come from space

A key molecular building block for self-replicating organisms may have originated in deep space, scientists say. 

Phosphorus, particularly in the form of phosphates, is one of a handful of elements crucially important for life. Not only is it a critical component of DNA, the molecule that encodes our genes and allows everything from plants to people to pass them on to their offspring, but it’s important for other cellular processes, ranging from the formation of cell membranes to the shuttling of energy within cells via adenosine triphosphate (ADP) and adenosine triphosphate (ATP). 

On modern Earth, phosphates are an important component of fertilisers. They can also be a problem when excess amounts wash into lakes and rivers, creating ecosystem-destroying algae blooms. But in the prebiotic universe, phosphates were in short supply, and scientists have long been unsure where they came from. 

Phosphorus itself, astrophysicists believe, formed in the heart of exploding stars, and then spread to the gas and dust clouds from which the next generation of stars and solar systems condensed. But how that phosphorus turned into phosphates has been a mystery.{%recommended 4911%}

In a paper published in the journal Nature Communications, however, a team led by Ralf Kaiser, a physical chemist at the University of Hawaii at Manoa, US, thinks it’s found an answer.

The process, Kaiser says, begins with phosphine (PH3), a chemical found not only in interstellar gas clouds, but also in the atmospheres of Jupiter and Saturn. In addition, it has been found in gas jets from comet 67P/Churyumov-Gerasimenko, visited between 2014 and 2016 by the European Space Agency’s Rosetta probe. 

Phosphine, however, is highly toxic, a deterrent to prior astrobiological studies. “If you breathe it, you die,” Kaiser says. 

But with proper precautions, he adds, it’s easily manageable for chemists accustomed to working with such things.  

Using a vacuum chamber designed to mimic outer space, and working at temperatures as cold as a few degrees above absolute zero, Kaiser’s team created nanometre-sized particles that replicated interstellar grains, coated with carbon dioxide, water, and phosphine. They then bombarded them with high-energy electrons that simulated the effect of galactic cosmic rays. 

Analysing the products of this, they found three important phosphate-like chemicals, phosphonic acid (H₃PO₃), phosphoric acid (H₃PO₄), and pyrophosphoric acid (H₄P₂O₇). 

Once formed in the cold reaches of interstellar space, Kaiser says, such nanoparticles could have coalesced into larger objects, such as comets and ice-rich asteroids. These could then have rained down on Earth, late in the planet’s formation, seeding the world with these important precursors for life.  

Phosphine isn’t the only precursor from which these chemicals might have been formed. Some scientists believe they were generated on Earth in warm-water reactions with another primordial phosphorous substance, a mineral known as schreibersite, commonly found in meteorites and, presumably, in ancient asteroids. 

“Thirteen years ago, we had a paper that showed that if you put this mineral in water, the water reacts to create very similar compounds to what we see in this paper,” says Matthew Pasek, a cosmo-chemist at the University of South Florida, US. 

But the new study, he notes, is exciting because it indicates that these compounds could also have formed fairly in space, without the need for liquid water. “It shows there’s something pretty neat with phosphorus as it evolves, especially as you start looking off the surface of the Earth,” he says.

The next step, Kaiser suggests, is to look for ways in which these primordial phosphates might have become linked to carbon-containing molecules to form more complex phosphates, such as ADP, ADP, and the building blocks of DNA and cell membranes.

Meanwhile, the finding is also of interest for scientists hoping to find life on other worlds.

Chris McKay, an astrobiologist at NASA Ames Research Centre in California, US, notes that a paper to be published in the October 2018 issue of The Astronomical Journal has found that phosphorus might be a limiting factor for the possibility of finding life on worlds with sub-surface oceans, such as those believed to exist on several outer solar system moons, including Enceladus and Europa.

“Anything to do with phosphorous is of interest in terms of the search for life,” McKay says.