Another piece of evidence that comets delivered molecules for life to Earth has been unveiled by an instrument on the Rosetta spacecraft.
The Rosetta Orbiter Spectrometer for Ion and Neutral Analysis, or ROSINA, sniffed the dusty halo around Comet 67P/Churyumov-Gerasimenko and found the amino acid glycine as well as phosphorus, necessary for DNA and cell membranes.
“This is the first unambiguous detection of glycine in the thin atmosphere of a comet,” says Kathrin Altwegg, principal investigator of the ROSINA instrument at the University of Bern.
How Earth’s early life originated is a matter of debate. Some scientists think simple organic compounds were ferried to the planet’s oceans on comets and asteroids barrelling around the Solar System, so missions to comets have sniffed for these prebiotic molecules.
It doesn’t help that the amino acid glycine, one of the building blocks of proteins and DNA, is also very difficult to detect. It may the be simplest amino acid, but it sublimates (switches from solid to gas phases) at a little under 150 °C – much warmer than the frozen nucleus of a comet.
So scientists’ best bet is to see if glycine molecules are found in a comet’s coma – the nebulous dusty cloud that forms when it moves close enough to the Sun.
As a comet approaches and warms, chunks of its central solid part, called the nucleus, sublime and form a thin atmosphere. The coma can extend to diameters as wide as the Sun, and reach temperatures much higher than the frozen nucleus.
But the search for cometary glycine has largely been fruitless. No signs were found in two of the most widely studied comets, Hale-Bopp and Hyakutake.
Excitement built in 2009, when it was reported that samples brought back from comet Wild 2 by NASA’s Stardust spacecraft contained traces of glycine, along with glycine’s precursor molecules methylamine and ethylamine.
But doubts were raised when the researchers declared other amino acids brought back by Stardust were probably contaminants from Earth.
ROSINA which sorts and “weighs” molecules to determine their atomic makeup, has none of those contamination issues. It makes measurements in orbit and shoots data back to Earth.
In October 2014, just before Rosetta dropped the Philae lander onto the comet’s surface, ROSINA detected glycine 10 kilometres from the comet’s surface. At that point, the comet was three times further from the Sun as Earth (three astronomical units).
But when Comet 67P moved to two astronomical units from the Sun in March 2015, glycine was picked up by Rosetta again – this time, 15 kilometres from the comet’s surface.
As as the comet neared the Sun, its coma grew. Rosetta was forced to retreat to safety and orbit at greater distances.
In July and August 2015, when the comet’s orbit was as close to the Sun as it would take it (1.2 astronomical units), ROSINA sniffed glycine 200 kilometres from the solid nucleus.
What about other amino acids? Past studies have found 80 different amino acids in meteorites. But ROSINA didn’t pick up any other amino acids from Comet 67P.
This isn’t surprising, the researchers write, because glycine is the only amino acid that can form without liquid water. As comets spend most of their time in the cold reaches of the Solar System, opportunities to make other amino acids are exceptionally rare.
Along with glycine, ROSINA detected ionised phosphorus – an integral part of cell membranes, cellular energy sources and genetic code.
Phosphorus is another molecule that’s been inconsistently seen on comets: traces were found in the dusty signature of Halley’s comet, but not in Stardust grains.
While ROSINA measurements showed a distinct peak in the mass of ionised phosphorus, the researchers admit they don’t know how it formed. They saw no clear signs of chemical compounds such as phosphorus nitride or methylidynephosphane, but speculate phosphine, PH3, may be ionised phosphorus’ parent molecule on Comet 67P.
And there’s more. Hydrogen sulphide and hydrogen cyanide were also among the molecules detected in Comet 67P’s coma.
“Demonstrating that comets are reservoirs of primitive material in the Solar System, and vessels that could have transported these vital ingredients to Earth, is one of the key goals of the Rosetta mission, and we are delighted with this result,” Rosetta project scientist Matt Taylor says.
The work was published in Science Advances.
Belinda Smith is a science and technology journalist in Melbourne, Australia.
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