A daily dose of particles sprayed from the young Sun could have produced molecules necessary for life on early Earth – and kept the planet warm enough for it to gain a foothold.
NASA scientists, led by Vladimir Airapetian at the Goddard Space Flight Centre in Maryland, analysed Kepler Space Telescope images of so-called superflares – vast blasts of energetic particles barrelling out of stars.
In a paper published in Nature Geoscience, they suggest the Sun – which was dimmer four billion years ago but more active than it is today – regularly belched superflares, each with the power of a million billion atomic bomb blasts, and peppered the Earth with energetic particles.
The blasts were strong and frequent enough that they penetrated the Earth’s protective magnetic field and interacted with molecules in the atmosphere, changing its chemistry and making it suitable for life.
This also explains the “faint young Sun” paradox – back then, our star was only around 70% today’s brightness, and would not have spat out enough heat to keep Earth warm enough so water was liquid on its surface.
“That means Earth should have been an icy ball,” Airapetian says. “Instead, geological evidence says it was a warm globe with liquid water.”
Some 3.8 billion years ago, scientists think the atmosphere was dominated by molecular nitrogen gas – around 80% – with 20% carbon dioxide and trace amounts of methane.
Molecular nitrogen is a very stable molecule. Only very energetic events, such as lightning, can split it in two – or solar particles in the form of protons, Airapetian and colleagues write.
As protons smashed into gases in the atmosphere, they split molecular nitrogen, carbon dioxide and methane to produce compounds such as nitrous oxide and hydrogen cyanide.
Nitrous oxide acted as a greenhouse gas, 300 times more powerful than carbon dioxide, keeping the planet warm. And hydrogen cyanide was a source of nitrogen for amino acids – the building blocks of DNA.
Superflares aren’t solely ancient phenomena. A tree ring study in Japan showed evidence for one such event around 1,200 years ago. But billions of years ago, the blasts would have been daily occurrences – so active was the Sun.
And if Earth was showered by superflare particles, so too was Mars. Ramses Ramirez, a planetary scientist at Cornell University in New York, writes in a News and Views article that the Red Planet was also warm and wet at around the same time and may have also profited from solar showers.
“The findings may have implications for the climate and potential biology of terrestrial exoplanets orbiting very young Sun-like stars.”