A stormy young Sun may have provided the early Earth with the ingredients and climate needed to kick-start life. That’s the claim of NASA scientists, who say that powerful solar eruptions may have warmed the Earth at a time when the Sun was relatively cool. They also say that the Earth’s life-giving supply of nitrogen was synthesized by energetic particles from the Sun.

Having a clear idea of the necessary conditions for life to emerge on Earth is a key scientific goal – both to trace our own origins and to better gauge which of the many thousands of known exoplanets may hold life. A particular sticking point in developing a clear picture of Earth’s early evolution was that four billion years ago, when life-friendly conditions were developing, the young Sun wasn’t luminous enough to warm our planet. Despite its storminess, the Sun was 30% dimmer then than it is today.

Cool and stormy

“Back then, Earth received only about 70% of the energy from the Sun than it does today,” says solar scientist Vladimir Airapetian at NASA’s Goddard Space Flight Center in Maryland. “That means Earth should have been an icy ball. Instead, geological evidence says it was a warm globe with liquid water. We call this the ‘faint young Sun paradox’.”

Another problem lies with the fact that a key component for the building blocks for life is nitrogen (N) – but at that time, only unreactive molecular nitrogen (N 2 ) was present in the atmosphere. A very energetic process would have been necessary to break apart the molecular nitrogen into atomic nitrogen, allowing it to recombine into more biologically suitable forms. The latest research by Airapetian and colleagues shows that charged particles from the solar storms could have both broken apart the nitrogen and provided the heat required for life.

For clues on how the young Sun behaved, scientists study Sun-like stars in our galaxy at different ages. Apart from confirming that the young Sun would have been relatively faint, the studies also show that young stars frequently produce powerful flares. These are giant bursts of light and other radiation that are similar to the flares we see on the Sun today. Such flares are often accompanied by huge clouds of solar material, called coronal mass ejections (CMEs), being shot out into space.

Superflare showers

NASA’s Kepler mission has found Sun-like stars that are young, and many of these are seen to produce “superflares” – enormous explosions so rare today that we only experience them once every 100 years or so. But Kepler’s data show these young stars producing as many as 10 superflares a day. Based on these observations, Airapetian and colleagues say that clouds of charged particles ejected due to a young Sun’s stormy outbursts triggered changes in the early Earth’s atmospheric chemistry.

The team simulated how the superflares would interact with our planet, and found that they would have distorted the Earth’s magnetic field – which was also weaker at the time – by creating large gaps around the poles. These gaps provided gateways for the energetic solar particles to penetrate the atmosphere. “Our calculations show that you would have regularly seen auroras all the way down in South Carolina,” says Airapetian.

Hotting up

The charged particles would travel down the magnetic-field lines and collide with the molecular nitrogen as well as the carbon dioxide, which was split into carbon monoxide and oxygen. The free nitrogen and oxygen atoms would have then combined to form nitrous oxide (N 2 O) – a powerful greenhouse gas – and hydrogen cyanide (HCN). Indeed, nitrous oxide is some 300 times more powerful at warming the atmosphere than carbon dioxide. The team’s calculations showed that if even 1% of the carbon dioxide in the atmosphere was N 2 O, it would be sufficient to warm up the Earth’s surface to a temperature that could support liquid water, as well as the beginnings of life. “Changing the atmosphere’s chemistry turns out to have made all the difference for life on Earth,” says Airapetian.

The researchers also believe that the HCN could have provided a nitrogen source for biological molecules such as amino acids. Indeed, the daily dose of solar particles may also have provided the huge amount of energy needed to create complex molecules such as RNA and DNA that eventually seeded life.

At the same time, constant solar showers and radiation could also be quite detrimental. The magnetic onslaught could even rip off a planet’s atmosphere if its magnetosphere is too weak. Determining where the balance lies will help us to determine which extrasolar star systems could potentially harbour life. “We want to gather all this information together – how close a planet is to the star, how energetic the star is, how strong the planet’s magnetosphere is – to help search for habitable planets around stars near our own and throughout the galaxy,” says team-member William Danchi. Working with others in related fields, the researchers hope to come up with a “robust description of what the early days of our home planet looked like – and where life might exist elsewhere”.

The work is published in Nature Geoscience.