It was only a matter of 700 million years or so after Earth formed and its surface cooled and solidified that life began to flourish on Earth. All studies suggest that life requires water – and we know from rocks on Earth that the climate in this distant past was sufficiently warm for liquid water to be present. But therein lies a mystery.
The young sun emitted only about 70% of the radiation it emits today, making it unlikely that it could have heated the Earth enough for liquid water to exist on the surface. But now new research, published in Nature Geoscience, suggests that the sun must have been a lot more active than we previously thought. The work has significant implications for theories about the production of the simple molecules required for life – on Earth and beyond.
The puzzle of how water could exist on the Earth’s surface at so early a time is dubbed the faint young sun paradox and has been debated for over 40 years. There have been many different solutions proposed, but they all require significant levels of greenhouse gases to be available in the early atmosphere. However, this is not entirely consistent with what has been observed in the geological record. For instance, the level of carbon dioxide (CO2) required in the atmosphere to enhance surface temperatures is greater than seems to have been present based on evidence from fossil soils. Nevertheless, assuming that these gases were present, researchers have developed a number of computational models outlining how atmospheric chemistry at the time could have helped kick-start life.
But the new research has gone about doing this modelling in a different way by changing the starting assumption about the sun’s activity. The sun was usually considered to be pretty similar to how it is today, apart from having a slightly lower heat flux. But the new study used data from the exoplanet-hunting mission Kepler, which has also been recording the activity of different types of stars, to work out that this was probably not the case.
In fact, the number and frequency of solar flares from young sun-like stars seen by Kepler indicate that our newly-formed sun must have been much more active than previously thought. Although generally radiating at about 70% of its current level, the early sun would have been subject to frequent and violent solar flares or coronal mass ejections (CME). During such an event, there is the potential for the energetic particles emitted from the sun to power through the magnetic field which usually protects the Earth.
While there has only been one such recorded event in the past 30 years, the researchers calculated that the young sun may have produced at least one CME per day that was in the direction of Earth. This means that the amount of energy dumped into the Earth’s atmosphere would have been sufficient to fuel a whole series of chemical reactions.
The cocktail of life
It is widely assumed that the atmosphere at the time – as it is now – was dominated by nitrogen. The enhanced flux of energetic particles from the sun would split the relatively unreactive nitrogen molecule into two highly reactive nitrogen atoms. These could then go on to react with almost any molecule proposed to be in the early atmosphere – including carbon monoxide (CO), carbon dioxide (CO2), water (H2O), methane (CH4), ammonia (NH3) and others.
The final products of many of these reaction pathways are hydrogen cycanide (HCN) and nitrous oxide (N2O). The latter, also known as laughing gas, is a powerful greenhouse gas – important for allowing liquid water to exist. The former, hydrogen cycanide, is a key precursor molecule for the formation of amino acids.
In this way, the research solves the problems of maintaining the temperature of the Earth’s surface, and the production of a biologically-significant feedstock for life on Earth at the same time. The authors go on to suggest that a rain of other nitrogen-bearing molecules onto the surface would also provide fertiliser for a new biology.
The result is exciting and definitely a case of solving two problems for the price of one – with the bonus of it being applicable to some of the exoplanets being discovered by the Kepler Space Telescope.