It’s not clear how life started on Earth, although we do have some solid leads. It’s certain that with a little heat and some rather simple and common chemicals, amino acids – a vital cornerstone of biology – can quickly form. With this in mind, the most probable crucible for DNA and simple bacteria-like lifeforms are deep-sea hydrothermal vents, where today microbes thrive without sunlight.
A new Geoscience Frontiers study has come out of the blue to challenge this popular hypothesis. According to its Japanese co-authors, a natural nuclear reactor kickstarted life on Earth. This may sound a little outlandish, but it’s actually one of the most plausible ideas to date – and one that could explain alien life elsewhere in the Solar System too.
Shortly after the Earth formed 4.5 billion years ago, it began to cool, but still remained a genuinely hellish landscape. The oceans were an age away from taking shape – water mostly remained as a superheated gaseous vapor for several hundred million years – and asteroids and comets pummeled the surface. This was appropriately named the Hadean Eon.
Back then, though, the Earth had far more uranium-235 than it does now. This decays, which releases radiation and heat at a rather constant tick.
This heat is not insubstantial. It has a huge influence on alien worlds and moons like Pluto and potentially Saturn’s Enceladus, both of which have cryovolcanic features. On Earth, around half of the heat that goes into generating volcanism and the movement of tectonic plates comes from radioactive decay.
During the Hadean, this plentiful supply of U-235 meant that it was “cooking” the basic carbon, potassium, and nitrogen-based constituents of Earth’s primitive volcanic atmosphere. In fact, the radiation given off by these isotopes was the perfect type to promote chemical chain reactions, the sort that lead to the formation of amino acids, RNA and DNA.
According to the research team from the Riken and the Tokyo Institute of Technology, this heat source would have fueled geysers rich in organic chemistry – the type that you also get in deep-sea hydrothermal vents.
The advantage of a nuclear-powered geyser though is that the temperature of the water column never reaches boiling point. In many cases, long-chain organic compounds break down in water that greatly exceeds this temperature, which makes hydrothermal vents a touch-and-go place for early, fragile life to develop.
So how did the researchers work this out? Essentially, they came up with a mathematic model that looked at the best available evidence.
One piece of evidence they referred to were the famous Miller-Urey experiments. Conducted from 1952 onwards, these two pioneering researchers added several gases that were found in the early Earth’s atmosphere – water, methane, ammonia and hydrogen.
Adding a spark to simulate lightning strikes, they found that amino acids spontaneously appeared. In 2007, after Miller died, the original vials were examined and found to contain more amino acid types than have ever been detected in natural conditions.
This new study notes that the energy of these “lightning strikes” is identical to that of their proposed U-235 nuclear reactor, at least at the peripheries – and unlike lightning, which is fairly sterilizing, radiation provides a slow burn heat source.
There’s more to the model than this, but the premise is undoubtedly solid. Radiation in the minds of most is thought of as a life-killer, but it’s rather remarkable to think that it may have started life in the first place.
Of course, if this model is correct, then astrobiologists may look to the icy geysers and seas of other worlds, and wonder if the same story of life played out there.