03 August 2018 News

Plate tectonics not needed for life, but UV light is

An artists impression of Kepler 452b - Earth's 'cousin.' Image: NASA
An artists impression of Kepler 452b - Earth's 'cousin.' Image: NASA

Searching for life in the Universe just got a lot easier as a study published this week has identified key conditions that could help scientists narrow down the best places to search, while another study opens the possibilities of finding more habitable planets than previously thought.

It stands to reason that one of the best places to look for life elsewhere is on a planet that is similar to Earth, as let’s face it, once it got going on our world, life has been pretty successful at clinging to existence whatever has been thrown at it. But life on our planet is not just made possible by conditions on Earth, it is made possible by the universal average G-type main sequence star we orbit.

By looking at the type and strength of light given off by its host star, researchers from the University of Cambridge and the Medical Research Council Laboratory of Molecular Biology (MRC LMB) have identified a group of planets where the same chemical conditions that may have led to life on Earth exist.

The type of light is ultraviolet (UV) light - the wavelength of light that burns your skin if you don’t cover up sufficiently on hot days. UV light has a shorter wavelength than visible light and is therefore a more harmful type of radiation (to humans at least), but given enough of it, it could kick-start life on other planets by powering a series of chemical reactions that produce the building blocks of life as we know it; the same way it likely gave life on Earth a push in the right direction.

The reactions that gave rise to the first RNA – the close relative of DNA which most biologists believe was the first molecule of life to carry information – springs from an usual source; hydrogen cyanide – a colourless, extremely poisonous and flammable acid that is used for fumigation, in the manufacture of plastics and nitrites and in conflict as a chemical warfare agent.

Hydrogen cyanide on Earth was thought to be produced when nitrogen in the atmosphere interacted with carbon that slammed into the young Earth in the form of meteorites. Powered by the UV light from the sun, the chemicals created from these interactions generated the building blocks of ribonucleic acid (RNA) which is present in all living cells.

Stars that are around the same temperature as our sun emit enough UV light for these reactions to be effective, while cool stars do not, except if they have frequent powerful solar flares to jolt the chemistry forward step by step.

Presently, several planets detected by the Kepler telescope, including Kepler 452b, a planet that has been nicknamed Earth's 'cousin', reside in a zone favourable to these conditions I.e they receive enough light to activate the chemistry and could have liquid water on their surfaces. This zone is called the abiogenesis zone and although they are too far away at the moment to probe with current technology, next-generation telescopes, such as NASA's TESS and James Webb Telescopes, will hopefully find many more just like them.

"There's an important distinction between what is necessary and what is sufficient. The building blocks are necessary, but they may not be sufficient: it's possible you could mix them for billions of years and nothing happens. But you want to at least look at the places where the necessary things exist,” said Dr Paul Rimmer, the paper's first author.

Meanwhile, another paper published this week by geoscientists at Penn State University, US, say that plate tectonics – the driving force that shapes our planet and a feature long assumed to be a requirement for suitable conditions for life – are in fact not required.

Temperatures on Earth are kept to a reasonable habitable level by the release and capture of carbon dioxide (CO2). After it is released from the ground, it is then returned to the subsurface when it has been sequestered into surface rocks and sediment and then pushed underground at certain plate boundary locations (convergent boundaries).

The movement of the plates across Earth’s outer shell is what keeps the CO2 on our planet in equilibrium. Too much of it and we all boil and too little and we all freeze. Earth is the only planet we know of with plate tectonics, so without them, how does a planet recycle its CO2?

Planets where the crust is one giant, spherical plate floating on a mantle rather than being divided up into several plates like ours are known by the rather apt but unattractive name of stagnant lid planets.

To understand how these planets function compared to our own, two researchers, Bradford Foley and Andrew Smye, created a computer model and ran hundreds of simulations to vary a planet's size and chemical composition. This determined how much heat a planet’s climate could retain based on its initial heat budget, or the amount of heat and heat-producing elements present when a planet forms.

Their results showed that stagnant lid planets can sustain conditions for liquid water for billions of years. Some as many as 4 billion years, roughly Earth's life span to date.

"You still have volcanism on stagnant lid planets, but it's much shorter lived than on planets with plate tectonics because there isn't as much cycling," said Smye. "Volcanoes result in a succession of lava flows, which are buried like layers of a cake over time. Rocks and sediment heat up more the deeper they are buried."

Degassing, the process whereby carbon dioxide gas can escape from rocks and make its way to the surface, still occurred if there was enough heat and pressure and was also found to increase depending on what types and quantities of heat-producing elements are present in a planet up to a certain point, said Foley.

"There's a sweet spot range where a planet is releasing enough carbon dioxide to keep the planet from freezing over, but not so much that the weathering can't pull carbon dioxide out of the atmosphere and keep the climate temperate," he said.

"One interesting take-home point of this study is that the initial composition or size of a planet is important in setting the trajectory for habitability," Smye said. Without plate tectonics, knowing what and how much of a certain element is present could help determine a planet's potential to sustain life as its fate is set from the outset of its birth.

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