Meteorites' mechanical energy might have created building blocks of life on Earth

Did a cosmic impact kick-start amino acid formation?

It is one of the big unsolved mysteries: how did life on Earth start? Researchers in Germany are now arguing that a meteorite impact could have created the molecules that gave life its big break. Their experiments are the first to show that the mechanical energy released during an impact could have transformed simple chemicals into amino acids.1

There are various theories on how simple chemicals were transformed into amino acids, lipids, carbohydrates and nucleic acids on ancient Earth. These include submarine hydrothermal vents 3.5 billion years ago, or volcanic activity even earlier on, during the chaotic Hadean era.

However, José Hernández from Aachen University and colleagues found that a meteorite strike could have generated the first amino acids from a few simple components. Mechanical forces and friction generated during such an event can catalyse chemical reactions – a concept exploited in mechanochemistry. Here, ingredients are ground together without any solvent or heating, usually in a ball mill.

The idea that friction between rocks could have started reactions on early Earth came to Hernández a few years ago during a holiday in Rome. ‘I was standing in front of the Coliseum. Everybody was taking pictures but I was looking at the floor and saw these very round stones,’ he recounts. He took a few of them back to Germany and found that they could replace metal milling balls in his mechanochemical experiments.2

‘There have been many studies that have shown meteorites as shuttles for material that ends up on Earth,’ Hernández says. ‘But we were interested if the mechanical energy of those landings could have activated certain chemical systems.’ The team simulated this by milling an aldehyde and an amine with potassium cyanoferrate and silicon dioxide. An hour and a half later, the reagents had converted into α-aminonitriles, precursors to amino acids.

At first, the researchers were surprised that the reaction worked at all, since it was missing one key ingredient: cyanide. Although potassium cyanoferrate is a potential source that could have existed on early Earth, it is very stable and only releases hydrogen cyanide at extreme temperatures.

‘But mechanical milling is able to challenge this high activation energy,’ enthuses Judit Šponer, who researches prebiotic chemistry at the Academy of Sciences of the Czech Republic, but wasn’t involved in the study. ‘I loved the concept,’ says Šponer. ‘The beauty is in its simplicity.’ Although accurately reproducing the whole prebiotic environment in laboratory experiments remains challenging, she adds that ‘the mechanochemical approach is a piece of the bigger picture that has never been considered before’.

Hernández and colleagues now want to see if other key prebiotic molecules, like nucleobases or cyanamide, can be made by mechanochemical means.

Ozone still declining in the lower stratosphere

An unexplained decrease in stratospheric ozone – revealed by satellite measurements – appears to have been offsetting the ozone layer’s overall recovery over the past 20 years.

The finding flies in the face of atmospheric models, which predict ozone levels in all parts of the atmosphere should be recovering in the years following the Montreal Protocol agreement which banned the use of ozone-depleting chemicals such as chlorofluorocarbons (CFCs) in 1987.

Some of the damage caused by CFC use has been reversed – for example, the infamous ozone hole over Antarctica has shrunk. But new data reveals that elsewhere this is not the case. A collection of precise satellite measurements covering the tropics and mid-latitudes – between 60° north and 60° south – from 1998 to 2016 shows that levels in the lower stratosphere have continued to decline. This is offsetting increasing levels in the upper stratosphere, and overall there has been no significant increase or decrease in the total amount of ozone since 1998.

The reasons for the decline in lower stratospheric ozone are unclear, and the international team who made the discovery say that finding out why it is happening must be a priority. Recently researchers warned that increasing use of the industrial solvent dichloromethane may be threatening ozone recovery.

What is Ozone

Ozone (O3) is a relatively unstable molecule made up of three atoms of oxygen (O). Although it represents only a tiny fraction of the atmosphere, ozone is crucial for life on Earth.

Depending on where ozone resides, it can protect or harm life on Earth. Most ozone resides in the stratosphere (a layer of the atmosphere between 10 and 40 km above us), where it acts as a shield to protect Earth's surface from the sun's harmful ultraviolet radiation. With a weakening of this shield, we would be more susceptible to skin cancer, cataracts, and impaired immune systems. Closer to Earth in the troposphere (the atmospheric layer from the surface up to about 10 km), ozone is a harmful pollutant that causes damage to lung tissue and plants.

The amounts of "good" stratospheric and "bad" tropospheric ozone in the atmosphere depend on a balance between processes that create ozone and those that destroy it. An upset in the ozone balance can have serious consequences for life on Earth, as scientists are finding evidence that changes are occurring in ozone levels—the "bad" tropospheric ozone is increasing in the air we breathe, and the "good" stratospheric ozone is decreasing in our protective ozone layer. This article describes processes that regulate "good" ozone levels.