How did life originate? High-energy particles from the sun may be the key

How did life on Earth begin? This is one of the greatest mysteries of modern science. Recently, a study found that high-energy particles from the sun may be the key.

Early theories of the origin of life

When the Earth was first formed about 4.5 billion years ago, it was a barren ball of rock, frequently hit by meteorites and dotted with erupting volcanoes. Based on geophysics and existing fossil records, scientists speculate that conditions suitable for life had developed on Earth during the Hadean period, 4.5 billion to 3.8 billion years ago. However, what we have not been able to understand is what kind of environmental conditions led to the emergence of complex chemical properties at that time.

An artistic conception of the early Earth. (Photo/NASA)

In the process of exploring the origin of life, one of the most well-known experiments is called the Miller-Urey experiment. In 1952, Stanley Miller of the University of Chicago and Nobel Prize winner Harold Urey conducted an experiment in which they injected water, ammonia, hydrogen and methane into a closed glass container to simulate the atmosphere believed to be the early Earth. Then, they repeatedly connected electric sparks to the container to simulate lightning.

A week later, they found that glycine, alanine and aspartic acid had formed in the container. As we know, amino acids are the raw materials of proteins and all cellular life. In 1953, Miller and Urey officially published the results of this experiment, which had a huge impact once they were released. It showed that the complex organic molecules necessary for the emergence of life could be formed from some basic components in the early Earth's atmosphere. Many scientists therefore believe that life originated from such a "primordial soup" rich in basic molecules of life, which is the "warm little pond" described by Darwin.

However, the answer to the origin of life is not that simple. In the past 70 years, a number of new discoveries have made this explanation foggy. One of the main problems is that scientists have found that ammonia and methane were not as abundant as previously thought in the early atmosphere of the Earth. Instead, the early Earth's atmosphere was full of carbon dioxide and nitrogen. Although carbon dioxide and nitrogen can also produce amino acids, the decomposition of these two molecules requires more energy and the number of amino acids produced will be greatly reduced.

A more efficient energy source

So, what other events can provide energy for the decomposition of these molecules? On April 28, 2023, in a new study published in the journal Life, scientists discovered a new source of amino acid formation while analyzing data collected by NASA's Kepler Project: high-energy particles from the sun!

We know that the Sun, like all other stars, has a life cycle. During its lifetime, its brightness and size will vary. A 2016 study showed that about 4 billion years ago, the Sun was only about three-quarters as bright as it is today. Although the young Sun was dimmer than it is now, its surface was turbulent due to huge eruptions, and it often produced powerful "superflares", which are huge bursts of light and radiation. Such superflares are usually accompanied by huge coronal mass ejections that erupt into space.

Solar eruption events, which include solar flares, coronal mass ejections and solar energetic particle events. (Photo/NASA's Goddard Space Flight Center) (GIF: https://www.nasa.gov/science-research/heliophysics/a-stormy-active-sun-may-have-kickstarted-life-on-earth/)

Nowadays, such a powerful superflare only occurs once every hundred years or so. But once it occurs, it will have a major impact on modern technology, such as power grids, satellites, communications, navigation, etc. However, 4 billion years ago, superflares would erupt every few days, so they would emit a large number of particles moving at nearly the speed of light. These high-energy particles would rush into the Earth's atmosphere and trigger chemical reactions.

In the new study, in order to test whether these high-energy particles from the sun are the energy source for the production of amino acids, the researchers created a gas mixture that matches our current understanding of the early Earth's atmosphere. However, since it is not clear how much methane there was in the early Earth's atmosphere, only that it should be very small, they mixed a small amount of methane with carbon dioxide, molecular nitrogen, and water.

Then, in their experiment, they fired protons into the gas mixture to simulate the sun's high-energy particles, and at the same time, they ignited the gas mixture by switching on an electrical spark discharge to simulate lightning.

The results showed that when the proportion of methane exceeded 0.5%, the gas mixture bombarded with protons (a simulated solar particles) could produce detectable amounts of amino acids. However, the formation of amino acids by electric spark discharge (a simulated lightning) required a methane concentration of about 15%.

Even when the methane content reaches 15%, the rate at which spark discharge produces amino acids is only one millionth of that of protons. Not only that, compared with spark discharge, proton impacts also tend to produce more carboxylic acids, which are the precursors of amino acids. So it seems that, under the same conditions, solar particles seem to be a more efficient energy source than lightning.

Of course, this study does not put an end to the mystery of the origin of life. The origin of life is a grand and complex proposition. It is difficult for us to accurately describe where and how it all happened. All we can do is keep exploring and looking for the most appropriate possibility to restore the starting point that created trillions of species on Earth.

Paper link:

https://www.mdpi.com/2075-1729/13/5/1103

This article is a work supported by Science Popularization China Starry Sky Project

Author: Principle

Reviewer: Tao Ning, Associate Researcher, Institute of Biophysics, Chinese Academy of Sciences

Produced by: China Association for Science and Technology Department of Science Popularization

Producer: China Science and Technology Press Co., Ltd., Beijing Zhongke Xinghe Culture Media Co., Ltd.