Amino acids were found in the samples brought back by Hayabusa 2. Does this mean that alien life has been discovered?

Recently, there were news reports that the Japan Aerospace Exploration Agency's asteroid probe Hayabusa 2 discovered the presence of amino acids in samples brought back to Earth from the asteroid Ryugu. This news once again triggered people's eager desire to find life outside the Earth.

In fact, the existence of organic matter in the universe is not a rare thing. As early as 50 years ago, scientists discovered that organic matter is widely present in the universe. Moreover, under the modern theory of the chemical origin of life, the widespread existence of organic matter in the universe is not surprising.

But the breakthrough this time is that "the existence of amino acids has been confirmed outside the Earth for the first time." In other words, this is the first time that humans have obtained asteroid samples in outer space on their own and discovered amino acids in asteroid samples, not the first time we have discovered amino acids outside the Earth.

01

Don't be surprised, organic matter is widely present in the universe

In 1969, a bright fireball fell from the sky near Murchison, Victoria, Australia. It broke into three large fragments and countless small fragments before falling. People picked up more than 100 kilograms of meteorite fragments within an area of ​​13 square kilometers, the two largest of which weighed 7 kilograms and 680 grams respectively. These meteorites were named Murchison meteorites.

Murchison meteorite Image source: Wikipedia

Since 1971, scientists have been studying the chemical composition of the Murchison meteorite and discovered that the meteorite contains amino acids, and there are more than 70 kinds of amino acids, including glycine, alanine, glutamic acid, isovaline, pseudoleucine, etc. Since amino acids are one of the necessary organic substances that make up life, this discovery has aroused scientists' fanatical interest in searching for extraterrestrial life. In the following decades, they have found a variety of amino acids and other chemical precursors necessary for the birth of various life in other meteorites that fell to Earth.

These chemicals include various amino acids, polyols, nucleobases (purine, pyrimidine, etc.), aromatic compounds, etc. In 2009 and 2015, NASA and ESA's comet probes discovered a variety of organic compounds from comets. In February 2020, some scientists even claimed to have found a protein containing iron and lithium in the Acfer 086 and Allende meteorites [1] (controversial).

With the advancement of science and technology, in addition to searching for organic matter in meteorites, scientists have also used telescopes with spectrometers to observe the spectra of various organic matter in nebulae, molecular clouds, and cosmic dust in the universe, such as polycyclic aromatic hydrocarbons, fullerenes, aromatic-aliphatic mixed organic matter, acetaldehyde alcohol, etc. There are hundreds of types of these organic matter.

Judging from these findings, organic matter, including amino acids, is actually widely distributed in the universe.

So why are there so many organic substances in the universe?

This is actually a natural chemical reaction. NASA scientists believe that this chemical reaction began billions of years after the birth of the universe. They often occurred after the stars appeared in a nebula and before the planets appeared. The carbon elements in the nebula began to form polycyclic aromatic hydrocarbons under the action of cosmic radiation and stars. This is the beginning of the formation of all organic matter. Polycyclic aromatic hydrocarbons are actually some carbon rings. Here are some examples:

Naphthalene, black is carbon atoms, gray is hydrogen atoms Image source: Wikipedia

Fluorene, same as above. In short, polycyclic aromatic hydrocarbons are carbon rings that are repeatedly connected. Image source: wikipedia

In addition to the formation of polycyclic aromatic hydrocarbons (PAHs) due to chemical reactions around stars, aliphatic hydrocarbons are also formed (the carbon atoms in PAHs form rings, while here the carbon atoms form chains). Under the influence of various atoms, ions, molecules and radiation in the interstellar space, they will be hydrogenated, oxidized and hydroxylated, thereby transforming into more complex organic substances, such as the precursors of amino acids and nucleotides [2].

After these compounds are formed in space, they will be attached to various rocky or icy asteroids and float in the universe. This is how the amino acids in the various meteorites we have found on Earth and the samples from the asteroid Ryugu brought back by Hayabusa2 come from.

02

Alien amino acids discovered, how far are we from alien life?

In every natural history museum or science and technology museum, the process of the birth of life is always shown with an experimental device: the Miller experiment. Ammonia, methane, hydrogen, water, carbon dioxide and other gases are heated in a bottle with continuous discharge, and amino acids appear in the liquid obtained by cooling. In subsequent experiments, people have obtained peptides, ribose, nucleic acid bases and other substances necessary for the formation of life.

Miller's experimental process Image source: Wikipedia

The Miller experiment simulated the chemical evolution process on Earth, and what we talked about earlier was the chemical evolution process in the universe. These have all been proven to be true, and in nature, whether on Earth or in the universe, chemical reactions naturally occur to form organic matter. But this is actually just the first step in the birth of life, and it is still far from true life.

After that, there are three more steps: small organic molecules assemble into organic macromolecules; organic macromolecules assemble into multi-molecular systems; and multi-molecular systems assemble each other (this is a decisive step, the most important of which is the formation of the cell membrane system and the entry of nucleic acids into the membrane). So it is still a long way from amino acids to life.

Image source: Reference [3]

In addition, although these amino acids in the universe show that organic matter is widely present in the universe, it is difficult for the universe to provide a sufficiently stable environment for the formation of life. Because scientists have found that the configuration of amino acids found in meteorites is different from that of amino acids that make up life.

First, there are differences in molecular structure between meteorites and the same organic matter on Earth. Many of them are isomers (same molecular formula, different structure)[4]

Schematic diagram of trichlorobiphenyl isomers Image source: Wikipedia

Secondly, the amino acids from meteorites and those on Earth differ in chirality. What is chirality? You can understand it by doing a small experiment. Raise your left and right hands, palm to palm, and they can fit together. But if you put your left and right hands together with your palms facing down, you will find that they cannot fit together. This is chirality. Amino acids on Earth only have left-handed structures, but amino acids in meteorites have both left-handed and right-handed structures [4].

Chirality of amino acids Image source: Wikipedia

The third question is that in the process of chemical evolution to form life, more organic matter may be needed. Can these meteorites provide enough organic matter for the evolution of life? Only on larger planets similar to the early Earth can there be a stable hydrothermal environment for the continued chemical evolution.

03

Does alien life exist?

In 2017, scientists discovered some microbial fossils on the coast of Quebec, Canada. These microorganisms were thermophilic bacteria that lived near the deep-sea hydrothermal vents on Earth at that time [5]. This discovery was published in Nature.

Photo of fossil microorganisms on early Earth. Image source: Reference [5]

They lived between 3.77 billion and 4.22 billion years ago. What does that mean? The Earth was only about 300 million to 800 million years old at that time, and it was going through a very brutal period: the Heavy Bombardment. During the Heavy Bombardment, the Earth was frequently bombarded by asteroids. It is estimated that the volume of the Earth increased by about 10% during this period because of the intensive bombardment of asteroids.

Image credit: NASA

In such a harsh environment, life still quickly appeared on Earth.

Although our current discovery of organic matter on meteorites has not been able to prove that life on Earth originated from outside the Earth, it does show one thing, that is, the existence of organic matter is relatively common in space, and as long as the conditions are right, various organic matter can be formed. So, we can make a small speculation that as long as there are habitable planets like the Earth, the possibility of life existing is very high, or to be more radical, extraterrestrial life must exist.

Therefore, in the process of exploring extraterrestrial life, scientists will first consider the factor of the habitable zone. Within the habitable zone, the water on the planet can exist in liquid form. This liquid water can undoubtedly provide a stable environment for chemical evolution and provide a living environment for primitive life.

The habitable zones of different stars. Large, hot stars have far-reaching and wide habitable zones, while small, cold stars have close and narrow habitable zones. Image source: NASA

However, these exoplanets are too far away for our current technology. Instead of spending a lot of effort to find life on exoplanets, it is better to look for extraterrestrial life in the solar system now. In the solar system, there is a better chance of finding life on Mars, which is located on the outer edge of the habitable zone of the solar system. Although it is now dry, according to scientists, it actually had surface water 3 billion years ago. There is still groundwater and underground lakes now, and there are even vast ice caps at the North and South Poles.

Image source: Reference [6]

If we compare the evolutionary history of the Earth and Mars, we will find that according to the theory of chemical evolution, there is very likely life on Mars, but this life form is still relatively primitive, and may be ancient life similar to bacteria and archaea on Earth.

Image source: Homemade

Martian life may still exist in some corner of Mars, and we can get evidence from Earth. Since 1970, scientists have discovered many microorganisms living in extreme environments on Earth, including extreme thermophiles, psychrophiles, halophiles, acidophiles, alkaliphiles, etc., and even in rock crevices more than 10 kilometers underground where there is no sunlight, extremely high temperature and pressure, there are microorganisms. These extreme microorganisms have expanded the scope of life and give us reason to believe that life still exists on Mars.

The archaea on Earth look like these. If there is life on Mars, it may be similar to these organisms. Image source: Wikipedia

Author: Gravity

Producer: China Science Expo

References:

[1]McGeoch M, Dikler S, McGeoch JE M. Hemolithin: a meteoritic protein containing iron and lithium[J]. arXiv preprint arXiv:2002.11688, 2020.

[2]Gudipati, Murthy S.; Yang, Rui (September 1, 2012). "In-Situ Probing Of Radiation-Induced Processing Of Organics In Astrophysical Ice Analogs – Novel Laser Desorption Laser Ionization Time-Of-Flight Mass Spectroscopic Studies".The Astrophysical Journal Letters.756(1): L24.Bibcode:2012ApJ...756L..24G.doi:10.1088/2041-8205/756/1/L24.

[3] Jiang Nan. Synthesis of amino acids before the origin of life[D]. Jilin University, 2013.

[4] Yang Jing, Lin Yangting, Ouyang Ziyuan. Extraterrestrial organic compounds[J]. Earth Science Frontiers, 2014, 21(06): 165-187.

[5]Dodd, M., Papineau, D., Grenne, T. et al. Evidence for early life in Earth's oldest hydrothermal vent precipitates. Nature 543, 60–64 (2017).

[6] Zhao Jiannan. Geological characteristics of ancient lakes in southern Mars and their implications for paleoclimate environment[D]. China University of Geosciences, 2017.