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Photo Source: Pixabay "Aliens" is a magnificent fantasy of children, an enduring theme of film and television, and an object that astrobiologists are trying to find. If there are clues in space exploration, how can we be sure that it comes from the activities of extraterrestrial life? A new study that points directly to the nature of "life" may open up new ideas for the search for extraterrestrial life.

In the search for extraterrestrial life, astrobiologists are always looking for the simplest and most robust forms of life because they are most likely to survive in harsh alien environments. However, chemicals related to simple organisms can also be produced through abiotic pathways. As a result, scientists sometimes think that extraterrestrial life has been discovered, but there is a lack of hard evidence.

Not only that, extraterrestrial life may be very different from life on Earth. For example, Silicon-based Life is a regular in science fiction, and we suspect that their long-chain molecules are mainly made up of silicon rather than carbon, so their chemical composition is very different from that of life as we are familiar with. Such being the case, how should we look for extraterrestrial life? is there anything special about "life" so that we can accurately know its existence?

Yes, but not in the mid-1970s, two NASA Viking probes flew to Mars in search of life, but the conclusions were controversial. The detection results show that there may be life on Mars, the evidence comes from an isotope labeling experiment: the food needed by microbes is labeled with carbon 14 and added to Martian soil samples; if microbes ingest the labeled food, these microbes release radioactive carbon dioxide, which is detected by the instrument.

The results showed that two Viking landers 6500 kilometers apart on Mars detected radioactive carbon dioxide in the soil of the experimental group, but not in the soil of the control group after heating and sterilization, suggesting that microbial metabolism took place in the Martian soil. However, other life detection experiments carried out by the two Viking ships did not find any signs of life.

Images of Mars taken by the Viking 2 lander (image source: NASA / Wikipedia) in 1996, scientists discovered a Martian meteorite in Antarctica, in which microfossils (microfossil, microfossils that can only be studied using a microscope) seem to add to the evidence for the existence of life on Mars. But subsequent studies have pointed out that several abiotic pathways are also easy to produce so-called microfossil traces.

Recently, some scientists say large amounts of phosphine have been found in the atmosphere of Venus, and on Earth, phosphine is mainly produced by microbes. But other scientists question the results, arguing that even if phosphine exists in the atmosphere of Venus, it may come from some strange form of volcanic activity of Venus, rather than from life.

These stories of the search for extraterrestrial life have a similar development: at first the clues are exciting, then the doubts are suspected, and finally the hypothesis of the existence of life is rejected. Time and again, astrobiologists seem to be able to find only biosignature, but frustratingly, biological signs are not conclusive evidence of the existence of extraterrestrial life. Is there any indicator that we can confirm the discovery of extraterrestrial life?

New ideas brought about by complexity A study published in Natural Communications (Nature Communications) has proposed a new idea called Assembly Theory (assembly theory). Assembly theory no longer focuses on simple biological signs, but on the complexity of the nature of life (complexity). It is based on the idea that any form of life in the universe will encode life information in a complex combination of molecules, which is very different from inanimate matter.

In the field of astrobiology, calls for attention to "complexity" have been going on for some time. NASA gave a complex definition of "life" in 1994: life is a self-sustaining chemical system capable of Darwinian evolution. The problem is that the key concepts contained in this definition are inherently complex and difficult to test and quantify. As Jim Green, chief scientist of NASA, puts it: "I can't build a machine that can look for 'evolution', 'reproduction' or 'metabolism'."

The assembly theory provides a clearer and more universal definition of life. The assembly theory assumes that for any object in any environment, when its abundance and complexity increase, the possibility that it comes from life activities will increase. Abundance refers to how often the object appears in the environment, and complexity can be measured by estimating the steps required to assemble such an object.

Research co-author Sarah Walker (Sara Walker), a biophysicist at Arizona State University, believes that assembly theory is a milestone in astrobiology because it proposes operational complexity measurements for the first time, giving theories about the nature of life the opportunity to combine with experimental observations.

Although the assembly theory of complex molecules is suitable for objects of various scales, researchers focus on its application at the molecular level. Because molecules are the most basic components of biology, both in the laboratory and in the universe. To measure the complexity of molecules, the team defined the "material assembly index" (mass assembly number, MA), which uses algorithms to assign values to different molecules.

MA refers to the number of steps required to build a molecule ideally. We know that a molecule can usually be synthesized in many ways, and MA corresponds to the shortest assembly path. It considers only the valence rules, regardless of other limitations, including chemical reaction conditions, and the objects created in each step can be reused in subsequent steps. Therefore, the molecules with fewer kinds of chemical bonds and higher symmetry have lower MA values, and vice versa.

Analyze examples of the principles of assembly steps (photo source: original paper) the researchers assigned MA values to 2.5 million molecules in a chemical database. Phosphine, which is regarded by some scientists as the biological signature of Venus, consists of one phosphorus atom and three hydrogen atoms, connected by a symmetrical phosphorus-hydrogen single bond, and its MA is only 1. In contrast, tryptophan is composed of 11 carbon atoms, 12 hydrogen atoms, 2 nitrogen atoms and 2 oxygen atoms, and its structure is more complex, with a MA of 12.

Molecular structure of tryptophan (photo source: NEUROtiker / Wikipedia) to verify the effectiveness of MA, the researchers tested it with real molecules. Because molecules with high MA have more chemical bonds and relatively lower symmetries, the researchers predict that they will produce more peaks in mass spectrometry (each peak represents different ions in the mixture), while molecules with low MA will do the opposite. The experimental results are consistent with their prediction-there is a linear relationship between the number of peaks and MA, and the correlation is 0.89.

There are three molecules as examples on the left and their corresponding mass spectra on the right. It can be seen that the more complex the molecule is, the higher the MA is and the more peaks are shown in the mass spectrometry. (photo Source: original paper) after the "threshold" of life established the connection between theory and practice, the researchers further tested their core hypothesis: high-MA molecules can only be produced by organisms. They examined the mass spectra of samples of a variety of mixtures, including E. coli, plant alkaloids, coal, granite and even beer, and estimated their MA values based on linear relationships. The researchers found that only samples with living organisms had a MA higher than 15.

Li Cronin (Lee Cronin), a chemist at the University of Glasgow who led the study, said that when a molecule's MA is greater than 15:00, the probability of it being produced in an abiotic process under terrestrial conditions is extremely low (less than 1 / 6 × 1023). As a result, molecules with MA values greater than or equal to 15 can almost only be produced by life. In other words, we can find life through mixtures where the MA is greater than a certain threshold.

So, is the MA value of 15 the absolute criterion for judging life and non-life? No. First of all, many molecules with low MA values may also be biological signs, such as the simple molecular structure of oxygen released by organisms into the Earth's atmosphere through photosynthesis. Second, Cronin points out that although whether MA is greater than 15 seems to be the critical condition for the existence of life on Earth, the critical value may be different in a planetary environment that is very different from Earth.

To test their theory, co-author Heather Graham, an astrobiologist at the NASA Goddard Space Flight Center, sent Cronin a set of blind samples. One is a fossil from millions of years ago, and the other is a sample of the Murchison meteorite. The Merchison meteorite is a meteor that fell to Earth in 1969. It is rich in organic carbon compounds, but not living things.

The Merchison meteorite specimen of the American Museum of Natural History (photo: Basilicofresco / Wikipedia) Cronin found that the fossil sample has a high MA value and has been confirmed to have signs of life, while the Merchison meteorite is rich in molecules, but its MA value is still less than 15. This is a verification of assembly theory, and the results are exciting, indicating that only complex sample composition does not represent the participation of life, but "complex molecules" that can reflect the complexity of chemical organization are the key elements of life.

Put into practice NASA's previous interstellar missions have collected some mass spectrum data from other planets. Green and NASA scientists wondered if assembly theory could be used to look for signs of life.

Green's first consideration was the Cassini vehicle that collected water vapor samples from Enceladus. Unfortunately, Cassini's mass spectrometer can only detect molecules less than 100 atomic mass units (amu), but the assembly theory is only applicable to molecules greater than 150 amu. NASA's Curiosity and Perseverance Mars probes are equipped with mass spectrometers that can detect molecules of more than 150 amu, but they lack the ability to study single molecules and are not enough to analyze MA values.

Green said that future space exploration missions should be equipped with mass spectrometers that can measure larger molecules and conduct more accurate analysis. Dragonfly, Saturn's moon Titan, which will arrive in 2034, is expected to achieve this goal. It will explore the atmosphere and surface of Titan, looking for the components of life. Although the dragonfly mass spectrometer does not have all the functions of a laboratory mass spectrometer, it has the ability to detect complex molecules.

Artistic imagery of dragonfly's mission in Titan (photo: NASA / Wikipedia) in the solar system, there are places where life may still be waiting for humans to explore, and large telescopes are also looking for potentially habitable planets in the vast universe. Assembly theory provides a new perspective to look at the universe on a molecular scale and guides us to pay attention to the complexity of life when looking for extraterrestrial life. At this moment, there are countless complex molecules synthesizing, flowing, and working in our bodies that make us so different in the universe.

Original text link:

Https://www.scientificamerican.com/article/life-is-complicated-literally-astrobiologists-say/

Links to papers:

Https://www.nature.com/articles/s41467-021-23258-x

Reference link:

Https://doi.org/10.1089/ast.2015.1464

Https://www.nasa.gov/feature/dragonfly-launch-moved-to-2027

This article comes from the official account of Wechat: global Science (ID:huanqiukexue), written by Natalie Elliot (Natalie Elliot), compiled by Zheng Yuhong, revised by Li Shiyuan

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