In the American movie “Avengers”, the home of the big villain Thanos is a planet called Titan. In fact, Titan really exists in the universe. It is the largest satellite orbiting Saturn. Not only that, Titan is also called “another earth” by many people. Because it is one of the four rocky planets with an atmosphere in the solar system (the other three are Earth, Venus and Mars), and there are lakes and rivers made of liquid methane on Titan. Liquid methane plays a role similar to the water on the planet on Titan. They continuously evaporate into the air, cool into methane cloud, and then form methane rain, returning to lakes and rivers, forming a complete circulation system.
So some people began to think that the earth is full of life, a large part of the reason is the existence of liquid water. But in other parts of the universe, life is likely to be made up of different chemical substances, which may be dissolved and aggregated in liquids like methane. Not only that, there are more and more newly identified exoplanets by astronomers. Some of them are likely to have oceans composed of hexane, ethers, chloroform or other liquids, which are likely to be the basis of unknown life.
At present, it is not clear to humans whether there are new life forms on these planets, but researchers are exploring some possibilities in laboratories on Earth.
The most basic life module
What is the most basic part of a simple life?
The answer should be a combination of molecules (DNA or RNA) that can carry genetic information and molecules that can form cell membranes. Because genetic information is the core of life, it is also the medium for the continuation and transmission of life, and the operation of life requires a series of controllable chemical reactions. Cell membranes can separate living substances from the external environment to ensure the controllability of these biochemical reactions. Therefore, when researchers look for new life forms, they must first start with cell membranes and genetic factors.
However, the chemical mechanism of life on Earth cannot be applied to the ocean formed by hydrocarbons (materials composed of carbon and hydrogen, such as methane, ethane, chloroform, etc.) like those on Titan.
In the water molecule, the end where the oxygen atom is located has a little negative charge, while the hydrogen end has a little positive charge. The water molecule is not neutral but polar. This polarity affects the combination of water and various biological components. For example, water can bind to proteins through hydrogen bonds (the interaction between slightly positively charged hydrogen and other slightly negatively charged atoms), affecting the protein’s Structure and biological function.
Most of the hydrocarbons are non-polar. The molecules are not charged at both ends and are neutral. It is difficult to form special chemical bonds with other chemicals through the mutual attraction of charges (for example, make water and protein Combined hydrogen bonds). So if there is life in the Methane Lake of Titan, it will be a completely different set of construction systems.
Various experiments on new life forms
If scientists want to discover life with heterogeneous structures on other planets, they must first try to create something heterogeneous on the earth. At present, chemists and biologists all over the world are exploring new life molecules and structures through computer simulations and practical laboratory operations.
In 2015, Jonathan Lanier, a planetary scientist at Cornell University in the United States, conducted a study with two other chemical engineers to explore what life forms look like in Titan’s low-temperature environment full of methane? They used a computer digital model to determine an organic compound called acrylonitrile. This compound can be combined in liquid methane to form a “hollow sphere,” similar to the membrane structure necessary to compose living organisms. Then in 2017, NASA scientists used spectroscopic telescopes to detect large amounts of acrylonitrile in Titan’s atmosphere, which increased the possibility of “alternative life” on Titan.
In addition to the above-mentioned digital simulation methods for finding cell membranes, there are also scientists who are searching for compounds that may constitute cell membranes in experiments. Sarah Maurer, a biochemist at Connecticut State University in the United States, is one of them. She said that cell membranes participate in the chemical reactions of animal and plant cells. In addition to keeping cells separated from the outside world, membranes also need to selectively transport substances in and out.
Although methane and ethane filled Lake Titan, these substances only become liquid at extremely low temperatures, so the researchers replaced them with hexane, which can remain liquid at room temperature. As for how to verify whether a certain substance is suitable as a cell membrane? After they made the test substance into a membrane, they then observed whether phosphate (the small molecule that makes up the backbone of DNA and RNA), a slightly negatively charged molecule, could pass through the cell membrane. In general, as an important part of genetic material, phosphate in cells should not be able to pass through cell membranes.
Although Titan is the biggest focus for finding life on other planets in the solar system, life may also exist on some ice-covered planets:
Europa, Enceladus, Enceladus and Triton are candidate planets for possible existence of life.
Lauren Williams, a molecular biologist at the Georgia Institute of Technology in the United States, also plans to do similar experiments. He hopes to replace the liquid substrate with chloroform and silicate instead of phosphate, so that the molecules can be more easily dissolved in hydrocarbons. . His team is also preparing to use other chemicals to replace familiar adenine and cytosine (a base in DNA nucleotides, one of the components), etc., to test new and possible genetic molecules.
There are actually many experiments on new DNA molecules. Complex DNA is actually composed of only four bases permutation and combination, namely guanine, cytosine, adenine and thymine, usually represented by the letters G, C, A and T. The permutations and combinations of four different types of bases store the genetic information of all life on earth.
In February 2019, a research team led by Steven Benner, founder of the Alakua Applied Molecular Evolution Foundation in Florida, USA, published an important article in the journal Science. The researchers successfully combined four artificially synthesized bases with four naturally-occurring bases to create a DNA molecule composed of eight bases, and the DNA molecule can store and transcribe information just like natural DNA. The study proved that synthetic bases can recognize and bind to each other, and the double helix structure formed by them can maintain a stable structure. It also confirmed that there may indeed be other life gene systems in the universe.
In addition, the Brecher research group of St. Louis University in the United States is studying a new kind of molecule that may constitute a genetic gene. The bases of this molecule do not rely on hydrogen bond pairing, but are paired by sharing a molecule called thiolipid.
Looking for alien life
The above experiments are all scientists’ exploration of new life forms. In these experiments, they will not actually create alien creatures, but the experiment may answer the question “whether there is a new form of life that can survive in the liquid hydrocarbon ocean?”
At present, people looking for extraterrestrial life are based on water. If the answer to the above question is yes, then this means that there is a huge blind spot in our current exploration of life. By proving that the most basic life chemistry can be produced in other liquids, scientists can open up new areas of exploration, or guide us on how to comb through the large amounts of data collected by space probes.
For example, in 2005, Chris McKay, an astrobiologist at NASA’s Ames Research Center, pointed out that there may be organisms on Titan that may survive by metabolizing acetylene or ethane, and said that they would live in the atmosphere of Titan. It leaves obvious traces in it. However, after screening the data collected by the Cassini space probe that orbits Saturn and explores Saturn, the researchers found that there is neither an ethane layer nor acetylene on the surface of the satellite, which largely overthrows it. This statement.
Perhaps the significance of exploring extraterrestrial life in the laboratory lies in this. It can guide us where to go and how to find and verify extraterrestrial life.