We know that on Earth, carbon atoms are the building blocks of life, and all known organisms use carbon compounds as their basic molecular structure. An important reason is that each carbon atom can form bonds with up to four other atoms at the same time. This property makes carbon suitable for the formation of long molecular chains, which in turn form the "vital parts" of life, such as proteins and DNA.
Although life on Earth is all carbon-based life. However, the environment of the alien planet is completely different from that of the Earth, so could extraterrestrial life have a completely different chemical basis from that of life on Earth?For example, they may not need carbon atoms at all to create biomolecules, or they may not need liquid water at all.
Now, let's see which chemical elements can form an extraterrestrial life, and what are the various forms of extraterrestrial life.
Silicon-based life
In the Star Trek film series, you'll be impressed by the rock-eating Horta, a highly intelligent alien creature that looks like a "carrion" that feeds on rocks and is able to move freely through them, while human instruments simply can't detect them because they're silicon-based life.
Silicon-based life is a common alien form in science fiction, so is it possible that aliens are really made of "silicon"?
No matter on any planet, an organism must complete a series of life activities in order to survive and reproduce, such as absorbing nutrients and food, converting food into energy, excreting waste, repairing body damage, reproduction, etc., all of these complex tasks need to start from the basic life unit cell, that is, the molecular machinery that makes up the cell needs to operate efficiently to complete these tasks. And to function efficiently, it is necessary to build up the chemical elements of life to form macromolecules. Silicon and carbon have many very similar chemical characteristics, for example, silicon atoms can also bond with up to four other atoms at the same time, and can also produce molecules large enough to carry biological information.
An organic macromolecule composed of ammonia molecules.
In addition, silicon is one of the most common elements in the universe. Silicon, for example, accounts for nearly 30% of the mass of the earth's crust and 150 times more carbon than the earth's crust. This means that, unlike carbon, there will likely be a lot of silicon on most small rocky planets. Scientists have long known that life on Earth has the ability to manipulate silicon. For example, silicate bodies, which are microscopic particles of silica, can be found in grasses and other plants. Photosynthetic algae, known as diatoms, are able to synthesize silica into their silicon shells.
Although scientists have not found organosilicon compounds on Earth for a long time, this has now changed. Researchers at the California Institute of Technology in the United States discovered a marine red thermophilic halogen in a hot spring in Iceland, and were pleasantly surprised by the bacterium's cytochrome C protein, which is commonly used to transfer electrons to other proteins, which can synthesize small amounts of organosilicon compounds.
It was then coded, tested, and significantly enhanced in its ability to create silicone compounds. Eventually, the mutant enzyme produced at least 20 different organosilicon compounds, 19 of which were completely new to scientists. In addition to this, scientists have also shown that genetically modified E. coli can also create organosilicon compounds. This suggests that microbes may have naturally evolved the ability to create these organosilicon compounds, and that extraterrestrial life could indeed be silicon-based.
Can it survive on liquid ammonia?
On Earth, the molecules in the earth's organisms are inseparable from water, and there is very little liquid water in the universe, but the liquid is abundant. Studies have shown that there will be at least one liquid on the surface of many planets. For example, Saturn's moon has ammonia in the clouds, and Neptune's moon Triton has liquid nitrogen geysers, can extraterrestrial life depend on these liquids to survive?
Before answering this question, let's take a look at the essential characteristics of the fluid that supports the existence of life. First of all, the liquid needs to be a good solvent, and it is especially important to have the ability to dissolve large molecules. Because living molecules tend to be very large, proteins, for example, usually contain at least a few thousand atoms each. Secondly, the flow range needs to be very large, which refers to the temperature difference between its melting and boiling points, which means that cells in the organism that use solvents will be less susceptible to climate change. Finally, it also needs to have the characteristics of weak viscosity and strong surface tension, because low viscosity is conducive to promoting cell interaction, and high surface tension improves adsorption efficiency and helps to concentrate biomolecules on the cell surface.
It is likely that there is methane-dependent life on Titan.
All things considered, scientists believe that ammonia is the most likely alternative solvent because it is chemically similar to water, and it can also form strong hydrogen bonds, which can dissolve most organic molecules that water can dissolve. It is also the fourth largest molecule in the universe and is present on many planets.
However, ammonia also has some weaknesses of its own. For example, ammonia has a weak dielectric constant, only one-third of water, the dielectric constant is the ability of the solvent to dissolve charged ions and molecules, and a large dielectric constant is very important for dissolving salts, supporting acid-base chemistry and dissolving large charged molecules, which shows that in ammonia solution, it may be difficult to preserve large and complex molecules. Secondly, the surface tension of ammonia is weak, and the ability of the cell surface to adsorb biomolecules will also be weak.
In addition to the fact that ammonia has many deficiencies as a life solvent, ammonia actually has a lot of problems. For example, ammonia reacts violently with oxygen and eventually burns, so amino life, if it exists, will almost certainly live in an oxygen-free environment. Ammonia is also powerless to provide any barrier in the face of ultraviolet light. When water reacts with high-energy ultraviolet light, it decomposes into O2, which is further converted into ozone (O3). Ozone absorbs ultraviolet light, which prevents water molecules from breaking down further. In contrast, ammonia breaks down into nitrogen (N2), provides no protection, and eventually disintegrates itself in the face of intense radiation.
What would methane life look like?
If you're lucky enough to travel through space, you'll find Saturn's largest moon, Titan, a very interesting place. This satellite is very cold, with an average temperature of 1795 , but it has a vast ocean, which is made up of methane. Normally, methane is in gaseous form, but it becomes a liquid in very cold environments. There are also many active volcanoes on Titan, which erupt methane into the atmosphere, forming clouds, rain or snow.
Methane is a very common substance in the universe, and perhaps there are more cold planets like Titan. Scientific research has shown that there are organisms on Earth that depend on methane to survive. High-energy radiation causes methane to react with atmospheric nitrogen, creating a more complex array of molecules. Under certain conditions, organic compounds such as methane can combine with water molecules and interact to form amino acids, which are the building blocks of life. So, does this mean that there will be life on Titan, which depends on methane to survive?
A very big challenge is that methane has a very low melting and boiling point, it has a melting point of -1825, the boiling point is -1615, with only 21 in the liquid range, which means that methane life can hardly withstand drastic changes in ambient temperature. The second major challenge is how to form something similar to a cell membrane. Because methane is a non-polar molecule, certain polar organic molecules are excluded.
Despite these challenges, scientists at Cornell University in the United States have come up with a unique methane life form.
Artist's rendering of silicone-based life.
We know that on Earth, cells have an outer membrane – a bilayer of phospholipid cell membranes. These strong biofilms are water-based, allowing substances to pass through them, and the vesicles they form provide a place for the organic matter in every cell. Small vesicles made up of such a membrane are called liposomes. The researchers hypothesize that life in Titan may also be made up of cells, but that cell membranes will be made of methane compounds.
The envisioned cell membrane, named nitrogenous, is made up of nitrogen, carbon, and hydrogen molecules that can all be found in Titan's oceans of ultra-cold methane. Through computer simulations, scientists have also found the most stable nitrogenous body, which is composed of three carbon atoms, three hydrogen atoms and one nitrogen atom, which is very small and simple in structure compared to the phospholipids found on Earth that make up cell membranes, but it is very stable, does not decompose easily, and has similar flexibility to the phospholipid membrane of Earth cells, and more importantly, it is also present in Titan's atmosphere.
The next step will be to test how such cells survive in the methane environment, especially how they multiply and metabolize.