Carbon is a nonmetallic element. carbon on Mars is abundant in the the form of carbon dioxide. Elemental carbon can be produced via the bosch reaction. One of the important uses of carbon in a colony would be in the production of plastics and hydrocarbons.
Carbon makes up about 0.39% of the matter in the solar system.
Massive stars (more than about half a solar mass) are capable of burning helium in the so-called triple-alpha process:
In brief, two helium-4 nuclei are fused to create highly unstable beryllium-8 nuclei. While most of these nuclei simply decay back to helium, a small fraction will fuse with another helium-4 nucleus to form yet another unstable nucleus, an excited state of carbon-12 (here denoted ). While once again most of these nuclei will simply decay back to helium-4 and beryllium-8, a tiny fraction will instead randomly decay to the ground state of carbon-12, where they will remain. Over time, this process produces a lot of energy and carbon.
Role of carbon-formation in the future of Mars
Our sun, while massive enough to fuse helium, has not yet begun this process. When the helium core ignites in the distant future, the core will become very hot and dense, causing the outer layers of the sun to expand and cool. This "red giant" phase of the sun's life will almost certainly destroy all life on Earth, quite possibly evaporating the planet, but Mars is likely to survive. Whether conditions on Mars would be tolerable for any Earth-origin lifeforms at that time depends on less accurately known aspects of the process, mainly how much mass the sun loses and how much drag the planet experiences, but it seems likely that the planet will be reasonably tolerable for hundreds of millions, if not billions of years.
Because there are no stable atomic nuclei of mass numbers 5 and 8, the triple-alpha process is the only way in which stars can create elements beyond helium on a large scale. As a result, carbon is the fourth most common element in the universe. (It lies after oxygen because the same stars that create carbon-12 mostly convert it into oxygen-16 by the addition of another helium-4 nucleus.)
Carbon is readily available on Mars in the form of CO2 from the atmosphere, and from carbonate deposits in the martian regolith.
Carbon is an essential element in organic molecules.
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