Carbon, periodic table C, 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.
The most common isotope is carbon 12, carbon 14 is used for radiocarbon dating.
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.
Once C12 has been created in a star, it can speed the fusion process using the CNO cycle (where C12 absorbs successive protons, becoming first Nitrogen then Oxygen, which then absorbs another proton and splits off a Helium nuclei and returns to C12). The CNO cycle acts as a catalyst, allowing stars to burn Hydrogen into Helium faster, increasing their temperature.
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. Elemental Carbon may be produced via the bosch reaction. The availability of Elemental Carbon as diamonds or graphite will need to be evaluated by exploration. Elemental carbon as graphene, carbon nanotubes or Buckyballs may be useful for certain applications.
Carbon is an essential element in organic molecules. Atmospheric CO2 and CO should be adequate as sources for all hydrocarbons and organic chemistry requirements for a settlement.
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