Popular Science: The journey of Mars to methane and (much) farther
The carbon cycle on Earth is well-established but it greatly relies on biosphere (living organisms). The cycle in space can be similar, just without the influence of biosphere. However, it is quite complicated to study space bodies directly so experiments involving their chemistry are mostly carried out in laboratories under space-like conditions. Certain models of the carbon cycle have been published but they were all based on geochemistry (they mainly considered the geological composition of Mars as a reactive environment). The new carbon network lifts UV radiation as the main element. This is possible since Mars has no ozone layer to protect the planet from UV rays. The presence of an ozone layer is also one reason why this cycle is thought to be of little to no significance on present-day Earth, yet of great significance for modulating the atmosphere of Mars and other similar planets.
The new carbon network consists of four parts. It all starts with carbon dioxide, the main compound of the atmosphere on Mars. Through exposure to UV radiation and some of the minerals on the surface, carbon dioxide can be converted into methane. Methane also requires a source of protons to form, which can be water or acid. As hydrochloric acid was recently discovered on Mars, scientists used this particular chemical in the experiment. The reaction produced certain side products, which served as further proof of the mechanism, since they were detected on Mars and their origin has yet to be reliably explained.
The following parts of the process are atom exchange between carbon dioxide molecules and the mineral surface and the regeneration of carbon dioxide through the destruction of any product. The atom exchange can be important to the chemical composition of both the atmosphere and the planetary surface. Reaction products can be destroyed at any time and this process is the reason for methane loss. Even though there are many reasons why reaction products are destroyed, with the main ones being fires, lightnings, atmospheric discharges and reactions with other molecules, the methane loss is still slower than methane production.
The newly suggested cycle is significant not only on Mars but on any space body with similar conditions – the abundance of carbon dioxide, the presence of hydrogen (in the form of water or acid), UV radiation and a suitable mineral surface. Because of these requisites, this process cannot take place on Venus for example, as it has a thick and opaque atmosphere blocking the UV rays coming from the Sun. For similar reasons, the process is of little to no significance to present day Earth, but this has not always been the case. The Earth surface is not as suitable as the Mars surface, but before there was an ozone layer and an abundance of oxygen in the atmosphere, the conditions for methane production may have been much more favourable. This carbon cycle could have been very important for prehistoric Earth.
All the reactions mentioned above are taking place in reduced environments which are also highly conducive to forming more complicated organic molecules in an abiotic way (without living organisms). Such molecules are mainly amino acids and nucleic acid bases, both of which are fundamental groups of compounds for life as we know it. Scientists simulated the conditions of an asteroid clash and the above-mentioned molecules formed as expected. This proves that the entire network is much more complex, and it might not only explain the origin of methane on Mars but also touch on the issues of forming life.
Scientists thus managed to propose a new carbon network that could explain how Mars got its methane, which was recently detected by the Curiosity rover. The only problem here is that another rover (Trace Gas Orbiter) did not detect any traces of methane. Therefore, the question of the presence and origins of methane or the red planet remains open.