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written by Kian Cochrane on September 02, 2001
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A form of lichen.
A form of lichen.
Credit: University of Hawaii
Lichen [Editor's note: Lichen is not a moss (or even a plant). It is a symbiosis between fungi and algae or a cyanobacteria.] are a combination of fungi and algae. Lichen are important for the terraformation of Mars because they can survive very adverse conditions, like those on a partially terraformed Mars. Lichen, like all other plants, utilize the light energy in sunlight to turn carbon dioxide and water into sugars and oxygen.

Even though lichen can survive very adverse conditions, the current Mars cannot support even the hardiest of lichen. Although the temperatures in the tropical regions would be high enough to support lichen to some extent, there is nearly no water, so any lichen planted on Mars would quickly die. Although the water problem could be solved by artificial irrigation, there is not enough carbon dioxide currently in the atmosphere to make enough oxygen for human life.

Partial terraformation is the name given to the first stages of terraforming that raise the temperature of Mars so that carbon dioxide and water are released from the ice caps and regolith. Only a small man-made rise in temperature is needed, because of a phenomenon called positive-feedback greenhousing. Once the average temperature of Mars is raised by just 4 K, the carbon dioxide contained in the southern ice cap will sublime. Since carbon dioxide is a greenhouse gas, the extra carbon dioxide will raise the temperature further, releasing even more carbon dioxide from the regolith, which raises the temperature even further.

Estimates from terraforming scientists state that once the above process has completed itself, the temperature will be high enough to melt some of the underground permafrost, mainly in the tropical regions. Since water vapor is also a greenhouse gas, another positive-feedback process starts itself, creating enough water to support the lichen. Depending on the technology used, partial terraformation could take anywhere from 20 to 10,000 years.

As said before, lichen are extremely hardy. They can grow, reproduce and photosynthesize in tempertatures as low as 249 K, and can hibernate in extreme cold, allowing themselves to survive temperatures as low as 173 K. Lichen can grow on rocks as well as Martian soil. They also form a water reserve, so if there is a shortage of water, they hibernate and tap off the reserve at a very low rate. This allows the lichen to survive the occasional drought. It is not known for certain whether lichen are radiation-resistant plants, but since a partially terraformed Mars would have a relatively thick atmosphere and an adequate ozone layer formed by the carbon dioxide, radiation would not be too much of a problem after a while, except during solar storms. Another less obvious advantage of lichen is that, unlike most other plants, they have no oxygen requirement and a very low nitrogen requirement, shortening the effective partial terraformation length.

The main disadvantage of lichen is that they grow and reproduce very slowly. It would take a very long time for the lichen to cover the planet. Genetic engineering could probably speed up the growth and reproduction rate, making lichen much more feasible, although they would only prosper in conditions well suited to them if such engineering were done. There are trade-offs to everything. The second disadvantage is not exclusive to lichen, but rather to all plants. Plants (lichen included) are somewhat inefficient. Even the most efficient plant (sugar cane) can only convert 3.5% of the Sun's energy into sugar and oxygen. Because of this, it will take a very long time to convert the carbon dioxide in the atmosphere into oxygen. Black chlorophyll can theorectically increase the efficiency, but it is unclear how this can be applied.

If all goes well, the Martian atmosphere will become breathable a few millenia after partial terraformation. If the calculation turns out to be flawed, and there is little useable water on Mars, the lichen can start out using an artificial irrigation system that would utilize the small amount of water in the atmosphere. Then, the sugar produced can be dissociated to recycle the water used. However, this would result in an extremely low production rate, causing the oxygenation stage to last much longer.

Lichen is certainly not the quickest way to go, but it is certainly the easiest, cheapest, and the most technologically feasible. So whenever you see some lichen, just remember that what seems like an insignifigant organism might someday help to achieve one of the most grandiose goals of science fiction.

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