Can Life exist on Mars?
Although the issue of whether there is life on Mars has not been resolved yet and probably won't be for several years, some intellectual speculation can be prrformed as to the potential for the Red Planet to eventuallyharbor microbial or other higher forms of life.
The following study is adapted from "Terraforming : Engineering Planetary Environments", a fascinating book by Martyn Fogg.
It is essential to define the limits to habitability of the entire range of life, in terms of those environmental parameters directly relevant to physiology, such as gravity, temperature, pressure, atmospheric composition, illumination etc.
Importance of Temperature
Temperature is an important factor determining if life can exist on a planet. Different types of living beings have different adaptations for survival in varying temperatures. The temperature range tolerable by microorganisms is remarkable and the microbial ecosystems of Antarctica typify the sort of life that can survive where extreme cold is a limiting factor. Other bacteria, for example thermophilic bacteria, prefer scalding hot temperature and thrive within hot springs, both on land and on the ocean floor. However, their metabolic optima are usually very close to their upper temperature limit - a few degrees above and they are killed.
The range of tolerable temperatures for multicellular organisms is much more restricted. Plants and most animals fall under this category. Many plants not specifically adapted to cold and suffer fatal frost damage by one night's exposure to 0 degree Celsius. In general, productive photosynthesis requires temperatures of >0 degrees Celsius and preferably >10 degrees. The optimum range for germination of most seeds and growth of most plants is 10-40 degrees Celsius. Thus, while microorganisms can survive and reproduce successfully in extreme environments, plants cannot. These temperatures ranges are very similar to those tolerable by animal life, which is hardly surprising since plants are the basis of the food chain.
Sapiens have shown a particular ability to cope with temperature variations and are thus found in more varied locales than any other animal species. This is, of course, because of one of our earliest technological inventions- clothing. At high temperatures, the relative humidity of the air becomes an equally important factor; above human body temperature (37ºC) evaporative heat loss via perspiration must take on the entire burden of thermostasis and this cannot occur if the air is already saturated with water vapor (100% relative humidity). As temperature increases, relative humidity must be reduced and the time of exposure lessened to avoid hyperthermia. For instance, people can just about withstand 50% for 24 hours, but any longer than this would require lowering the temperatures, humidity, or both. Humans cannot exist remote from the ecosystems that provide life support. Taking this into account, a mean average temperature range of 0-30ºC seems reasonable for sustainable human habitability, so long as most extreme of seasonal variations do not take daily average temperatures much in excess of 40ºC or far below -10ºC.
Importance of Atmospheric Pressure and Chemical Composition
There is expected to be a wide range of atmospheric pressure consistent with habitability; the total barometer pressure consisting of tolerable partial pressures of essential constituents, such as nitrogen, oxygen, carbon dioxide and water vapor. This is especially so for microorganisms with their extraordinary adaptability and metabolic variety. For example, bacteria have demonstrated the ability to tolerate and metabolize gaseous compound that would be dangerous to higher organisms such as hydrogen, ammonia, methane, and hydrogen sulfide. For a sustainable bacterial ecology, nitrogen fixation will be required and it has been estimated that the organisms responsible could carry out this process at pN2 between 1-10 mbar, much less than the 790 mbar in the Earth's present atmosphere. The minimum total atmospheric pressure that might be consistent with bacterial habitability would be made up from water vapor in equilibrium with an average planetary temperature of ~0ºC (~6 mbar) plus essential amounts of N2, O2, and CO2. More salubrious conditions are needed by higher plants which are complex aerobic organisms that evolved in concert with animals. They are limited in their function by both pCO2 for photosynthesis and pO2 for respiration-the necessity for the latter is sometimes overlooked as it is easy to assume that since plants evolve O2 as they grow, they are never limited by oxygen deficiency.
Other Conditions for Habitability
There are other physiologically limiting factors which must be taken into account when considering planetary habitability, and the most important of these are listed in the following table:
For humans to be able to settle other planets, it will first be essential to establish the range of gravitational acceleration consistent with normal development and a healthy life. Unfortunately, there is very little evidence to go on. Terrestrial life has evolved under the uniform and constant gravitational acceleration of 9.8 m/s2 ; astronauts and other laboratory organisms have spent extended times in orbit subject to ~0 m/s2 - there is unfortunately no data of long-term survival at other accelerations and the only experience we have at all comes from experiments in aircraft and centrifuges. However, it does appear that there are serious problems with prolonged human life at zero gravity.
From the above analysis it may be concluded that the Martian conditions are not very favorable for housing any form of life as we know it. Obviously humans wopuld perish from incidence o UV rays, extremely cold temperatures and lack of oxygen added to a very low atmospheric pressure. The data also shows (see Composition of the Martian atmosphere) that even bacterial colonies would not be able to prosper in the Martian atmosphere, incapable of any nitrogen fixation process and with a pressure that is too low. The only serious possibility of finding life on Mars would be in ecosystems buried deep under the Martian permafrost, similar to those found in the dry valleys of the Anctartic.
The evidence presented in 1996 referred to the study of the Martian meteorite points to a possibly extinct form of life, belonging to another epoch in which Mars was a warmer and wetter planet.