Modern day Mars is not dissimilar to the most extreme deserts on Earth. It is a barren and desolate place, yet is still the subject of intense study and curiosity. Why is there such interest in this planet, and why do so many space scientists believe it may once have harbored life?
Essentially, the presence of life on any planet or satellite is determined by five key factors:
1. Temperature.
For life to exist, the temperature of a planet must allow water to exist in a liquid state. Too low and water will freeze (rupturing cells of an organism as it expands), too high and it will simply evaporate as water vapor. Space scientists often refer to the ‘Goldilocks zone’ – the distance from the Sun at which the temperature is just right for liquid water. Earth is actually just outside this zone, but compensates via warming from the greenhouse effect.
Due to its distance from the Sun and tenuous atmosphere, the average surface temperature of Mars is -63°C although maximum temperatures of around 20 °C have occasionally been recorded in some locations. Water would almost certainly be found as solid ice in most areas of Mars.
2. Atmosphere.
The atmosphere fulfills several roles. The presence of ozone in Earth’s atmosphere means that most of the Sun’s harmful ultraviolet radiation is absorbed before it reaches organisms on the surface. The atmosphere also provides sufficient air pressure to keep water in a liquid state – water will boil if the surrounding pressure is reduced too far. Finally, the atmosphere provides crucial elements that are used for key life processes (e.g. oxygen is necessary for organisms that release energy via aerobic respiration).
The Martian atmosphere consists almost entirely of carbon dioxide (95%), with smaller amounts of nitrogen and argon also present. There is a notable absence of ozone, which means that ultraviolet radiation can reach the surface of the planet unimpeded. This has grim consequences for life, as ultraviolet radiation can break apart the very molecules of living organisms (including DNA, proteins, carbohydrates, etc.). The Martian surface is therefore sterile, although there is a possibility that life may still exist underground where it is shielded from harmful radiation.
As Mars does not possess a magnetic field (due its lack of geological activity), the atmosphere has been steadily eroded by the Solar Wind. The atmosphere of Mars is now very insubstantial with very low air pressure at the surface. On Earth, the mean air pressure at sea level is quoted as around 101 kPa; by comparison the mean air pressure on the surface of Mars is only 0.6 kPa. This is important because of the relationship between air pressure and the boiling point of water: as air pressure decreases, the boiling point of water also decreases. At the extremely low pressures currently experienced on Mars, any liquid water would immediately boil and turn to water vapor.
3. Liquid water.
Water must exist in a liquid phase in order to support life. The main reason is that it acts as a universal solvent – many substances can dissolve in liquid water, therefore acting as a medium in which biochemical reactions can occur. Without liquid water, many of the crucial reactions necessary for life would simply never happen.
Because of the issues of low temperature and low pressure, large bodies of liquid water cannot exist on the surface of Mars. Although substantial amounts of water have been identified in the permafrost and polar caps, it is always found as ice. Water could not exist as a liquid on the surface of Mars for the reasons of low pressure mentioned previously.
However, observations of Martian strata and surface features indicate that this may not always have been the case. There are many valleys and ridges on Mars which appear to have been formed from ancient rivers and streams. One such area is Gale Crater, which NASA’s Mars Science Laboratory and Curiosity rover are currently investigating. The crater shows the deposition of several layers of sedimentary rock – something which could only have happened due to the action of liquid water. If liquid water once existed on Mars, it follows that there must once have been a more substantial atmosphere to provide the necessary pressure.
4. Energy.
There must be an energy source to power the reactions necessary for life. Most commonly, the Sun is the ultimate source of energy for ecological food chains, the energy being absorbed and stored in sugars by organisms capable of photosynthesis. However, it should be noted that there are some organisms (various species of bacteria) that can derive their energy from minerals found deep underground in the Earth’s crust.
Like Earth, Mars receives energy from the Sun and in enough quantity to support ecological food webs. However, Mars often experiences global dust-storms which drastically reduce sunlight intensity for several months at a time. Life forms which derive energy from sources other than the Sun would clearly have an advantage in such an environment. One example of such an organism on Earth is Ferrobacillus ferroxidans, an ‘iron-eating’ bacterium which derives energy from oxidizing iron (present in abundance on the surface of Mars as iron oxide). Methanogenic bacteria also exist on Earth and derive energy through reactions involving carbon dioxide and hydrogen, releasing methane gas as a waste product. Substantial quantities of methane have been detected in the Martian atmosphere, suggesting release from an underground source. Whether this could be from methanogenic bacteria is open to debate, but the scientific community generally agrees that it warrants further investigation.
5. Nutrients.
Nutrients supply the energy needed by organisms and also provide elements needed as ‘building blocks’ for complex biological molecules. There must be a way of cycling nutrients between atmosphere, terrain and oceans in order for organisms to make use of them. On Earth, this happens as geological activity exposes elements to the surface where they may then be cycled between organisms and the environment.
On Mars there is no discernible geological activity, although huge extinct volcanoes such as Olympus Mons and tectonic fault lines such as Valles Marineris indicate that this was not always the case. Being only half the size of Earth, the interior of Mars has cooled rapidly and lost heat – so much so that it is thought to have solidified, causing the cessation of any significant geological activity. Without geological cycling of elements, it is unlikely that any complex ecology could be supported.
Without geological activity, there is a further consequence: the loss of a protective magnetic field. On Earth, substantial convective currents within the molten mantle result in a ‘dynamo’ effect with a powerful magnetic field being produced around our planet. This protects our atmosphere from the Sun’s radiation and stops it from being eroded and depleted. Mars does not currently possess a magnetic field, probably because of a cold and dormant core. As a result, its atmosphere is exposed to erosion from the Solar Wind.
In summary
The surface of Mars can no longer support life but may have done so in the past. Being smaller than the Earth, Mars is likely to have lost heat more rapidly and cooled internally. This would have caused the internal dynamo effect within Mars’ interior to cease and the protective magnetic field to ebb away. Without this protection the atmosphere would have been steadily eroded by the Solar Wind, with air pressure dropping so low that water could no longer exist in liquid form.
Although the surface of Mars is no longer supportive of life, this does not mean that life cannot be found elsewhere. If life did become established in the distant past, it is perhaps now sheltering deep underground. Plumes of methane gas in the Martian atmosphere appear to be a strong bio-indicator for primitive organisms such as bacteria, and there is still the very real possibility that life may be found on this fascinating planet. Discovering life on an alien planet would raise profound questions about the origins and evolution of life on Earth. It is for this reason that space agencies such as NASA and ESA continue to invest such enormous sums of time and money in satisfying mankind’s curiosity.
