| 1

HOW TO BUILD A HABITABLE PLANET

LIFE SUPPORT SYSTEMS OF THE EARTH AND MARS

1. INTRODUCTION

The purpose of this paper is to identify the factors that make a planet habitable by observing the structures and processes that occur on the Earth and comparing them to those on Mars so as to conclude whether life currently is or could be made possible on our neighbouring planet.

2. THE EARTH

There are four different biogeochemical systems on Earth that allow chemicals to flow between reservoirs and interact with the environment. They are the atmosphere, lithosphere, hydrosphere and biosphere and each plays a key role in maintaining the balance that allows life to flourish on the planet. The cycles in which the chemicals move from one system to another occur at different speeds (ranging from a single nutrient cycle to geological timescales) and generally go through organic (interacting with living organisms) and inorganic (interacting with the physical environment) phases.

2.1 Atmosphere

The Earth’s atmosphere is made up of various gases such as nitrogen, oxygen, carbon dioxide, argon, neon, helium, water vapour, methane and others.

The amount of carbon dioxide present in the atmosphere has changed dramatically since the early days of the planet. Today it makes up only 0.03% of the atmosphere and is a key element in keeping the Earth’s systems in dynamic equilibrium by recycling oxygen (photosynthesis) and helping maintain a comfortable temperature for life to exist on the planet (greenhouse effect). The greenhouse effect is a natural process through which part of the heat energy emitted by the Sun enters the atmosphere, is partially reflected from the Earth’s surface back into the troposphere and is reflected back to the Earth by the ‘barrier’ formed by greenhouse gases (carbon dioxide, water vapour, methane and others). In this way, the heat energy from the Sun is kept in the lower atmosphere and is redistributed across the planet by convection currents.

A few billion years ago however, the high concentration of carbon dioxide (90%) meant the greenhouse effect was far more powerful and that the Earth was a very hot place. Over time however, life forms appeared that began storing the energy from the Sun through the process of photosynthesis and thus absorbed the carbon dioxide, releasing oxygen into the atmosphere. Over the next few million years, the quantities of oxygen have fluctuated: it reacted with the iron from the seas and rocks but when the excess quantities could no longer be absorbed by the iron, oxygen levels rose and caused

| 2

wildfires which in turn created more carbon dioxide, eventually bringing down the concentration of oxygen in the atmosphere to a steady 21%. Oxygen plays a key role in aerobic respiration, chemical weathering, soil formation and the oxidation of volcanic gases.

Also present in the Earth’s atmosphere is argon, making up 0.9% of it. Although the exact role of this element is not known, scientists have identified that it serves as a dilutant, as it is relatively inert.

The largest concentration of gas in the atmosphere is that of nitrogen at 78%, a gas that also acts as a dilutant to the other atmospheric gases, as its triple (covalent) bonds make it very stable and inert. Certain groups of bacteria are able to process the nitrogen found in the atmosphere to produce ammonia, which is more reactive and this can then be converted into nitrites and nitrates by other bacteria. As a consequence, we find nitrogen compounds in plants and animals.

The Earth’s atmosphere can be divided into four different layers according to altitude: the troposphere, the stratosphere, the mesosphere and the thermosphere.

(The Environment: Principles and Applications, Chris Park, 2001)

The lowest layer, reaching up to 10-12km above the Earth’s surface, is the troposphere: a warm, moist layer containing most of the dust, vapour and mass of the atmosphere (90% of its gases). This layer is responsible for the different climates of the planet and it

stratopause

tropopause

mesopausethermosphere

mesosphere

stratosphere

troposphere

| 3

is where most weather processes and biological activity (including respiration) take place. The lower levels of the layer obtain their heat due to their contact with the Earth’s surface. The troposphere is where the greenhouse effect occurs and the layer in which convection currents redistribute the heat energy sourced from the Sun. It is also where atmospheric pressure is at its highest, as due to gravity, the atmosphere is pulled towards the Earth and most of the mass of the atmosphere is found in the troposphere.

Above the troposphere (alt. 15-50km) is the stratosphere: a relatively thin, dry, stable and weather-free layer which also includes the ozone layer. This very thin layer is made up of triatomic oxygen molecules which absorb the incoming UV radiations from the sun and use up the energy so that the UV waves do not reach the Earth’s surface. As the energy from the UV waves is absorbed, a reversible process takes place: diatomic oxygen molecules are broken up into single atoms, which then bond with other diatomic molecules to produce triatomic molecules (ozone); this in turn gets broken up into O and O2 and the process is repeated over and over.

O2 → O + O ; O + O2 → O3 ; O3 → O + O2

Because solar radiation is absorbed in this process and due to the fact that the stratosphere is heated from above, the temperatures in this layer increase with altitude.

The mesosphere is the atmospheric layer between the altitudes of 50 and 85km. It protects the planet from meteorites by burning them up either completely or so as to make them smaller in size (and thus relatively harmless) before they reach the surface of the Earth. Due to the decreasing density of the atmosphere in this higher layer, solar radiation cannot be absorbed and temperatures decrease with altitude.

The outer most layer of the atmosphere is the thermosphere or ionosphere, between 85 and 100km above the ground. Short-wave solar radiation energizes the ions in the thermosphere, making them move at very great speeds and an increase in temperature is recorded. This layer is virtually a vacuum and it protects the Earth from solar cosmic particles.

2.2 Hydrosphere

About 3.5 billion years ago, soon after the formation of the planet, temperatures began to stabilize and the global rainstorms formed by condensing volcanic steam gave birth to a global ocean of water. It is this same water that is continuously circulating through the atmosphere, lithosphere and biosphere today that forms the hydrosphere. The cycle in which water flows from one system to another and is distributed around the planet (though unevenly in time and space) is the hydrological or water cycle.

Water moves from one reservoir to another by various processes and in doing so, it passes through all its three physical states (solid, liquid, gaseous) and interacts with the three other biogeochemical systems. The processes involve heat energy exchange which leads to a change in temperature.

| 4

Water reaches the atmosphere through evaporation from oceans, surface storage or soil and through transpiration from vegetation. It accumulates in the atmosphere, gets redistributed by global and regional wind systems and through condensation it is released back to the surface by precipitation. Here, water can either flow into rivers or lakes. Water stored in ice and snow can either sublimate into water vapour and accumulate in the atmosphere or it can melt and infiltrate into groundwater reserves, eventually reaching the ocean by discharge.

(http://en.wikipedia.org/wiki/Water_cycle)

Major natural stores of water

in the global hydrological cycleStore PercentageOceans 97.41Ice caps and glaciers 1.9Groundwater 0.5Soil moisture 0.01Lakes and rivers 0.009Atmosphere 0.0001

The global water cycle: storage amounts and timesStore Size (km2) Typical residence timePlants and animals 700 1 weekAtmosphere 13000 8-10 daysRivers 1700 2 weeksSoil 65000 2 weeks – 1 yearLakes, reservoirs, wetlands 125000 yearsRock (groundwater) 7000000 days – thousands of yearsIce 26000000 thousands of yearsOceans 1370000000 thousands of years

| 5

(The Environment: Principles and Applications, Chris Park, 2001)

What is striking about water is not simply its presence, but its abundance: 70% of the surface of the planet is covered with water, it is found in the bodies of most living organisms and also accounts for 5% of the volume of rocks. It plays a vital role in biological and physical processes and metabolism - its importance to life on Earth cannot be overstated.

After its formation and stabilization, the planet was able to retain water thanks to its gravitational field, adequate atmospheric pressure and relatively stable temperatures. In nature, water can be found in all its three states: solid, liquid and gaseous form.

Water vapour is known to be one of the major contributors to the naturally occuring greenhouse effect and helps keep the Earth at a comfortable temperature for life.

As a liquid, water acts as a medium that influences climate in that it redistributes the excess energy from the tropics to the poles via ocean currents.

Its unusual chemical properties make water almost an ‘impossible’ substance and is an excellent medium for chemical reactions:

transparency: allows certain wavelengths of light to enter it, so that aquatic organisms can photosynthesise;

it is a good solvent, which means it allows a large range of chemical reactions to take place, it can transport a variety of chemicals, as well as erode materials such as limestone;

incompressibility: can build a hydrostatic structures such as a hydrostatic skeleton;

viscosity: allows free flowing transport;

surface tension: certain insects can walk on water;

cohesion: provides insulation;

liquidity: makes conditions for aquatic life possible (medium, temperature, etc.), acts as a medium for cells, allows soil drainage and sediment flow and reshapes the geological features of the Earth, feeds plant and animal life.

As water only exists on the planet in a limited, fixed amount which gets transported, recycled and redistributed in the four biogeochemical cycles. Solar energy as well as gravity act as fuel for these processes and the hydrological cycle forms a closed system offering an inexhaustible resource that is synonymous with life.

2.3 Lithosphere

Most of what we know about the interior of the Earth has been concluded from indirect evidence. Seismic and gravity studies as well as the study of meteorites, particularly chondrites, which are representative of the material that accumulated to form the Earth, give scientists some idea of how the Earth is structured.

| 6

The Earth appears to be made up of a series of concentric spherical layers, each with its own unique set of roles and properties.

The inner most part of the planet is the core: we can distinguish between an inner and an outer core. The inner core is a solid sphere rich in iron and nickel with a radius of 1255km. Temperatures range between 4500 and 5500°C, making it as hot as the surface of the Sun. The pressure in the inner core is extremely high, about 3.5 million times higher than the atmospheric pressure. The outer core envelops the inner core and has a thickness of 2220km. It, too, is rich in iron and nickel, but the material is in its liquid state due to the fact that it is not subjected to such enormous amounts of pressure as the material of the inner core. The outer core becomes cooler as we move farther away from the inner core, the minimum temperature being 2000°C. This outer layer of the core is believed to be rotating around the inner part similar to a dynamo, and therefore generates the Earth’s magnetic field whose role it is to protect the planet from meteorites.

Enveloping the outer core is the mantle: this is a 2900km thick layer of dense rock. It is composed mainly of minerals ( silicate compounds), iron and magnesium. Although it is in solid form, the material has a fluid behaviour. The deepest ‘shell’ of the mantle is the asthenosphere. Because of the heat and pressure of the inner Earth, as the core and mantle materials interact, convection currents are generated which carry the partially melted rocks of the mantle and are responsible for tectonic plates’ movement. The next layer of the mantle, referred to by geologists as the lithosphere, is a 75km thick layer of rigid, solid material that merges with the moho. The moho, or Mohorovicic discontinuity, is the boundary between the mantle and the crust. This is the outer most shell with a varying thickness of 8-65km. The crust is made up of the least dense rocks representing the surface of the Earth (includes all of the landforms and oceans) and appears to be floating on the denser rocks of the mantle. We can distinguish two different types of crust:

sima (oceanic crust): made of thin, dense, heavy material that tends to deform rather than break when subjected to geological stress

sial (continental crust): made of brittle, thick, light material which will fracture along individual faults when subjected to geological stress

The phenomenon of plate tectonics is associated with the movement of large fragments of the planet’s crust (plates) over the ‘flowing’ rocks of the mantle. Various geological activities (volcanic eruptions, earthquakes, the shaping of mountains, folding and faulting ) occur along the margins of plates, as this is where material from the mantle and heat energy can escape to the surface.

Volcanoes form at converging or diverging plate boundaries and thanks to eruptions, the magma from within the mantle can surface and be deposited on top of the Earth’s crust as igneous rock. Volcanic ash is rich in minerals and volcanic soil is very fertile. All volcanic rocks contain various quantities of silica and can be divided into three groups: mafic (silica-poor rock such as basalt), intermediate(such as andesite) and felsic (silica-

| 7

rich rock such as granite).The igneous rock gets weathered in time and is transported and redistributed along the Earth’s surface, where it undergoes lithification and becomes sedimentary rock. This can be clastic (composed of fragments of other pre-existing rocks like sandstone), organic (composed of dead organic materials such as limestone or chalk) or chemical precipitates (formed by deposition of materials that were previously dissolved in water, such as dolomite). As more layers accumulate, the rock is buried and due to factors such as heat and pressure, it is compacted to form metamorphic rock (such as gneiss, schist or slate). This occurs when one plate slides under another: the rock overlying the mantle is moved by convection currents, gets pushed back into the mantle (subduction) where it is melted and turns back into magma and is transported by the same currents within the Earth until it resurfaces again through volcanic activity, thus completing the rock cycle.

Volcanic activity is also particularly significant as during eruptions, the carbon dioxide from the rock is released back into the atmosphere (thus linking the CO2 from the lithosphere with the CO2 in the atmosphere).

2.4 Biosphere

Carbon is to be one of the most widely distributed elements on the planet and, due to its versatility, one of the basic building blocks of life. A carbon atom has only four electrons on its outer most shell, which gives it the ability to form a very wide range of complex and intricate bonds with other chemicals found on Earth. This makes it an excellent candidate as a fundamental building block for all forms of life. It can form up to four covalent bonds with other elements, it has the capacity to bond with itself to create long

burial, compaction ( (heat, pressure)

melting (heat)

cooling, uplifting(magma crystallization)

weathering, erosion, transport (denudation)

| 8

molecules, and it can also form double or triple bonds with other carbon atoms. The versatility of this element allows it to link together the four life support systems of the Earth.

Carbon is constantly circulated and recycled, and the processes it undergoes involve interaction with living organisms and their surrounding environment (organic and inorganic phases).

Carbon is stored in the atmosphere as carbon dioxide. This is used by plants for photosynthesis and consequently oxygen is released into the atmosphere. The carbon compounds produced by the plants are passed along in the food chain, and the carbon eventually returns to the atmosphere by respiration, natural combustion (such as wildfires) or decay (through death and decomposition). Carbon dioxide can also enter water directly (diffusion), dissolve into it and bond with the water molecules to form carbonic acid. This is converted into calcium carbonate, which accumulates in sediments. Dead aquatic organisms can also end up on the bottom of the ocean and undergo lithification. It is thanks to chemical weathering and volcanic eruptions that the carbon trapped in the rocks is released back into the atmosphere.

Carbon is also important in the form of carbon dioxide as a greenhouse gas: it is responsible for the regulation of global temperature.

eaten by

herbivorehydrocarbons in fossil fuels

carbon compounds in micro organisms

photosynthesis

plant respiration

excretion or death

deathdecomposition by bacteria and fungi

fossilisation

sedimentation

erosion by acids in rain water

combustion

microbial respiration

carbon compounds in animals

| 9

3. MARS

Among the several unmanned missions that have reached Mars in the past, it was the two Viking missions (1976),the more recent Mars Pathfinder (1997) and Phoenix (2008) that have provided us with the most important information we now have about our neighbouring planet Mars. In addition to images, these missions have added to our knowledge with atmospheric, geophysical, geochemical and thermal data.

3.1 Atmosphere

The atmosphere of Mars is similar to what scientists believe the early Earth’s atmosphere was like: predominantly carbon dioxide (95%), nitrogen (2.7%), argon (1.6%) with traces of oxygen (0.13%), water vapour and methane.

Since Mars lost most of its magnetic field about 4 billion years ago, solar winds and cosmic rays can interact directly with the Martian ionosphere and thus keep the atmosphere thinner than it would normally be as atoms from the outer layers are easily stripped away.

Compared to the early Martian atmospheric composition, “Most of the original volatiles (elements and molecules that normally exist in gaseous form) probably remain, but they are locked in the porous rock of the surface layer. This implies that the planet’s atmosphere has probably changed in the course of its history.” (Peter Cattermole, 1994)

Presently, even though due to its large concentration of carbon dioxide the Martian atmosphere should have greater potential to trap heat from the sun through the greenhouse effect, because of its 227.9 million km from the Sun (as opposed to the Earth’s 149.60 mil km), as well as its comparatively low atmospheric pressure (due to low gravity), the average temperature on the planet ranges between 20°C and -140°C (the greenhouse effect only warms the planet by 10°C, as opposed to the Earth where temperatures increase by 33°C due to the same phenomenon).

3.2 Lithosphere

The Martian surface is largely iron oxide dust (rust), giving the planet its reddish appearance. North of the Martian equator, the landscape consists mostly of volcanic plains and huge shield volcanoes concentrated on a swelling in the lithosphere (Tharsis Bulge). Although currently we do not observe any geologic or tectonic activity, there are high chances of them having occurred in the past.

Mars shows signs of past volcanic activity and it is assumed that its iron sulphite core surrounded by a silicate mantle is much cooler than that of the Earth. The comparatively small mass of Mars means it grew cooler more rapidly than our planet and so volcanic activity grew smaller in intensity. However, “it is possible that some [volcanoes] are still active” (Simon Lamb, David Sington, 1998)

| 10

Scientists today assume that 4-3.5 billion years ago, when water was still flowing on the Martian surface, most of the CO2 from the atmosphere formed an acid with the rain, reached the surface water through precipitation and became trapped in the rocks. This, along with hydrogen fluxing (the presence of water vapour in the upper atmosphere dissociating under the influence of solar radiation causing a steady leakage of atmospheric gases into space) means that the level of atmospheric CO2 sank and the planet became cooler. It had no way of replenishing its atmosphere, and the intensity of volcanic eruptions was also decreased. Some scientists speculate that this must have brought about a Martian ice age about 3 billion years ago. (Simon Lamb, David Sington, 1998)

Evidence of the stability of the Martian crust can be found in huge volcanoes such as Olympus Mons, which rises 26km above the surface and is 500km across at its base. It strongly suggests that there is no tectonic activity on Mars, meaning the rock cycle is inactive (there is no recycling of rocks).

Plate tectonics do not currently operate on Mars, therefore the CO2 trapped in the rocks cannot be released back into the atmosphere.

3.3 Hydrosphere

It is assumed that Mars formed through processes similar to those of the Earth, and that water was either released from the cooling planet as vapour or that it was brought by comets and other colliding heavenly bodies.

South of the equator, the surface is heavily cratered by impacts and shows extensive valley networks and outflow channels. The Martian landscape suggests that our neighbouring planet once had flowing rivers (the equatorial canyon system Valles Marineris is supporting evidence), however it seems to have lost most of its water due to factors such as low atmospheric pressure, low gravity and a very thin atmosphere.

Although scientists are able to observe polar ‘ice’ caps on Mars (made up of frozen carbon dioxide and water), there appears to be no liquid water on Mars even though sometimes the surface temperature of the planet rises above the freezing point (0°C). Mars’ atmospheric pressure is one hundredth that of the Earth and because the lower the air pressure, the more rapidly any liquid evaporates, water cannot exist in liquid form on this planet (it would evaporate instantly).

One of the rocks that Pathfinder (1997) examined in an old Martian water channel proved to be rich in silica, making it very similar to Earth’s rocks that have undergone some sort of differentiation, hinting at the past existence of liquid water on the surface of Mars.

“Further evidence that liquid water once existed on the surface of Mars comes from the detection of specific minerals such as hematite and goethite, both of which sometimes form in the presence of water.” (http://en.wikipedia.org/wiki/Mars)

| 11

4. CONCLUSIONS

According to the current understanding of planet habitability, there are four major factors that determine whether life as we know it can exist on another planet: an energy source, liquid water, a suitable atmosphere with the basic building blocks of life and relatively stable temperatures that would allow life to exist and evolve.

4.1 Energy source

As opposed to the Earth, Mars is too far away from the Sun, resulting in a low influx of solar radiation. Its thin atmosphere (in which CO2 is high in proportion but not in quantity) is incapable of trapping and transferring heat energy across the surface of the planet and thus Martian surface temperatures do not favour the development of living organisms.

4.2 Liquid water

The low atmospheric pressure inhibits Mars from retaining the water so integral to life and metabolism in liquid form for long periods of time. This is possible only at very low altitudes and for very short periods of time. On Mars, liquid water does not exist above the surface.

The water frozen at Mars’ polar ice caps, if melted, could cover the entire planet in an ocean 11m deep. The atmospheric factors however make such an event highly unlikely or difficult even if it were attempted artificially.

4.3 Atmosphere, temperature and chemical building blocks

Although the sun may have given off less heat in the past, high levels of CO2 contributed to a comfortable temperature on the Earth’s surface, allowing life to develop.

Mars’ present-day atmosphere very much resembles Earth’s early atmosphere in this respect as far as ratio is concerned, however quantitatively it does not favour terrestrial life. In spite of the high concentration of carbon dioxide, the Martian greenhouse effect is far too weak to maintain suitable temperatures.

The nitrogen and oxygen levels would be much higher in the presence of living organisms, although “evidence suggests that single-cell plants and animals developed from inert materials in the Earth’s crust.” (Chris Park, 2001) It was these first organisms that have, in essence, created an atmosphere that was suited for the birth and evolution of other more advanced organisms. So, if we were to make Mars habitable and restructure its atmosphere, this would be among the first priorities: we would need to make conditions suitable for these early organisms so that they in turn can further induce the changes necessary to recreate an environment that will harbour terrestrial life.

Scientists have suggested that certain micro organisms (methanogens) may already be present on Mars, as they could account for the quantities of methane found on the planet. Traces of methane and formaldehyde were recently discovered on Mars. The

| 12

existence of these chemicals in view of the fact that they would normally break down quickly in the atmosphere, hints at the existence of life. It has also been suggested that these chemical compounds may also be the result of volcanic activity. If however it is indeed living organisms that are responsible for the presence of these compounds, we would need to look for Martian life far below the surface as liquid water may exist there due to milder temperatures.

Phoenix (2008) has found that Martian soil is highly alkaline and indicates the presence of soil nutrients (high levels of magnesium, sodium, potassium and chloride were detected). But even though organisms have readily available nutrients, they would still be confronted with exposure to solar radiation: Mars has no equivalent of an ozone layer. We again conclude that conditions are not suitable for organic life to develop at the surface: “In 2007, it was calculated that DNA and RNA damage by cosmic radiation would limit life on Mars to depths greater than 7.5 metres below the planet's surface.” (http://en.wikipedia.org/wiki/Life_on_Mars). This information suggests that the best possible locations for discovering life on Mars could be beneath the surface, in areas that have not been studied yet.

4.4 Plate tectonics and volcanic activity

The relative geological inertia of Mars means that it no longer generates a magnetic field to protect from harmful solar radiation, and that there is no more inner heat and motion to form the convection currents needed for tectonic movement. This implies that the all the Martian CO2 is trapped partly in the atmosphere and partly in rocks (the lack of tectonic and volcanic activity means the rock cycle has been broken and the carbon dioxide in the rock cannot be renewed and released back into the atmosphere).

Most of the research and exploration done on Mars in search of extraterrestrial life is based on the assumption that these potential living organisms will be similar to those found on Earth and therefore necessitate similar conditions. Yet there is another possibility: life forms may evolve around different metabolic mechanisms. Silicon has been suggested as an alternative to our main building block, carbon, and ammonia may very well be an alternative to water.

The evidence we have gathered so far indicates that Mars may not boast the conditions necessary for life, at least not at the surface, or not as we know it: it may harbour life in forms that we do not recognize. We may very well be wrong in looking to different planets either searching for or trying to create a mirror of our own.

As a species whose apparition and evolution was made possible by a plethora of favourable factors, cycles and systems (and some would argue, intelligent design), we are responsible for the Earth and must understand its workings and know our own history well before we can seek out new horizons.

| 13

5. REFERENCES AND BIBLIOGRAPHY

· Atmospheric Processes and Human Influence (Paul Warburton)

· The Environment: Principles and Applications (Chris Park, 2001)

· A Short History of Planet Earth (J. D. MacDougall, 1996)

· Water: Its Global Nature (Michael Allaby, 1992)

· The Encyclopaedia of Earth and Other Planets (Peter Cattermole, 1994)

· Earth Story – The Shaping of Our World (Simon Lamb, David Sington, 1998)

· http://en.wikipedia.org/wiki/Mars (retrieved 30/09/2010)

· http://en.wikipedia.org/wiki/Life_on_Mars (retrieved 30/09/2010)