Earth is a vibrant blue planet, the only place that we can be sure that life exists. As A Perfect Planet shows, a complex and interconnected set of systems operate to form our environment. Every planet except one in the Solar System has a gaseous atmosphere, as do some moons and even some dwarf planets. Why does the Earth’s atmosphere make it so perfect as a home for life?
The Earth’s atmosphere
The atmosphere is thin and light, a gas that is only bound to the solid mass of the Earth by gravity. The mass of the atmosphere is about 250 times less than that of the oceans and accounts for less than one millionth of the total mass of the planet. The atmospheric pressure and density of air decrease exponentially with height above the surface. The term ‘exponentially’ means that for every increase in height by the same distance, the atmospheric density falls by the same factor. In the case of the Earth, density halves for every increase of around 5.9 km in altitude. At the top of Mount Everest (8.85 km above sea-level), about 70% of the atmosphere is already beneath your feet. Contrast this with the radius of the Earth itself, around 6,371 km. The atmosphere is really a very thin skin.
The image above was taken by an astronaut on the International Space Station as the Sun sets behind the outer edge of the Earth. Astronauts aboard the International Space Station experience sixteen sunsets and sunrises per day as it orbits Earth! The lowest layer of the atmosphere is mostly illuminated red and is cloudy in this image. This is the troposphere, where the atmosphere churns as it carries heat upwards by convection from the surface, like water in a saucepan when heated from below. The troposphere is where nearly all of our familiar weather happens. Long-distance passenger aircraft fly in the upper troposphere. The troposphere extends to about 10–20 km from the surface; it is up to 20 km deep near the equator, where the atmosphere is convecting strongly, and can be as little as half that at the poles. Within the troposphere the temperature falls by about 6°C for every kilometre of altitude gained, so by the top of the troposphere the temperature can fall well below -80°C and the pressure and density are only a tenth of those at the surface. These conditions require a pressurised and heated cabin or a space suit.
The atmosphere above the troposphere is called the stratosphere and appears a clear, blue colour in the image. In the stratosphere the temperature begins to rise again with increasing height. In contrast to the troposphere, the stratosphere is very stable, or stratified, as implied by the name. Although 90% of the mass of the atmosphere is below in the troposphere, the stratosphere still makes a vital contribution to the environment beneath. Only a few aircraft and research balloons are built to be able to fly in the stratosphere in the thin air. Most of the sky would already be dark, rather than blue, for a passenger in such an aircraft. The top of the stratosphere lies at about 50 km altitude. Although there are more rarefied atmospheric layers above, for practical purposes the environment is already much more like ‘space’ by this point. By convention ‘space’ is said to start at an altitude of 100 km, although this is a rather arbitrary round number and there is no ‘top’ to the atmosphere as such, rather atoms and molecules simply become increasingly rare, but a very few reach out even beyond the Moon’s orbit and are lost to Earth.
The connected environment
A Perfect Planet emphasises how different elements of our environment are interconnected. The atmosphere responds to varying heating by sunlight, driving movement. These motions in the troposphere are what we experience as wind. The oceans supply water vapour, which absorbs and releases heat, forms clouds and scatters sunlight. The patterns of continents, oceans and sea ice at the surface determine the heating that drives the troposphere from below. Volcanic activity has outgassed the atmosphere itself from the body of the forming planet and continues to supply many trace gases and small particulates to the mix. Variations in volcanic output are connected to sometimes huge climate changes. Life has modified the composition of the atmosphere, resulting in the present range of gases, which is very different to the carbon dioxide-dominated atmospheres of Venus and Mars. Now humans are leaving our own mark in pollutants and greenhouse gases.
But despite being so thin and light, the atmosphere is far from insignificant. The influence is not all in one direction and the atmosphere plays a vital role in the evolution of the other elements. Winds drive ocean currents and rainfall alters ocean salinity and so density, which drives other, deep circulation patterns. Atmospheric transport spreads volcanic outputs over wide areas. The climate change resulting from the atmospheric response to volcanic gases and aerosols is thought to have melted the ice in periods when Earth was deep-frozen and to have caused mass extinctions several times in the past. The atmosphere today is both a consequence of, and essential for, life.
A deep breath
A basic necessity for humans is to breathe oxygen at a sufficient pressure for our respiratory system to work. Complex, multicellular life on Earth needs an atmosphere for either respiration or photosynthesis. Even aquatic species rely on dissolved oxygen from the air. Photosynthesis, in contrast, requires carbon dioxide, also from the atmosphere. To survive, both animals and plants need a considerably larger mass of air per day than they do food or drink, typically by about five times for humans.
The oxygen in our atmosphere was first produced in large quantities by simple, single-celled cyanobacteria about 2–3 billion years ago. After this ‘Great Oxygenation Event’ many more complex, multi-celled organisms were able to evolve to take advantage of the changed environment.
One way that the atmosphere impacts us all is through ‘weather’. Weather is simply the day-to-day change in the atmosphere around us. The atmosphere changes rapidly because it has a relatively low density and, as a gas, it is able to flow freely. Change in the oceans or the rocky surface of the Earth happens over longer timescales. Weather affects nearly all human occupations. Agriculture, construction, travel and the leisure industries, all depend on the weather in various ways. Hence, reliable weather prediction is vital. It is difficult to think of any occupation that might not benefit from a weather forecast: even astronauts require detailed forecasts for launch and re-entry.
The weather has been a source of power for human civilizations, from windmills and sailing ships to modern turbines for renewable energy. The atmosphere itself converts heat energy from the Sun into motion as winds. People have learned to convert some of this wind energy into electricity or use it for other mechanical work, such as pumping or milling. Of course, we are not the only life form to make use of wind energy. Plants can use it to disperse their seeds more widely, for example.
The water transporter
The atmosphere is vital for all life on the planet, not only humans, and for reasons other than supplying gases needed for respiration and photosynthesis. Winds also transport water into the interiors of large areas of land, which would otherwise rapidly become parched desert. This supply of fresh water is essential to life on Earth. An illustration of the importance of rainfall patterns is clearly seen in the image below, obtained from a geostationary weather satellite.
The central region of Africa, around the equator, is bright green with lush vegetation. The reason is clearly visible as it is also almost covered by white, convective clouds in the image. These grow during the day, as the surface warms in the sunlight, and typically produce rain in the afternoons, supplying water to the plants and animals which live below. A similar process occurs to the left of the disc in South America, where the rainforest is drained by the great Amazon basin. To the south and north of the equatorial region lie vast areas of dry, yellowish brown desert. Most obvious is the Sahara Desert in North Africa, just above the centre of the image. These areas are relatively cloud-free and there is little rainfall. The reason is that air which has risen from the surface at the equator has already lost much of its water content as the vigorous equatorial clouds condense. This dry air reaches the top of the troposphere, turns north or south, and then descends to the surface again. This circulation pattern is known as a Hadley cell. The deserts appear in the regions of dry, descending air, which no longer has sufficient water content to form clouds.
Further north, Europe is also supplied with water, perhaps in the form of snow on mountains at this time of year, from a bank of clouds in the image. These occur as a result of a different weather process as weather systems move from west to east across the North Atlantic. Low pressure, cyclonic systems can be seen as spirals of cloud, rotating in an anti-clockwise sense in the northern hemisphere, under the influence of the Earth’s rotation.
The different weather patterns at different latitudes on the Earth are all part of the atmosphere’s response to solar heating. The consequent patterns of rainfall determine what type of life can flourish in different regions. Without resupply by rain or snow, the centres of continents would be arid, and life would struggle to survive.
The atmosphere also shields life. Many small rocks are littered throughout the Solar System, left over from the violent processes that occurred during its formation or formed in subsequent collisions between objects. Small rocks, called meteoroids, frequently encounter the Earth. Most burn up when they enter the atmosphere and do not reach the ground, often called ‘shooting stars’ or meteors. Even rocks as large as one kilometre across tend to break up and only smaller fragments (meteorites) reach the ground, with less devastating consequences than might otherwise occur.
The shielding role of the atmosphere crucially extends to electromagnetic radiation. Electromagnetic waves occur on a spectrum ranging from very short wavelength waves, such as gamma- and X-rays, to long wavelengths, such as microwaves and radio waves. One very small region of the electromagnetic spectrum is known as ‘light’, at wavelengths that our eyes can detect. The wavelength of visible light ranges from about 0.4 mm (which we see as violet) to about 0.7 mm (which we see as red). The colours of the rainbow fall between these two wavelengths. The symbol mm means a millionth of a metre and is called a ‘micrometre’, or often just a ‘micron’. For comparison, a bacterium is about 1 mm across, a human red blood cell is about 10 mm and a human hair is typically 100 mm wide.
We think of air as transparent, but actually it is almost opaque to electromagnetic waves at many wavelengths. The visible region is an exception, often called a ‘window’. This is why animal eyesight normally operates on wavelengths around this region. Other ‘windows’ exist and are used for radio signals, for example, but the atmosphere protects life from many shorter wavelengths that are often harmful to cell structures.
The Sun emits a variety of electromagnetic waves, but most of its energy is in the visible range and at shorter ultraviolet wavelengths. Ultraviolet waves, with wavelengths of about 0.2–0.4 mm, can be extremely dangerous, damaging genetic material and causing cancers. Atoms and molecules in the atmosphere scatter short wavelengths and some gases, mainly ozone, absorb ultraviolet radiation in the most damaging wavelength region. Almost no solar radiation at wavelengths shorter than about 0.3 mm reaches the surface as a result of this scattering and ozone absorption. Were this not the case, as on planets without a substantial amount of ozone, life might only be possible underground. It is this absorption of ultraviolet light by ozone that explains why the Earth’s stratosphere gets warmer with increasing altitude, as described earlier. The scattering of shorter wavelengths also explains the beautiful blue colour of a clear sky.
The natural greenhouse
The Earth also emits electromagnetic waves, but at its cooler temperature these are at a longer wavelength of around 10 mm, known as the thermal infrared region. The atmosphere is not completely transparent to thermal infrared radiation, and several gases (including methane, nitrous oxide, water vapour and carbon dioxide) absorb energy in the wavelength range of 1–20 mm. This energy is re-emitted by the atmosphere and causes the well-known ‘greenhouse effect’ (confusingly, this is not how garden greenhouses actually work, they mainly act to suppress vertical convection and block the escape of air and heat from the warm surface).
The natural greenhouse effect from the present-day atmosphere on Earth is equivalent to about 33 °C of warming at the surface. The average (over time of day, season and all locations) surface temperature of the Earth is around 14 °C. If the atmosphere were truly transparent at all wavelengths, the surface temperature would fall to -18 °C on average. Lakes and seas would remain frozen over. It seems unlikely that complex life would have developed without easy access to liquid water at the surface. That the Earth supports water in all three phases (solid ice, liquid water and water vapour), at various locations on the surface of the planet, is a unique feature in the present-day Solar System. Water occurs throughout the Solar System, but on no other planet or moon can liquid water persist at the surface without freezing or boiling.
The heat engine
It is interesting to compare temperatures on the Earth and the Moon’s surface. The Moon is, on average, the same distance from the Sun and is made of dark rock. In the day, the surface temperature can reach 130 °C and then fall to -175 °C at night. This huge cycle of over 300 °C is far greater than is seen anywhere on Earth, and it might appear surprising even given the Moon’s longer diurnal cycle (the Moon rotates once every 27 days). That we experience a far more moderate day/night difference in temperature is mostly due to the effect of the atmosphere. The movement of air can transport heat into a region in darkness, and back-scattering of infrared radiation can keep the surface there warmer than if it all escaped to space. One familiar example is that surface frosts more often appear after a clear night compared to a cloudy one, when more infrared radiation is scattered back by cloud particles.
Although the atmosphere is only a small proportion of the mass of the Earth, its rapid response to heating variations means that it transports an enormous amount of heat. The weather systems discussed in relation to water transport above exist because of the thermal contrast between the equator and poles. They transport heat from warmer to cooler regions. A heat engine is simply a device that converts heat energy to kinetic energy and this is the same process that powers winds. In total, the atmosphere transports about 5 PW (5 petawatts or 5,000,000,000,000,000 watts) of power. This is roughly 250 times the total power consumed by humans today in all forms. This heat transport means that the equator is cooled and the polar regions are warmed very significantly. The Moon lacks similar heat transport. Temperatures of -247 °C have been recorded in craters near the poles of the Moon that get very little sunlight. This temperature is low compared even to Pluto. Nitrogen freezes at about -210 °C and oxygen at -219 °C. If Earth’s poles were to get this cold, then the atmosphere itself would freeze into ice around the poles, precipitating a calamitous atmospheric collapse. Less dramatically, the enormous poleward heat transport by the existing atmosphere keeps high latitude regions warm enough, and equatorial regions cool enough, to remain hospitable environments for life.
The atmosphere: friend or foe?
This article has discussed why the atmosphere is beneficial for life, but weather can also be deadly. Even apparently destructive aspects may have benefits to life on the planet as a whole; life evolves under the stresses of a changing environment. The ‘Great Oxygenation Event’ of the atmosphere led to the development of respiration, employed by animals today. But it was also a ‘Great Oxygenation Catastrophe’ that poisoned many other simple lifeforms at the time. Great extinctions of life are linked to past climate changes. It is a sobering thought that we owe our entire existence, and that of all the large plants and animals with which we are familiar, to a tenuous layer of gas only a few kilometres thick above our heads.