Fortunately for life on Earth, our planet is protected from the worst vagaries of the solar wind by its strong magnetic field. Only the strongest solar storms, and the most energetic particles, can penetrate that shielding.
But what if the Earth had no magnetic field? The situation would be grave. Our nearest planetary neighbour, Mars, lacks a strong magnetic field and most likely has been unshielded since the early youth of the solar system. As stars age they usually mellow but remain capable of slowly stripping planets of their gaseous shrouds.
It is clear, therefore, that for a planet to be considered a promising target in the search for life, it must possess a strong magnetic shield to protect its atmosphere. But how does a planet like the Earth maintain a magnetic field sufficiently strong, and for sufficiently long, to offer that kind of protection? But therein lies a problem — how can you maintain convection inside a planet over periods of billions of years?
The interior of the Earth is very hot, primarily as a result of the decay of radioactive elements trapped in the interior. In order to setup and maintain convection, you need a large temperature difference between two locations. It is thought that this is precisely what happened to Mars.
At first, the red planet may well have been sufficiently hot inside for a certain degree of tectonic activity to occur. Certainly, the crust of Mars bears evidence of an ancient magnetic field, frozen into the rocks. But Mars is smaller than Earth and so lost its interior heat to space much more quickly. It follows that the earth system is only able to continue to function because it is constantly replenished with a sufficient supply of energy mainly from the sun.
The dominant flows of energy at the global scale occur as a result of the large discrepancies that occur between the amounts of solar radiation received and re-emitted at different points on the earth's surface.
Such discrepancies are most clearly apparent in the wide variations in surface temperature that exist between the equator and the poles. Those temperature variations drive the global energy circulation which acts to redistribute heat from the warm to the cold parts of the earth's surface. An overall poleward transfer of energy occurs by means of a variety of processes: the transfer of heat by winds and warm air masses; the transfer of latent heat associated with water vapour; the movement of heat in ocean currents; and the returning counter-flows of cooler air and water.
The three main processes of energy transfer at the global scale may be summarised as:. It is important to acknowledge that pronounced latitudinal variations occur in these three processes. Overall, however, these processes of energy transfer maintain a state of equilibrium in the earth system: they remove energy from areas of surplus in lower latitudes and transfer it to areas of deficit in higher latitudes.
The earth system contains several 'great cycles' in which key materials are transported through the environment. In general, cycles occur in closed systems; at the global scale, many systems may be assumed to be closed because the earth receives negligible quantities of minerals from space as a result of meteorite impacts and because only limited quantities of materials can escape the earth's atmosphere.
The key materials that cycle through the major biogeochemical cycles are carbon, oxygen, hydrogen, nitrogen, phosphorous and sulphur - all of which are essential for life. The biogeochemical cycles operate at the global scale and involve all of the main components of the earth system; thus materials are transferred continually between the geosphere, atmosphere, hydrosphere and biosphere. However, since the biogeochemical cycles involve elements that are essential for life, organisms play a vital part in those cycles.
Typically then, the biogeochemical cycles involve an inorganic component the abiotic part of the cycle, including sedimentary and atmospheric phases and an organic component comprising plants and animals, both living and dead. Like other environmental systems, biogeochemical cycles involve the flow of substances between stores also known as reservoirs in the geosphere, atmosphere, hydrosphere and biosphere.
Water plays a vital role in mediating many of the flows between stores. Three of the key biogeochemical cycles are the nitrogen, carbon and sulphur cycles, whose main features are described here. Of course, biogeochemical cycles have been substantially modified by human activities - a fact that has enormous implications for the understanding and management of environmental issues.
This planet, known as Alpha Centauri Bb,is about as massive as Earth, but its hot surface may be covered with molten rock — its orbit takes it about 25 times closer to its star than Earth is from the sun. There are a few key ingredients that scientists often agree are needed for life to exist — but much debate remains as to what limits there actually might be on life.
Even Earth hosts some strange creatures that live in extreme environments. Here's what makes life able to thrive on our home planet and likely for alien life to arise on other worlds :. In such a soup, the ingredients for life as we know it, such as DNA and proteins, can swim around and interact with each other to carry out the reactions needed for life to happen.
The most common contender brought up for this solvent is the one life uses on Earth: water. Water is an excellent solvent, capable of dissolving many substances. It also floats when it is frozen, unlike many liquids, meaning that ice can insulate the underlying fluid from freezing further. If water instead sunk when frozen, this would allow another layer of water to freeze and sink, and eventually all the water would get frozen, making the chemical reactions behind life impossible.
Astronomers looking for extraterrestrial life most often focus on planets in the so-called habitable zones of their stars — orbits that are neither too hot nor too cold for liquid water to persist on the surfaces of those worlds. Earth happened to hit the Goldilocks mark, forming within the sun's habitable zone. Mars and Venus lie outside it; if Earth's orbit had been just a bit further inside or outside of where it is, life may likely never have arisen and the planet would be a cold desert like Mars or a cloudy furnace like Venus.
Astrobiologists increasingly suggest looking beyond conventional habitable zones. For instance, while liquid water might not currently persist on the surface of Mars or Venus, there may have been a time when it did.
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