Granted, scientists believe that there may be microbial or even aquatic life forms living beneath the icy surfaces of Europa and Enceladus, or in the methane lakes on Titan. But for the time being, Earth remains the only place that we know of that has all the right conditions for life to exist.
One of the reasons for this is because the Earth lies within our sun 's Habitable Zone aka. This means that it is in right spot neither too close nor too far to receive the sun's abundant energy , which includes the light and heat that is essential for chemical reactions.
But how exactly does our sun go about producing this energy? What steps are involved, and how does it get to us here on planet Earth? The simple answer is that the sun, like all stars, is able to create energy because it is essentially a massive fusion reaction.
Scientists believe that this began when a huge cloud of gas and particles i. This not only created the big ball of light at the center of our solar system, it also triggered a process whereby hydrogen, collected in the center, began fusing to create solar energy. Technically known as nuclear fusion, this process releases an incredible amount of energy in the form of light and heat.
But getting that energy from the center of our sun all the way out to planet Earth and beyond involves a couple of crucial steps. In the end, it all comes down to the sun's layers, and the role each of them plays in making sure that solar energy gets to where it can help create and sustain life.
It is here, in the core, where energy is produced by hydrogen atoms H being converted into nuclei of helium He. This is possible thanks to the extreme pressure and temperature that exists within the core, which are estimated to be the equivalent of billion atmospheres The net result is the fusion of four protons hydrogen nuclei into one alpha particle — two protons and two neutrons bound together into a particle that is identical to a helium nucleus.
Two positrons are released from this process, as well as two neutrinos which changes two of the protons into neutrons , and energy. The core is the only part of the sun that produces an appreciable amount of heat through fusion.
The rest of the sun is heated by the energy that is transferred from the core through the successive layers, eventually reaching the solar photosphere and escaping into space as sunlight or the kinetic energy of particles. The sun releases energy at a mass—energy conversion rate of 4. To put that in perspective, this is the equivalent of about 9. This is the zone immediately next to the core, which extends out to about 0. There is no thermal convection in this layer, but solar material in this layer is hot and dense enough that thermal radiation is all that is needed to transfer the intense heat generated in the core outward.
Basically, this involves ions of hydrogen and helium emitting photons that travel a short distance before being reabsorbed by other ions.
Temperatures drop in this layer, going from approximately 7 million kelvin closer to the core to 2 million at the boundary with the convective zone. Density also drops in this layer a hundredfold from 0. Here, the temperature is lower than in the radiative zone and heavier atoms are not fully ionized. As a result, radiative heat transport is less effective, and the density of the plasma is low enough to allow convective currents to develop.
Because of this, rising thermal cells carry the majority of the heat outward to the sun's photosphere. UV levels are not constant over the course of a day, or even over the course of a year.
An obvious factor is the position of the sun in the sky. At noon, for example, the electromagnetic waves emitted from the sun travel a much shorter path through the earth's atmosphere then they would at, say, 5 pm, and thus noon-time intensity is stronger. A second important parameter determining UV at the ground is the amount of ozone present in the stratosphere. Low ozone correlates with much UV. However, there are many other features of the environment that contribute to UV radiation variability.
Most important are clouds. On cloudy days, UV levels are usually lower than during clear skies as clouds can deflect rays up into space.
Clouds can, however, also lead to increased UV levels. This happens, for example, when the sun is not obscured by clouds but clouds in the vicinity of the sun reflect additional radiation to the ground. So a general rule is not to feel save from UV radiation just because it's cloudy! The amounts of UV one is exposed to also varies with altitude. This increase has nothing to do with being closer to the sun - any elevation you might gain would be miniscule in comparison to the distance from the earth to the sun, and so would have an insignificant outcome on UV levels.
Instead, the increase is the result of a thinner atmosphere with a smaller number of molecules being present to absorb or scatter UV. Examples of such molecules are tropospheric ozone commonly associated with smog and aerosols , molecules that remain suspended in the air. Aerosols can be a multitude of substances - dust, soot, sulfates, etc.
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