Background
Understanding the concept, when scientists consider the question of life on other worlds within our Solar System, they are thinking of microscopic bacteria-like life rather than multi-cellular plants and animals. Life as we know today, such as human life, is not the kind of life that scientist are searching for beyond Earth in the Solar System. It is microorganisms, rather than large, complex life forms, that are able to live in extreme environments. On Earth, microorganisms were the only form of life for billions of years. So what does life, any form, need to be able to survive on the Earth and elsewhere in our Solar System? Life requires a few basic things in order to survive, grow, and reproduce which include liquid water, a source of energy, and a breathable atmosphere although some anaerobic bacteria can dispense of the latter. Liquid water can be present in small amounts, but every thing we know about life on Earth suggests that access to some liquid water is essential. Sunlight makes a good energy source for life on Earth.
Although our own food supply is entirely dependent upon plants using the process of photosynthesis, many communities of microorganisms on the Earth thrive off the energy produced by natural chemical reactions that occur between water and rock, frequently in the presence of hot water such as in the vents at the bottom of the ocean. Rocks, water, and gases from the atmosphere or interior of a planet can also provide the elements necessary to generate living organisms.
Although the evidence for past life on Mars from Martian meteorites is still considered inconclusive by most of the scientific community, the announcement a few years ago of the discovery of evidence for past life in a Martiam meteorite found in Antarctica fueled a renewed interest in the idea of life on Mars. The announcement came about because of Mars' active volcanic history, as well as evidence that water did exist there in the past. Mars is a promising candidate in the search for life beyond the Earth. Of all the other planets in our Solar System, Mars is the most Earth-like in many respects. Despite the similarities between our world and our celestial neighbor, the surface of Mars is cold, very dry, and unlikely to support life. Billions of years ago, water flowed on the surface of Mars, indicating Mars had a wetter, and probably warmer, climate. However, in the present atmosphere of Mars, the melting and boiling temperatures of water are both zero degrees Celsius, so liquid water cannot exist. Despite the inhospitable surface, the discovery of life in extreme environments on the Earth suggests that life could also live underground on Mars today. The Martian subsurface probably contains a significant amount of water ice, and "perhaps" liquid water, which cannot exist on the surface. The presence of past volcanoes on Mars, which could melt ground ice and circulate water in the subsurface, would have provided an excellent environment for microorganisms. At very least, in the past, Mars has had all of the requirements for life that includes liquid water, an energy source, and the chemical building blocks of life.
Another possible candidate for life in the Solar System is the ice-covered moon Europa. Though it is slightly smaller than our own Moon, Europa is a major moon of Jupiter. The appearance and youth of the surface suggest that an ocean of liquid water may exist below a frozen shell of ice. If so, Europa fulfills one of the major requirements for life, which is access to liquid water. Below the surface of ice, and possibly ocean, Europa is a rocky moon. Even if an ocean no longer exists on Europa, it almost certainly did in the past. This leads scientist to understand that there could not be life on Europa.
The smog covered world of Titan, the largest moon of Saturn, is larger than the planets Mercury and Pluto. Titan is the only moon in the Solar System with a thick atmosphere; the surface pressure is more than one and a half times greater than that of the Earth's atmosphere at sea level. Titan is a fascinating world, but is it possible for life? The answer would have to be no because life must have liquid water to survive. Mars and Europa remain the best candidates beyond the Earth for finding either extinct or possibly living life in our Solar System. The atmosphere of Jupiter, which has water clouds and some organic molecules, has also been suggested as a place where life could exist. However, access to the chemical building blocks of life is limited and the unstable nature of the atmosphere would likely destroy any organisms that might be present. It is also possible that life could once have existed on Venus which is now a dry world with a surface temperature hot enough to melt lead. Unfortunately, any traces of ancient life would have been long destroyed by the completely hostile environment leaving no evidence that life ever existed on our neighboring planet. Io, the volcanically active moon of Jupiter, also may once have had water and the possibility for life like Venus, any water Io possessed in its past is now long gone and no traces of life can be expected to be found.
When faced with the question, "what about life beyond the Solar System?" Astronomers are searching for extra-solar planets and are developing instruments and techniques to make planets like those in our Solar System easier to detect. NASA is also planning space-based missions, which may have the chance of not only detecting Earth-like planets around other stars but also of gathering information about the atmospheres of such planets. Life has changed the composition of the Earth's atmosphere over time. Examining the atmospheric composition of an extra-solar Earth-like planet may allow us to detect life on distant worlds. Planetary systems around other stars hold the promise of not only microscopic life, but perhaps larger life forms. Of course, if technologically advanced life were to exist on planets beyond the Solar System, they could be sending out radio messages indicating their presence. It is that possibility that is being pursued by means of radio searches for intelligent signals such as the SETI (Search for Extra Terrestrial Intelligence) project.
Most scientists believe the Solar System began as a cloud of dust and gas. Gravity pulled the cloud in, making it smaller and denser. As it shrank, the cloud heated up and the Sun formed. The outer region of the cloud flattened out and shaped into a disc, which spun around the new Sun. Particles of gas and dust within the disc bumped each other, which made them stick together and form large lumps. These lumps grew into planets, all of them revolving around the Sun. One group of rocky lumps never grew into planets and these are what we call asteroids, or perhaps they did, but a large collision broke it up.
Our Sun is a star. It is the star closet to the Earth. It is the star we see in the daytime. It is the center of our Solar System. The Sun is so big that more than a million Earths would fit inside it. It looks small because it is so far from Earth. Like all stars, our Sun is a ball of very hot gases. It gives off light and heat. The Sun makes energy in its center. Nuclear conversions of Hydrogen into Helium take place, releasing energy. The outermost 30% layer of the Sun is boiling gases. This is why the Sun looks like a ball of fire. We couldn't live without the Sun. It gives us heat to stay warm. It gives us light to see by. Plants need the sunlight too. It helps plants make food for us to eat and Oxygen for us to breathe.
Mercury is the nearest planet to the Sun and is also the smallest, except for Pluto. It can be seen for a short time just before sunrise or after sunset because it is so close to the Sun. Mercury has almost no atmosphere. With no air, and no clouds, the weather forecast on Mercury would be very simple; unbearably hot by day and freezing cold at night.
Venus is easy to see in the sky because it is very bright. Only the Sun and Moon are brighter than Venus. Venus is often called the sister planet to the Earth. It is almost the same size as the Earth and the nearest planet to us. But thick clouds completely cover it. The thick atmosphere makes Venus a very dangerous place to explore. This is because it mainly consists of a gas called carbon dioxide which we cannot breathe. It also traps the heat so that Venus is much hotter than Mercury, but on its entire surface.
Earth is a beautiful blue planet with white swirling clouds and is unique in many ways. This is the planet where humans live in. It is the only planet that has liquid water on the surface. About two-thirds of the Earth is covered with oceans of water. In colder parts, the water freezes into ice and there are droplets of water in the clouds. Most elements of the Earth are very rare including Nitrogen and Oxygen. The abundance of Noble Gases is low and do not combine with any other elements which leads to an element being lost. Earth coupled with water of volcanic activity emits Oxygen, Carbon Dioxide, and Water Vapor. This allows more CO2 to be released into the atmosphere which causes Earth's temperature to rise. Most of the CO2 is absorbed by rocks and some by the trees of Earth. Every time a tree dies the CO2 is released back into the atmosphere.
Mars has a thin and wispy atmosphere made almost entirely of carbon dioxide. At the north and south poles of Mars, there may be ice-caps. This kind of ice is known as dry ice. The surface of Mars is a stony desert with sand that is so tiny that you would need a microscope to see them. The winds of Mars easily stir up vast dust storms, which cause the pinkish glow in the Martian sky.
Asteroids are small, have very little gravity, and are hard to see in space because their rocky surfaces don't reflect a lot of light. The space between Mars and Jupiter is filled with a population of irregularly shaped chunks of debris called asteroids. Objects in this asteroid belt are made of rock and metal, mostly nickel and iron. Scientists believe the asteroids are pieces of a planet that never formed on a planet that broke up. An ongoing gravitational tug-of-war between Jupiter and Mars may have prevented the pieces from bonding together. Other asteroids are not part of this belt and some have paths which cross the Earth's orbit.
Jupiter is not a solid planet. Unlike the solid planets closest to the Sun, Jupiter is a huge ball of gas. Jupiter rotates very rapidly, once every ten hours. This is so fast that the planet bulges at the equator. The rapid rotation also causes high wind speeds in the upper atmosphere, where the clouds are stretched out into colorful bands. Different parts rotate at slightly different rates, and this speed difference causes the bands and their various colors.
Saturn is a frigid world, with a cloud composition and a wind system similar to that of Jupiter. Saturn's beautiful rings do not touch the planet. The rings are tilted at a 29% angle. This means that they slowly change their appearance when viewed from Earth. The rings are mainly made of billions of tiny particles and each speck is orbiting Saturn. They all orbit the planet as if they were unrelated satellites.
Uranus is mainly made of hydrogen and helium, but one-seventh of its atmosphere is methane. This gas makes it appear bluish in a large telescope. The axis of Uranus is tilted by more than a right angle, which means its north pole actually points below the planet's orbit. This gives Uranus rather strange seasons. One pole faces the Sun and has a constant sunlight for about forty years. This end of the planet then goes into complete darkness for about another forty years while the other pole of the planet faces the Sun. It also appears to rotate backwards, in the opposite direction of all other planets.
Neptune is a similar planet to Uranus, though it is not tipped over on its side and is a little smaller than Uranus. Like Jupiter, Neptune has a wind spot, although Neptune's is darker. There is also a smaller dark wind spot that turns around in the opposite direction to the larger one. Both of the wind spots are swept along by winds blowing as fast as 2,100 km an hour, the fastest winds in the Solar System.
Pluto, the farthest planet from the Sun, is an oddity. It is the smallest of all the planets, smaller than our Moon and less than a third as big as the largest moons in the Solar System. We do not know very much about Pluto because it is very small and very far away. Pluto is the only planet that has never been visited by a space probe from Earth.
Occasionally a strange "star" seems to appear bright in the night sky, only to disappear a few weeks later. This is not really a star. It is a comet. Actual stars are made of gas. Nevertheless, comets are large lumps of snow and dust, more like enormous dirty clumps of snow than stars. Comets may look bright, but unlike stars they do not make their own light. Comets are seen only when they come close enough to the Sun for its light to shine on them. When a comet does get close to the Sun, its snow becomes extremely hot and turns into a glassy cloud that streams out behind the shooting comet and is known as the tail of the comet.
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