   #copyright

Nature

2007 Schools Wikipedia Selection. Related subjects: General Biology

   Galunggung in 1982, showing a combination of natural events.
   Enlarge
   Galunggung in 1982, showing a combination of natural events.
   A more steady-state view of nature: Hopetoun Falls, Victoria,
   Australia. Much attention has been given to preserving the flora and
   other natural characteristics of the view, while allowing ample access
   for visitors to this much-visited site.
   Enlarge
   A more steady-state view of nature: Hopetoun Falls, Victoria,
   Australia. Much attention has been given to preserving the flora and
   other natural characteristics of the view, while allowing ample access
   for visitors to this much-visited site.

   Nature, in the broadest sense, is equivalent to the natural world,
   physical universe, material world or material universe. "Nature" refers
   to the phenomena of the physical world, and also to life in general.
   The term generally does not include manufactured objects and human
   interaction unless qualified in ways such as, e.g., "human nature" or
   "the whole of nature". Nature is also generally distinguished from the
   supernatural. It ranges in scale from the subatomic to the galactic.

   The word "nature" derives from the Latin word natura, or "the course of
   things, natural character." Natura was a Latin translation of the Greek
   word physis (φύσις), which originally related to the innate way in
   which plants and animals grow of their own accord, and to the Greek
   word for plants generally. The concept of nature as a whole, the
   physical universe, is a more recent development that gained
   increasingly wide use with the advent of modern scientific method in
   the last several centuries.

   Within the various uses of the word today, "nature" may refer to the
   general realm of various types of living plants and animals, and in
   some cases to the processes associated with inanimate objects – the way
   that particular types of things exist and change of their own accord,
   such as the weather and geology of the Earth, and the matter and energy
   of which all these things are composed. It is often taken to mean the "
   natural environment" or wilderness – wild animals, rocks, forest,
   beaches, and in general those things that have not been substantially
   altered by human intervention, or which persist despite human
   intervention. This more traditional concept of natural things which can
   still be found today implies a distinction between the natural and the
   artificial, with the latter being understood as that which has been
   brought into being by a human or human-like consciousness or mind.

Earth

   View of the home planet, taken in 1972 by the Apollo 17 crew. This
   image is the only photograph of its kind to date, showing a fully
   sunlit hemisphere of the Earth.
   Enlarge
   View of the home planet, taken in 1972 by the Apollo 17 crew. This
   image is the only photograph of its kind to date, showing a fully
   sunlit hemisphere of the Earth.

   Earth (or, "the Earth") is the fifth largest planet in the solar
   system, third in order of distance from the Sun. It is the largest of
   its planetary system's terrestrial planets and the only place in the
   universe known to support life.

   The most prominent features of the earth's climate are its two large
   polar regions, two relatively narrow temperate zones, and a wide
   equatorial tropical to subtropical region. Precipitation patterns vary
   widely according to location, ranging from several meters of water per
   year to less than a millimeter. About 70 percent of the surface is
   covered by salt-water oceans. The remainder consists of continents and
   islands, with the vast majority of the habitable land in the Northern
   Hemisphere.

   Earth has evolved through geological and biological processes that have
   left traces of the original conditions. The outer surface is divided
   into several tectonic plates that gradually migrate across the surface
   over geologic time spans, which at least several times have changed
   relatively quickly. The interior of the planet remains active, with a
   thick layer of molten Earth mantle and an iron-filled core that
   generates a magnetic field. The atmospheric conditions have been
   significantly altered from the original conditions by the presence of
   life forms, which create an ecological balance that stabilizes the
   surface conditions. Despite the wide regional variations in climate by
   latitude and other geographic factors, the long-term average global
   climate is quite finely regulated, and variations of a degree or two of
   average global temperature have historically had major effects on the
   ecological balance, and on the actual geography of the Earth.
   Phylum Pediastrumboryanum. Plankton have been part of nature on the
   Earth for at least the past 2 billion years.
   Enlarge
   Phylum Pediastrumboryanum. Plankton have been part of nature on the
   Earth for at least the past 2 billion years.

   Based on the available evidence, scientists have reconstructed detailed
   information about the planet's past. Earth is believed to have formed
   about 4.55 billion years ago out of the solar nebula, along with the
   Sun and other planets. The moon formed relatively soon afterwards
   (roughly 20 million years later, or 4.53 billion years ago). Initially
   molten, the outer layer of the planet cooled, resulting in the solid
   crust. Outgassing and volcanic activity produced the primordial
   atmosphere. Condensing water vapor, augmented by ice delivered by
   comets, produced the oceans. The highly energetic chemistry is believed
   to have produced a self-replicating molecule around 4 billion years
   ago.

   Continents formed, then broke up and reformed as the surface of Earth
   reshaped itself over the course of hundreds of millions of years,
   occasionally combining to form a supercontinent. Roughly 750 million
   years ago, the earliest known supercontinent Rodinia, began to break
   apart. The continents later recombined to form Pannotia which broke
   apart about 540 million years ago, then finally Pangaea, which broke
   apart about 180 million years ago.
   Land-based plants and fungi have been part of nature on Earth for about
   the past 400 million years. These have needed to adapt and move many
   times as the continents and climates changed.
   Enlarge
   Land-based plants and fungi have been part of nature on Earth for about
   the past 400 million years. These have needed to adapt and move many
   times as the continents and climates changed.

   There is significant evidence, still being discussed among scientists,
   that a severe glacial action during the Neoproterozoic era covered much
   of the planet in a sheet of ice. This hypothesis has been termed the
   "Snowball Earth", and it is of particular interest as it precedes the
   Cambrian explosion in which multicellular life forms began to widely
   proliferate about 530-540 million years ago.

   Since the Cambrian explosion there have been five distinctly
   identifiable mass extinctions. The last mass extinction occurred some
   65 million years ago, when a meteorite collision probably triggered the
   extinction of the non-avian dinosaurs and other large reptiles, but
   spared small animals such as mammals, which then resembled shrews. Over
   the past 65 million years, mammalian life diversified.

   Several million years ago, a species of small African ape gained the
   ability to stand upright. The subsequent advent of human life, and the
   development of agriculture and further civilization allowed humans to
   affect the Earth in a relatively short timespan like no other life form
   had before, affecting both the nature and quantity of other life forms
   as well as global climate.

   A 1998 survey by the American Museum of Natural History found that 70%
   of biologists view the present era as part of a mass extinction event,
   the Holocene extinction event, the fastest to have ever occurred. Some,
   such as E. O. Wilson of Harvard University, predict that human
   destruction of the biosphere could cause the extinction of one-half of
   all species in the next 100 years. The extent of the current extinction
   event is still being researched, debated and calculated by biologists.

Weather and climate

   The atmosphere of the Earth serves as a key factor in sustaining the
   planetary ecosystem. The thin layer of gases that envelops the Earth
   are held in place by the planet's gravity. It consists of 78% nitrogen,
   21% oxygen, and traces of other gases. The atmospheric pressure
   declines steadily with altitude, and has a scale height of 8.5
   kilometres: the height at which the atmospheric pressure has delined by
   a factor of e. The ozone layer of the Earth's atmosphere plays an
   important role in depleating the amount of ultraviolet radiation that
   reaches the surface. As DNA readily damaged by UV light, this serves to
   protect life at the surface. The atmosphere also retains heat during
   the night, thereby reducing the daily temperature extremes.

   Terrestrial weather occurs almost exclusively in the lower part of the
   atmosphere, and serves as a convective system for redistributing heat.
   Ocean currents are another important factor in determining climate,
   particularly the major underwater thermohaline circulation which
   distributes heat energy from the equatorial oceans to the polar
   regions. These currents help to moderate the differences in temperature
   between winter and summer in the temperate zones. Also, without the
   redistributions of heat energy by the ocean currents and atmosphere,
   the tropics would be much hotter, and the polar regions much colder.

   Weather can have both beneficial and harmful effects. Extremes in
   weather, such as tornadoes or hurricanes and cyclones, can expend large
   amounts of energy in their path and produce devastation. Surface
   vegetation has evolved a dependence on the seasonal variation of the
   weather, and sudden changes lasting only a few years can have a
   dramatic effect, both on the vegetation and on the animals dependent on
   the growths for food.

   The planetary climate is a measure of the long-term trends in the
   weather. Various factors are known to influence the climate, including
   ocean currents, surface albedo, greenhouse gases, variations in the
   solar luminosity, and changes to the planet's orbit. Based on
   historical records, the Earth is known to have undergone drastic
   climate changes in the past, including ice ages. The climate of a
   region is dependent on a number of factors including latitude. A
   latitudinal band of the surface with similar climatic attributes forms
   a climate region. There are a number of such regions, ranging from the
   tropical climate at the equator to the polar climate in the northern
   and southern extremes. Weather is also influenced by the seasons, which
   result from the Earth's axis being tilted to its orbital plane. Thus,
   at any given time during the summer or winter, one part of the planet
   is more directly exposed to the rays of the Sun (see Fig. 1). This
   exposure alternates as the Earth revolves in its orbit. At any given
   time, regardless of season, the northern and southern hemispheres
   experience opposite seasons.

   Weather is a chaotic system that is readily modified by small changes
   to the environment, so accurate weather forecasting, is currently
   limited to only a few days. Overall, two things are currently happening
   worldwide: (1) temperature is increasing on the average; and (2)
   weather patterns are changing and becoming increasingly chaotic.

Life

   Female mallard and ducklings - reproduction is essential for continuing
   life
   Enlarge
   Female mallard and ducklings - reproduction is essential for continuing
   life

   The advent of photosynthesis in very basic forms of plant life
   worldwide allowed the sun's energy to be harvested to create conditions
   allowing for more complex life. The resultant oxygen accumulated in the
   atmosphere and gave rise to the ozone layer. The incorporation of
   smaller cells within larger ones resulted in the development of yet
   more complex cells called eukaryotes. Cells within colonies became
   increasingly specialized, resulting in true multicellular organisms.
   With the ozone layer absorbing harmful ultraviolet radiation, life
   colonized the surface of Earth.

   Although there is no universal agreement on the definition of life,
   scientists generally accept that the biological manifestation of life
   is characterized by organization, metabolism, growth, adaptation,
   response to stimuli and reproduction. Life may also be said to be
   simply the characteristic state of organisms. Properties common to
   terrestrial organisms (plants, animals, fungi, protists, archaea and
   bacteria) are that they are cellular, carbon-and-water-based with
   complex organization, having a metabolism, a capacity to grow, respond
   to stimuli, and reproduce. An entity with these properties is generally
   considered life. However, not every definition of life considers all of
   these properties to be essential. Human-made analogs of life may also
   be considered to be life.

   The biosphere is the part of Earth's outer shell — including air, land,
   surface rocks and water — within which life occurs, and which biotic
   processes in turn alter or transform. From the broadest
   geophysiological point of view, the biosphere is the global ecological
   system integrating all living beings and their relationships, including
   their interaction with the elements of the lithosphere (rocks),
   hydrosphere (water), and atmosphere (air). Currently the entire Earth
   contains over 75 billion tons (150 trillion pounds or about 6.8 x10^13
   kg) of biomass (life), which lives within various environments within
   the biosphere. Over nine-tenths of the total biomass on Earth is plant
   life, on which animal life depends very heavily for its existence. More
   than 2 million species of plant and animal life have been identified to
   date, and estimates of the actual number of existing species range from
   several million to well over 50 million. The number of individual
   species of life is constantly in some degree of flux, with new species
   appearing and others ceasing to exist on a continual basis. The total
   number of species presently is in rapid decline.
   A confluence of "natural" and a "made" environment.
   Enlarge
   A confluence of "natural" and a "made" environment.

   The distinction between plant and animal life is not sharply drawn,
   with some categories of life that stand between or across the two.
   Originally Aristotle divided all living things between plants, which
   generally do not move, and animals. In Linnaeus' system, these became
   the Kingdoms Vegetabilia (later Plantae) and Animalia. Since then, it
   has become clear that the Plantae as originally defined included
   several unrelated groups, and the fungi and several groups of algae
   were removed to new kingdoms. However, these are still often considered
   plants in many contexts. Bacterial life is sometimes included in flora,
   and some classifications use the term bacterial flora separately from
   plant flora.

   Among the many ways of classifying plants are by regional floras,
   which, depending on the purpose of study, can also include fossil
   flora, remnants of plant life from a previous era. People in many
   regions and countries take great pride in their individual arrays of
   characteristic flora, which can vary widely across the globe due to
   differences in climate and terrain. Regional floras commonly are
   divided into categories such as native flora and agricultural and
   garden flora, the latter of which are intentionally grown and
   cultivated. Some types of "native flora" actually have been introduced
   centuries ago by people migrating from one region or continent to
   another, and become an integral part of the native, or natural flora of
   the place to which they were introduced. This is an example of how
   human interaction with nature can blur the boundary of what is
   considered nature. Another category of plant has historically been
   carved out for weeds. Though the term has fallen into disfavor among
   botanists as a formal way to categorize "useless" plants, the informal
   use of the word "weeds" to describe those plants that are deemed worthy
   of elimination is illustrative of the general tendency of people and
   societies to seek to alter or shape the course of nature. Similarly,
   animals are often categorized in ways such as domestic, farm animals,
   wild animals, pests, etc. according to their relationship to human
   life.
   Wildebeast in Ngorongoro Conservation Area, Tanzania. Note the tendency
   to congregate, one of nature's displays of what is sometimes called the
   herding instinct or herd behavior.
   Enlarge
   Wildebeast in Ngorongoro Conservation Area, Tanzania. Note the tendency
   to congregate, one of nature's displays of what is sometimes called the
   herding instinct or herd behaviour.

   Animals as a category have several characteristics that generally set
   them apart from other living things, though not traced by scientists to
   having legs or wings instead of roots and leaves. Animals are
   eukaryotic and usually multicellular (although see Myxozoa), which
   separates them from bacteria, archaea and most protists. They are
   heterotrophic, generally digesting food in an internal chamber, which
   separates them from plants and algae. They are also distinguished from
   plants, algae, and fungi by lacking cell walls. With a few exceptions,
   most notably the sponges (Phylum Porifera), animals have bodies
   differentiated into separate tissues. These include muscles, which are
   able to contract and control locomotion, and a nervous system, which
   sends and processes signals. There is also typically an internal
   digestive chamber. The eukaryotic cells possessed by all animals are
   surrounded by a characteristic extracellular matrix composed of
   collagen and elastic glycoproteins. This may be calcified to form
   structures like shells, bones, and spicules, a framework upon which
   cells can move about and be reorganized during development and
   maturation, and which supports the complex anatomy required for
   mobility.

   Although humans currently comprise only about one-half of one percent
   of the total living biomass on Earth, the human effect on nature is
   disproportionately large. Because of the extent of human influence, the
   boundaries between what we regard as nature and "made environments" is
   not clear cut except at the extremes. Even at the extremes, the amount
   of natural environment that is free of discernible human influence is
   presently diminishing at an increasingly rapid pace, or, according to
   some, has already disappeared.

Ecosystems

   An aerial view of a human ecosystem. Pictured is the city of Chicago
   Enlarge
   An aerial view of a human ecosystem. Pictured is the city of Chicago

   All forms of life are highly interactive with the environment in which
   they exist, and also with other life forms. In the 20th century this
   premise gave rise to the concept of ecosystems, which can be defined as
   any situation where there is interaction between organisms and their
   environment. Ecosystems are composed of a variety of abiotic and biotic
   components that function in an interrelated way. Some of the more
   important components are: soil, atmosphere, radiation from the sun,
   water, and living organisms. Each living organism has a continual
   relationship with every other element that makes up its environment.
   Within the ecosystem, species are connected and dependent upon one
   another in the food chain, and exchange energy and matter between
   themselves as well as with their environment. Michael Pidwirny, in
   Fundamentals of Physical Geography, describes the concept as follows:

     Ecosystems are dynamic entities composed of the biological community
     and the abiotic environment. An ecosystem's abiotic and biotic
     composition and structure is determined by the state of a number of
     interrelated environmental factors. Changes in any of these factors
     (for example: nutrient availability, temperature, light intensity,
     grazing intensity, and species population density) will result in
     dynamic changes to the nature of these systems. For example, a fire
     in the temperate deciduous forest completely changes the structure
     of that system. There are no longer any large trees, most of the
     mosses, herbs, and shrubs that occupy the forest floor are gone, and
     the nutrients that were stored in the biomass are quickly released
     into the soil, atmosphere and hydrologic system. After a short time
     of recovery, the community that was once large mature trees now
     becomes a community of grasses, herbaceous species, and tree
     seedlings.

   Every species has limits of tolerance to factors that affect its
   survival, reproductive success and ability to continue to thrive and
   interact sustainably with the rest of its environment, which in turn
   may have effects on these factors for many other species or even on the
   whole of life. The concept of an ecosystem is thus an important subject
   of study, as such study provides information needed to make decisions
   about how human life may interact in a way that allows the various
   ecosystems to be sustained for future use rather than used up or
   otherwise rendered ineffective. For the purpose of such study, a unit
   of smaller size is called a microecosystem. For example, an ecosystem
   can be a stone and all the life under it. A macroecosystem might
   involve a whole ecoregion, with its drainage basin.

   The following ecosystems are examples of the kinds currently under
   intensive study:
     * "continental ecosystems", such as "forest ecosystems", "meadow
       ecosystems" such as steppes or savannas), or agro-ecosystems,
     * systems in inland waters, such as lentic ecosystem"s such as lakes
       or ponds; or lotic ecosystems such as rivers,
     * oceanic ecosystems.

   Another classification can be made by reference to its communities,
   such as in the case of a human ecosystem. The broadest classification,
   today under wide study and analysis, and also subject to widespread
   arguments about its nature and validity, is that of the entire sum of
   life seen as analogous to a self-sustaining organism, known as the Gaia
   theory.

Human relationship with nature

   The development of technology by the human race has allowed the greater
   exploitation of natural resources and has helped to alleviate some of
   the risk from natural hazards. In spite of this progress, however, the
   fate of human civilization remains closely linked to changes in the
   enviroment. There exists a highly complex feedback-loop between the use
   of advanced technology and changes to the environment that are only
   slowly becoming understood.

   Humans employ nature for both leisure and economic activities. The
   acquisition of natural resources for industrial use remains a primary
   component of the world's economic system. Some activities, such as
   hunting and fishing, are used for both sustenance and leisure, often by
   different people. the adoption of agriculture was first adopted around
   the 9th millennium BCE. Ranging from food production to energy, nature
   influences economic wealth.

   Human beings have also made medicinal use of plants for thousands of
   years. Plant extracts can treat cramps, heal rhumatism and cure lung
   inflammation .. While science has enabled us to process natural
   substances into pills, tinctures, powders and essential oils., the
   market economy and the 'authority' status attributed to the medical
   fraternity have made their use less popular. The term alternative
   medicine is often attributed to the use of natural plants and extracts
   for healing purposes.

   Manmade threats to nature include pollution, deforestation, and
   disasters such as oil spills. Humankind has contributed to the
   extinction of some plants and animals.

Wilderness

   The Daintree Rainforest, a wilderness area in Queensland, Australia.
   Enlarge
   The Daintree Rainforest, a wilderness area in Queensland, Australia.

   Wilderness is generally defined as a natural environment on Earth that
   has not been directly modified by human activity. Ecologists consider
   wilderness areas to be an integral part of the planet's self-sustaining
   natural ecosystem (the biosphere).

   The word, "wilderness", derives from the notion of wildness; in other
   words that which is not controllable by humans. The word's etymology is
   from the Old English wildeornes, which in turn derives from wildeor
   meaning wild beast (wild + deor = beast, deer). From this point of
   view, it is the wildness of a place that makes it a wilderness. The
   mere presence or activity of people does not disqualify an area from
   being "wilderness." Many ecosystems that are, or have been, inhabited
   or influenced by activities of people may still be considered "wild."
   This way of looking at wilderness includes areas within which natural
   processes operate without very noticeable human interference.

   Looked at through the lens of the visual arts, nature and wildness have
   been important subjects in various epochs of world history. An early
   tradition of landscape art occurred in the Tang Dynasty (618-907). The
   tradition of representing nature as it is became one of the aims of
   Chinese painting and was a significant influence in Asian art. Artists
   learned to depict mountains and rivers “from the perspective of nature
   as a whole and on the basis of their understanding of the laws of
   nature … as if seen through the eyes of a bird.” In the 13th century,
   Shih Erh Chi listed "scenes lacking any places made inaccessible by
   nature,” as one of the 12 things to avoid in painting.

   In the Western world the idea of wilderness having intrinsic value
   emerged in the 1800s, especially in the works of the Romantic movement.
   British artists John Constable and JMW Turner turned their attention to
   capturing the beauty of the natural world in their paintings. Before
   that, paintings had been primarily of religious scenes or of human
   beings. William Wordsworth’s poetry described the wonder of the natural
   world, which had formerly been viewed as a threatening place.
   Increasingly the valuing of nature became an aspect of Western culture.

Beauty in nature

   Salmon fry hatching. One of the original roots of the Latin word natura
   was natus, from nasci translated as "to be born".
   Enlarge
   Salmon fry hatching. One of the original roots of the Latin word natura
   was natus, from nasci translated as "to be born".

   Beauty in nature is a common theme in modern life and in art, and books
   emphasizing beauty in nature fill large sections of libraries and
   bookstores. That nature has been depicted and celebrated by so much
   art, photography, poetry and other literature shows the strength with
   which many people associate nature and beauty. Why this association
   exists, and what the association consists of, is studied by the branch
   of philosophy called aesthetics. Beyond certain basic characteristics
   that many philosophers agree about to explain what is seen as
   beautiful, the opinions are virtually endless.

   Many scientists, who study nature in more specific and organized ways,
   also share the conviction that nature is beautiful; the French
   mathematician, Jules Henri Poincaré (1854-1912) said:

     The scientist does not study nature because it is useful; he studies
     it because he delights in it, and he delights in it because it is
     beautiful.
     If nature were not beautiful, it would not be worth knowing, and if
     nature were not worth knowing, life would not be worth living. Of
     course I do not here speak of that beauty which strikes the senses,
     the beauty of qualities and of appearance; not that I undervalue
     such beauty, far from it, but it has nothing to do with science; I
     mean that profounder beauty which comes from the harmonious order of
     the parts and which a pure intelligence can grasp.

   A common classical idea of beautiful art involves the word mimesis, the
   imitation of nature. Also in the realm of ideas about beauty in nature
   is that the perfect is implied through symmetry, equal division, and
   other perfect mathematical forms and notions.
   The first few hydrogen atom electron orbitals shown as cross-sections
   with color-coded probability density
   Enlarge
   The first few hydrogen atom electron orbitals shown as cross-sections
   with colour-coded probability density

Matter and energy

   Some fields of science see nature as "matter in motion", obeying
   certain "laws of nature" which science seeks to understand. For this
   reason the most fundamental science is generally understood to be
   "physics"—the name for which is still recognizable as meaning that it
   is the study of nature.

   Matter is commonly defined as the substance of which physical objects
   are composed. It constitutes the observable universe. According to the
   theory of special relativity, there is no unchangeable distinction
   between matter and energy, because matter can be converted to energy
   (see annihilation), and vice versa (see matter creation). The visible
   components of the universe are now believed to compose only 4 percent
   of the total mass. The remainder is believed to consist of 23 percent
   cold dark matter and 73 percent dark energy. The exact nature of these
   components is still unknown and is currently under intensive
   investigation by physicists.

   The behaviour of matter and energy throughout the observable universe
   appears to follow well-defined physical laws. These laws have been
   employed to produce cosmological models that successfully explain the
   structure and the evolution of the universe we can observe. The
   mathematical expressions of the laws of physics employ a set of twenty
   physical constants that appear to be static across the observable
   universe. The values of these constants have been carefully measured,
   but the reason for their specific values remains a mystery.

Nature beyond Earth

   NGC 4414, a typical spiral galaxy in the constellation Coma Berenices,
   is about 56,000 light years in diameter and approximately 60 million
   light years distant.
   Enlarge
   NGC 4414, a typical spiral galaxy in the constellation Coma Berenices,
   is about 56,000 light years in diameter and approximately 60 million
   light years distant.
   The deepest visible-light image of the universe, the Hubble Ultra Deep
   Field. Image Credit: NASA, ESA, S. Beckwith (STScI) and the HUDF team.
   Enlarge
   The deepest visible-light image of the universe, the Hubble Ultra Deep
   Field. Image Credit: NASA, ESA, S. Beckwith (STScI) and the HUDF team.

   Outer space, also simply called space, refers to the relatively empty
   regions of the universe outside the atmospheres of celestial bodies.
   Outer space is used to distinguish it from airspace (and terrestrial
   locations). There is no discrete boundary between the Earth's
   atmosphere and space, as the atmosphere gradually attenuates with
   increasing altitude. Outer space within the solar system is called
   interplanetary space, which passes over into interstellar space at what
   is known as the heliopause.

   Although outer space is certainly spacious, it is far from empty. Outer
   space is sparsely filled with several dozen organic molecules
   discovered to date by microwave spectroscopy, blackbody radiation left
   over from the big bang and the origin of the universe, and cosmic rays,
   which include ionized atomic nuclei and various subatomic particles.
   There is also some gas, plasma and dust, and small meteors.
   Additionally, there are signs of human life in outer space today, such
   as material left over from previous manned and unmanned launches which
   are a potential hazard to spacecraft. Some of this debris re-enters the
   atmosphere periodically.

   The planet Earth is currently the only known body within the solar
   system to support life. However, current evidence suggests that in the
   distant past the planet Mars possessed bodies of liquid water on the
   surface. For a brief period in Mars' history, it may have also been
   capable of forming life. At present though, most of the water remaining
   on Mars is frozen. If life exists at all on Mars, it is most likely to
   be located underground where liquid water can still exist.

   Conditions on the other terrestrial planets, Mercury and Venus, appears
   to be too harsh to support life as we know it. But it has been
   conjectured that Europa, the fourth-largest moon of Jupiter, may
   possess a sub-surface ocean of liquid water and could potentially host
   life.
   Retrieved from " http://en.wikipedia.org/wiki/Nature"
   This reference article is mainly selected from the English Wikipedia
   with only minor checks and changes (see www.wikipedia.org for details
   of authors and sources) and is available under the GNU Free
   Documentation License. See also our Disclaimer.
