THE SUN
The Solar System
The solar system consist of a central star (the Sun) and the bodies that
orbit it. These bodies include eight planets and their more than 160 known
moons; dwarf planets; Kuiper Belt objects; asteroids; comets; and meteoroids.
The Solar System also contains interplanetary gas and dust. The planets fall
into two groups: four small rocky planets near the Sun (Mercury, Venus, Earth ,
and Mars); and four planets further out, the giants(Jupiter, Saturn, Uranus,
and Neptune). Between the rocky planets and giants is the asteroid belt, which
contains thousands of chunks of rock orbiting the Sun. Beyond Neptune is the
Kuiper Belt and, more distant, the Oort Cloud. Most of the bodies in the planetary
part of the Solar System move around the Sun in elliptical orbits located in a
thin disc around the Sun's equator. All the planets orbit the sun in the same
direction (anticlockwise when viewed from above) and all but Venus and Uranus
also spin about their axes in this direction. Moons also spin as they, in turn,
orbit their planets. The entire Solar System orbits the center of our galaxy,
the Milky Way.
The sun is the star at the center of the Solar System. It is about five billion years old and will continue to shine as it does now as it do now for about five billion years. The Sun is a yellow main sequence star about 1.4 million kilometres in diameter. It consist almost entirely of hydrogen and helium. In the Sun’s core, hydrogen is converted to helium by nuclear fusion, releasing energy in the process. The energy travels from the core, through the radiative and convective zones, to the photosphere (visible surface), where it leaves the Sun in the form of heat and light. On the photosphere there are often dark, relatively cool areas called sunspots, which usually appears in pairs or groups and are and are caused by the cooling effect of the magnetic field. Other types of solar activity are flares, which are usually associated with sunspots, and prominences. Flares are sudden discharges of high-energy radiation and atomic particles. Prominences are huge loops or filaments of gas extending into the solar atmosphere; some last for hours, others for months. Beyond the photosphere is the chromosphere (inner atmosphere) and the extremely rarified corona (outer atmosphere), which extends millions of kilometres into space. Tiny particles that escape from the corona give rise to the solar wind, which streams through space at hundreds of kilometres per second. The chromosphere and corona can be seen from Earth when the Sun is totally eclipsed from the Moon
Mercury
Mercury is the nearest planet to the Sun,
orbiting at an average distance of about 58 million kilometres. Because Mercury
is the closest planet to the Sun, it moves faster than any other planet,
travelling at an average speed of nearly 48 kilometers per second and
completing an orbit in just under 88 days. Mercury is very small (only 40 per
cent bigger than the Moon) and rocky. Most of the surface has been heavily
cratered by the impact of meteorites, although there are also smooth, sparsely
cratered lava-covered plains. The Calories Basin is the largest crater,
measuring about 1,300 kilometers across. It is thought to have been formed when
a 60-kilometer-diameter asteroid hit the planet and is surrounded by concentric
rings of mountains thrown up by the impact. The surface also has many
cliff-like ridges (called rupes) that are thought to have been formed when the
hot core of the young planet cooled and shrank about four billion years ago,
buckling the planet’s surface in the process. The planet rotates about its axis
very slowly, taking nearly 59 Earth days to complete one rotation. As a result,
a solar day (sunrise to sunset) on Mercury is about 176 Earth days-twice as
long as the 88-day Mercurian year. Mercury has extreme surface temperatures,
ranging from a maximum of 430 degree celsius on the sunlit side to -170 degree
celsius on the dark side. All nightfall, the temperature drops very quickly
because the planet’s atmosphere is almost non-existent. It consists only of
minute amounts of helium and hydrogen captured from the solar wind, plus traces
of the other gases.
Venus
Venus is a rocky planet and the second planet from the
Sun. Venus spins slowly backwards as it orbits the Sun, causing its rotational
period to be the longest in the Solar System, at about 243 Earth days. It is
slightly smaller than Earth and probably has a similar internal structure,
consist of a semi-solid metal core, surrounded by a rocky mantle and crust.
Venus is the brightest object in the sky after the Sun and moon because its
clouds reflects sunlight strongly. The main component of the atmosphere is
carbon dioxide, which traps heat in a greenhouse effect far stronger than that
on Earth. As a result, Venus is the hottest planet, with a maximum surface
temperature of about 480 degree celsius. The thick cloud layers contain
droplets of sulphuric acid are driven around the planet by winds at speeds of
360 kilometres per hour. Although the planet takes 243 Earth days to rotate
once, the high-speed winds cause the clouds to circle the planet in only four
Earth days. The high temperature, acidic clouds, and enormous atmospheric
pressure (about 90 times greater at the surface than that on Earth) make the
environment extremely hostile. However, space probes have managed to land on
Venus and photograph its dry, dusty surface. The Venusian surface has also been
mapped by the probes with radar equipment that can “see” through the cloud
layers. Such radar maps reveal a terrain with craters, mountains, volcanoes,
and areas where craters have been covered by plains of solidified volcanic
lava. There are two large highland regions called Aphrodite Terra and Ishtar Terra.
The Earth
The earth is the third of the eight planets that orbit
the Sun. It the largest and the densest rocky planet, and the only one known to
support life. About 70 per cent of the Earth’s surface is covered by water,
which is not found in liquid form on the surface of any other planet. There are
four main layers: the inner core, the outer core, the mantle, and the crust. At
the heart of the planet the solid inner core has a temperature of about 6,600
degree celsius. The heat from this inner core causes material in the molten
outer core and mantle to circulate in convection currents. It is thought that
these convection currents generate the Earth’s magnetic field, which extends
into space as the magnetosphere. The Earth’s atmosphere helps screen out some of
the harmful radiation from the Sun, stops most meteoroids from reaching the
planet’s surface, and trap enough heat to prevent extremes of cold. The Earth
has one natural satellite, the Moon, which is thought to have formed when a
huge asteroid impacted Earth in the distant past.
The Moon
The moon is the earth’s only natural satellite. It is
relatively large for a moon, with a diameter of about 3,470 kilometers – just
over quarter that of the Earth. The Moon takes the same time to rotate on its
axis as it takes to orbit the Earth (27.3 days), and so the same side (the near
side) always faces us. However, the amount of the surface we can see – the
phase of the Moon – depends on how much of the near side is in sunlight. The
Moon is dry and barren, with negligible atmosphere and water. It consist mainly
of solid rock, although its core may contain molten rock and or iron. The
surface is dusty, with highlands covered in craters caused by meteorite
impacts, and lowlands in which large craters have been filled by solidified
lava to form dark areas called maria or “seas”. Maria occur mainly on the near
side, which has a thinner crust than the far side. Many of the craters are
rimmed by mountain ranges that form the crater walls and can be thousands of
meters high.
Mars
Mars, known as the red planet, is the fourth planet from the Sun and the outermost rocky planet. In the 19th century, astronomers first observed what were thought to be signs of life on Mars. These signs included apparent canal-like lines on the surface, and dark patches that were thought to be vegetation. It is now known that the “canals” are an optical illusion, and the dark patches are areas where the red dust that covers most of the planet has been blown away. The fine dust particles are often whipped up by winds in to dust storms that occasionally obscure almost all surface. Residual fine dust in the atmosphere. The northern hemisphere of Mars has many large plains formed of solidified volcanic lava, whereas the southern hemisphere has many craters and large impact basins. There are also several huge, extinct volcanoes, including Olympus Mons, which, at 600 kilometers across and 25 kilometers high, is the largest known volcano in the Solar System. The surface also has many canyons and branching channels. The canyons were formed by the movements of the surface crust, but the channels are thought to have been formed by flowing water that has now dried up. The Martian atmosphere is much thinner than Earth’s, with only a few clouds and morning mists. Mars as two tiny, irregularly shaped moons called Phobos and Deimos. Their small size indicates that they may be asteroids that have been captured by the gravity of Mars.
Jupiter
Jupiter is the fifth planet from the Sun and the innermost of
the four giant planets. It is the largest and the most massive planet, with a
diameter about 11 times that of the Earth and a mass about 2.5 times the
combined mass of the seven other planets. Jupiter is thought to have a small
rocky core surrounded by an inner mantle of metallic hydrogen (liquid hydrogen
that acts like a metal). Outside the inner mantle is an outer mantle of liquid
hydrogen and helium that merges in to the gaseous atmosphere. Jupiter’s rapid
rate of rotation causes the clouds in its atmosphere to form belts and zones that
encircle the planet parallel to the equator. Belts are dark, low-lying,
relatively warm cloud layers, and zones are bright, high-altitude, cooler cloud
layers. Within the belts and zones, turbulence causes the formation of cloud
features such as white ovals and red spots, both of which are huge storm
systems. The most prominent cloud feature is a storm called the Great Red Spot,
which consists of a spiralling column of clouds three times wider than the
Earth that rises about eight kilometers above the upper cloud layer. Jupiter
has a thin, faint, main ring, inside which is a tenuous halo ring of tiny
particles. Beyond the main ring’s outer edge is a broad and faint two-part
gossamer ring. There are 63 known Jovian moons. The four largest moons (called
the Galileans) are Ganymede, Callisto, Io, and Europa. Ganymede and Callisto
are cratered and icy. Europa is smooth and icy and is thought to have a
subsurface water ocean. Io is covered in bright red, orange, and yellow
splotches. This coloring is caused by sulphurous material from active volcanoes
that shoot plumes of lava hundreds of kilometers above the surface.
Saturn
Saturn is the sixth planet from the Sun. It is a gas giant
almost as big as Jupiter, with an equatorial diameter of about 120,500
kilometers. Saturn is thought to consist of a small core of rock and ice
surrounded by an inner mantle of metallic hydrogen (liquid hydrogen that acts
like a metal). Outside the inner mantle is an outer mantle of liquid hydrogen
that merges into a gaseous atmosphere. Saturn’s clouds form belts and zones
similar to those on Jupiter, but obscured by overlying haze. Storms and eddies,
seen as red or white ovals, occur in the clouds. Saturn has an extremely thin
but wide system of rings that is less than one kilometer thick but extends
outwards to about 420,000 kilometers from the planet’s surface. The main rings
comprise thousands of narrow ringlets, each made of icy rock lumps that range
in size from tiny particles to chunks several meters across. The D, E, and G
rings are very faint, the F ring is brighter, and the A, B, and C rings are
bright enough to be seen from Earth with binoculars. In 2009, a huge dust ring
was discovered 6 kilometers (4 million miles) beyond the main system. Saturn has
more than 60 known moons, some of which orbit inside the rings and are thought
to exert a gravitational influence on the shapes of the rings. Unusually, seven
of the moons are co-orbital- they share an orbit with another moon. Astronomers
believe that such co-orbital moons may have originated from a single satellite
that broke up.
Uranus
Uranus is the seventh planet from the Sun and the third
largest, with a diameter of about 51,000 kilometers. It is thought to consist
of a dense mixture of different types of ice and gas around a solid core. Its atmosphere
contains traces of methane, giving the planet a blue-green hue, and the
temperature at the cloud tops is about -210 degree celsius. Uranus is the most
featureless planet to have been closely observed: only a few icy clouds of
methane have been seen so far. Uranus is unique among the planets in that its axis
of rotation lies close to its orbital plane. As a result of its strongly tilted
rotational axis, Uranus rolls on its side along its orbital path around the
Sun, whereas other planets spin more or less upright. Uranus is encircled by
main rings that consist of rocks interspersed with dust lanes and too distant
outer rings made of dust. The rings contain some of the darkest matter in the
Solar System and are extremely narrow, making them difficult to detect: most of
them are less than 10 kilometers wide, whereas most of Saturn’s rings are
thousands of kilometers width. There 27 Uranium known moons, all of which are
icy and most which are further out than the rings. The 13 inner moons are small
and dark, with diameters of less than 160 kilometers, and the five major moons
are between about 470 and 1,600 kilometers in diameter. The major moons have a
wide variety of surface features. Mirinda has the most varied surface, with
cratered areas broken up by huge ridges and cliffs 20 kilometers high. Beyond these
are nine much more distant moons with diameters less than 150 km.
Neptune and Pluto
Neptune is the
furthest planet from the Sun, at an average distance of about 4,500 million
kilometers. Neptune is the smallest of the giant planet and is thought to
consist of a small rocky core surrounded by a mixture of liquids and gases.
Several transient cloud features have been observed in its atmosphere. The
largest of these were the Great Dark Spot, which was as wide as the Earth, the
Small Dark Spot, and the Scooter. The Great and Small Dark Spots were huge
storms that were swept around the planet by winds of about 2,000 kilometers per
hour. The Scooter was a large area of cirrus cloud. Neptune has six tenuous
rings and 13 known moons. Triton is the largest Neptunian moon and the coldest
object in the Solar System, with a temperature of -240 degree celsius. Unlike
most moons in the Solar System, Triton orbits its mother planet in the opposite
direction to the planet’s rotation. The region extending out from Neptune’s
orbit is populated by Kuiper Belt objects and dwarf planets they make a
dough-nut shaped belt called the Kuiper Belt. The Kuiper Belt objects are a mix
of rock and ice, irregular in shape, and less than 1,000 kilometers across. The
larger dwarf planets, which include Pluto, are almost round bodies. Pluto was
the first object discovered beyond Neptune and was considered a planet until the
dwarf planet category was introduced in 2006. It is made of rock and ice and is
2,274 kilometers across. It has three known moons. The largest, Charon, is
about half Pluto’s size and the two probably had a common origin.
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