NCERT Summary: Our Solar System - 1 | Geography for UPSC CSE PDF Download

Understanding Our Solar System

• The solar system is a vast and complex system that includes the Sun and all the celestial bodies that are bound to it by gravity. This includes planets, moons, asteroids, comets, and a variety of smaller objects.
• The Sun is the central star of our solar system and is the primary source of energy for all the objects within it. It is a massive ball of hot gases, primarily hydrogen and helium, undergoing nuclear fusion which produces immense amounts of energy.
• Around the Sun, there are eight major planets, each with its own unique characteristics and orbits. These planets are further categorized into two groups:
• Inner Planets: Also known as terrestrial planets, they are composed mainly of rock and metal. They have solid surfaces and are denser than the outer planets. The inner planets are:
  • Mercury: The closest planet to the Sun, Mercury has a thin atmosphere and is known for its extreme temperature fluctuations.
  • Venus: Often called Earth’s twin due to its similar size, Venus has a thick, toxic atmosphere and is the hottest planet in the solar system.
  • Earth: The only planet known to support life, Earth has a diverse range of environments and a protective atmosphere.
  • Mars: Known as the Red Planet, Mars has a thin atmosphere and is home to the largest volcano and canyon in the solar system.
• Outer Planets: These planets are much larger than the inner planets and are primarily composed of gases. They have thick atmospheres and lack solid surfaces. The outer planets are:
  • Jupiter: The largest planet in the solar system, Jupiter is known for its Great Red Spot, a massive storm, and its many moons.
  • Saturn: Famous for its stunning rings, Saturn is the second-largest planet and is made mostly of hydrogen and helium.
  • Uranus: An ice giant with a unique tilt, Uranus has a blue color due to methane in its atmosphere.
  • Neptune: The farthest planet from the Sun, Neptune is known for its deep blue color and strong winds.

Moons: Many of the planets in our solar system have natural satellites, known as moons, that orbit them. These moons vary greatly in size, composition, and the number of craters on their surfaces. Some moons, like Jupiter’s Europa and Saturn’s Enceladus, are of particular interest because they may have subsurface oceans and the potential for supporting life.

• Asteroids: These are small, rocky bodies that orbit the Sun, primarily found in the asteroid belt between Mars and Jupiter. They vary in size and shape and are remnants from the early solar system.
• Comets: Comets are composed of ice, dust, and rocky material. They originate from the outer regions of the solar system and, when they approach the Sun, the heat causes their ice to vaporize, creating a glowing coma and tail.
• The Formation of the Solar System: The solar system formed about 4.6 billion years ago from a rotating disk of gas and dust left over from the formation of the Milky Way galaxy. The Sun formed at the center of this disk, and the remaining material gradually coalesced to form the planets, moons, and other objects.
• The Nebula: The solar system began as a nebula, a vast cloud of gas and dust in space. This nebula started to collapse under its own gravity, leading to the formation of the Sun at its core and the surrounding material that would become the planets. This process is believed to have occurred around 5-5.6 billion years ago, with the planets forming later, about 4.6 billion years ago.

Light Year

A light year is a measure of distance and not of time. Light travels at a speed of 3,00,000 km/second. Considering this, the distances the light will travel in one year is taken to be one light year. This equals 9.461x10¹² km. The mean distance between the sun and the earth is 149,598,000 km. In terms of light years, it is 8.311 minutes of a year.

The Moon

The big splat

• Scientists have not reached a consensus on earlier theories regarding the Moon's formation. However, the prevailing view is that the Moon became Earth's satellite due to a "giant impact," often referred to as "the big splat."
• This theory suggests that a body measuring between one and three times the size of Mars collided with the Earth shortly after the planet's formation.
• The force of this collision expelled a substantial portion of the Earth's material into space.
• The ejected debris subsequently entered orbit around the Earth and eventually coalesced to form the Moon, approximately 4.44 billion years ago.

Development of Lithosphere

Formation of the Earth

• In the beginning, the Earth was very unstable. Over time, its density increased, which raised the internal temperature.
• This increase in temperature caused materials to separate based on their density. Heavier materials, like iron, sank towards the centre, while lighter materials rose to the surface.
• As time went on, the Earth cooled down, solidified, and shrank in size. This cooling process led to the formation of the outer layer called the crust.
• During the Moon's formation, a significant impact further heated the Earth. The process of differentiation allowed the Earth's materials to form distinct layers: the crust, mantle, outer core, and inner core.
• The density of materials increases from the crust towards the core.

Minerals and Rocks

• The earth is made up of different elements, with the outer layer being solid and the inside being hot and molten.
• Around 98% of the earth's crust consists of eight elements. oxygen, silicon, aluminium, iron, calcium, sodium, potassium, and magnesium.
• The rest of the elements include titanium, hydrogen, phosphorus, manganese, sulphur, carbon, nickel, and others.
• These elements are called minerals when they are in solid form.
• A mineral is a substance that occurs naturally, is inorganic, and has a specific atomic structure along with particular chemical and physical properties.
• Minerals can be made up of two or more elements, but some minerals like sulphur, copper, silver, gold, and graphite are made of a single element.
• Elements in the earth's crust are not usually found alone; they combine with other elements to form various substances.
• Even though there are not many elements in the lithosphere, they are combined in many different ways, leading to a wide variety of minerals.
• More than 2,000 minerals have been found in the earth's crust, but the ones that occur most commonly belong to six main groups known as rock-forming minerals.
• The main source of all minerals is the hot magma found inside the earth.
• When magma cools down, mineral crystals begin to form, creating a series of minerals that eventually solidify into rocks.
• Minerals like coal, petroleum, and natural gas are organic substances found in solid, liquid, and gaseous forms, respectively.
• Besides these primary minerals, others such as chlorite, calcite, magnetic minerals, hematite, bauxite, and barite are present in varying amounts in some rocks.

The Major Elements of the Earth’s Crust

Metallic Minerals

These minerals contain metal content and can be sub-divided into three types:

• Precious metals: gold, silver, platinum etc.
• Ferrous metals: iron and other metals often mixed with iron to form various kinds of steel.
• Non-ferrous metals: include metals like copper, lead, zinc, tin, aluminum etc.

Hardness- relative resistance being scratched; ten minerals are selected to measure the degree of hardness from 1-10. They are: 1. talc; 2. Gypsum; 3. calcite; 4. fluorite; 5. apatite; 6. felspar; 7. quartz; 8. topaz; 9. corundum; 10. Diamond. Compared to this for example, a fingernail is 2.5 and glass or knife blade is 5.5.

Non-Metallic Minerals

These minerals do not contain metal content. Sulphur, phosphates and nitrates are examples of non-metallic minerals. Cement is a mixture of non-metallic minerals.

Rocks

The earth’s crust is composed for rocks. A rock is an aggregate of one or more minerals. Rock may be hard or soft and in varied colours. For example, granite is hard, soapstone is soft. Gabbro is black and quartzite can be milky white. Rocks do not have definite composition of mineral constituents. Feldspar and quartz are the most common minerals found in rocks.

As there is a close relation between rocks and landforms, rocks and soils, a geographer requires basic knowledge of rocks. There are many different kinds of rocks which are grouped under three families on the basis of their mode of formation.
They are:

• Igneous Rocks- solidified from magma and lava;
• Sedimentary Rocks- the result of deposition of fragments of rocks by exogenous processes;
• Metamorphic Rocks- formed out of existing rocks undergoing recrystallisation.

Igneous Rocks

• These rocks are formed from the solidification of magma and lava.
• Igneous comes from the Latin word Ignis, which means 'fire.'
• When magma cools and solidifies, it becomes igneous rock.
• Examples of igneous rocks include granite, gabbro, pegmatite, basalt, volcanic breccias, and tuff.
• Magma is molten rock found beneath the Earth's surface. When it rises and cools, it forms igneous rock.
• Cooling and solidification can happen either within the Earth's crust or on its surface.
• Igneous rocks are classified based on their texture, which depends on the size and arrangement of mineral grains.
• Slow cooling at great depths results in larger grains, while rapid cooling at the surface leads to smaller, smoother grains.

Sedimentary Rocks

• Sedimentary comes from the Latin word sedimentum, meaning settling. These rocks form from fragments of other rocks that are deposited by natural processes.
• Lithification is the process by which these fragments compact and form rocks over time. Rocks (igneous, sedimentary and metamorphic) of the earth’s surface are exposed to denudational agents, and are broken up into various sizes of fragments. Such fragments are transported by different exogenous agencies and deposited. These deposits through compaction turn into rocks. 
• In many sedimentary rocks, the layers of deposits retain their characteristics even after lithification. Hence, we see a number of layers of varying thickness in sedimentary rocks like sandstone, shale etc.

Sedimentary rocks are classified into three main groups based on their formation:

• Mechanically formed. Examples include sandstone, conglomerate, limestone, shale, and loess.
• Organically formed. Examples include geyserites, chalk, limestone, and coal.
• Chemically formed. Examples include chert, limestone, halite, and potash.

Metamorphic Rocks

• The term metamorphic refers to the change of form. These rocks form under changes in pressure, volume, and temperature (PVT).
• Metamorphism occurs when rocks are pushed deeper into the Earth by tectonic processes or when molten magma rises and interacts with crustal rocks.
• This process causes existing rocks to undergo recrystallisation and reorganisation of their materials.

Interior of the Earth

• The radius of the Earth is approximately 6,370 kilometers.
• It is currently impossible for anyone to physically reach the center of the Earth to observe or collect samples. Under such conditions, you may wonder how scientists tell us about the earth’s interior and the type of materials that exist at such depths.
• Scientists have developed methods to understand what lies beneath the Earth's surface using tools like seismic data and geological evidence. Most of our knowledge about the interior of the earth is largely based on estimates and inferences. Yet, a part of the information is obtained through direct observations and analysis of materials.

Direct Sources

• The most accessible material from the Earth's interior is surface rock, which is obtained through mining activities.
• Gold mines in South Africa extend to depths of 3-4 kilometers, and further exploration is limited by extreme heat conditions at greater depths.
• Besides mining, researchers have launched various initiatives to investigate deeper into the Earth's crust.
• Among these, the "Deep Ocean Drilling Project" and the "Integrated Ocean Drilling Project" are prominent.
• The deepest drilling site, located at Kola in the Arctic Ocean, has reached a depth of 12 kilometers.
• These deep drilling endeavors have provided a wealth of information through the analysis of materials extracted from different depths.
• Volcanic eruptions also provide direct information about the Earth's interior.
• When magma is expelled to the surface during an eruption, it becomes available for scientific analysis.
• However, it is difficult to ascertain the depth of the source of such magma.

Indirect Sources

• Research indicates that with increasing depth below the Earth's surface, both temperature and pressure rise. This information is gathered from mining activities.
• Meteorites that fall to Earth provide another valuable source of information about the planet.

Other significant indirect sources of information include:

• Gravitational data, which helps scientists understand the distribution of mass within the Earth.
• Magnetic field measurements, which offer insights into the Earth's interior and core.
• Seismic activity observations, which provide data on the Earth's structure and composition based on the behavior of seismic waves.
• Meteorites also offer indirect evidence regarding the composition of the Earth, helping scientists infer what materials make up our planet.