Ⅰ. The Sun

Image source - Google
Image by - Science Facts

Physical Features 

It is a star at the centre of the solar system & is a nearly perfect sphere of hot plasma, radiating energy as visible, UV, IR radiations. Its diameter is about 1.39 mn Km & weighs almost 2 x 10^30 Kg & accounts for about 99.86% of the total mass of the solar system. 

It consists of 73% of hydrogen, 25% of helium & 2% of heavier elements like carbon, iron, etc. It is at a mean distance of 150 mn Km from the Earth. The temperature of its surface is about 5772 K & of its atmosphere is 5 x 10^6 K. 

The Sun is a G - type main-sequence star (G2V) & was formed approximately 4.6 bn years ago from the gravitational collapse of matter within a large molecular cloud. Most of this gathered in the centre, whereas the rest flattened into an orbiting disk that became the Solar System. 

The Sun's core fuses about 600 mn tons of hydrogen into helium per second, converting 4 mn tons of matter into energy as a result which takes about 10,000 years to escape the core after which the sunlight takes just 8 min 20 sec to reach the Earth from the surface.

The Sun rotates faster at its equator than at its poles because of the convection motion caused by heat transport & Coriolis force due to its rotation. The rotational period is approximately 25.6 days at the equator & 33.5 days at the poles anticlockwise around its axis. It has four natural satellites : 

  1. 3122 Florence
  2. 130 Elektra
  3. 225088 Gonggong  
  4. 90482 Orcus. 

The structure of the Sun contains the following layers :

1. Core 

  • The innermost layer where nuclear fusion occurs
  • 20 - 25% of the Sun's radius. 
  • Temperature is 15.7 mn K. 
  • Two types of nuclear reactions called the p-p chain & CNO cycle takes place in the core that produces 99% of the power by fusion. 
  • The p-p chain occurs around 9.2 x 10^37 times/sec converting about 3.7 x 10^38 protons into alpha particles which release 3.846 x 10^26 W of energy. 

2. Radiative zone 

  • Lies between 25 - 70% of the radius of the Sun 
  • Energy is transferred by radiation instead of convection. 
  • Temperature drops from 7 mn K to 2 mn K 
  • Density decreases from 20 g/cm^3 to 0.2 g/cm^3 of the Sun. 

3. Tachocline 

  • Boundary region between the radiative & convection zones 
  • Basically a transition layer. 
  • Successive horizontal layers slide past each other due to the rotational difference  
  • It is hypothesized that this layer creates the Sun's magnetic field due to the presence of a magnetic dynamo in it. 

4. Convective zone 

  • Lies between 70% of the Sun's radius & photosphere 
  • Plasma is not dense & hot enough to transfer energy via radiation hence, it is transferred by convective currents to the surface.
  • Its thermal columns form an imprint on the photosphere 
  • Gives a granular appearance called solar granulation & super granulation at the smallest & largest scales. 
  • These columns are called Bénard cells & are hexagonal prism-shaped. 

5. Photosphere 

  • The layer below which the Sun becomes opaque to visible light. 
  • Photons from this layer escape the Sun through the corona & become sunlight. 
  • Upper part is cooler than the lower part hence the Sun appears brighter at the centre than on the edge, this is known as limb darkening.
  • Has a particle density of about 10^23/m^3 
  • Not fully ionized, leaving almost all the hydrogen in atomic form. 

6. Atmosphere 

  • A gaseous layer surrounding the Sun 
  • Comprises the chromosphere, solar transition region, corona & heliosphere. 
  • These can be seen during a solar eclipse 
  • Are much hotter than the photosphere due to Alfvén waves that have enough energy to heat the corona. 

  1. The chromosphere is a 2000 Km thick layer with a maximum temperature of about 20000 K & is visible as a coloured flash at the beginning and end of the total solar eclipse. 
  2. Above this, is a thin transition region of 200 Km where the temperature increases to 1 mn K. 
  3. The next layer is the corona where the temperature increases to 2 mn K. It is the extended atmosphere of the Sun which has a greater volume than the photosphere. 
  4. After this, the heliosphere is the outermost layer filled with solar wind plasma which travels outward continuously & gives the Sun's magnetic field a spiral shape until it impacts the heliopause. 


II. Mercury 

Image source - Google | Image by - National Geographic

It is the smallest planet in the Solar System & closest to the Sun. It has a revolutionary period of 87.97 days & a rotational period of 176 days. It is a terrestrial planet with a radius of 2439.7 Km & is at a distance of 57.91 Km from the Sun. It consists of approximately 70% metallic & 30% of silicate materials. 

It is named after the Roman god 'Mercurius' who was a messenger of gods. It can only be seen near the western horizon after the sunset or the eastern horizon before sunrise during twilight. 

Structure of Mercury 

1. Core 

  • Occupies about 55% of the volume 
  • Has a solid iron sulfide outer layer
  • Deeper liquid layer & a solid inner core. 

2. Mantle 

  • 500-700 Km thick layer surrounding the core 
  • Consists of silicates. 

3. Crust 

  • 35 Km thick layer of solid silicates. 
  • Chemically heterogeneous. 
  • Low in iron but high in sulfide. 
  • Dominated by iron-poor pyroxene & olivine
  • Along with sodium-rich plagioclase & minerals of mixed Mg, Ca & FeS. 

Geographic Features of Mercury 

1. Craters 

  • Small bowl-shaped cavities to multi-ringed impact basins hundreds of km across. 
  • Appear in all states from fresh rayed to highly degraded remnants. 
  • Largest crater - Caloris Planitia or Caloris Basin of diameter 1550 Km. 
  • At its antipode, there is a large region of hilly terrain called the 'Wierd Terrain'. 

2. Plains 

There are two types of plains regions on Mercury : 

1. Gently rolling, hilly plains between the craters are the oldest visible surfaces predating the heavily cratered terrain. These intercrater plains have obliterated many craters & show a scarcity of smaller craters below 30 Km in diameter. 

2. Smooth plains are widespread flat areas that fill craters of different sizes. These have the same albedo as the older intercrater plains. Their localisation & rounded shapes strongly support volcanic origins. 

3. Compression Folds

  • Numerous compression folds or 'rupes' crisscross the plains of Mercury. 
  • As the planet's interior cooled, it contracted & deformed the surface
  • It created wrinkle ridges & lobate scrapes associated with thrust faults. 
  • Can be 1000 Km long & 3 Km high. 
  • It has reduced Mercury's radius by 1-7 Km. 
  • Less than 50 mn years old. 

4. Volcanoes 

  • Pyroclastic flows from low profile shield volcanoes have been found on Mercury 
  • 51 deposits have been identified where 90% of them are within the impact craters. 
  • A "rimless depression" inside the Caloris Basin consists of at least nine overlapping volcanic vents that are 8 Km in diameter. 
  • It is thus a compound volcano. 
  • The vent floors are 1 Km deep created by explosive eruptions. 

Atmosphere of Mercury 

Mercury has a very tenuous & highly variable atmosphere containing hydrogen, helium, oxygen, etc. These elements originate either from the Solar wind or from the planet's crust. Solar light pushes the atmospheric gases away creating a comet-like tail behind the planet. The exosphere is not stable i.e. gases are continuously lost & replenished. 

  • Hydrogen & Helium come from the Solar wind  
  • Get diffused into Mercury's magnetosphere & escape back to space. 
Water vapour is released by a combination of processes such as : 

  • Comets striking the surface
  • Sputtering creates water out of hydrogen from the solar wind  
  • Oxygen from the rocks 
  • Sublimation from the reservoirs of water ice in the polar craters. 

Magnetic Field of Mercury 

Mercury has a significant dipolar & global magnetic field. It is about 1.1% of Earth's magnetic field strength. The poles are nearly aligned with the planet's axis. 

The magnetic field is generated by a dynamo effect from the circulation of the planet's iron-rich liquid core. Strong tidal heating effects caused by the planet's high orbital eccentricity help the core to stay in the liquid state that is necessary for this dynamo effect. 

Magnetosphere of Mercury 

Mercury's magnetic field is strong enough to deflect the solar wind creating a magnetosphere that traps solar wind plasma. Bursts of energy particles in the planet's magnetotail indicate a dynamic quality to the planet's magnetosphere. 

Mercury's magnetosphere hosts numerous magnetic tornadoes which are twisted bundles of magnetic field connecting the planet's magnetic field to space that are 800 Km wide. These tornadoes are known as flux transfer events that form open windows in the magnetic shield through which the solar wind can enter & impact the Mercury's surface.