International Space Agencies – Missions and Discoveries

Why does Mercury have such a big iron core?


From UPSC perspective, the following things are important :

Prelims level: Our planetary system

Mains level: Not Much

Researchers have developed a model showing that the density, mass and iron content of a Mercury’s core is influenced by its distance from the Sun’s magnetic field.

About Mercury

  • Mercury is the first and the smallest planet in our solar system.
  • It is also the closest planet to Earth.
  • Like the other three terrestrial planets, Mercury contains a core surrounded by a mantle and a crust.
  • But unlike any other planet, Mercury’s core makes up a larger portion of the planet.
  • MESSENGER was a NASA robotic space probe that orbited the planet Mercury between 2011 and 2015, studying Mercury’s chemical composition, geology, and magnetic field.
  • It was the analysis from the MESSENGER mission that tells: Mercury’s core is solid.

Mystery over the core

  • It has long been known that Mercury’s core composition is made of liquid metal.
  • The core itself is about 3,600 km across. Surrounding that is a 600 km thick mantle.
  • And around that is the crust, which is believed to be 100-200 km thick.
  • The crust is known to have narrow ridges that extend for hundreds of kilometres.
  • This large core has long been one of the most intriguing mysteries about Mercury.

Why does Mercury have a large core?

  • A new study reveals that the sun’s magnetism is the reason.
  • The sun’s magnetic field influences the density, mass, and iron content of Mercury’s core.
  • The four inner planets of our solar system—Mercury, Venus, Earth, and Mars—are made up of different proportions of metal and rock.
  • A gradient in which the metal content in the core drops off as the planets get farther from the sun.
  • The researchers explain how this happened by showing that the sun’s magnetic field controlled the distribution of raw materials in the early forming solar system.

What are the key propositions?

  • During the early formation of the solar system, when a swirling dust storm and gas encircled the sun, iron’s grain was drawn toward the centre by the sun’s magnetic field.
  • At the time of planet formation from clumps of that dust and gas, planets nearer to the sun consolidated more iron into their centres than those farther away.
  • Scientists also found that the density and proportion of iron in the planet’s core correlate with the strength of the magnetic field around the sun during planetary formation.
  • Existing models on planetary formation were used to determine the speed at which gas and dust were pulled into the centre of our solar system during its formation.
  • The magnetic field that the sun would have generated as it burst into being and calculated how that magnetic field would draw iron through the dust and gas cloud.

Cooling led solidification

  • As the early solar system began to cool, dust and gas that were not drawn into the sun started to clump together.
  • The clumps closer to the sun would have been exposed to a stronger magnetic field and thus would contain more iron than those farther away from the sun.
  • As the clumps coalesced and cooled into spinning planets, gravitational forces drew the iron into their core.

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