NASA’s Gravity Recovery and Interior Laboratory (GRAIL)  mission data reveals new insights into why the Moon’s two hemispheres differ in appearance.

What are the two sides of the moon?

• Nearside: It is that part which is always visible from the Earth.
  • Covered by vast plains, known as mare, formed by volcanic activity. 
  • Crust on this side is thinner which allows magma to rise more easily. 
  • This side is warmer and more radioactive due to radioactive elements like thorium and titanium. 
• Farside: This side is never visible from earth. 
  • It has more rugged terrain than plains. 
  • Crust on this side is thicker which prevents lava flows. 

Reasons Behind the Lunar Asymmetry

• Tidal Deformation and Gravitational Influence
  • The Moon’s nearside is more geologically active and flexible due to Earth’s gravitational pull.
  • Tidal deformation causes the nearside to experience greater internal movement, shaping its surface differently from the farside.
• Volcanic Activity and Heat Distribution
  • The nearside was once much more volcanically active than the farside, resulting in the formation of large basaltic plains (mare).
  • Heat-producing radioactive elements like thorium and titanium accumulated on the nearside, making it hotter than the farside (by around 100-200°C).
  • This heat imbalance influenced the geological features of both hemispheres.
• Crust Thickness and Surface Composition
  • GRAIL data revealed that the nearside has a thinner crust, allowing magma to flow more easily in the past.
  • The farside has a thicker crust, preventing widespread lava flows, which explains its rugged, cratered surface.

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