Q. Which of the following are sources of Earth’s internal heat?

1. Radio-active decay
2. Gravitation
3. Orbital characteristics of the Earth-Moon & Earth-Sun system

Select the correct option from the codes given below:
Answer: 1 & 2
Notes: Earth’s internal heat is mainly derived from radioactive decay and residual heat due to gravitational accretion. Radioactive decay produces about 80% of Earth’s heat, while gravitational energy from accretion accounts for about 20%. Orbital characteristics contribute to tidal energy, not Earth’s internal heat.