- The sun transfers heat energy from itself to the planets / earth through sunlight
- The sun is the source of energy for life on earth
- So, the source of heat energy on earth is transferred from the sun to earth

So, the source of heat energy on earth is transferred from the sun to earth
Q: What is the main source of heat for Earth's surface?
Context:
- The sun transfers heat energy from itself to the planets / earth through sunlight 
- The sun is the source of energy for life on earth