The thermosphere and exosphere are the warmest layers of the Earth’s atmosphere due to their unique position and the absorption of high-energy solar radiation.
In the thermosphere, which extends from about 85 kilometers (53 miles) to 600 kilometers (372 miles) above the Earth, temperatures can soar as high as 2,500 degrees Celsius (4,500 degrees Fahrenheit) or more. This extreme warmth is primarily a result of the absorption of ultraviolet (UV) and X-ray radiation from the sun. In this layer, solar energy is absorbed by the sparse gas molecules that exist there. Although it is very hot, the gas is so thin that it would not feel warm to a human; the few air molecules present can’t transfer heat effectively.
The exosphere lies above the thermosphere, starting around 600 kilometers (372 miles) and extending out to about 10,000 kilometers (6,200 miles). In this region, the atmosphere becomes even thinner. Like the thermosphere, the exosphere contains very few particles. However, these sparse molecules can gain a lot of energy from solar radiation, leading to high temperatures as well. Most of the particles in this layer are hydrogen and helium, and they can travel at very high speeds.
So, the key reason the thermosphere and exosphere are the warmest layers is due to the direct interaction with solar radiation, which raises the temperature of the sparse gas particles found at those altitudes. Despite being extremely hot in terms of temperature, due to the rarity of particles, these layers do not have a significant thermal effect on spacecraft or satellites moving through them.