While the atmosphere doesn't have a clear-cut boundary, the exosphere, the outermost layer, extends roughly 10,000 kilometers (6,200 miles) from Earth's surface. However, the thermosphere, the layer responsible for absorbing most of the Sun's ultraviolet radiation, stretches from about 80 to 1,000 kilometers (50 to 620 miles).

Therefore, the thermosphere is the closest layer that extends about 17 km above Earth's surface.