Earth's airglow. Image showing the different colours of airglow that appear at varying altitudes in the Earth's atmosphere (from top to bottom: exosphere, thermosphere, mesosphere, stratosphere, troposphere). Airglow is a faint emission of light by a planetary atmosphere that occurs when atoms and molecules in the upper atmosphere, excited by sunlight, emit light to shed their excess energy. Airglow constantly shines throughout Earth's atmosphere creating a 'bubble' of light that encases the entire planet. The composition of the atmosphere at different altitudes causes different colours of light to be emitted and this effect can be used to study the different layers of the atmosphere, such as temperature, density, and composition.