Earth Day 2020: Our planet’s atmosphere

Our atmosphere (made up of 78 per cent nitrogen and 21 per cent oxygen, with other gases making up the last per cent) is held in place by gravity and consists of a number of different layers that work together to protect us from solar radiation and to keep consistent temperatures. The atmosphere gets thinner with altitude, with 80 per cent of its mass in the first layer closest to the Earth’s surface. 

There are five main layers that make up the atmosphere. The troposphere is the first layer (and is where our weather occurs), followed by the stratosphere, mesosphere, thermosphere and exosphere. There is no definite boundary between where the atmosphere ends and outer space begins, though the Kàrmàn line at 100km above sea level is often regarded as a the boundary.

There are other layers that exist alongside the five main layers. The lowest of these is the ‘planetary boundary’, which is within the troposphere and closest to the Earth’s surface and its depth can vary widely between 100m to 3,000m as it is directly affected by conditions on the surface. The ozone layer is the one that most of us will be familiar with and this is contained within the stratosphere, in its lower portion. Around 90 per cent of the ozone in the atmosphere lies here. The ionosphere is what causes auroras, such as the northern lights, as it is ionised by solar radiation and stretches from 50 to 1,000km, overlapping the exosphere and thermosphere.

Finally, the homosphere and the heterosphere run from the Earth’s surface to around 80km and from 80km upwards respectively. They are so-named because of the way the gases within them are mixed. The heterosphere has a chemical composition that changes with height, whereas the homosphere’s make-up remains more constant. The five main layers are based on the thermal structure of the atmosphere, whereas the additional layers mentioned here are classified according to composition.