Science Q4 - G7

• The outermost Earth system is an invisible layer of gases that surrounds the planet.
• The force of Earth's gravity pulls molecules of gases into a layer surrounding the planet.
• This mixture of gases forms a layer around Earth called the atmosphere.
• The atmosphere is denser near Earth's surface and becomes less dense farther from Earth.
• Ozone is a molecule composed of three oxygen atoms, (O3). Ozone in the upper atmosphere absorbs high-energy ultraviolet (UV) radiation coming from the Sun.
• The highest concentration of ozone is in the ozone layer in the lower stratosphere.
• Nitrogen and oxygen together make up 99% of the planet’s atmosphere.
• Nitrogen makes up the bulk of the atmosphere but is not involved in geological or biological processes in its gaseous form.
• Humidity is the amount of water vapour in the air. Humidity varies from place to place and season to season.
• When the air is very humid, it feels heavy or sticky. Dry air usually feels more comfortable. When humidity is high, water vapour makes up only about 4% of the atmosphere.
• Greenhouse gases trap heat in the atmosphere. Important natural greenhouse gases include carbon dioxide, methane, water vapour, and ozone.
• Carbon dioxide (CO2) is released into the atmosphere by volcanoes and through respiration and combustion of fossil fuels.
• Water vapour (H2O) is produced when water evaporates from lakes, rivers, and oceans.
• Particles of dust, soil, faecal matter, metals, salt, smoke, ash, and other solids make up a small percentage of the atmosphere and are called particulates.
• The troposphere is the lowest and most important layer of the atmosphere.
• The upper boundary of the troposphere is the tropopause.
• The stratosphere is above the troposphere. The ozone layer is found from 15 to 30 km, and the thickness varies by season and latitude. UV radiation splits an O2 molecule into two. Each atom combines with another oxygen molecule to create ozone, O3.
• The mesosphere is above the stratosphere. The mesosphere is extremely cold; especially at its top, about -90 degrees celsius.
• The air in the mesosphere has an extremely low density: 99.9% of the mass of the atmosphere is below the mesosphere.
• The exosphere is above the thermosphere. It extends up to 10,000km (6,200 miles) or more. At this height, there are very few particles left so it's almost empty space. There is not enough gravity to hold gases here, so they escape into space.