The Earth is the only known planet capable of supporting life. It has four major subsystems that sustain life:
1) The geosphere is the solid portion of the planet including rocks, minerals and landforms. Below the crust, temperatures are high enough for rock to flow.
2) The hydrosphere includes all water on Earth's surface and underground in liquid, vapor and ice form, including oceans, lakes, rivers, groundwater and aquifers.
3) The atmosphere is made up of layers of gases held in place by Earth's gravity. It protects life from solar radiation and provides breathable air.
4) The biosphere is the global ecosystem comprising all living things and their