Earth science is the scientific study of our planet, the Earth. It encompasses all sciences which focus on the Earth, and uses physics, geology, geography, meteorology, mathematics, chemistry and biology. The Earth sciences generally recognize four "spheres" of study of the Earth: the lithosphere, the hydrosphere, the atmosphere, and the biosphere, corresponding to rocks, water, air, and life. Sometimes the cryosphere is considered as a distinct portion of the hydrosphere, and the pedosphere (soil) is considered a subset of the lithosphere.
Earth science has established many simple but important facts about the makeup of our planet. One significant set of facts are the relative chemical abundances of our air (78% nitrogen, 21% oxygen, 1% water vapor, .93% argon, .03% carbon dioxide, .002% other) and crust (made up mainly of oxides, including 60 silica or sand). Earth scientists have accurately measured our planet's diameter (12,756 km or 7,926 mi) and mass (5.9736 × 1024 kg). They have also measured the path of the Earth around the Sun and its implications for seasonal variations in temperature and weather.
An important part of the Earth's makeup is its biosphere, or all the Earth's life. The Earth sciences regularly study the relationship between the biosphere and the rest of the planet, especially the atmosphere. Plants regularly convert atmospheric carbon dioxide into oxygen while animals do the opposite. The Earth's current oxygen-rich atmosphere was created billions of years ago when photosynthesis first evolved. Photosynthesis produces oxygen as a byproduct. The evolution of photosynthesis was such a significant chemical event for the Earth's atmosphere that it has a name — the Oxygen catastrophe, named because the massive release of oxygen was toxic to many of the organisms existing on the planet at the time.
One of the most significant findings of Earth science was in the 1950s, when it was proved that the continents are large rock plates floating on a liquid mantle underneath. This is now known as plate tectonics, and Earth scientists have found that at points in the Earth's distant past, all the continents were merged into one supercontinent known as Pangaea. This is important from the perspective of paleontology: when all the Earth's landmass was condensed into a single continent, the interior of the continent would have been subjected to such temperature extremes that it would have been difficult for complex life to survive there.
