Environmental Science
The Universe
The universe is the totality of existence: all of space, time, matter, and energy. It’s approximately billion years old.
Our home galaxy, the Milky Way, contains our Solar System, which has planets and hundreds of moons orbiting them.
The Earth
The Earth is the third planet from the sun in the Solar System. It spins on its own axis once every hours, creating our day and the appearance of the sun rising and setting.
While rotating on its axis, the Earth also orbits the sun in an elliptical pattern, taking approximately days and creating our calendar year.
The Earth’s rotational axis is tilted about , causing different parts of the planet to get more or less sun exposure throughout the year. This creates our changing seasons and is why the Northern and Southern Hemispheres have opposite seasons to one another. Near the equator, sunlight strikes almost straight on all year, so the tilt changes sun exposure there very little, and the weather stays warm year-round. Seasons come from this tilt, not from Earth’s distance to the sun - Earth is actually closest to the sun in January, during the Northern Hemisphere’s winter.
Moons and satellites
A moon spins along its own axis while also orbiting its planet. Our moon orbits the Earth in approximately days and also completes one full rotation in the same amount of time. Because both actions take the same amount of time, this means that we only ever see one side of the moon from Earth.
This kind of joint rotation is called “synchronous rotation” or being “tidally locked.”
The moon’s gravity also pulls on Earth’s oceans, creating tides. When the moon passes directly between the Earth and the sun, it blocks sunlight from part of Earth, creating a solar eclipse. When the Earth passes between the sun and the moon, the moon moves through Earth’s shadow, creating a lunar eclipse.
Moon phases
The moon doesn’t produce its own light. It appears bright because it reflects the sun’s light, and, depending on the positions of the moon, Earth, and sun, it can appear dark, fully lit, or somewhere in between.
The various stages of the moon’s appearance are called the “phases of the moon,” and they cycle through a pattern that takes approximately days to complete.
Moons aren’t distributed evenly among the planets. Mercury and Venus have none, while larger planets tend to have more because their stronger gravity can capture and hold them.
Weather and climate
Weather describes a location’s temperature, humidity, precipitation (rain, snow, sleet, and hail), cloud cover, wind, and other atmospheric conditions.
Climate describes a location’s long-term weather patterns, usually looking at data of the past years or more.
Climates are grouped in a variety of different ways, but we can roughly divide the Earth by latitude into polar, temperate, subtropical, and tropical zones.
Polar zones
Polar zones are at the furthest northern and southern parts of the Earth and include the Arctic and Antarctic. These regions get the least amount of direct sunlight because sunlight strikes them at a low angle, so the climate is typically frigid year-round, with extensive ice and snow coverage in many areas. These regions also get the most varied amounts of light over the seasons, with hours of sunlight in parts of the summer and hours of darkness in parts of the winter.
Temperate zones
Temperate zones are warmer than polar zones and get more direct sunlight. They typically have four distinct seasons (spring, summer, autumn, and winter) as well as moderate rainfall. Due to the Earth’s tilt, day length in these zones varies more than it does near the equator.
Subtropical zones
Subtropical zones are characterized by hot summers and mild winters, with little frost or snowfall.
Tropical zones
Tropical zones are areas around the equator. They get the most direct sunlight throughout the year and so are typically warm to hot year-round. These zones have the least variation in temperature between seasons and the least variation of sunlight over the seasons. They average hours of sunlight in both summer and winter.
Geosphere (rock cycle)
The geosphere includes all of Earth’s rocks, minerals, soil, and fossils, spanning from deep in the Earth’s interior to its surface.
The rock cycle is the continuous transformation and recycling of rocks within the geosphere, a process that can take millions of years.
There are three main types of rocks, and all of them can transform into one another, depending on the conditions and pressures being placed on them.
Igneous rock: igneous rocks are formed when molten rock, such as lava or underground magma, cools and crystallizes.
Sedimentary rock: sedimentary rocks are formed when existing rocks or minerals become fragmented through weathering and are then deposited, compacted, and cemented together over time.
Metamorphic rock: metamorphic rocks are formed when pressure and heat are applied to existing rocks, changing their structure without fully melting them.
The rock cycle is driven by a combination of Earth’s internal and external processes. Internal processes include tectonic plate movement, heat, and pressure. External processes include weathering, which breaks rocks down over time, and erosion, which carries the pieces away and deposits them elsewhere, where they eventually form new rocks.
Tectonic plates: Tectonic plates are massive, rigid sections of Earth’s crust and upper mantle that fit together to form the planet’s outer shell. They float on the slow-moving mantle beneath them, and over time they collide, pull apart, or slide past one another. This movement drives volcanic activity, earthquakes, and the formation of mountains and valleys.
Mechanical weathering: the physical breakdown of rock caused by forces like wind, water, and ice, which wear down or break apart rock without changing what it’s made of.
Chemical weathering: the breakdown of rock through chemical reactions, such as oxidation or acid rain, which change the rock’s mineral makeup over time.
Hydrosphere (water cycle)
The hydrosphere includes all of Earth’s water in both fresh and salt forms, and in all three states: liquid, solid, and gas.
The water cycle is the constant movement of water between Earth’s surface and the atmosphere, through evaporation, condensation, and precipitation.
Evaporation: the sun’s heat and energy causes water on Earth’s surface to turn from its liquid form into its gaseous form. This water vapor is then carried upward into the atmosphere.
Condensation: as the water vapor travels higher in the atmosphere, the temperature becomes cooler. This cooler temperature transforms some of the water vapor back into its liquid form. The condensed water can collect to form clouds, which are a combination of miniature liquid water droplets and/or ice crystals.
Precipitation: eventually, the liquid water droplets or ice crystals become large enough that they are pulled back to Earth’s surface by gravity. If the water stays in liquid form, it falls to the Earth as rain. If it becomes cool enough to transform into solid form or partly solid form, it falls to the Earth in the form of hail, sleet, or snow.
Atmosphere
The atmosphere is the layer of gases, aerosols, and particulates that surrounds Earth. It is held in place by gravity and separates Earth’s surface from outer space. It provides three key functions: supplying the oxygen animals need and the carbon dioxide plants need to survive, reducing harmful solar radiation like ultraviolet light, and helping maintain steady temperatures through the greenhouse effect.
The atmosphere is primarily nitrogen (), oxygen (), and argon (), with trace amounts of other gases and a constantly cycling amount of water vapor (, depending on location and weather).
The atmosphere is made up of five main layers, each with their own characteristics.
Troposphere: this is the layer closest to Earth’s surface, containing almost all of the atmosphere’s water and about 80% of its total mass. Almost all of Earth’s weather happens here. Temperature drops as you rise in altitude.
Stratosphere: the second layer from Earth’s surface, containing the ozone layer, which absorbs most of the sun’s harmful ultraviolet radiation. Temperature rises with altitude here, because the ozone absorbs that ultraviolet energy.
Mesosphere: this is the third layer of atmosphere. Temperature drops again with altitude here, making the mesosphere the coldest layer of Earth’s atmosphere. It is also the layer where most meteors burn up upon entry.
Thermosphere: this is the fourth layer of Earth’s atmosphere. The thermosphere is the hottest layer, though it doesn’t feel hot due to its low density. Many low-Earth-orbit satellites orbit in this layer, and natural light displays like the aurora borealis and aurora australis primarily occur here.
Exosphere: this is the fifth layer of Earth’s atmosphere, though some scientists consider it to be part of open space rather than the atmosphere. Air density is so low that molecules rarely collide, and some escape Earth’s gravity into space entirely. Like the thermosphere, many satellites orbit in this layer.