General Science Review: Earth and Space Science

Geology
Minerals are naturally occurring, inorganic solids with a definite chemical composition and an orderly internal crystal structure. A polymorph is two minerals with the same chemical composition, but a different crystal structure. Rocks are aggregates of one or more minerals, and may also contain mineraloids (minerals lacking a crystalline structure) and organic remains. The three types of rocks are sedimentary, igneous, and metamorphic.

Rocks are classified based on their formation and the minerals they contain.

Minerals are classified by their chemical composition. Geology is the study of the planet Earth as it pertains to the composition, structure, and origin of its rocks. Petrology is the study of rocks, including their composition, texture, structure, occurrence, mode of formation, and history. Mineralogy is the study of minerals.

Sedimentary rocks are formed by the process of lithification, which involves compaction, the expulsion of liquids from pores, and the cementation of the pre-existing rock. It is pressure and temperature that are responsible for this process. Sedimentary rocks are often formed in layers in the presence of water, and may contain organic remains, such as fossils. Sedimentary rocks are organized into three groups: detrital, biogenic, and chemical. Texture refers to the size, shape, and grains of sedimentary rock. Texture can be used to determine how a particular sedimentary rock was created. Composition refers to the types of minerals present in the rock. The origin of sedimentary rock refers to the type of water that was involved in its creation. Marine deposits, for example, likely involved ocean environments, while continental deposits likely involved dry land and lakes.



Igneous rock is formed from magma, which is molten material originating from beneath the Earth's surface.

Depending upon where magma cools, the resulting igneous rock can be classified as intrusive, plutonic, hypabyssal, extrusive, or volcanic. Magma that solidifies at a depth is intrusive, cools slowly, and has a coarse grain as a result. An example is granite. Magma that solidifies at or near the surface is extrusive, cools quickly, and usually has a fine grain. An example is basalt.

Magma that actually flows out of the Earth's surface is called lava. Some extrusive rock cools so quickly that crystals do not have time to form. These rocks have a glassy appearance. An example is obsidian. Hypabyssal rock is igneous rock that is formed at medium depths.



Metamorphic rock is that which has been changed by great heat and pressure. This results in a variety of outcomes, including deformation, compaction, destruction of the characteristics of the original rock, bending, folding, and formation of new minerals because of chemical reactions, and changes in the size and shape of the mineral grain.

For example, the igneous rock ferromagnesian can be changed into schist and gneiss. The sedimentary rock carbonaceous can be changed into marble. The texture of metamorphic rocks can be classified as foliated and unfoliated.

Foliation, or layering, occurs when rock is compressed along one axis during recrystallization. This can be seen in schist and shale. Unfoliated rock does not include this banding. Rocks that are compressed equally from all sides or lack specific minerals will be unfoliated. An example is marble.



Fossils are preservations of plants, animals, their remains, or their traces that date back to about 10,000 years ago. Fossils and where they are found in rock strata makes up the fossil record. Fossils are formed under a very specific set of conditions. The fossil must not be damaged by predators and scavengers after death, and the fossil must not decompose.

Usually, this happens when the organism is quickly covered with sediment. This sediment builds up and molecules in the organism's body are replaced by minerals. Fossils come in an array of sizes, from single-celled organisms to large dinosaurs.

Plate Tectonics
The Earth is ellipsoid, not perfectly spherical. This means the diameter is different through the poles and at the equator. Through the poles, the Earth is about 12,715 km in diameter. The approximate center of the Earth is at a depth of 6,378 km. The Earth is divided into a crust, mantle, and core. The core consists of a solid inner portion. Moving outward, the molten outer core occupies the space from about a depth of 5,150 km to a depth of 2,890 km. The mantle consists of a lower and upper layer. The lower layer includes the D' (D prime) and D" (D double-prime) layers. The solid portion of the upper mantle and crust together form the lithosphere, or rocky sphere. Below this, but still within the mantle, is the asthenosphere, or weak sphere. These layers are distinguishable because the lithosphere is relatively rigid, while the asthenosphere resembles a thick liquid.

The theory of plate tectonics states that the lithosphere, the solid portion of the mantle and Earth's crust, consists of major and minor plates. These plates are on top of and move with the viscous upper mantle, which is heated because of the convection cycle that occurs in the interior of the Earth. There are different estimates as to the exact number of major and minor plates. The number of major plates is believed to be between 9 and 15, and it is thought that there may be as many as 40 minor plates. The United States is atop the North American plate. The Pacific Ocean is atop the Pacific plate. The point at which these two plates slide horizontally along the San Andreas fault is an example of a transform plate boundary. The other two types of boundaries are divergent (plates that are spreading apart and forming new crust) and convergent (the process of subduction causes one plate to go under another). The movement of plates is what causes other features of the Earth's crust, such as mountains, volcanoes, and earthquakes.

Volcanoes can occur along any type of tectonic plate boundary. At a divergent boundary, as plates move apart, magma rises to the surface, cools, and forms a ridge. An example of this is the mid-Atlantic ridge. Convergent boundaries, where one plate slides under another, are often areas with a lot of volcanic activity. The subduction process creates magma.

When it rises to the surface, volcanoes can be created. Volcanoes can also be created in the middle of a plate over hot spots. Hot spots are locations where narrow plumes of magma rise through the mantle in a fixed place over a long period of time. The Hawaiian Islands and Midway are examples. The plate shifts and the island moves. Magma continues to rise through the mantle, however, which produces another island. Volcanoes can be active, dormant, or extinct.

Active volcanoes are those that are erupting or about to erupt. Dormant volcanoes are those that might erupt in the future and still have internal volcanic activity. Extinct volcanoes are those that will not erupt.

Geography
For the purposes of tracking time and location, the Earth is divided into sections with imaginary lines. Lines that run vertically around the globe through the poles are lines of longitude, sometimes called meridians.

The Prime Meridian is the longitudinal reference point of 0. Longitude is measured in 15-degree increments toward the east or west. Degrees are further divided into 60 minutes, and each minute is divided into 60 seconds. Lines of latitude run horizontally around the Earth parallel to the equator, which is the 0 reference point and the widest point of the Earth. Latitude is the distance north or south from the equator, and is also measured in degrees, minutes, and seconds.

Tropic of Cancer: This is located at 23.5 degrees north. The Sun is directly overhead at noon on June 21st in the Tropic of Cancer, which marks the beginning of summer in the Northern Hemisphere.
Tropic of Capricorn: This is located at 23.5 degrees south. The Sun is directly overhead at noon on December 21st in the Tropic of Capricorn, which marks the beginning of winter in the Northern Hemisphere.
Arctic Circle: This is located at 66.5 degrees north, and marks the start of when the Sun is not visible above the horizon. This occurs on December 21st, the same day the Sun is directly over the Tropic of Capricorn.
Antarctic Circle: This is located at 66.5 degrees south, and marks the start of when the Sun is not visible above the horizon. This occurs on June 21st, which marks the beginning of winter in the Southern Hemisphere and is when the Sun is directly over the Tropic of Cancer.



Latitude is a measurement of the distance from the equator.

The distance from the equator indicates how much solar radiation a particular area receives. The equator receives more sunlight, while polar areas receive less. The Earth tilts slightly on its rotational axis. This tilt determines the seasons and affects weather. There are eight biomes or ecosystems with particular climates that are associated with latitude. Those in the high latitudes, which get the least sunlight, are tundra and taiga. Those in the mid latitudes are grassland, temperate forest, and chaparral. Those in latitudes closest to the equator are the warmest. The biomes are desert and tropical rain forest. The eighth biome is the ocean, which is unique because it consists of water and spans the entire globe. Insolation refers to incoming solar radiation. Diurnal variations refer to the daily changes in insolation. The greatest insolation occurs at noon.

The tilt of the Earth on its axis is 23.5°. This tilt causes the seasons and affects the temperature because it affects the amount of Sun the area receives. When the Northern or Southern Hemispheres are tilted toward the Sun, the hemisphere tilted toward the sun experiences summer and the other hemisphere experiences winter. This reverses as the Earth revolves around the Sun. Fall and spring occur between the two extremes.

The equator gets the same amount of sunlight every day of the year, about 12 hours, and doesn't experience seasons. Both poles have days during the winter when they are tilted away from the Sun and receive no daylight. The opposite effect occurs during the summer. There are 24 hours of daylight and no night. The summer solstice, the day with the most amount of sunlight, occurs on June 21st in the Northern Hemisphere and on December 21st in the Southern Hemisphere. The winter solstice, the day with the least amount of sunlight, occurs on December 21st in the Northern Hemisphere and on June 21st in the Southern Hemisphere.

Weather, Atmosphere, Water Cycle
Meteorology is the study of the atmosphere, particularly as it pertains to forecasting the weather and understanding its processes. Weather is the condition of the atmosphere at any given moment. Most weather occurs in the troposphere. Weather includes changing events such as clouds, storms, and temperature, as well as more extreme events such as tornadoes, hurricanes, and blizzards. Climate refers to the average weather for a particular area over time, typically at least 30 years. Latitude is an indicator of climate. Changes in climate occur over long time periods.

The hydrologic, or water, cycle refers to water movement on, above, and in the Earth. Water can be in any one of its three states during different phases of the cycle. The three states of water are liquid water, frozen ice, and water vapor. Processes involved in the hydrologic cycle include precipitation, canopy interception, snow melt, runoff, infiltration, subsurface flow, evaporation, sublimation, advection, condensation, and transpiration.

Precipitation is when condensed water vapor falls to Earth.
Examples include rain, fog drip, and various forms of snow, hail, and sleet. Canopy interception is when precipitation lands on plant foliage instead of falling to the ground and evaporating. Snow melt is runoff produced by melting snow. Infiltration occurs when water flows from the surface into the ground. Subsurface flow refers to water that flows underground. Evaporation is when water in a liquid state changes to a gas. Sublimation is when water in a solid state (such as snow or ice) changes to water vapor without going through a liquid phase. Advection is the movement of water through the atmosphere. Condensation is when water vapor changes to liquid water. Transpiration is when water vapor is released from plants into the air.



The ocean is the salty body of water that encompasses the Earth. It has a mass of 1.4 x 1024 grams. Geographically, the ocean is divided into three large oceans: the Pacific Ocean, the Atlantic Ocean, and the Indian Ocean. There are also other divisions, such as gulfs, bays, and various types of seas, including Mediterranean and marginal seas. Ocean distances can be measured by latitude, longitude, degrees, meters, miles, and nautical miles. The ocean accounts for 70.8% of the surface of the Earth, amounting to 361,254,000 km2. The ocean's depth is greatest at

Challenger Deep in the Mariana Trench. The ocean floor here is 10,924 meters below sea level. The depths of the ocean are mapped by echo sounders and satellite altimeter systems. Echo sounders emit a sound pulse from the surface and record the time it takes to return. Satellite altimeters provide better maps of the ocean floor.

The atmosphere consists of 78% nitrogen, 21% oxygen, and 1% argon. It also includes traces of water vapor, carbon dioxide and other gases, dust particles, and chemicals from Earth. The atmosphere becomes thinner the farther it is from the Earth's surface. It becomes difficult to breathe at about 3 km above sea level. The atmosphere gradually fades into space. The lowest layer of the atmosphere is called the troposphere. Its thickness varies at the poles and the equator, varying from about 7 to 17 km. This is where most weather occurs. The stratosphere is next, and continues to an elevation of about 51 km. The mesosphere extends from the stratosphere to an elevation of about 81 km. It is the coldest layer and is where meteors tend to ablate. The next layer is the thermosphere. It is where the International Space Station orbits.

The exosphere is the outermost layer, extends to 10,000 km, and mainly consists of hydrogen and helium.

Earth's atmosphere has five main layers. From lowest to highest, these are the troposphere, the stratosphere, the mesosphere, the thermosphere, and the exosphere. Between each pair of layers is a transition layer called a pause. The troposphere includes the tropopause, which is the transitional layer of the stratosphere. Energy from Earth's surface is transferred to the troposphere. Temperature decreases with altitude in this layer.  In the stratosphere, the temperature is inverted, meaning that it increases with altitude. The stratosphere includes the ozone layer, which helps block ultraviolet light from the Sun. The stratopause is the transitional layer to the mesosphere. The temperature of the mesosphere decreases with height. It is considered the coldest place on Earth, and has an average temperature of -85 degrees Celsius. Temperature increases with altitude in the thermosphere, which includes the thermopause. Just past the thermosphere is the exobase, the base layer of the exosphere. Beyond the five main layers are the ionosphere, homosphere, heterosphere, and magnetosphere.



Most clouds can be classified according to the altitude of their base above Earth's surface. High clouds occur at altitudes between 5,000 and 13,000 meters. Middle clouds occur at altitudes between 2,000 and 7,000 meters. Low clouds occur from the Earth's surface to altitudes of 2,000 meters. Types of high clouds include cirrus (Ci), thin wispy mare's tails that consist of ice; cirrocumulus (Cc), small, pillow-like puffs that often appear in rows; and cirrostratus (Cs), thin, sheetlike clouds that often cover the entire sky. Types of middle clouds include altocumulus (Ac), gray-white clouds that consist of liquid water; and altostratus (As), grayish or blue-gray clouds that span the sky. Types of low clouds include stratus (St), gray and fog-like clouds consisting of water droplets that take up the whole sky; stratocumulus (Sc), low-lying, lumpy gray clouds; and nimbostratus (Ns), dark gray clouds with uneven bases that indicate rain or snow. Two types of clouds, cumulus (Cu) and cumulonimbus (Cb), are capable of great vertical growth. They can start at a wide range of altitudes, from the Earth's surface to altitudes of 13,000 meters.



Astronomy
Astronomy is the scientific study of celestial objects and their positions, movements, and structures. Celestial does not refer to the Earth by itself, but does include its movement through space. Other celestial objects include the Sun, the Moon, planets, satellites, asteroids, meteors, comets, stars, galaxies, the universe, and other space phenomena. The term astronomy has its roots in the Greek words 'astro' and 'nomos,' which means
'laws of the stars.'

What can be seen of the universe is believed to be at least 93 billion light years across. To put this into perspective, the Milky Way galaxy is about 100,000 light years across. Our view of matter in the universe is that it forms into clumps. Matter is organized into stars, galaxies, clusters of galaxies, superclusters, and the Great Wall of galaxies. Galaxies consist of stars, some with planetary systems. Some estimates state that the universe is about 13 billion years old. It is not considered dense, and is believed to consist of 73 percent dark energy, 23 percent cold dark matter, and 4 percent regular matter. Cosmology is the study of the universe. Interstellar medium (ISM) is the gas and dust in the interstellar space between a galaxy's stars.
The solar system is a planetary system of objects that exist in an ecliptic plane. Objects orbit around and are bound by gravity to a star called the Sun. Objects that orbit around the Sun include: planets, dwarf planets, moons, asteroids, meteoroids, cosmic dust, and comets. The definition of planets has changed. At one time, there were nine planets in the solar system. There are now eight. Planetary objects in the solar system include four inner, terrestrial planets: Mercury, Venus, Earth, and Mars. They are relatively small, dense, rocky, lack rings, and have few or no moons. The four outer, or Jovian, planets are Jupiter, Saturn, Uranus, and Neptune, which are large and have low densities, rings, and moons. They are also known as gas giants. Between the inner and outer planets is the asteroid belt. Beyond Neptune is the Kuiper belt. Within these belts are five dwarf planets: Ceres, Pluto, Haumea, Makemake, and Eris.

The Sun is at the center of the solar system. It is composed of 70% hydrogen (H) and 28% helium (He). The remaining 2% is made up of metals.

The Sun is one of 100 billion stars in the Milky Way galaxy. Its diameter is 1,390,000 km, its mass is 1.989 x 1030 kg, its surface temperature is 5,800 K, and its core temperature is 15,600,000 K. The Sun represents more than 99.8% of the total mass of the solar system. At the core, the temperature is 15.6 million K, the pressure is 250 billion atmospheres, and the density is more than 150 times that of water. The surface is called the photosphere. The chromosphere lies above this, and the corona, which extends millions of kilometers into space, is next. Sunspots are relatively cool regions on the surface with a temperature of 3,800 K. Temperatures in the corona are over 1,000,000 K. Its magnetosphere, or heliosphere, extends far beyond Pluto.

Mercury: Mercury is the closest to the Sun and is also the smallest planet. It orbits the Sun every 88 days, has no satellites or atmosphere, has a Moon-like surface with craters, appears bright, and is dense and rocky with a large iron core.

Venus: Venus is the second planet from the Sun. It orbits the Sun every 225 days, is very bright, and is similar to Earth in size, gravity, and bulk composition. It has a dense atmosphere composed of carbon dioxide and some sulfur. It is covered with reflective clouds made of sulfuric acid and exhibits signs of volcanism. Lightning and thunder have been recorded on Venus's surface.

Earth: Earth is the third planet from the Sun. It orbits the Sun every 365 days. Approximately 71% of its surface is salt-water oceans. The Earth is rocky, has an atmosphere composed mainly of oxygen and nitrogen, has one moon, and supports millions of species. It contains the only known life in the solar system.

Mars: Mars it the fourth planet from the Sun. It appears reddish due to iron oxide on the surface, has a thin atmosphere, has a rotational period similar to Earth's, and has seasonal cycles. Surface features of Mars include volcanoes, valleys, deserts, and polar ice caps. Mars has impact craters and the tallest mountain, largest canyon, and perhaps the largest impact crater yet discovered.



Jupiter: Jupiter is the fifth planet from the Sun and the largest planet in the solar system. It consists mainly of hydrogen, and 25% of its mass is made up of helium. It has a fast rotation and has clouds in the tropopause composed of ammonia crystals that are arranged into bands sub-divided into lighter-hued zones and darker belts causing storms and turbulence. Jupiter has wind speeds of 100 m/s, a planetary ring, 63 moons, and a Great Red Spot, which is an anticyclonic storm.

Saturn: Saturn is the sixth planet from the Sun and the second largest planet in the solar system. It is composed of hydrogen, some helium, and trace elements. Saturn has a small core of rock and ice, a thick layer of metallic hydrogen, a gaseous outer layer, wind speeds of up to 1,800 km/h, a system of rings, and 61 moons.

Uranus: Uranus is the seventh planet from the Sun. Its atmosphere is composed mainly of hydrogen and helium, and also contains water, ammonia, methane, and traces of hydrocarbons. With a minimum temperature of 49 K, Uranus has the coldest atmosphere. Uranus has a ring system, a magnetosphere, and 13 moons.

Neptune: Neptune is the eighth planet from the Sun and is the planet with the third largest mass. It has 12 moons, an atmosphere similar to Uranus, a Great Dark Spot, and the strongest sustained winds of any planet (wind speeds can be as high as 2,100 km/h). Neptune is cold (about 55 K) and has a fragmented ring system.



The Earth is about 12,765 km (7,934 miles) in diameter. The Moon is about 3,476 km (2,160 mi) in diameter. The distance between the Earth and the Moon is about 384,401 km (238,910 mi). The diameter of the Sun is approximately 1,390,000 km (866,000 mi). The distance from the Earth to the Sun is 149,598,000 km, also known as 1 Astronomical Unit (AU). The star that is nearest to the solar system is Proxima Centauri. It is about 270,000 AU away.

Some distant galaxies are so far away that their light takes several billion years to reach the Earth. In other words, people on Earth see them as they looked billions of years ago.

It takes about one month for the Moon to go through all its phases. Waxing refers to the two weeks during which the Moon goes from a new moon to a full moon. About two weeks is spent waning, going from a full moon to a new moon. The lit part of the Moon always faces the Sun. The phases of waxing are: new moon, during which the Moon is not illuminated and rises and sets with the Sun; crescent moon, during which a tiny sliver is lit; first quarter, during which half the Moon is lit and the phase of the Moon is due south on the meridian; gibbous, during which more than half of the Moon is lit and has a shape similar to a football; right side, during which the Moon is lit; and full moon, during which the Moon is fully illuminated, rises at sunset, and sets at sunrise. After a full moon, the Moon is waning. The phases of waning are: gibbous, during which the left side is lit and the Moon rises after sunset and sets after sunrise; third quarter, during which the Moon is half lit and rises at midnight and sets at noon; crescent, during which a tiny sliver is lit; and new moon, during which the Moon is not illuminated and rises and sets with the Sun.