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Why is Earth called a terrestrial planet even though it is 70% covered by water?
There really isn't a lot of water on the Earth. The image below, from the USGS science school, graphically illustrates the volume of the water on (and in) the crust of the Earth, relative to the volume of everything else.
The bigger blue ball is all the salt water, from all the oceans & seas, the small one is all the fresh water, from the rivers, lakes and aquifers. You may need to zoom in to see the fresh water ball…
There is more water dissolved in the mantle, but we can't get at it (with current technology), apart from what's released in volcanic eruptions.
From John Dvorak's comment, the word "terrestrial" derives from the Latin "terrestris", meaning "of or pertaining to the Earth", which itself is related to the Latin word "terra" meaning "Earth". So Earth is by definition a terrestrial planet.
As PM 2Ring notes, the quantity of water in the Earth is relatively small, corresponding to roughly 0.02% of the mass of the planet. There may be a similar amount in the mantle that's not accounted for in the estimate linked there, but it is still a very small proportion of the mass. The majority of the Earth is in the form of rock, with the second component being the iron/nickel core accounting for 33% of the mass.
The term "ocean planet" is often used to refer to planets that are scaled-up versions of Ganymede or Pluto that have migrated into the habitable zone, see for example Léger et al. (2004) 'A new family of planets? "Ocean-Planets"'. These planets contain far greater quantities of water: the case considered in that paper has 50% water by mass, similar to Ganymede. In particular the base of the ocean on such planets is not the water/rock boundary as it is on Earth: instead the ocean is separated from the rocky core by a layer of high-pressure ice.
Why is Venus called Earth's "sister planet"?
Venus is often called Earth's "sister planet" or "twin" because of 3 major similarities:
- Mass, i.e. how much they weigh
- Composition, i.e. what material they're made of
Further, Venus is the closest planet to Earth within the solar system.
Earth’s diameter is #"12,756.2 km"# , and Venus's is #"12,103.6 km"# . Earth is only #
5%# larger than Venus, which, in comparison to other size differences between other surrounding planets, is a relatively minute difference in size.
In this image, you have to look carefully to notice that Earth (right) is slightly larger:
As with their sizes, the mass of Venus is only slightly lesser than that of Earth—Earth weighs only #
19%# more than Venus does. Even the force of gravity on the 2 planets is close you'd experience #90%# of gravity's force on Earth if you visited Venus.
Both Venus and Earth have metal cores surrounded by a mantle of silica rock (also common in rocky moons and asteroids), and then a thin crust.
Despite these similarities, Earth and Venus are also quite different, causing some to nickname Venus "Earth's evil twin."
The major differences are those that make Venus too hostile to support life as Earth does: temperature and atmosphere.
Venus's average temperature is #461.85 °"C"# (higher than that of molten lead!) and the atmospheric pressure is 90 times that on Earth’s surface.
While Earth's atmosphere is composed of oxygen and nitrogen (with only small amounts of #"CO"_2# ), Venus’ atmosphere is #96.5%# carbon dioxide. Unlike Earth's fluffy cumulus clouds, Venus has clouds of sulphuric acid that rain down on the planet's surface, adding to the planet's inhospitable environment.
Exploring Venus: Earth’s Twin Planet
Despite the grave differences between the two planets, Venus is commonly called Earth’s twin planet. (Image: NASA images/Shutterstock)
Venus is the brightest object in the sky after the Sun and Moon. Names such as Morning Star and Evening Star were derived from this brightness. The name “Venus” has roots in Roman mythology: Venus is the goddess of love and beauty. This goddess of beauty is so similar to Earth that some even call it Earth’s twin planet.
Earth’s diameter and the average density is only five percent higher than Venus. Venus’s orbit is also closest to Earth’s, and its distance from the Sun is 72% the distance of Earth. That means travel time from Earth to Venus could be less than travel time from Earth to Mars. Maybe the closer distance to the Sun causes a higher temperature, but how much hotter is it than the deserts here? Also, how do we know how hot it is?
Space Missions to Venus
Space missions to Venus began in the 1960s. After 12 unsuccessful attempts and landing failures, the 13 th spacecraft was successfully launched. Passing Venus’s atmosphere was especially difficult due to the winds much stronger than our biggest hurricanes and the thick density. Spacecraft 13 was the Venera 4 mission by the Soviet Union in 1967. However, no photo of Venus’s surface could be captured and sent back to Earth, until Venera 9 and 10 landed on Venus in 1975. They captured black and white photos that showed Venus’s surface is covered in rocks, but not much dust. The Venera 13 and 14 missions captured the full-color panorama views in 1982.
Spacecrafts do not last long under the extreme heat and pressure of Venus. (Image: Jurik Peter/Shutterstock)
Why did it take so many years and so many missions to capture some images? The first reason is the extreme heat on Venus. The temperature reaches 850 o F, due to the greenhouse effect of Venus’s atmosphere. Next, the pressure on Venus’s surface is 92 times higher than sea-level pressure on Earth. Thus, the landers were destroyed easily. Venera 13 returned data for the longest period ever, and that was 127 minutes. The last attempt was Vega 2 in 1985, which transmitted data for almost 60 minutes. Radio waves later helped gather all the information we have today of Venus.
This is a transcript from the video series A Field Guide to the Planets. Watch it now, on Wondrium.
Venus orbits the Sun at 72% the distance of Earth but is the only planet that spins opposite the direction that all the planets orbit around the Sun. It is unusual because all planets in the solar system were accreted from a disk of material surrounding the Sun, revolving and rotating in one direction. Hence, all planets spin in the same direction, except for Venus.
Planets spin and orbit at different angles, but Venus has an axial tilt of 177°. This is while Earth, Mars, Saturn, and Neptune have a 20-30° tilt. The reason might be an object hit Venus so strongly that it changed the direction and tilted the planet. Another theory is that the strong winds around Venus’s thick atmosphere pushed the planet in the same direction for so long that they finally flipped it over. Taking the orbit and spinning into account, one might wonder how long one year in Venus is.
One Year in Venus
Venus orbits the Sun at a very low rotation rate. In fact, one year in Venus passes quicker than one sidereal day. A sidereal day is the length of time it takes for a planet to complete a full 360-degree rotation, and equals 243 Earth days. A year on Venus is 225 Earth days. The solar day is shorter than the sidereal day: 117 Earth days thus, one year equals 1.92 Venus solar days. Every 24 years, Venus has a leap year with three days.
The Atmosphere, Temperature, and Pressure in Venus
Venus’s atmosphere is made up of carbon dioxide (96%), nitrogen (3.5%), and less than 1% carbon monoxide, argon, sulfur dioxide, and water vapor. These gases in the atmosphere turn Venus into a huge greenhouse with a temperature of 850°F, which is too much considering its distance from the Sun and the fact that Venus reflects the light much stronger than Earth. As there are no plants and liquid water on Venus, nothing absorbs the carbon dioxide, and the greenhouse effect is significantly stronger than on Earth.
The dominance of carbon dioxide turns Venus into a greenhouse giant. (Image: Peter Simoncik/Shutterstock)
The pressure 50 kilometers above Venus’s surface is similar to the pressure on Earth, but on the surface, it is 92 bars. The pressure on Earth at sea level is one bar.
Apparently, the Earth’s twin has much more significant differences than expected.
Common Questions About Venus
Venus sometimes called Earth’s “sister planet” or Earth’s twin . It is a terrestrial planet because of similar size, mass, proximity to the Sun, and bulk composition to those of the Earth’s. However, in other aspects, it is significantly different and has no living conditions for humans.
Venus makes a complete orbit around the Sun in a year in Venusian time – equal to 225 Earth days, which is less than two day-night cycles on Venus. Its orbit around the Sun is the most circular of any planet, but Venus orbits the Sun almost upside down. This is one of the biggest dissimilarities with the Earth, despite being Earth’s twin planet.
Venus, or the Earth’s twin , is often the brightest object in the sky, other than the moon. The reason is the atmosphere of Venus and how much it reflects the Sun’s light and heat. Despite similarities with Earth, Venus can reflect Sun rays much stronger than Earth.
Venus has an extremely hot surface and very high atmospheric pressure. The most common elements in Venus’s atmosphere are carbon dioxide (96%), nitrogen (3.5%), and less than 1% carbon monoxide, argon, sulfur dioxide, and water vapor. Even though it is Earth’s twin , there is no oxygen in Venus’s atmosphere.
While shares of Gamestop are now coming down to earth at about $50 apiece, the former rally’s impact has been far from negligible.
In recent standoffs, the NBA has been willing to move heaven and earth to maximize its television revenue.
With only one mining firm currently producing rare earth s in the US, Round Top—which the company hopes to have in operation by 2023—would play a significant role in helping diversify supplies.
Sophie Murguia, assistant editorThere are some writers you discover and subsequently decide you must follow to the ends of the earth .
Over the past several decades, China has built up and cemented its dominance in global rare earth s, and at its peak the country accounted for almost 98% of the world’s raw rare earth s production.
The questions going through my mind are: How on earth are there Kalashnikovs and rocket launchers in the heart of Paris?
One is forced to ask, what on earth was Andrew doing hanging out with scantily clad teenagers?
They carved a refuge out of the wilderness and then, in 200 years, built it into the most powerful nation on earth .
Once giants walked this earth , and some of them were Democrats.
Woods were shredded, the earth trembled and the ground exploded in showers of stone and red-hot metal splinters.
The most High hath created medicines out of the earth , and a wise man will not abhor them.
The Majesty on high has a colony and a people on earth , which otherwise is under the supremacy of the Evil One.
All things that are of the earth , shall return to the earth again, and all waters shall return to the sea.
It was difficult, with the mean appliances of the time, to wring subsistence from the reluctant earth .
He felt himself the meanest, vilest thing a-crawl upon this sinful earth , and she—dear God!
having to do with the surrounding area or environment.
layers of gases surrounding a planet or other celestial body.
type of dark volcanic rock.
oldest underlying rock formation in any region.
line separating geographical areas.
physical, cultural, or psychological feature of an organism, place, or object.
seismic boundary between the continental crust and oceanic crust.
thick layer of Earth that sits beneath continents.
area where two or more tectonic plates bump into each other. Also called a collision zone.
the extremely hot center of Earth, another planet, or a star.
old, stable part of continental crust, made up of shields and platforms.
rocky outermost layer of Earth or other planet.
spotted, or having areas of differently colored shades or tones.
having parts or molecules that are packed closely together.
area of land that receives no more than 25 centimeters (10 inches) of precipitation a year.
a barrier, usually a natural or artificial wall used to regulate water levels.
always changing or in motion.
our planet, the third from the Sun. The Earth is the only place in the known universe that supports life.
to explode or suddenly eject material.
at some point in the future.
volcano that will no longer erupt.
located or formed outside Earth's atmosphere.
having to do with the physical formations of the Earth.
type of hard, igneous rock.
type of hard, igneous rock.
tint or general variety of color.
large chunks of ice that break off from glaciers and float in the ocean.
rock formed by the cooling of magma or lava.
in geochemistry, an element that stays in a liquid phase during the melt or crystallization process.
chemical element with the symbol Fe.
equilibrium of Earth's crust, where the forces tending to elevate landmasses balance those tending to depress them. Also called isostatic equilibrium.
molten rock, or magma, that erupts from volcanoes or fissures in the Earth's surface.
outer, solid portion of the Earth. Also called the geosphere.
having to do with Earth's moon or the moons of other planets.
molten, or partially melted, rock beneath the Earth's surface.
chemical element with the symbol Mg.
flexible and capable of reforming itself without breaking when under stress.
middle layer of the Earth, made of mostly solid rock.
category of elements that are usually solid and shiny at room temperature.
rock that has transformed its chemical qualities from igneous or sedimentary.
underwater mountain range.
inorganic material that has a characteristic chemical composition and specific crystal structure.
point between Earth's crust and the mantle below. Also called the Moho.
solid material turned to liquid by heat.
depression in the Earth's surface located entirely beneath the ocean.
thin layer of the Earth that sits beneath ocean basins.
a long, deep depression in the ocean floor.
remnant of oceanic crust (certain igneous rocks) embedded in continental crust.
the way mountains are formed.
to release a gas that was dissolved, trapped, frozen or absorbed in another material.
large, spherical celestial body that regularly rotates around a star.
ancient rocks that formed as part of continental crust, now overlain with sediment and sedimentary rock, located in the interior of continents.
natural substance composed of solid mineral matter.
solid material transported and deposited by water, ice, and wind.
rock formed from fragments of other rocks or the remains of plants or animals.
ancient rocks that formed as part of continental crust and are located in the interior of continents.
rocks, mostly silicates and aluminum, making up most of Earth's continental crust.
chemical compound (SiO2) that makes up most of the Earth's rocks.
most common group of minerals, all of which include the elements silicon (Si) and oxygen (O).
process of one tectonic plate melting, sliding, or falling beneath another.
area where one tectonic plate slides under another.
small submarine used for research and exploration.
(S 2- ) negatively charged ion of sulfur, or a chemical compound containing such an ion.
movement of tectonic plates resulting in geologic activity such as volcanic eruptions and earthquakes.
massive slab of solid rock made up of Earth's lithosphere (crust and upper mantle). Also called lithospheric plate.
degree of hotness or coldness measured by a thermometer with a numerical scale.
one of the four planets closest to the sun: Mercury, Venus, Earth, or Mars.
areas in the Earth's interior between the upper mantle, near the Earth's crust, and the lower mantle, near the Earth's core.
exactly the same in some way.
all known matter, energy, and space.
liquid that is thick and sticky.
an opening in the Earth's crust, through which lava, ash, and gases erupt, and also the cone built by eruptions.
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Jeannie Evers, Emdash Editing
Caryl-Sue, National Geographic Society
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Ocean currents are the continuous, predictable, directional movement of seawater driven by gravity, wind (Coriolis Effect), and water density. Ocean water moves in two directions: horizontally and vertically. Horizontal movements are referred to as currents, while vertical changes are called upwellings or downwellings. This abiotic system is responsible for the transfer of heat, variations in biodiversity, and Earth&rsquos climate system. Explore how ocean currents are interconnected with other systems with these resources.
Weathering is the process of the weakening and breakdown of rocks, metals, and manmade objects. There are two main types of weathering: chemical and physical. An example of chemical weathering is acid rain. Caused mostly by the burning of fossil fuels, acid rain is a form of precipitation with high levels of sulfuric acid, which can cause erosion in the materials in which it comes in contact. An example of physical weathering is wind blowing across the desert playas. This process causes rocks to form a specific pyramid-like shape and they are called ventifacts. Select from these resources to teach about the process of weathering in your classroom.a
The Earth&rsquos surface may seem motionless most of the time, but it&rsquos actually always moving, ever so slowly, at a scale that is difficult for humans to perceive. The Earth&rsquos crust is broken up into a series of massive sections called plates. These tectonic plates rest upon the convecting mantle, which causes them to move. The movements of these plates can account for noticeable geologic events such as earthquakes, volcanic eruptions, and more subtle yet sublime events, like the building of mountains. Teach your students about plate tectonics using these classroom resources.
The Rock Cycle
Many of Earth&rsquos key processes function in cycles and rock cycle is no exception. The rock cycle is a web of processes that outlines how each of the three major rock types&mdashigneous, metamorphic, and sedimentary&mdashform and break down based on the different applications of heat and pressure over time. For example, sedimentary rock shale becomes slate when heat and pressure are added. The more heat and pressure you add, the further the rock metamorphoses until it becomes gneiss. If it is heated further, the rock will melt completely and reform as an igneous rock. Empower your students to learn about the rock cycle with this collection of resources.
According to the United States Geologic Survey, there are approximately 1,500 potentially active volcanoes worldwide. Most are located around the Pacific Ocean in what is commonly called the Ring of Fire. A volcano is defined as an opening in the Earth's crust through which lava, ash, and gases erupt. The term also includes the cone-shaped landform built by repeated eruptions over time. Teach your students about volcanoes with this collection of engaging material.
Seafloor spreading is a geologic process in which tectonic plates&mdashlarge slabs of Earth's lithosphere&mdashsplit apart from each other.
In 1977, after decades of tediously collecting and mapping ocean sonar data, scientists began to see a fairly accurate picture of the seafloor emerge. The Tharp-Heezen map illustrated the geological features that characterize the seafloor and became a crucial factor in the acceptance of the theories of plate tectonics and continental drift. Today, these theories serve as the foundation upon which we understand the geologic processes that shape the Earth.
Earth is the planet we live on, the third of eight planets in our solar system and the only known place in the universe to support life.
The planet is recycled
The ground you're walking on is recycled. Earth's rock cycle transforms igneous rocks to sedimentary rocks to metamorphic rocks and back again.
The cycle isn&rsquot a perfect circle, but the basics work like this: Magma from deep in the Earth emerges and hardens into rock (that's the igneous part). Tectonic processes uplift that rock to the surface, where erosion shaves bits off. These tiny fragments get deposited and buried, and the pressure from above compacts them into sedimentary rocks such as sandstone. If sedimentary rocks get buried even deeper, they "cook" into metamorphic rocks under lots of pressure and heat.
Along the way, of course, sedimentary rocks can be re-eroded or metamorphic rocks re-uplifted. But if metamorphic rocks get caught in a subduction zone where one piece of crust is pushing under another, they may find themselves transformed back into magma.
The boring billion went bye-bye when a big supercontinent ripped apart 750 million years ago, triggering a global chill called the Snowball Earth. This model suggests the planet was a mushy "snowball" nearly completely covered with glaciers. The volcanic eruptions and rock weathering that accompanied the supercontinent breakup had trapped carbon dioxide, massively cooling the planet. Geologists have found evidence of glaciers on every continent from this time, even at spots that were at tropical latitudes.
The atmosphere's oxygen levels started rising again roughly 650 million years ago, about the time when the first animals appeared. The first hard parts on animals appear during the Cambrian Period 545 million years ago. While researchers have yet to agree on the reason for this explosion of life, many think a combination of factors spurred this extraordinary jump from single cells to complex creatures. For instance, the spreading continents sent a surge of nutrients into the oceans and opened up new habitats. And an evolutionary arms' race set off as animals fought to chow down on each other and protect themselves from predators.
- Arney et al. - "Atmos: Studies of Exoplanet Atmospheres Enabled by a Versatile 1-D Photochemical-Climate Model"
- Glocer et al. - "Dynamics of Upper Atmospheres of Terrestrial Exoplanets Around Active K to M dwarfs as a Factor of Habitability"
- Guzewich et al. - "Simulating Factors Influencing Habitable Exoplanets with ROCKE3D"
- Kiang et al. - "Land Planets: Foundations for Understanding the Distribution of Surface Habitability and Life Inside the Habitable Zone"
- Kuang et al. – "Habitability of Magnetic Exo Terrestrial Planets"
- Schnittman et al. - "Modeling eccentricity effects with chemistry-coupled GCM simulations"
- Way et al. - "Impact of Extreme Space Weather on Climates of Terrestrial-Type Exoplanets"
Key Questions Guiding SEEC Research
This is a fairly simple trope. It is simply the habit of calling Earth "Terra" in Sci Fi. The word is adopted from the Latin word for, well, earth.
It is used to make the planet Earth follow the Roman naming systems for the planets of the Solar System and also because "Terrans" is a more respectable description for the inhabitants of the planet than "Earthlings". Another advantage is that it is language-neutral, since it is by far the most common word for the planet&mdashfour world languages call this planet Terra or some variant thereof, note French "Terre", Portuguese and Italian "Terra" and Spanish "Tierra" all come from the Latin word &mdash which gives French SF authors the convenient advantage of referring to the inhabitants as "Terriens", or "Terrans". as do many of the other Romance (i.e. Latin-derived) languages with fewer speakers.
Also, Terra provides a convenient standardization in that Earth is otherwise one of the only two planets in the Solar System that are not named for Roman deities, with Uranus, named for a Greek one, as the other, and one of the few things in general in the Solar System that are not named for Roman or Greek mythology, along with a few others, such as the likely dwarf planet Makemake, named for the creator in Easter Island folklore, or the Uranian moon Puck, named for a character in Shakespeares A Midsummer Nights Dream.
This is often accompanied by reference to Earth's sun as "Sol", and the moon as "Luna", to differentiate them from other suns or moons.
Calling Earth "Terra" may be a result of an Earth That Was scenario.
Due to the evolution of the pronunciation of Latin itself, the older form "Tella" also fits here. SF authors of the 1930s also applied an alternative version of the word, "Tellus," which means the same thing.
For whatever reason "Terran" is frequently used as an alternate name for humans even when the planet is still called Earth.
Planetary geologists divide crust into three categories, based on how and when they formed. 
Primary crust / primordial crust
This is a planet's "original" crust. It forms from solidification of a magma ocean. Toward the end of planetary accretion, the terrestrial planets likely had surfaces that were magma oceans. As these cooled, they solidified into crust.  This crust was likely destroyed by large impacts and re-formed many times as the Era of Heavy Bombardment drew to a close. 
The nature of primary crust is still debated: its chemical, mineralogic, and physical properties are unknown, as are the igneous mechanisms that formed them. This is because it is difficult to study: none of Earth's primary crust has survived to today.  Earth's high rates of erosion and crustal recycling from plate tectonics has destroyed all rocks older than about 4 billion years, including whatever primary crust Earth once had.
However, geologists can glean information about primary crust by studying it on other terrestrial planets. Mercury's highlands might represent primary crust, though this is debated.  The anorthosite highlands of the Moon are primary crust, formed as plagioclase crystallized out of the Moon's initial magma ocean and floated to the top  however, it is unlikely that Earth followed a similar pattern, as the Moon was a water-less system and Earth had water.  The Martian meteorite ALH84001 might represent primary crust of Mars however, again, this is debated.  Like Earth, Venus lacks primary crust, as the entire planet has been repeatedly resurfaced and modified. 
Secondary crust is formed by partial melting of silicate materials in the mantle, and so is usually basaltic in composition. 
This is the most common type of crust in the Solar System. Most of the surfaces of Mercury, Venus, Earth, and Mars comprise secondary crust, as do the lunar maria. On Earth, we see secondary crust forming primarily at mid-ocean spreading centers, where the adiabatic rise of mantle causes partial melting.
Tertiary crust is more chemically-modified than either primary or secondary. It can form in several ways:
- Igneous processes: partial-melting of secondary crust, coupled with differentiation or dehydration 
- Erosion and sedimentation: sediments derived from primary, secondary, or tertiary crust
The only known example of tertiary crust is the continental crust of the Earth. It is unknown whether other terrestrial planets can be said to have tertiary crust, though the evidence so far suggests that they do not. This is likely because plate tectonics is needed to create tertiary crust, and Earth is the only planet in our Solar System with plate tectonics.
The Earth's crust is a thin shell on the outside of the Earth, accounting for less than 1% of Earth's volume. It is the top component of lithosphere: a division of Earth's layers that includes the crust and the upper part of the mantle.  The lithosphere is broken into tectonic plates that move, allowing heat to escape from the interior of the Earth into space.
A theoretical protoplanet named "Theia" is thought to have collided with the forming Earth, and part of the material ejected into space by the collision accreted to form the Moon. As the Moon formed, the outer part of it is thought to have been molten, a "lunar magma ocean." Plagioclase feldspar crystallized in large amounts from this magma ocean and floated toward the surface. The cumulate rocks form much of the crust. The upper part of the crust probably averages about 88% plagioclase (near the lower limit of 90% defined for anorthosite): the lower part of the crust may contain a higher percentage of ferromagnesian minerals such as the pyroxenes and olivine, but even that lower part probably averages about 78% plagioclase.  The underlying mantle is denser and olivine-rich.
The thickness of the crust ranges between about 20 and 120 km. Crust on the far side of the Moon averages about 12 km thicker than that on the near side. Estimates of average thickness fall in the range from about 50 to 60 km. Most of this plagioclase-rich crust formed shortly after formation of the moon, between about 4.5 and 4.3 billion years ago. Perhaps 10% or less of the crust consists of igneous rock added after the formation of the initial plagioclase-rich material. The best-characterized and most voluminous of these later additions are the mare basalts formed between about 3.9 and 3.2 billion years ago. Minor volcanism continued after 3.2 billion years, perhaps as recently as 1 billion years ago. There is no evidence of plate tectonics.
Study of the Moon has established that a crust can form on a rocky planetary body significantly smaller than Earth. Although the radius of the Moon is only about a quarter that of Earth, the lunar crust has a significantly greater average thickness. This thick crust formed almost immediately after formation of the Moon. Magmatism continued after the period of intense meteorite impacts ended about 3.9 billion years ago, but igneous rocks younger than 3.9 billion years make up only a minor part of the crust.