Category: Guide to Space

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The density of the Earth is 5.513 g/cm³. This is an average of all of the material on the planet. Water is much less dense than iron, hence an average is needed for ease of use. Earth is the most dense planet in the Solar System; however, if gravitational compression where factored out, the second most dense planet, Mercury, would be more dense. The density of Earth is calculated by dividing the planet’s mass by its volume, then simplifying from kg/km to g/cm cubed.

Here is the density of the other planets in our Solar System so you can compare to Earth’s.

Mercury	5.43 g/cm3
Venus	5.243 g/cm3
Mars	3.934 g/cm3
Jupiter	1.326 g/cm3
Saturn	0.687 g/cm3
Uranus	1.270 g/cm3
Neptune	1.638 g/cm3
The Sun	1.408 g/cm3

Just knowing the density of a planet is not much information. It sort of only gives a partial picture. Here are a few more interesting facts about the Earth that may help you understand our planet a little more.

The Moon is thought to have been formed when a large asteroid or a planetesimal impacted Earth. The Moon is the portion that was thrown back into space and the particles that accreted to it. Scientist think that other planets may have obtained some of their moons in a similar manner. The Earth is the only planet with a single Moon, but has two quasi-satellites 3753 Cruithne and 2002 AA₂₉.

The Sun is constantly evolving. In a few billion years it will begin to heat up on its way to the red giant phase of a star’s life. Along the way it will become hot enough to destroy life on Earth. The question will become how will humans survive. Colonizing other celestial objects is one option. Some scientists have developed a theoretical way to move the entire planet. It would require finding an asteroid large enough to perturb Earth’s orbit and push away from the Sun. Colonizing another planet could actually be easier.

Despite a lot of internet hype, there is no credible threat to the Earth that will coincide with the end of the Mayan calendar. The Mayan calender does not even end, 2012 marks the end of the current long-count period. 2013 marks the beginning of another.

The density of Earth is one of thousands of interesting facts that you find about your home planet. Here at Universe Today, we hope that you want to find many more and continue to research the world around you.

We have written many articles about density for Universe Today. Here’s an article about the density of the Sun, and here’s one about the density of Mars.

Want more resources on the Earth? Here’s a link to NASA’s Human Spaceflight page, and here’s NASA’s Visible Earth.

We have also recorded an episode of Astronomy Cast about Earth, as part of our tour through the Solar System – Episode 51: Earth.

Sources:
NASA
Physics Forums

You might not realize it, but you’re standing on a thin shell of solid rock encasing a vast quantity of molten rock. This is the Earth’s crust, and it’s the part of the planet that has cooled down enough to solidify. But just a few kilometers below your feet, it’s molten rock, extending for thousands of kilometers down to the planet’s superheated iron core.

Here on solid ground, on the continental shelves, the crust of the Earth is about 30 km thick. In the mid-ocean, the thickness of the crust can be as little as 5 km. The entire crust occupies just 1% of the Earth’s volume.

The crust is composed of a variety of igneous, metamorphic and sedimentary rocks gathered together into tectonic plates. These plates float above the Earth’s mantle, and it’s believed that convection of rock in the mantle causes the plates to slide around. On average, rocks in the crust last about 2 billion years before they slide underneath another plate and are returned to the Earth’s mantle. New rocks are formed in the mid-ocean regions where new material wells out of the Earth in between spreading plates. In comparison, rocks in the oceans are only 200 million years old.

The temperature of the crust increases as you go deeper into the Earth. It starts out cool, but can get up to 400 degrees C at the boundary between the crust and the mantle.

Scientists really know very little about internal structure of the Earth. The crust is the only part that we have any information about. And we’ve barely explored it at all. The deepest hole ever dug was the Russian Kola Superdeep Borehole. Started in 1970, the hole eventually reached a depth of 12.3 km. They eventually had to quit because temperatures in the hole became too hot to go any further. Other plans are in the works to bore into the crust in the ocean, where the thickness is much less.

We have written many articles about the Earth for Universe Today. Here’s an article about how the Earth’s core rotates faster than the crust, and here’s an article about how potassium could be heating up the Earth’s interior.

Want more resources on the Earth? Here’s a link to NASA’s Human Spaceflight page, and here’s NASA’s Visible Earth.

We have also recorded an episode of Astronomy Cast about Earth, as part of our tour through the Solar System – Episode 51: Earth.

Sources:
http://earthquake.usgs.gov/research/structure/crust/index.php
http://en.wikipedia.org/wiki/Crust_%28geology%29

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How many are in the Milky Way, though? There could be billions, many of them habitable and Earth-like, according to some astronomers. Our ability to detect planets orbiting other stars has been around for less than 20 years, and most of the planets discovered to date are much larger than Jupiter (in fact, extrasolar planets are commonly measured in “Jupiter masses”.)

There are a few different methods for detecting exoplanets. The primary techniques are astrometry and radial velocity measurements. Astrometry is basically measuring the gravitational influence of a planet as it orbits its star. How much it pulls the star side to side can give a lot of information as to the amount of mass the planet has. Measuring radial velocity is much like astrometry, only with this method the amount the star moves toward and away from the Earth is measured by observing the Doppler shift of the light coming from the star.

Another technique is called the transit method. As a planet orbits in front of its star, the light coming from the star is dimmed, and by observing the star for long periods of time, and taking the spectrum of the light both when the planet is in front of the star and behind, much can be known about the makeup of the planet’s atmosphere (if there is any). The transit method is often used in combination with astrometry and radial velocity measurements to estimate the mass of the planet.

Other methods for detecting planets are explained on the European Space Agency’s website and Curious About Astronomy. If you want a complete list of all planets detected so far, NASA’s PlanetQuest site is a great place to start, as well as The Extrasolar Planets Encyclopaedia.

Direct imaging of extrasolar planets is very difficult, as the overwhelming amount of light coming from the star a planet is orbiting completely washes it out. However, Hubble has imaged the planet Fomalhault b, and the system HR8799, which consists of three planets, was imaged using the Keck and Gemini telescopes.

There are currently a number of NASA missions working on the discovery of extrasolar planets, including Hubble, and the Spitzer Space Telescope. The Kepler mission, launched on March 6th of 2009, will monitor a section of the sky containing over 100,000 stars and use the methods described above in an effort to detect an exoplanets in that region. The Terrestrial Planet Finder mission is another mission to study all aspects of extrasolar planets in rather great detail, though it is still in the concept phase as of this writing.

Exoplanets were discussed on Astronomy Cast in  Episode 34: Discovering Another Earth and Episode 125: A Zoo of Extrasolar Planets.

Source: NASA

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The ground under your feet might seem solid, but you’re standing on a relatively thin crust of rock above a vast ocean of rock. This molten rock is the Earth’s mantle, and it comprises the largest part of the Earth’s volume.

The crust we stand on is only about 30 km thick. Out in the oceans, it’s even thinner, getting down to 5 km in places. Beneath this crust is the mantle of the Earth; a region that extends down a depth of almost 2,900 km.

Although the mantle is largely hidden from our view, we do see it in places where cracks open up, allowing the molten rock to escape. These are volcanos, of course, and the liquid rock we see pouring out is the same as you’d find in the mantle.

The Earth’s mantle is mostly composed of silicate rocks that are rich in iron and magnesium. Although it’s mostly solid, it’s hot enough that it can flow over long timescales. The upper mantle flows more easily than the lower mantle because of the increasing temperature and pressures as you descend into the Earth.

The Earth’s tectonic plates float on top of the mantle. In some places, the plates are sliding under one another, returning rock back to the interior of the Earth. In other places, the plates are spreading apart, and fresh volcanic material is welling up to fill the cracks.

Inside the mantle, convection is slowly taking place – like in a lava lamp. Hotter material, heated by the core of the Earth rise slowly to the surface of the mantle. Material cools near the crust and then sinks back down to the core, to repeat the process all over again. It’s believed that this convection helps drive the motions of Earth’s tectonic plates.

We have written many articles about the Earth for Universe Today. Here’s an article that talks about how scientists might study the interior of the Earth using neutrinos, and here’s one about how rising temperatures could shut down plate tectonics.

Want more resources on the Earth? Here’s a link to NASA’s Human Spaceflight page, and here’s NASA’s Visible Earth.

We have also recorded an episode of Astronomy Cast about Earth, as part of our tour through the Solar System – Episode 51: Earth.

Sources:
http://en.wikipedia.org/wiki/Mantle_%28geology%29
http://www.schools.utah.gov/curr/science/sciber00/7th/earth/sciber/erlayers.htm

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Deep within the Earth, thousands of kilometers below your feet is the core of the Earth. Once thought to be a single ball of iron, scientists now know that the Earth’s core contains a solid inner core, surrounded by a liquid outer core. Let’s take a look at the outer core of Earth.

The discovery that the core of the Earth contains a solid inner core surrounded by a liquid outer core was made by seismologist Inge Lehmann, who was studying how seismic waves bounce off the interior of the Earth. Instead of bouncing off a solid core, Lehmann observed that the liquid outer core caused the waves to reflect differently from how they bounced off the inner core.

Further studies have refined the size of the outer core. The inner core is thought to be 2,440 km across, and when you include the liquid outer core of the Earth, the entire core measures 6,800 km across; about twice as big as the Moon.

It’s believed that the core of the Earth formed early on in our planet’s history, when the entire planet was made of molten rock and metal. Since it was a liquid, the heaviest elements, like iron, nickel, gold and platinum sunk down into the center, leaving the less dense elements on top.

Without the outer core, life on Earth would be very different. Scientists believe that convection of liquid metals in the outer core create the Earth’s magnetic field. This magnetic field extends outward from the Earth for several thousand kilometers, and creates a protective bubble around the Earth that deflects the Sun’s solar wind. Without this field, the solar wind would have blasted away our atmosphere, and Earth would be dead and lifeless like Mars.

The inner core is also known to rotate, turning approximately 0.3 to 0.5 degrees per year relative to the rotation of the surface. In other words, the inner core makes an extra rotation every 700-1000 years compared to the surface.

We have written many articles about the Earth for Universe Today. Here’s an article about the recent discovery that the Earth has an inner, inner core.

Want more resources on the Earth? Here’s a link to NASA’s Human Spaceflight page, and here’s NASA’s Visible Earth.

We have also recorded an episode of Astronomy Cast about Earth, as part of our tour through the Solar System – Episode 51: Earth.

Sources:
http://www.windows2universe.org/earth/Interior_Structure/interior.html
http://www.amnh.org/education/resources/rfl/web/essaybooks/earth/p_lehmann.html
http://en.wikipedia.org/wiki/Outer_core

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Deep beneath the Earth lies the core. This is a ball of solid metal surrounded by liquid metal. The solid part is the inner core of Earth, and the liquid part is known as the Earth’s outer core.

Scientists have long suspected that the interior of the Earth is much denser than the rest of the planet. That’s because the average density of the planet is 5.5 g/cm³, while the surface is only 3 g/cm³. In other words, if the surface is less dense than the Earth, on average, then the core must be much denser.

During the formation of the Earth, 4.6 billion years ago, the planet was a molten ball of rock and metal. Because it was a liquid, however, the heavier elements like iron and nickel were able to sink down into the center. In fact, the inner core of the Earth probably has vast amounts of the heaviest elements, like gold, platinum and uranium.

But the fact that the Earth had two cores, inner and outer, was first discovered in 1936 by seismologist Inge Lehmann. He observed that seismic waves created by earthquakes on its surface would bounce off the two cores differently. This is similar to how light waves refract differently as they pass through liquids. By measuring these seismic waves, scientists have been able to map out the size of the inner core.

The inner core of the Earth is thought to be about 2,440 km across; about 70% the size of the Moon. It’s very hot, probably 3,000 to 5,000 Kelvin.

Scientists once believed that the inner core was possibly a single, solid object; maybe even a single crystal of iron. But recent evidence has found that it has detailed structures, and even has an inner, inner core.

We have written many articles about the Earth for Universe Today. Here’s a full article about the discovery of the Earth’s inner, inner core.

Want more resources on the Earth? Here’s a link to NASA’s Human Spaceflight page, and here’s NASA’s Visible Earth.

We have also recorded an episode of Astronomy Cast about Earth, as part of our tour through the Solar System – Episode 51: Earth.

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Scientists believe that deep down inside the Earth, there’s a huge ball of liquid and solid iron. This is the Earth’s core, and it protects us from the dangerous radiation of space.

When the Earth first formed, 4.6 billion years ago, it was a hot ball of molten rock and metal. And since it was mostly liquid, heavier elements like iron and nickel were able to sink down into the planet and accumulate at the core. The core is believed to have two parts: a solid inner core, with a radius of 1,220 km, and then a liquid outer core that extends to a radius of 3,400 km. The core is through to be 80% iron, as well as nickel and other dense elements like gold, platinum and uranium.

The inner core is solid, but the outer core is a hot liquid. Scientists think that movements of metal, like currents in the oceans, create the magnetic field that surrounds the Earth. This magnetic field extends out from the Earth for thousands of kilometers, and redirects the solar wind blowing from the Sun. Without this magnetic field, the solar wind would blow away the lightest parts of our atmosphere, and make our environment more like cold, dead Mars.

Although the Earth’s crust is cool, the inside of the Earth is hot. The mantle is only about 30 km beneath our feet, and it’s hot enough to melt rock. At the core of the Earth, temperatures are thought to rise to 3,000 to 5,000 Kelvin.

Since the core is thousands of kilometers beneath our feet, how can scientists know anything about it? One way is to just calculate. The average density of the Earth is 5.5 grams per cubic cm. The Earth’s surface is made of less dense materials, so the inside must have something much more dense than rock. The second part is through seismology. When earthquakes rock the surface of the Earth, the planet rings like a bell, and the shockwaves pass through the center of the Earth. Monitoring stations around the planet detect how the waves bounce, and scientists are able to use this to probe the interior of the Earth.

We have written many articles about the Earth for Universe Today. Here’s an article about how the Earth might actually have an inner, inner core.

Want more resources on the Earth? Here’s a link to NASA’s Human Spaceflight page, and here’s NASA’s Visible Earth.

We have also recorded an episode of Astronomy Cast about Earth, as part of our tour through the Solar System – Episode 51: Earth.

Sources:
http://en.wikipedia.org/wiki/Structure_of_the_Earth
http://scign.jpl.nasa.gov/learn/plate1.htm

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The atmosphere we enjoy today is radically different from the atmosphere that formed with the Earth billions of years ago. And yet, the Earth’s early atmosphere somehow transformed into the life giving atmosphere we enjoy today.

The Earth formed with the Sun 4.6 billion years ago. At this point, it was nothing more than a molten ball of rock surrounded by an atmosphere of hydrogen and helium. Because the Earth didn’t have a magnetic field to protect it yet, the intense solar wind from the young Sun blew this early atmosphere away.

As the Earth cooled enough to form a solid crust (4.4 billion years ago), it was covered with active volcanos. These volcanos spewed out gasses, like water vapor, carbon dioxide and ammonia. This early toxic atmosphere was nothing like the atmosphere we have today.

Light from the Sun broke down the ammonia molecules released by volcanos, releasing nitrogen into the atmosphere. Over billions of years, the quantity of nitrogen built up to the levels we see today.

Although life formed just a few hundred million years later, it wasn’t until the evolution of bacteria 3.3 billion years ago that really changed the early Earth atmosphere into the one we know today. During the period 2.7 to 2.2 billion years ago, these early bacteria – known as cyanobacteria – used energy from the Sun for photosynthesis, and release oxygen as a byproduct. They also sequestered carbon dioxide in organic molecules.

In just a few hundred million years, this bacteria completely changed the Earth’s atmosphere composition, bringing us to our current mixture of 21% oxygen and 78% nitrogen.

We have written many articles about the Earth for Universe Today. Here’s an article about how the Earth’s early atmosphere was very different from the one we see today, and an another that describes how Titan’s atmosphere is probably similar to the Earth’s early atmosphere.

Want more resources on the Earth? Here’s a link to NASA’s Human Spaceflight page, and here’s NASA’s Visible Earth.

We have also recorded an episode of Astronomy Cast about Earth, as part of our tour through the Solar System – Episode 51: Earth.

Breathe in and you can appreciate that the Earth’s atmosphere has everything needed to support life on Earth. But what’s in it? Let’s take a look at the composition of the Earth’s atmosphere. Of course, things haven’t always been balanced they way they are today. But more of that in a second.

The Earth’s atmosphere is composed of the following molecules: nitrogen (78%), oxygen (21%), argon (1%), and then trace amounts of carbon dioxide, neon, helium, methane, krypton, hydrogen, nitrous oxide, xenon, ozone, iodine, carbon monoxide, and ammonia. Lower altitudes also have quantities of water vapor.

The atmosphere we have today is very different from the Earth’s early atmosphere. When the planet first cooled down 4.4 billion years ago, volcanos spewed out steam, carbon dioxide and ammonia, and it was 100 times as dense as today’s atmosphere.

The earliest bacteria, known as cyanobacteria, were probably the first oxygen-producing organisms on Earth. Approximately 2.7 to 2.2 billion years ago, they released large amounts of oxygen and sequestered the carbon dioxide. As oxygen was released, it reacted with ammonia to release nitrogen. The carbon dioxide in the atmosphere is exhaled by plants (and produced by human industry burning fossil fuels).

We have written many articles about the Earth for Universe Today. Here’s an article about how the Earth’s atmosphere is slowly leaking into space, and here’s an article about how the early Earth’s atmosphere was similar to Titan’s atmosphere.

Want more resources on the Earth? Here’s a link to NASA’s Human Spaceflight page, and here’s NASA’s Visible Earth.

We have also recorded an episode of Astronomy Cast about Earth, as part of our tour through the Solar System – Episode 51: Earth.

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The size of Earth, like the size of all of the celestial bodies, is measured in a number of parameters including mass, volume, density, surface area, and equatorial/polar/mean diameter. While we live on this planet, very few people can quote you the figures for these parameters. Below is a table with many of the pieces of the data used to measure the size of the Earth.

Mass	5.9736×1024kg
Volume	1.083×1012 km3
Mean diameter	12,742 km
Surface area	510,072,000 km2
Density	5.515 g/cm3
Circumference	40,041 km

Those numbers tell you the size of the Earth, but what about its other statistics? The atmospheric pressure at the surface is 101.325 kPa, average temperature is 14°C, the axial tilt is approximately 23°, and it has an orbital speed of 29.78 km/s. Earth orbits with a perihelion of 147,098,290 km, and an aphelion of 152,098,232 km, making for a semi-major axis of 149,598,261 km. Even though we need oxygen to survive, it is the second most abundant component of Earth’s atmosphere. Nitrogen accounts for 78% of the gases in the atmosphere and oxygen occupies 21%.

The Earth only has one moon. That is pretty uncommon in our Solar System. There are currently 166 recognized moon in our system. There is one asteroid that has a quasi relationship with Earth. 3753 Cruithne has a 1:1 orbital resonance with the Earth. It is a periodic inclusion planetoid that has a horseshoe orbit. It was discovered in 1986.

Since we occupy this planet, it is understandably the most extensively studied body in space. We have sent scientist to most of the corners of our world. Yet, we find dozens of new species each year and there are areas that have rarely seen a human’s footprints. There are aspects of our world that we do not understand and have theories too inadequate to explain. Science is light years ahead of where it was just 50 years ago. These advancements are exciting enough to make the possibilities of the near future seem boundless.

Now that you know the size of the Earth, you could look for information on extremophiles, the Mariana Trench, and the Tunguska event. Earth bound events are often taken for granted since we live here, but, with a little research, you may find much more excitement outside of your back door than you ever expected.

We have written many articles about the Solar System for Universe Today. Here’s an article about the size of Mars, and here’s one about the size of the Moon.

Want more resources on the Earth? Here’s a link to NASA’s Human Spaceflight page, and here’s NASA’s Visible Earth.

We have also recorded an episode of Astronomy Cast about Earth, as part of our tour through the Solar System – Episode 51: Earth.

Sources:
NASA Earth Facts
NASA Solar System Guide on Earth
NASA Solar System Orbit Diagram