Universe Today

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Take a look down beneath your feet. You’re standing on the Earth’s crust. Although it seems limitless, the Earth’s crust only accounts for less than 1% of the Earth’s interior. Let’s take a look at everything that’s inside the Earth.

The Earth’s crust is the outer shell of the Earth. This is the part that has cooled down enough to solidify into rock. The crust extends down 30 km to 80 km underneath the continents, and only 5 km beneath the oceans. As you travel down through the crust, temperatures increase. The crust is broken up into several tectonic plates which “float” on top of the Earth’s mantle. In some regions, plates are sliding underneath one another, recycling rocks into the Earth. The crust beneath the middle of the oceans is spreading apart, and new material is welling up.

Beneath the crust is the largest part of the Earth’s interior: the mantle, which makes up about 84% of the Earth’s volume. This region extends down to a depth of 2,890 km. As you travel down through the mantle, temperatures increase immensely; they start at 500 C near the crust, and get to well over 4000 C at the boundary to the core. The mantle is mostly solid, but it acts like a viscous fluid, and experiences convection. Hot blobs of rock rise up from regions around the core through the mantle, give up their heat, and then sink back down.

At the very center of the Earth lies the core. This is a solid sphere of metal 2,440 km across surrounded by a layer of liquid metal. Scientists think that mostly made of iron (80%), with the rest composed of other heavy metals, like nickel, gold, platinum and even uranium. The core is slowly rotating compared to the crust, so that the core completes one rotation every 1000 years or so. The Earth’s magnetic field is though to be generated by the convection of hot metal in the Earth’s outer core. This field protects the Earth from the Sun’s solar wind; there probably wouldn’t be life on Earth without this field.

We have written many articles about the Earth for Universe Today. Here’s an 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.

Sources:
http://www.portal.gsi.gov.in/portal/page?_pageid=127,687643&_dad=portal&_schema=PORTAL
http://en.wikipedia.org/wiki/Mantle_%28geology%29
http://www.windows2universe.org/earth/Interior_Structure/interior.html

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Don’t listen to the Flat Earth Society, they’re wrong; the Earth is round. But did you ever wonder why the Earth is round? It all comes down to gravity.

One of the effects of mass is that it attracts other mass. For small objects, like your computer, your car, and even a building, the force of gravity is tiny. But when you have millions, and even trillions of tonnes of mass, the effect of the gravity really builds up. All of the mass pulls on all the other mass, and it tries to create the most efficient shape… a sphere.

For smaller objects, like asteroids, the force of gravity trying to pull the object into a sphere isn’t enough to overcome the strength of the rock keeping it in shape. But once you get above a certain mass and size, the strength of the object can’t stop the force of gravity from pulling it into a sphere. Objects larger than about 1,000 km in size are able to pull themselves into a sphere.

In fact, the International Astronomical Union decided in 2006 that this ability was one of the requirements for an object to be considered a planet. They must orbit the Sun, they need to have cleared out all the smaller objects in their orbit, and they need to have enough gravity to pull themselves into a sphere.

When an object has the gravity to pull itself into a sphere, astronomers say that it’s in hydrostatic equilibrium. And that’s why the Earth is round.

Of course, the Earth isn’t perfectly round. Because it’s turning on its axis approximately once every 24 hours, the Earth’s equator bulges outwards. And there are mountains and valleys that make the Earth’s surface rough.

We have written many articles about the Earth for Universe Today. Here’s an article about how round the Earth really is.

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, answering a few questions, like why is Earth round. This was part of our tour through the Solar System – Episode 51: Earth.

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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.

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