Author: Fraser Cain

[/caption]
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.

[/caption]
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

[/caption]
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.

The Earth isn’t flat, that’s for sure. And if you look at a photograph, the Earth really looks round. But how round is it?

The actual shape of the Earth is actually an oblate spheroid – a sphere with a bulge around the equator. The Earth is bulged at its equator because it’s rapidly rotating on its axis. The centripetal force of the rotation causes the regions at the equator to bulge outward. And it actually makes a pretty big difference. The diameter of the Earth, measured across the equator is 43 km more than when you measure the diameter of the Earth from pole to pole.

This bulge has some interesting implications. For example, it means that the point on Earth furthest from the center isn’t actually Mount Everest, but Mount Chimborazo in Ecuador. Only because Chimborazo is closer to the Earth’s equator.

So how smooth is the Earth. When billiard balls are manufactured, they aim for a tolerance of 0.22%. The Earth has a tolerance of 0.17%, so it’s actually smoother than a billiard ball. If you could hold the Earth in your hands, it would feel smoother than a billiard ball.

But the Earth definitely isn’t flat.

We have written many articles about the Earth for Universe Today. Here’s a cool article about looking at the Earth as if it’s an extrasolar planet.

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.