OLN.B | Earth

“When I watched the horizon, I saw the abrupt, contrasting transition from the earth’s light-colored surface to the absolutely black sky. I enjoyed the rich color spectrum of the earth. It is surrounded by a light blue aureole that gradually darkens, becoming turquoise, dark blue, violet, and finally coal black.”

– Yuri Gagarin, Cosmonaut, First Human in Space

Earth isn’t the biggest planet, nor the smallest.  It’s not the closest to the sun, and certainly not the farthest.

The thing that makes Earth notable is that it’s the only place we know of that has life on it (whether that life is intelligent or not depends on who you ask, though most people agree that dolphins are pretty bright). There are currently over 7.5 billion people on Earth; scientists think that the total number of people who have ever lived on Earth is about 100 billion.

Where did all those people come from originally? Some scientists theorize that life on Earth might have actually started on another planet, or arrived here via asteroid. Not that little green men brought humans or any other kind of life here, but that the building blocks of life, like amino acids, were already present elsewhere first. Of course, it’s also possible that those building blocks occurred naturally here on Earth.

Although there are plenty of things we don’t yet know about our planet, there are plenty of others that we do. The structure, for instance. You could compare our planet to an egg: at the outermost layer, we have a thin outer shell called the crust. The crust is only about three to five miles thick under the oceans, and on average about 18 miles thick under land. Parts of the crust and upper mantle make up tectonic plates that periodically shift, causing earthquakes. This isn’t something that makes Earth unique, though. Scientists speculate that other planets have tremors as well, though they probably occur much less frequently.

Humans have never been able to dig beyond the crust, though we’ve certainly tried. The deepest hole ever dug is the Kola Superdeep Borehole, in one of the most northern parts of inhabited Russia. Over seven and a half miles deep, digging was started in 1970 by the Soviet Union (which Russia was a part of at the time). They dug until 1992, when unexpectedly high temperatures made it too difficult to go further. The hole is still there, but don’t get any ideas about visiting it to take a look waaaaay down — besides being only 9 inches across, it’s been welded shut.

Below the crust is the mantle, which is made up of hot magma and other semi-solid rocks and minerals; think of this as the egg white. The mantle is about 1800 miles thick and makes up 84% of our planet’s total volume.

At the center is the core, a sphere made mostly of iron and nickel that has a solid inner part and a fluid outer part. It’s the fluid iron that generates the Earth’s magnetic field as it flows. In our analogy, the core would be represented by the egg’s yolk.

Of course, we also have water, and a lot of it. About 70% of our planet is covered in water, most of it in the oceans. In fact, the Pacific Ocean alone covers almost a third of the entire planet. If you launch Google Earth and turn it to view the Pacific Ocean, you just might be surprised at how little land you actually see!

And then there’s the atmosphere – very helpful when it comes to breathing. Earth has an atmosphere that is about 78.09% nitrogen, 20.95% oxygen, 0.93% argon, 0.04% carbon dioxide, and even smaller amounts of other gases. You might think that the next thing beyond our atmosphere is our moon, but there’s a lot more out there before you get that far.

For decades, we’ve been launching machines into space.  Those machines tend to leave a lot of debris behind. “Space junk” might sound like a silly problem, but NASA takes it seriously, tracking more than 500,000 pieces of debris that are larger than a marble. Why is something so small such a big deal?  Well, when something is traveling at 17,500 miles per hour, even a tiny fleck of paint can damage a spacecraft.

If NASA thinks a piece of debris is close enough to potentially damage a spacecraft, they may decide to undertake “debris avoidance maneuvers.” And in the summer of 2018, the RemoveDebris Spacecraft was launched by astronauts on the International Space Station with the objective of harpooning or using a net to collect bits of junk.  It’s expected to de-orbit in March of 2019.

By now, you should know that Earth is one of eight major planets (or maybe nine…or possibly even 10, depending on Pluto’s status this month…or year… and/or the existence of the mysterious Planet X) that orbit around our sun, but there are hundreds of large objects and thousands and thousands of smaller ones that live in our neighbourhood.

In the last few units of the Hab Lessons, we’ve talked about the Earth in detail. Right now though, let’s talk about the other major parts of Our Local Neighbourhood.