Planetary Profile: Eccentric Earth

Image

I’ve been on a planet kick recently, doing profile posts for Mercury and Venus (mostly on the ways they’re weird), and have decided to just keep going. That means Earth is next. And it’s actually an interesting exercise to think about Earth no differently than the way one thinks about the other planets in the solar system. Earth is, after all, our everyday surroundings and familiarity breeds…well, not contempt in this case, but perhaps indifference?

But Earth is a planet and can viewed through the same lens as we would use for Mars or Neptune. So let’s take an astronomer’s view of our home and get to know Earth the planet—particularly all the ways it’s weird! And you know what? I’m going to ignore the fact that it has a biosphere. I think we all know that makes it bizarre in our solar system. Let’s focus on the other things. 

The Basics

When it comes to vital statistics, admittedly Earth isn’t terribly weird. Its orbit is pretty round, but not so round as Venus’s or Neptune’s. Its axial tilt is very similar to Mars’s and Saturn’s and not too far off from Neptune’s. There are a few titles that Earth can claim, such as largest rocky world and densest planet of the solar system, but it gets those by the skin of its planetary teeth (Venus is just a smidge smaller than Earth and Mercury is right on its heels for density).

Its reflectiveness (albedo, in science speak) is pretty middle of the pack as well. Venus, Jupiter, and Saturn are brighter, Mercury and Mars are dimmer, Uranus and Neptune are very similar to Earth. Even Earth’s spin time isn’t particularly remarkable, being very similar to the spin time of Mars. Or at least it is now…and that’s due to something that is unique to Earth.

Luna-cy

Image

Earth has a moon. That’s obviously not unique, but Earth’s relationship with its moon has singular aspects when compared to other worlds of the solar system.

First of all, Earth has a big moon, fifth biggest in the solar system. The moons that are bigger than ours orbit Saturn or Jupiter, planets that dramatically dwarf Earth in terms of mass. That means that Earth has the largest moon of any planet compared to its own size. 

Because of this, the Moon makes Earth move more than any other moon does to any other planet. When we say one thing orbits another, what we actually mean is that they gravitationally pull on each other and are both orbiting their common center of mass, known as their barycenter. When a small thing orbits a very big thing, like Earth orbiting the Sun, the barycenter is well inside the bigger object, and you can essentially just say the smaller object is going around the bigger one.

The closer in mass two objects are, the closer the barycenter comes to being outside the bigger object, and the more the bigger object moves as a result (it is possible to have the barycenter be completely outside the larger object—for instance, Pluto’s largest moon Charon is big enough compared to Pluto that, technically, Charon isn’t orbiting Pluto. Rather they’re both orbiting a point in space between them where their barycenter is).

Earth has enough more mass than the Moon that their barycenter is still inside the Earth, but not by a lot. It lies about a thousand miles below Earth’s surface, which sounds like a big distance but the barycenter sits about 75% of Earth’s radius away from the core. This means that Earth is wobbling a whole lot thanks to the pull of the Moon.

Image

And that pull is also causing the distortions on Earth’s surface that we refer to as ocean tides. One effect of having a big moon so close to us is that its gravity actually pulls differently on different parts of the Earth. The Moon pulls hardest on the side of Earth closest to it, slightly less hard on the center of the Earth, and even less hard on the side of Earth farthest from it.

The effect of this from the perspective of someone standing on the Earth is that Earth wants to bulge outward at two points: one very close to under the Moon and one on the exact opposite side. The crust is rigid enough that it’s not going to actually indulge in this wish to bulge—but the oceans are not so rigid. They bulge. Those two bulges in the ocean represent the daily high tides. As Earth rotates, it also spins under those bulges. From an Earthling’s perspective, it looks like the tides are moving around the planet (it should be noted that the Sun has a role in the tides, but it is much smaller than that of the Moon).

This pull of the Moon on the oceans is also having another effect on Earth that is, if not technically unique amongst the planets, then far more pronounced here than anywhere else: it’s slowing Earth down. Remember above when I said that Earth’s day length is pretty close the same as Mars’s now? It wasn’t always.

Essentially the drag of the tides on Earth is causing Earth’s rotation to slow and its day to get longer. It’s not a rapid process—we gain about 2 milliseconds every century, but it does mean that if you had been an early amphibian tooling around during the Mississippian Period around 340 million years ago, your day would only have been 22 hours long. You can thank the Moon and the oceans for literally adding extra hours to your day.

Liquidation

Image

Speaking of those oceans, that’s another pretty obvious way in which Earth differs from almost every other world in our solar system. There’s a whole lotta liquid on its surface. It is, again, not unique in this thanks to Titan’s methane lakes and seas, and if you could go back far enough in the solar system’s history all our research seems to suggest that there were at least one, and quite possibly two other planets with oceans (Mars and maybe Venus). And several outer solar system moons seem to have subsurface oceans.

But if you want to find surfaces that have liquid cycles these days, your options are Earth and Titan (and oceans of methane just don’t seem quite as appealing as water oceans). That liquid cycle plays a huge role in the shaping and changing of Earth’s surface. Earth undergoes erosion at an incredibly fast rate compared to most other rocky worlds in our solar system, a large part of which is driven by the planet’s water cycle.

This, of course, is helpful if you want to do something like maintain a biosphere, because it helps cycle nutrients through Earth’s systems, but it does make trying to figure out Earth’s geologic history difficult. It also does not help that Earth completely recycles its crust over time either.

Talkin’ Tectonics

Image

Earth is what we call an active lid planet. This means its surface is broken up into plates that are constantly, if slowly, moving towards, away, and against each other, also known as plate tectonics. Earth is the only world in the solar system today which can claim such status.

That’s not to say it’s the only active world—Venus and Io are notoriously volcanic, for instance. But while they may bury their crust with new volcanic layers, but the old crust is still there. If an astronaut geologist could get there and start digging, they’d be able to uncover many of the old layers (they will get subsumed into the mantle eventually).

But most of Earth’s old crust is long gone, pulled back under the surface as plates get subducted under one another and melted down to be recycled and eventually exuded as new crust. There are bits of continental crust that can be measured in billions of years old, but no ocean crust older than 200 million years.

Again this is helpful for a biosphere, because it also helps cycle minerals through Earth’s outer layers, but it is a serious drag for geologists who want those sweet old rocks (we wanted old solar system rocks so much we eventually went to the Moon for them).

Our Pale Blue Dot

Even without reckoning with the notorious infestation of biology cluttering up Earth’s surface, it’s a singular world. This king of the inner worlds struts its stuff, dancing with its massive moon, flashing its luxuriant oceans, and rumbling away with its shattered surface. 

But of course all of that is part of what makes it so suitable for life. If Earth wasn’t the unique planet that it is (in our own solar system at least—the jury remains out on how frequently worlds like Earth can be found elsewhere), it couldn’t have become our home because we would never have evolved. And it is home, the only one we have.

It’s our pale blue dot and we love it, foibles and all!

Image