Podcast: Play in new window | Download
Subscribe: RSS
Another week, another planet. This time we talk about our own home world: Earth. You might think you know the planet beneath your feet, but it’s actually one of the most interesting and dynamic places in the Solar System. Learn about our planet’s formation, weather, its changing climate, and life.
|
|
Transcript:
Dr. Pamela Gay: Hey Fraser, how’s it going?
Fraser: Good. I’ll bet you’re a little tired. We’re down in Seattle attending the Penny Arcade. This is where I’m recording right now. I’m a little sleepy.
Pamela: But you’re having a whole lot of fun.
Fraser: Absolutely. It’s a great convention and got a chance to meet Will Weaton so, if he’s listening, hi Wil. Next week you’re going to be at Dragon*Con.
Pamela: I’m going to Dragon*Con and hey Wil, if you’re there, I’d love to meet you too. [Laughter]
Fraser: But for all of our listeners, anyone who is interested Pamela is going to be at Dragon*Con next week and we’ll put links in the show notes. We’re going to be doing a live episode of Astronomy Cast. I won’t be there but Pamela will be there and we’ll have a special guest to do the episode with her.
If you’re going to be in the neighborhood, if you’re going to attend the conference, come by the Podcast area and meet Pamela.
Okay, let’s continue on then, another week; another planet. Last week we covered Venus so that means it’s our home planet’s turn. Let’s talk about Earth. All right Pamela, talk about Earth.
Pamela: Well, Earth is perhaps one of the geologically most interesting planets in our Solar System. There are a few moons that are out there contending for being most interesting geological object in general. As far as the planets go, Earth has the most going on.
We have plate tectonics. We have an active magnetic field. We’re not 100% stable in our orbit so all the little irregularities that cause twisting and turning, cause our atmosphere and our weather patterns to constantly be evolving and we have cycle within cycle within cycle of change constantly going on. And, we have life.
Fraser: Fortunately we live on the Earth so we are at good proximity to study it. So let’s start then with the formation. Once again, do you want to tell the story of our planet’s start?
Pamela: Well our planet like pretty much all the other planets in the solar system started out as a whole bunch of dust grains orbiting the sun while the sun was in the process of forming. These dust grains collided and merged and merged until we eventually got a nice big blob of stuff.
That blob of stuff was hanging out doing its best job to cool and solidify, cool and solidify and form a planet, when along came something Marsish in size and knocked the tar out of it.
In the process a lot of the light stuff that had been on the surface of that forming blob of planet Earth, got knocked into outer space. It re-solidified to form the moon and the heavy stuff got left behind to stay the planet Earth.
Fraser: We actually did a whole episode on the formation of the moon. So, although Pamela just went right past that really quickly you can hear the whole story that we will link from the show notes.
Okay so we have got the Earth as a blob and the moon as another blob of lighter elements. What next?
Pamela: So these blobs were really hot. They started off just not the type of place you would find liquid water. The moon stayed someplace where we still don’t have liquid water on the surface of the moon. The moon is too small to have an atmosphere so any liquid that is on its surface just instantly goes straight to vapor and escapes.
There’s ice on the moon, but that ice does not actually come from the stuff the moon formed out of. It comes from comets hitting the moon. And on Earth, all of the water on the planet Earth we think also comes from comets hitting the planet.
We start off with a hot lava blob that is mostly heavy stuff. As it cools, we enter a solar system period called the great bombardment. The planet gets totally blasted with comets. Comets hit the planet, melt and create the oceans.
The continents basically rose out the oceans. We had all sorts of volcanoes, and they burbled stuff to the surface. Because we have so much liquid, our planet’s plates (places where large amount of crustal material came to the surface) are able to slide around. The water actually works as a lubricant to help plate tectonics. Plate tectonics are a good thing.
Our planet is still working on cooling off from its original formation. Part of that cooling off means heat has to escape. It’s like when you have a pot of boiling spaghetti noodles with a lid on it. The lid will rattle and move around as blobs of steam try to come out of the surface.
Now if you locked that lid on, if you made your pot into a pressure cooker, it could actually explode from the pressure built up inside. That is kind of what Venus does when it resurfaces its entire surface in one foul volcanism swoop.
Here on Earth we have constant slow escape of the heat inside the planet. Some of that heat is not just coming from the planet still cooling off; it is actually being generated from nuclear materials inside our planet decaying. Our planet is actually a wee bit radioactive and any of you who live near granite quarries know that because you have radon building up in your basement from the decay of radioactive materials in granite.
That constant decay is also heating our planet, also escaping through the surface. The places where a lot of this heat is escaping, this forms the Pacific ring of fire where we have volcanoes in Hawaii, we have the Indonesian islands that get unfortunately hit with earthquakes on a regular basis as the planet shifts and moves.
We have one crustal layer plunging underneath another one near Peru, which recently caused a large earthquake. We have parts of Ethiopia actually tearing itself apart as our planet is constantly rearranging its surface.
Fraser: Is the plate tectonics we know today what’s been happening across the Earth for a long time or are we in a less active phase then what used to be? What does the future hold? How will things play out over time?
Pamela: As our planet cools, the plate tectonics is getting slower and slower. There is less heat that needs to escape so there is less pressure building up underneath the surface moving everything around.
Other things are less dramatic than they used to be. At different points in the Earth’s geologic history all the crust has been lumped up in one place. This actually causes the mass distribution of the Earth to be slightly different. You have all of the planet’s water on one side and all of the planet’s earth on the other side.
This can actually cause our planet to change how it is oriented relative to its north and south rotational poles. The north and south pole will stay roughly in the same alignment relative to the Sun, but the planet relative to that axis can actually pivot dramatically so that the big mass parts are along the equator and the low mass parts are near the poles. This is a more stable way for it to rotate.
Fraser: How much in the past has it changed?
Pamela: It is hard to go back and figure out exactly how much it has changed but we know that there have been some dramatic alignments. These are the type of alignments that if they occurred today would take Alaska and put them on the Equator. These rearrangements are documented in the magnetic rock. As rock comes up through lava vents (basically comes to the surface) it has things that can hold on to magnetic fields within it.
As that material cools the magnetic bits in the rock will align themselves relative to the Earth’s magnetic field. The magnetic field’s orientation is based somewhat on the rotation of the planet. What’s happening here is, we have (with the planet Earth) a solid core, it sits there. Around that solid core is molten iron. As that molten iron rotates, it has charged particles within it. Those rotating charges create a magnetic field.
As long as that rotation is going in roughly the same direction, the magnetic field stays roughly oriented the same. It tends to sloosh and move around a bit, but while its poles may switch, it generally stays oriented roughly along the rotation of the planet. North and South may switch but the direction of the poles stays roughly related to how the planet is rotating. You don’t end up with North coming out through equatorial Africa in general.
Fraser: But there are processes that do change the direction of the poles.
Pamela: There are processes that change the direction of the poles. Those processes generally keep the poles and the rotation axes of the planet roughly aligned with one another. Ninety-degree angles do not generally happen.
But when we are going through the sedimentary rock we occasionally find these places where the rock is just oriented wrong. We think that this is related to the planet actually slooshing. This is called true polar wander.
This happens when you end up with the planet’s water and mat crust oriented in strange ways and the planet has to realign itself so that its mass is distributed in a way that allows it to rotate stably. It is a really weird phenomenon.
Fraser: Can we talk a bit about periods of heating and cooling? I know that part of the polar tilt has a role to play in that as well.
Pamela: In addition to this really weird true polar wander that is a very rare and non-periodic occurrence, we do have these constant periodic changes going on. This is called Milankovitch cycles. The Milankovitch cycles are a bunch of different cycles layered on top of one another.
They cause these long-term periodicities in our climate. You have effects due to procession. This is how the Earth’s north and south poles stay at the same angle relative to our orbit around the sun but where that angle is pointed changes. So, our North Star doesn’t always stay our North Star.
Sometimes you actually end up with a southern polar star, which we don’t currently have. Depending on how our poles are aligned, you end up with different extremes in the seasons. Currently during the northern hemisphere winter, we are actually closest to the sun, so when the North Pole is pointed toward the sun the planet is also closest to the sun.
When we are closest to the sun we get about 6.8 percent more light than we do when we are furthest from the sun. Because of this, the northern winters aren’t as extreme as the southern winters. We can also at different points in the Earth’s procession have the South Pole pointed at the sun when we are closest to the sun.
We can end up with the poles not pointed at the sun, just at a wide angle when we are closest to the sun. All of this affects the extremes of the seasons. We also have affects that are caused by the eccentricity of the orbit. Jupiter, Saturn and all the other planets are constantly pulling on the planet Earth.
This causes the actual shape of our Earth to gradually change. Sometimes the Earth is going to be further from the sun; sometimes the Earth is going to be nearer to the sun, and how much nearer and how much further changes over time. Currently the difference between when we are closest and furthest to the sun is 6.8 percent more energy when we are closest. Sometimes we can get to the point that it is more than 20 percent more light when we are closest to the sun.
This also affects the seasons, making the seasons more extreme. Our sun itself is slowly changing. All of these different things are piling up on top of one another. Our axial tilt is also slowly changing. While we are more or less about 24 degrees, our axial tilt wanders from about 21.5 to 24.5 degrees. We are currently 23.44 degrees.
All of these different things pile up on top of one another to affect the seasons. This actually shows up in long-term understanding of climate where we take ice cores from glacial areas and we study; “How has the planet’s temperature varied over hundreds of thousands of years?â€
Fraser: We are going to throw all of that data into chaos this century with our human made global warming. [Laughter] But, that’s completely different.
Pamela: True. Unfortunately as we look at all these cycles we do see long-term cooling and heating trends and we are in what is supposed to be a normal heating trend, except we sort of shot off what is normally predicted.
It is kind of like if you are used to coasting your car down a hill and you know if you are going at 40 miles per hour at the top of the hill as you start going down the hill you just throw it into neutral and just let it go and you will be going 50 or 60 at the bottom of the hill. Imagine trying that experiment one day and all of a sudden you are going 300 miles per hour when you hit the bottom of the hill.
That’s just not normal and currently we are in one of these “the car is really accelerating far more than it should be†heating phases and we are trying to fully uncouple all of the different effects of our atmosphere.
Fraser: All right so we have talked about the ground itself, let’s talk a little bit about the weather. So, what are the processes that get our weather whipping up?
Pamela: Well, our weather comes from a lot of different things. It comes from the fact that as our planet rotates different sides of the planet are differently heated. Anytime you have one side cold, one side hot, you are going to end up with airflows.
We also have one side of the planet pointed more to the sun than the other, so you end up with day in one hemisphere is warmer than day in the other hemisphere. This ends up with different airflows. Then the planet is just rotating relative to the atmosphere. All of these different rotations cause the atmosphere, which is basically fluid, to end up convecting.
We also have oceans that hold onto the heat and transfer it themselves. There are mid-ocean conveyor belts that are driven by both changes in salinity and changes in temperature in different parts of the ocean. So you end up with cold air in the northern hemispheres oceans sinking down to the bottom, flowing down towards the equatorial regions where they heat up, rise to the surface. Then this warm water goes across the surface of the oceans back up to the northern extremes.
So, you have this constant conveyor belt of hot and cold water. As the cold water comes down to the mid latitudes it cools off the oceans and this helps temper our hurricanes. The warmer the water is, the more powerful the hurricanes can become. Then we also have this warm water going north and warm water actually helps warm the air allowing moderate climates to occur in nations like England.
If this conveyor belt shuts off, which it can do if you dump too much fresh water into the northern oceans, then you end up with extremes. You end up with the equatorial oceans getting hotter and hotter and hotter and that heat isn’t getting conveyed up to areas around Europe and Japan. So, those climates start to get a lot colder and you can end up with much more extreme conditions here on the planet Earth.
Fraser: Now we talk about with all of the planets that we have gone through so far (and I am sure when we get to Mars we will talk about this as well) and it’s about the search for life. Well, I think we found life here. I’m pretty sure.
Pamela: I think so. Not always sure. [Laughter]
Fraser: Okay. So, how do people think that got started?
Pamela: Well our planet is about 4.6 billion years old. We think that roughly 4 billion years ago, small single celled organisms started to pop out on our planet. From these single cells over time, things slowly got more and more complicated. We are still trying to sort out all the different states. How did this evolution take place? It is not something that we can resurrect in a lab. It is not something we can repeat and try over.
As near as we can tell, about 4 billion years ago single celled organisms began to exist. Originally there were probably things called methanogens that didn’t work the way life that we are used to worked. They didn’t require oxygen, in fact oxygen killed them, and they produced methane that probably helped warm our planet.
Methane is a greenhouse gas. This is why we are currently so concerned about things like moose in the Scandinavian nations that are giving off methane through processes best not talked about in an astronomy show.
So these methanogens gave off methane, helped keep the planet warm and then at some point they evolved. We ended up with new types of cells that instead used and produced oxygen. Our atmosphere radically changed.
We think about this same time, probably about 3 billion years ago, cells developed a way to use photosynthesis to better utilize energy from the sun. They used this photosynthesis to trap carbon and release oxygen into the atmosphere.
This killed off all methanogens and started creating an atmosphere that would be conducive to the type of life that exists now.
Fraser: I think it is amazing that the oxygen that we need to breathe today started out as a harmful by-product of that class of bacteria. [Laughter] It was a toxin.
Pamela: You know sometimes what is bad for one thing is really good for another thing. We have all had our own evil thoughts at different points. If only bad thing fu would happen then it would be really good for us.
Well in the case of life the processes that killed off all of the happily existing methanogens opened the door for different forms of bacteria and eukaryotic cells that were able to build up into the animal, plants and fungal cells that we have in all of our modern life forms.
Fraser: And the bacteria has spread to every corner of the Earth, not to mention us here up on the surface, but there is lots and lots of life just underground, bottom of the oceans, its everywhere.
Pamela: It is everywhere. Some of the cool things that we are discovering are just the ways that life has figured out to protect itself under hostile conditions. If you wander through the desert after a rainstorm, every little puddle you find is going to be teaming with life.
When these puddles dry up, where does that life go? It turns out that life stays right there but it’s able to encapsulate itself to protect itself to basically find ways to completely shut down and go dormant in some cases for hundreds of years waiting for the next puddle to form that it can live in and thrive in and reproduce in. So, no matter how hostile our planet gets, different forms of life find ways to survive within that hostility.
Fraser: I think these discoveries in the last few decades have made me feel a lot better about global catastrophes or even human made catastrophes. Like when another big asteroid hits the Earth and causes a mega extinction event, there are whole ecosystems that are completely disconnected from the sun and if it is cloudy for a hundred years they don’t care. [Laughter]
I think that is actually quite amazing that before we thought everything was so fragile, but in fact the Earth is quite durable and life has hit every niche and has many places that don’t rely on the sun for energy. So we less have to worry necessarily that life on Earth is going to survive and more just about ourselves.
Pamela: Yes, multi-cellular life, which has only been around for basically the past billion years, is fairly fragile. We die fairly easily.
Everyone has witnessed the death of an earthworm on a sunny day. We are fragile. The bacteria are here to stay. The microorganisms are here to stay. It’s the bigger things.
Animals as we know them have only been around for 600 million years. Things as simple as a fish have only been around for 500 million years. Insects started 400 million years ago on our 4.6 billion year old planet. Mammals are only 200 million years old and humans are only 2 million years old.
We are fragile, it took a long time to get to us, but I have faith those microbes; as long as there is a planet Earth, there is going to be some single celled organism living on it.
Fraser: So what does the future hold for our planet?
Pamela: Well, it depends on global warming. We live in an interesting time where the future of our planet is being defined today. But setting global warming aside, assuming that our atmosphere is able to return to the normal cycles of the Milankovitch cycle that it had gone through for the past billions of years, our planet should be okay for another 50 million years or so.
We are actually in the twilight of our planet. In the next tens of millions of years our sun is slowly going to warm and as it slowly warms it is going to slowly warm our planet. Eventually it is going to reach the point that our oceans start to boil and evaporate. When this happens we are going to have two different problems.
The first problem is it is too hot for life like us to exist. Once the oceans start evaporating, that water in the atmosphere is going to increase the heating of the planet. Water vapor is just another greenhouse gas.
Once our planet starts baking, the water is going to bake out of the Earth itself and we are going to have a problem a lot like Venus where the plate tectonics no longer has a lubricant to allow the plates to gradually shift. So the heat starts building up inside the planet as well where you can start getting catastrophic volcanism going on.
All in all, life is not going to be happy in about 50 million years. The sun is going to stay at a safe distance for another 5 billion years or so, but in about 5 billion years our sun itself is going to start going through changes in how it generates energy. This is going to affect the radius of the sun.
It could come out and get dangerously close and blast the planet Earth with amounts of radiation that are just going to zot away whatever is left of our atmosphere. So much radiation, so much light from the sun is going to blast our planet that our atmosphere is going to be toast.
It is not a good future, but we have had a good 4.6 billion year run.
Fraser: That’s true. That’s true. I think then, from what I understand, we are just far enough away that we probably won’t actually be consumed by the sun; so then the Earth will just rotate the burned out white dwarf for hundreds of billions of years.
Pamela: Exactly. Our planet isn’t likely to get destroyed, at least through any of the processes we currently understand. It is just life that is going to be destroyed.
Fraser: But I wonder though if our planet is going to continue to cool down it will still be a source of heat so all of the underground microbes will probably still be perfectly happy.
Pamela: Exactly. Exactly.
Fraser: Yeah that’s funny. All right, well that was great. So now we have covered the Earth, so next week we will proceed with the planetary excursion and we will do Mars, which I think is going to be a monster show because there is a lot to talk about there.
This transcript is not an exact match to the audio file. It has been edited for clarity. Transcription and editing by Cindy Leonard.