Earthquakes, Tsunamis and Volcanoes

I’m sure you’ve heard, ad nauseum, about the plate tectonics underlying the earthquake and tsunami in Japan. Indeed, it seems that plate tectonics, which produces earthquakes, volcanoes and tsunamis with devastating consequences is a force of destruction, pure and simple. But does it have a positive side as well?

The theory of plate tectonics, which at this point does the best job of explaining the earth’s geology, is based on the idea that the earth’s surface is made up of a number of semi-rigid plates which slide around over the earth’s surface. They interact primarily at their edges, where they may be pulling apart (as in the mid-Atlantic and the African rift valleys) sliding past each other (as in the San Andreas fault of California) or colliding.

Plates are made up of ocean crust, sometimes with relatively light continental crust on top. Ocean crust is dense enough to slide under other plates; the lighter continental rock above it resists being pulled under, and buckles or folds if it is on top of two colliding plates. Thus collisions of two plates with continents on top generally leads to mountain ranges such as the Himalayas.

Collisions between ocean plates and plates with light continental rock atop generally lead to subduction zones, such as the one off the west coast of South America, where the oceanic crust is pulled under the lighter continental crust. The sediments pulled down with the ocean crust are gradually heated and melted, reappearing as volcanic magma. Thus the volcanic spine of the Andes.

If two oceanic plates collide one is normally pulled under the other, but it is less obvious which will be subducted, and in fact this may change over time. The same melting of sediments occurs, and a line of volcanoes, such as the Aleutian Islands, normally develops next to the subduction zone.

Plates don’t slide past each other smoothly. They stick and then break loose, producing earthquakes. If they are just sliding past each other they may produce earthquakes but there is generally not much vertical movement. If one plate is being pulled under another, however, the sticking normally results in a bowing up of one plate, and when that sticking is released, there may be considerable vertical movement. If that movement is under water, a tsunami is created. This is what happened with the great Alaska earthquake, and has now happened off the coast of Japan.

But what would happen if the plates all just stopped? If there were no more plate tectonics? More, if there had never been any plate tectonics?

First, the earth would be flat and completely covered with water, if there were any water on the face of the earth. Mountains are constantly being eroded by the forces of weather. Given far less geologic time than has actually passed, any initial irregularities in the surface of the earth would have been smoothed out. Plate tectonics is and has been the main mountain builder on our planet.

Second, there is some question as to whether we would have an atmosphere. Certainly we’d have a hard time breathing the mixture of carbon dioxide, water vapor and other compounds put out by volcanoes, but then we’d have a hard time breathing the atmosphere prevailing when life evolved. Plants convert the gasses produced by volcanoes into an atmosphere we can breathe.

Third, plate tectonics is part of the way radioactive heating in the earth’s core is transferred to the surface. It’s one of the reasons we don’t have the radical resurfacing we think we see on Venus.

Plate tectonics can certainly produce devastation, but like weather, it’s something we have to live with. Japan has actually done a superb job of preparation, but there are prices we must pay for living on a dynamic planet, one which can support life. One of those prices has just come due.

Twelfth Day

On the twelfth day of Christmas, my true-love gave to me:
Twelve plates colliding,
Eleven vents erupting,
Ten glaciers surging,
Nine houses sinking,
Eight cars polluting,
Seven blizzards raging,
Six aurorae swirling,
Five solar flares.
Four chickadees,
Three mammoths,
Two ptarmigan
And a spruce hen in a spruce tree.

Momentum and Teleportation

Teleportation in Homecoming requires that energy, momentum, angular momentum and mass be conserved—all basic laws of physics. We’ll skip mass and angular momentum for right now, and just look at the situation where something is moving in a straight line.

Anything that is moving has both kinetic energy (energy of motion) and momentum, but the two are not the same. The difference is usually expressed mathematically: energy is half the mass times the square of the velocity and momentum is the mass times the vector velocity, but for many that just makes if more confusing. Let’s try this, instead. (If you don’t understand mass, think weight.)

Consider a car. Let it be a big, heavy car, say an SUV. Suppose it is coasting at a steady speed, say, 30 miles an hour to the west. Can a mosquito stop it by hitting the windshield? Not likely! The car’s resistance to having its steady motion changed is due to its momentum. This momentum has a direction—the direction the car is moving. Friction will slow it down, eventually, by transferring its momentum to the earth, but for the moment we’ll ignore that.

It also has kinetic energy. If the speed is doubled, the momentum will also double—but the kinetic energy will increase by a factor of four.

Remember momentum has a direction. Suppose we have another SUV moving 30 miles an hour to the east. Speed to the east and speed to the west cancel, so the momentum of the two-car system is zero. Their kinetic energy does not cancel, as can be seen if the two cars meet head-on—when the dust settles, they will be stationary at the point where they met. But the energy will have gone into crumpling metal (and whatever else makes up the cars) and ultimately into heat.

It is possible for two objects to bounce off of each other in such a way that energy, as well as momentum, is conserved. But if the momentum adds up to zero before the impact, it must also add up to zero after the impact. This is a common problem in billiards, though in this case the balls are most often moving at angles to each other so the vector sum of the momentum is not zero—but it will still be the same after the collision as it was before.

The conservation of momentum, in fact, nicely encapsulates Newton’s laws of motion.

Now consider Roi’s problem in teleporting to a very different location. He is moving with the planet under his feet. For illustration, let’s assume he is on the equator, at sea level, at sunrise, and wants to go to the opposite hemisphere, also on the equator at sea level, but at sunset.

Assuming he is on a planet like the Earth, he is moving toward the sun at around a thousand miles an hour, and the area he wants to teleport to is moving away from the sun at the same speed. No change in kinetic energy, but if he doesn’t do something about momentum, he’ll arrive moving about two thousand miles an hour relative to his surroundings—not a very survivable teleport!

My solution is strictly science fiction—I assume it is possible for a person (or a machine) to transfer or “swap” momentum from one mass to another. But they’d better remember to do it!

Winter Solstice

Sunset Dec 21 at Fairbanks, latitude 64 degrees 50 minutes. Photo taken about 2:40 pm, looking a little west of south.

Happy Southday! (Or, if you don’t follow time as measured on the planet Central, Happy Winter Solstice.) The days in the northern hemisphere are getting longer again!

Solstice has nothing to do with distance from the sun. In fact, we are rapidly approaching our closest approach to the sun, around January 3. But because the earth’s axis is tilted relative to its orbit around the sun, there are times (the solstices) when one pole or the other comes as close as it ever gets to pointing directly at the sun, while the other is as close as it can get to pointing away. That happened on Dec 21 this year with the north pole pointing as far as it could get away from the sun.

On the winter solstice, the sun never rises north of the Arctic circle, while it never sets south of the Antarctic circle. Closer to the equator it rises and sets, but the northern hemisphere days are at their shortest for the year, and the sun at noon is at its lowest in the sky. The low sun and short days combine to minimize the solar heating of the ground and water. The opposite is true in the southern hemisphere, where it is the first day of summer, and both day length and solar elevation are at their greatest for the year.

Our Earth’s axis of rotation is 23.5 degrees from axis of rotation of its orbit around the sun. What would happen if that angle were 0?

I actually invented such a planet, called Eversummer, for my second science fiction novel, Tourist Trap. It wasn’t exactly paradise!

The planet’s name, Marna thought, must have been picked out by a publicity agent.  Everspring would have been more accurate, or Everfall, or perhaps Constancy.  Maybe even Boredom.

The planet, with its rotational axis almost perpendicular to its orbital plane, had no seasons.  The poles were bitterly cold, glaciated wastelands where the sun forever rolled around the horizon.  The equatorial belt was an unchanging steam bath, the permanent home of daily tropical thunderstorms, varied by hurricanes along its poleward borders.  The desert belts, inevitable result of the conflict between the planet’s rotation and its unequal heating by its sun, were broad and sharply defined, with no transition zones where the rains came seasonally.  The temperate zones, between desert and polar ice, were swept year round by equinoctial storms, varied only by occasional droughts.  No monsoons, no seasonal blanket of snow to protect the dormant land, no regular alternation of wet and dry seasons.

Would you like to live on such a planet?

The Book Video is Here!

The video trailer for Homecoming is now up! It has its own page, but I’m putting it in the regular page stream, too.

Calendar (Homecoming Glossary)

CALENDAR: #scifi The Confederation calendar has slightly more than 364 days a year, with 4 of those days and whatever leap years are needed being considered outside the twelve 30-day months. “Day” and “year” are based on Central’s orbit and rotation rate, so the Confederation calendar does not quite match that of any other member planet of the Confederation. Most member planets have their own calendars, though year and day lengths are close enough to Central’s (and Earth’s) that matching is possible over a few fivedays’ time. The Confederation calendar is used to synchronize time across the Confederation as well as being the usual calendar on Central.

Fiveday (Homecoming Glossary)

FIVEDAY: #scifi Five days, and may be considered the Confederation equivalent of a week, though shorter. Six fivedays make up a month, which has nothing to do with a moon—Central, for instance, doesn’t have a moon.

Planet Building 10: #Sky Color

#scifi So far we have ignored one of the most important controls on the apparent color of a strange sun: the planetary atmosphere.  On the surface of a planet, we have to consider both absorption and scattering by molecules and particles in the atmosphere.  These processes strongly influence the apparent color of both sun and sky.  Since sky light falls on everything, while sun light falls only on non-shadowed areas. shadows will tend toward the sky color.  As a general rule, this is obvious only when the sun and sky colors are obviously different, as is true near sunrise and sunset on Earth, and is clearest on a light surface such as snow.  Shadows will not differ in color from illuminated areas unless the color of the sky differs from that of the sun.  So the color of shadows can be determined by sky color.

It is no coincidence that most common gasses are transparent in the visual wavelengths.  After all, our vision evolved to see through an atmosphere composed primarily of nitrogen, oxygen, argon, water vapor and carbon dioxide—eyes wouldn’t be much use in a band which was absorbed by the atmosphere.  There are a few colored gasses—Ozone is blue, for instance, while chlorine is green. But these gasses cannot explain sky colors which differ from those of the sun.  Consider ozone, which is frequently blamed for the blue of the sky.  This gas absorbs red wavelengths, allowing the blue through.  From the surface of a planet, it will absorb some of the red light from any light source above the ozone layer. But the light sources above the ozone layer are basically the sun and stars, both of which will appear bluer as a result.  If there is light scattered above the ozone, it will appear bluer also – but the effect of a colored gas is to color sky and sun by the same amount.  Given how readily our eyes see ambient light as white, we probably would not even notice the effect.

Furthermore, most colored gasses are extremely reactive, which makes them unlikely atmospheric constituents unless there is a constant source.  Ozone, for instance, is a part of the Earth’s atmosphere first because photosynthesis keeps producing free oxygen (O₂), and second because ultraviolet light acts on that diatomic oxygen to produce a layer of ozone.

Sky color is the result of scattering in the atmosphere.  If the scattering particles are large compared with the wavelength, such as dust particles or cloud droplets, the scattering process will be about the same across the visible spectrum, and the sky color will generally match the sun color if the sun is visible at all.  Even if the scattering particles absorb some wavelengths, such as red desert dust, the effect on sky color and star color will be quite similar – both will take on a reddish tinge.  The first photographs of the surface of Mars owe their sky color to this effect.

The sky color of Earth is due to scattering by the smallest available particles, the air molecules.  Very small particles scatter short wavelengths much more strongly than longer wavelengths, a process known as Rayleigh scattering.  Thus the shortest wavelengths produced by a star—the blue and violet—are scattered from the direct beam from the star and give the black sky an overlying blue radience while the star, because some of the blue has been removed, looks slightly reddish.  Because this is a true color contrast phenomenon, our eyes do not adjust to make everything look as if the illumination were white.   If the star has more blue and violet light (is more massive and hotter than the sun) there is more shortwave light to scatter, and the sky will be brighter and probably tend toward the violet in color.  A less massive, cool star, on the other hand, has very little blue or violet light to scatter, and will appear to shine in a relatively dark sky.  Such a sky could appear dark blue-green.  So even if the overall light appears “white” (in the sense that the light of an incandescent light bulb is white), sunlit areas will appear reddish and shadows green to blue.

Is a visually green sun possible?  Yes, but probably not as a common phenomenon on any planet.  Blue or green suns (and moons) are observed on rare occasions on Earth, when there are particles in the air which are uniformly of exactly the right size.  Far-traveled forest fire smoke, volcanic eruptions, and very fine desert dust have been known to produce blue or green suns in a orangy sky.  The process is called Mie scattering, and if the particles are just the right size they may scatter red light more than they scatter blue.  Thus the blue or green wavelengths from the sun reach our eyes directly, while the red and yellow wavelengths are scattered away, to reappear as sky color.  The needed particles, however, are of such a size that they do eventually fall out of the atmosphere.  Furthermore, there are not many processes that produce scattering particles all of exactly the same size.

Hmm – maybe some kind of life form that produced huge masses of spores of exactly the right size?  That could produce a green sun—but only during the season of spore production.

Next week the planet building series will end with a discussion of moons and their phases.

Author’s note: the entire planet building series, in order, will be put up on my author website, http://www.sueannbowling.com .

What Would an Alien Sun Look Like?

#scifi Now that we’ve discussed the role of water in affecting albedo, as a greenhouse gas and in moving energy around a planet, let’s go back to that alien planet we’re building. What will its sun look like?

In the first place, we do not need to consider all stars, or even all main-sequence stars. Massive stars have rather short lifetimes. Our sun has a projected lifetime of some 10 billion years. A star three times its mass would stay on the main sequence only half a billion years. Further, such massive stars are much rarer than sun-sized stars, and put out a very large fraction of their energy in ultraviolet (dangerous) wavelengths.

 

Apparent size of a star in the sky of one of its planets as a function of its color. The sun has a size of 1.

 

 

(In fact, named stars are almost all poor candidates for having livable planets. If they are named, they are relatively bright. Almost all bright stars are either very massive or are in the later, helium-burning stage of their existence.)

What about smaller stars? Lifetime is not a problem, nor is finding one—less massive stars are far more numerous than those of sun size. They are harder to find in the night sky because unless they are extremely close they are too dim to see. However, a planet would have to be quite close to a very small star to keep warm, and smaller stars are dangerous at that distance.

If we want an earth-like planet, with temperatures that allow water in liquid, solid and gaseous phases, we could take as a first approximation that the energy received from the star at the planet’s distance would be equal to that received by the earth from the sun—the solar constant. Let’s assume this in determining how the star’s color will affect its apparent size in the sky and the year length of the planet.

 

The length of a planet's year, in earth-years, as a function of the color of its sun.

 

First, we need to define color. Any star will look white, because our eyes automatically adjust to the color of any light containing a continuum of wavelengths in the visible part of the spectrum. But different stars have their light peaking in different parts of the spectrum. Our sun, for instance, peaks at a wavelength of about .5, which is green. Red is .7 and blue-violet is about .4, so the left side of each chart is ultraviolet and the right side is red—but all of the stars will look nearly white.

The difference is in the non-visible light. A star toward the left—blue—end of the chart will be putting out a lot of energy in the ultraviolet. We don’t see that part of the spectrum, though some other organisms can see in the near ultraviolet. But aside from a small amount needed to make vitamin D, ultraviolet is generally not good for living things.

Stars at the right—red—end of the spectrum again will look white to our eyes—they are in fact less red than an incandescent light bulb. But they will put out much more infrared radiation, and less ultraviolet. They would feel warmer than they looked, but human beings living under such a sun might well need to take supplemental vitamin D, and would probably evolve toward fair skin, just as Europeans have. Plant growth might also be slow.

If we assume a solar constant matching Earth’s, we can predict the apparent size of the sun in the sky and the length of the planetary year as functions of the star’s color. The charts show this, with the apparent size being scaled to that of the sun and the year lengths being scaled to ours. From this it is apparent that a redder star than our sun will appear larger in the sky and be associated with a shorter year, while a bluer star will appear smaller in the sky and the planet will have a longer year.

Next time: the sky color and the effect of the atmosphere on how things look.

Planet Building 8: Weather

We’ve talked about the need to transfer energy from the surface of a planet to high in the atmosphere, where greenhouse gasses can radiate it away. But solar energy is not absorbed uniformly over the face of a round planet, or uniformly in time if that planet is rotating. This means energy must also be transferred horizontally, and water plays an important role here, as well.

Since we’re close to the equinox, let’s look first at how incoming energy is distributed then. The sun at this time of year is almost directly over the equator, and on the horizon at the poles. This means that the incoming energy is at a maximum—in both time and space—at the equator, and zero at the poles.  Unless the poles are at absolute zero (- 460 degrees F) energy must be transferred from the equator to the poles.

Remember that liquid water can store an enormous amount of energy as heat, and about half of that transfer is actually carried out by the ocean currents. When you add the rotation of the planet, we have currents like the Gulf Stream and winds in temperate latitudes generally blowing from west to east. The result is that west coasts are generally warmer than east coasts—southern France, for instance, is at the same latitude as Maine. This will carry over to any planet with oceans, and because oceans are so good at storing energy, it is true at any time of year.

The atmosphere also carries energy from the equator to the poles. In fact, this transfer of energy is what drives the whole weather system. The eddies we call cyclones and anticyclones, the lows and highs on the weather map, carry energy from the regions of surplus near the equator to the regions that radiate energy away near the poles.

Some of this energy is carried by warm air moving poleward while cold air moves equatorward.  But a large part is carried by latent heat, just as in pure vertical transport. As the warm, moist air moves poleward, it tends to ride over colder air coming equatorward, and as it is lifted and cools, water condenses. Thus the latent heat added by evaporation over the tropical oceans is released to the air at much higher latitudes.

What about other times of year, such as the solstices?

The solstices are defined as the dates when one or the other of the planet’s poles are pointed most nearly directly at the sun. This affects day length as well as the angle at which the sun falls on the surface, with the surprising result that for the earth’s axial tilt (23 degrees 27 minutes) the maximum incoming energy at the top of the atmosphere is at the summer pole, and there is very little variation of incoming energy with latitude in the summer hemisphere.

Ice at the Earth’s poles keeps surface temperature low and the low angle of the sun means more energy is absorbed in the atmosphere, but high midsummer temperatures at the pole would be quite possible on a world without water. If the tilt were increased even farther, to around 70 degrees, the annual average incoming radiation at the poles would actually exceed that at the equator.

Another important way in which water affects temperature is its large capacity for storing heat, which means that in summer water is generally cooler than land. This leads to coastal climates being cooler in summer than those inland. It also leads to the summer monsoons, when hot air rising over the continents sucks in the cooler, moister air from the nearby oceans.

This is reversed in the winter hemisphere, where the water is usually warmer than land. Cold air flows out from the continents, often triggering storms when it moves over the warm, moist oceans.

Because the energy input near the equator is only a little less than during the equinoxes, while that of latitudes above the polar circles is zero, a great deal of energy must be transferred from the equator to the pole of the winter hemisphere. Thus the violence of winter storms, as energy is transferred both upward and toward the winter pole.

Finally, the daily cycle is hardly noticeable in water temperature, but it has a large effect on land temperature. The result is local winds from colder to warmer temperature near the surface—the sea breeze in the daytime and the land breeze at night. In some cases these moist, cool winds from the sea trigger thunderstorms when they move over the warm land.

All of these types of weather will be found on any rotating planet with oceans of liquid water. Don’t think they are “earth-only” effects in building a planet.