North Pole Weather, April 11

Sunrise at 6:31 am, sunset at 9:15 pm for 14 hours and 44 minutes of daylight. We’re now gaining about 6 min 50 sec a day, and we’ve lost astronomical night–the sun is less than 18 degrees below the horizon at solar midnight. By the end of the month we’ll have lost nautical night as well–the sun will never dip more than 12 degrees below the horizon. The snow is melting internally, though the water is still mostly refreezing within the snow pack. Visually, it’s only settling–but the snow stake now shows only 16″ on the ground.

Northwest corner of roof

Icicles  are beginning to form on the north corners of the roof.

Where snow has been plowed up and has dirt incorporated near the surface, it’s starting to look grimy. The paved roads are now pretty much ice free, except where snow melt runs over them in the daytime and then freezes at night. The highway department does a pretty good job of keeping the snow berms on main roads plowed back so they are below the level of the pavement, but minor roads and graveled roads are still a problem.  Snow on the gravel road I live on has barely started to melt.

End of my driveway, snowing dirt coming to the surface as the piled snow melts.

Next week? Based on climate history, the average daily temperature should now be above freezing. The forecast is less hopeful–daily highs in the low 30’s (if that high) until the weekend, when we might get some highs near 40 and lows around 20. The snow should continue to settle and the dirty snow will become more prominent, but I don’t expect major runoff until it warms up. No snow in the forecast, and this time of year it would only delay melting.

Since I don’t ski, it’s good weather to do the final editing on Tourist Trap!

Mass into Energy

“If you can Heal, you may eventually be able to exchange mass and energy if you conserve hadrons,” Derik told Roi. (from Homecoming.)

Exchange mass and energy? And what are hadrons?

Einstein’s famous equation—E=mc²—says that a very small amount of mass can be turned into a very large amount of energy or vice versa. If you could turn a gram of mass (that’s about the mass of a small gourmet jellybean) into energy, you’d get roughly 25 million kilowatt-hours. In practice, you have to conserve hadrons—essentially the total number of neutrons and protons in the cores of atoms, which make up most of their weight.

Thus it is possible, with enough heat and pressure, to force four hydrogen atoms (each with one proton and one electron) together to form one helium atom (with two protons, two neutrons and two electrons.) The four hydrogen atoms turn out to have about 1% more mass than the helium atom, and this extra mass, the binding energy, reappears as gamma rays and particles. This is the energy of the hydrogen bomb, and also the energy that powers the sun and most of the stars. The process is called fusion (coming together), but as of yet we cannot control it. (I don’t think you can call a hydrogen bomb controlled.)

A second way in which mass can be transformed into energy involves very heavy elements, such as uranium. It turns out that all of the protons and neutrons packed together in these elements aren’t really happy. They may split apart on their own, or because they are hit by some other particle, but the net effect is that the mass of the parts they split into is less than that of the original atom, so the split produces energy. This is nuclear fission (splitting.) This is the energy of the original atomic bomb, but it is also the energy that drives plate tectonics and volcanoes, and is occasionally tapped for geothermal energy. This one we can control to some extent, and it is the energy source for nuclear power plants—but those large, unstable atoms are rare in nature.

The middle-weight elements—such as iron—are stable. There is no way to extract binding energy from iron—it is sometimes called the nuclear ash of stars. (Note that many of the elements of all masses have isotopes in which an imbalance between neutrons and protons leads to instability, but these are very rare in nature. They are very important in man-made nuclear waste, however.)

Most of the energy we use today comes from sunlight past or present—fusion energy. Solar energy intercepted by the earth today fuels not only what is called solar energy but also wind power, hydropower, and biofuels. These are all “renewable” sources—we can use them at the rate present-day sunlight generates them. But they are limited by the amount of sunlight available.

Fossil fuels are storage for the sunlight of the past—and we are using them up far faster than they can be replenished. Only a tiny fraction of the sunlight falling on the surface of the earth is actually captured as biomass, and only a tiny fraction of that biomass is buried and eventually becomes coal, oil or natural gas. But ultimately, it is all nuclear energy.

So yes, mass can be turned into energy, and in fact almost all of the energy we use actually comes from such conversion. But there are limits.

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.

Planet Building 11: Suns and moons all over the place

#scifi Many planets have satellites, generally called moons when they are visible in the planet’s sky. These moons, like most other aspects of a planet, are subject to physical laws. Their apparent color, size, phase, periodicity and the strength of the tides they cause are all linked together.

To start with, let’s assume a planet with Earth-like gravity (which means Earth-like mass and size if it’s rocky) and a moon large enough that self-gravity has pulled it into a sphere. What will that moon (or moons) look like?

First, color. A moon shines by reflected sunlight. We’ve already discussed the fact that the light from just about any star will look white if there is no other color to compare it with, and the same will be true of moonlight. The color of the moon’s surface may have a slight effect, but only if it contrasts sharply with another moon in the sky at the same time. But what kind of contrast could we expect?

Water and free oxygen are very unlikely on the surface of a moon-sized object circling a planet that can be lived on by human beings, so the red iron oxides and blue water of Earth are out. Copper-containing rocks may be blue or green, but given the elemental abundance of copper in the universe it is unlikely that they would be present in such quantities as to color the entire surface. Sulfur is certainly a possibility–it gives Io its pizza-pie appearance—but it is so widespread on the surface of Io because intense volcanism, fueled by the enormous tidal pull of Jupiter, is continuously resurfacing that moon. Moons could certainly be brighter than ours (which is actually about the color of tar) but it is unlikely that they would look any color but white to our eyes.

How about size? Halve the diameter of our moon, while keeping it at the same distance, and it will look half the size in the sky. But putting it twice as far away, while keeping the diameter constant, will have exactly the same effect on apparent size. It will also, however, lengthen the orbital period—approximately the time between successive new moons. Doubling the distance will increase the period by a factor of 2.8.

Any moon will cause tides, and the tidal force will be proportional to the apparent size of the moon and its density. Thus our sun and moon look very nearly the same size in the sky, but the moon has over twice the tidal effect of the sun. Why? Because the sun, being largely compressed hydrogen, is less than half as dense as the moon. A moon the apparent size of ours but with more iron (denser) would produce stronger tides; an ice moon (virtually impossible at our distance from the sun) would produce weaker ones.

Moons have phases because they are lit by the sun on only one side. The phase thus depends totally on the angular distance between the moon and  the sun. In particular, a full moon rises at sunset and sets at sunrise. It will be relatively high in the midnight sky in winter; lower in summer. (This is really marked in interior Alaska, where the fact that the moon’s orbit is not quite parallel to that of the earth gives us times when the full moon, but not the sun, is circumpolar, and other times, in summer, when the full moon stays below the horizon.)

A half moon is always 90 degrees from the sun. A waxing half moon will rise around noon and set around midnight with the round side always toward the sun; the waning half will not rise until midnight. Crescents are always fairly near the sun in the sky. A waxing crescent sets a little after sunset; a waning crescent rises a little before the sun. In either case the horns point away from the sun.

Multiple moons in different parts of the sky will have different phases.

An example? Here’s one, from a trilogy I’m working on:

She opened her eyes to see the room, with a gibbous moon just above the horizon and a smaller half moon much higher in the blue morning sky.  Sunlight from the opposite window lay on the wall, and when she took a step forward she saw that a shabby-looking city lay below her, between the building and the prairie beyond.

Suns and moons all over, but they are in proper relationship to each other.

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.

Feastday (Homecoming Glossary)

Alaska a few days before the southward equinox

FEASTDAY: #scifi #time #calendar The intercalary day 182 days after Yearday, approximating the date of the southward equinox. As aphelion is near the southward equinox on Central, Feastday is generally within a day or two of the equinox, and is a holiday  on Central. Here on Earth, the southward Equinox is today, September 22 2010, at 7:09 pm Alaska Daylight Time. (That’s 8:09 PDT, 9:09 MDT, 10:09 CDT, 11:09 EDT, and September 23 for most of Eurasia and Africa.

Goodnews Cluster (Homecoming Glossary)

GOODNEWS CLUSTER: #scifi A group of three suns circled by five habitable planets. (The suns are far enough apart that they do not disturb the orbits of each other’s planets.) The cluster was settled by a number of different groups who wanted to practice their own beliefs without interference from others, and joined the Confederation as a group. Needless to say, the various groups have started to interfere with each other.

Planet Building and Astrophysics Part 2

Once hydrogen burning is well established,  a star settles to a remarkably consistent relationship between temperature, luminosity, diameter and mass known as the main sequence.  It will then very gradually increase its luminosity while decreasing its temperature, but the change is extremely slow.  Our sun has been on the main sequence for about four and a half billion years, and will remain on the main sequence for another 5 billion years.  During that first four and a half billion years, its luminosity increased by about 40% – enough that the early radiative equilibrium temperatures of its planets were about 9% below present values.  Observationally, the temperature of earth has remained in a range that allowed the presence of liquid water throughout this period, so the change in solar output was offset by changes in the Earth’s atmosphere or surface reflectivity.

More generally, stars spend most of their lifetimes, and by far the most stable part, on the main sequence.  A planet stable enough to support life, and especially one on which life has evolved, is most likely orbiting around a main sequence star.

How long a star remains on the main sequence again depends on its mass.  Very massive stars are prodigal of their energy, and exhaust their hydrogen fuel quickly.  A star five times the mass of the sun has ten thousand times the luminosity, and will exhaust its fuel in  fifteen million years – a mere eye-blink in geological and evolutionary terms.  A star half the mass of the sun would have a main-sequence lifetime of two hundred billion years – considerably more than the age of the universe.  It would also, however, have a luminosity a mere 3% of the sun’s.  A planet would have to be very close – and have a very short year – to get the amount of energy from that star that we do from the sun.

The end of life on the main sequence is catastrophic for any earth-like planets.  The star becomes both brighter and cooler – which means a very considerable increase in its diameter.  The sun, for instance, will expand to include the orbits of Mercury and Venus, and approach that of Earth.  The core temperature will rise until it reaches roughly a hundred million degrees Kelvin, at which point two other fusion reactions become possible – the combination of three helium nuclei to make carbon, and the collision of an additional helium nucleus with the carbon to make oxygen.  The resulting ed giants are elderly stars, either in the initial expansion phase or in the core helium burning stage   The transition to helium burning is again disastrous for any planets in the vicinity, and may cause part of the star’s mass to be blown off into space.  The star may go through a phase of periodic changes in luminosity.  In general a red giant is far less hospitable to planets than a main-sequence star – but it does have a very important role in that it produces heavier elements than helium – elements that did not exist shortly after the big bang.

Eventually, the star runs out of helium in its core.  Again it becomes unstable, often ejecting up to half its mass and thus adding two newly produced elements – oxygen and carbon – to the interstellar medium from which the next generation of stars will be born.  For a star with a mass less than four times that of the sun, there is no other possible fuel, and the star shrinks to a white dwarf – a dead star.

For larger stars, the core temperatures may rise high enough (600 million degrees Kelvin) that fusion reactions involving carbon atoms are possible, producing neon and magnesium as well as additional oxygen.  If the star has more than nine solar masses, the temperature may rise even higher, to a billion degrees Kelvin, at which point neon burning begins, producing still more magnesium and oxygen at the expense of neon, while at 1.5 million degrees oxygen burning begins.  Oxygen burning produces silicon, sulfur and phosphorous, as well as more magnesium.  Finally, at around 3 billion degrees Kelvin, silicon fusion begins, producing iron.  Iron is the most stable element with regard to nuclear reactions, and no further thermonuclear reactions are possible once all the other elements are burned to iron “ash”.

The star goes through these stages with increasing speed.  If it spent 7 million years on the main sequence, it will then spend 700 thousand years burning helium, 600 years burning carbon, a year burning neon, 6 months fusing oxygen, and a mere day to use up its silicon.

Without further fuel to keep up its temperature, the star is doomed.  It will either become a supernova or collapse into a black hole.  In the former case it returns a cornucopia of new elements to the interstellar medium for the production of the next generation of stars and planets, as well as sending out a shock wave that may well trigger the birth of new stars.

The elements returned to the interstellar medium are not only the ones produced by the primary reactions given above.  Some of the reactions produce neutrons, and these neutrons can interact with the elements produced earlier to produce new isotopes and elements.  In addition, the heat and pressure of the supernova explosion itself produces additional elements, some of which are intensely radioactive and no longer exist in the local part of our universe. But every element in our universe, except hydrogen and helium, is produced by the thermonuclear processes in stars.

Next week–planetary atmospheres.

Why Planets have Seasons

Why do planets have seasons?

The chance that a planet will rotate in exactly the same plane as its orbit is pretty remote.  So is the chance that its orbit will be exactly circular. (I have a planet in the sequel to Homecoming that is seasonless because its equator is in its orbital plane, but that’s fiction.)

The direction of the axis of rotation, the planet’s pole, plays a large role in its weather, and especially in the planet’s seasons.  Seasons on Earth are controlled primarily by the tilt of the earth’s equator relative to the plane of its orbit.  This tilt for the earth is 23 degrees and 27 minutes.

An increase in the tilt would enhance the contrast between the seasons, but reduce the difference in the total annual solar heating between the equator and the poles.  (If the tilt is greater than about 60 degrees, the poles actually get more annual average incoming energy than the equator, as is the case on Uranus.)

In contrast, if the equator were closer to being in the same plane as the planet’s orbit, there would be less difference in the seasons and more in the difference of the average annual energy budget between the equator and the poles.

In general the oceans, which hold heat and release it slowly, will be most influenced by the average annual energy budget, while the land areas are more influenced by the seasonal energy input.  Thus the progression of the seasons will be strongly influenced both by the distribution of land and water on the planet and by the inclination of its axis.

Water, by the way, is not a rare substance in the universe.  It is the simplest possible chemical compound of the most abundant element in the universe, hydrogen, and the third most abundant, oxygen.  (The second most abundant, helium, does not usually form chemical compounds and can be ignored for this purpose.)

Water is going to be lacking on a real planet only if it has been removed at some point in the planet’s history.  In our system, the near lack of water on Mercury and its scarcity on Venus is probably due to the fact that water, being relatively volatile, was blown out of the inner Solar system by the tantrums of the early sun.  Mars also seems short of free water, but that is likely due to its small size and correspondingly weak gravitational field and ice is still a possibility.

Getting water in liquid state is largely a function of planetary temperature and atmospheric pressure. Making the planet the right size (gravity) and putting the planet at the right distance from its sun are probably the critical factors here.

Solstices are defined as the dates when the planet’s axis points most nearly directly at the planet’s sun. In one hemisphere the sun will be at its highest in the sky and the days will be at their longest. In the opposite hemisphere the sun will be at its lowest and the days will be shortest. Close to the poles, the sun may appear to be above or below the horizon for 24 hours at a time. Summer and winter are defined as starting at the solstices.

Equinoxes are defined as the dates when the planet’s axis is at right angles to the line between the planet and its sun. At this time the difference in the radiation received at the pole and at the equator is at its maximum, but there is no difference between the northern and southern hemispheres. Spring and fall are defined as starting at the equinoxes.

There is another way of getting seasons on a planet, minor but perceptible on Earth, but possibly major on a planet whose equator is close to the plane of its orbit.  This is the ellipticity, or elongatedness, of its orbit.  The ellipticity of the Earth’s orbit is only about 0.016 at the present, though it has been as high as 0.06 in the past.  But ellipticity influences both the energy received from a planet’s sun, with maximum energy coming at perihelion (planet closest to sun) and the length of the seasons, since the planet moves fastest in its orbit when it is closest to its sun.  Seasons controlled by the orbital eccentricity differ from those due to axial tilt in several ways.

Firstly, pure tilt-controlled seasons are opposite in the two hemispheres, while pure perihelion-controlled seasons are the same in the northern and southern hemispheres.

Secondly, pure tilt-controlled seasons are equal in length–the summer half-year is the same length as the winter half-year, counting the equinoxes (when the sun is 90 degrees from its midsummer position relative to the stars) as the dividing points.  Perihelion-controlled seasons have a long winter and a short summer.

Thirdly, the length of time the sun is above the horizon each day varies markedly over a tilt-controlled seasonal cycle, as does the direction of sunrise and sunset at medium to high latitudes.  Tilt-controlled seasons at the equator have a double cycle, with the most incoming energy at the equinoxes.  In pure perihelion-controlled seasons, there is no change in the apparent path of the sun in the sky, and the equatorial seasons follow those nearer the poles.

Fourthly, in pure tilt-controlled seasons, there is no change in the apparent size of the sun in the sky.  In perihelion-controlled seasons, the sun appears larger in summer and smaller in winter.

Most real planets have both kinds of seasonality at once, with one dominating.  On Earth, tilt dominates, but summer is slightly longer than winter because Earth is at perihelion around January 4.  (This is why the number of days between the spring and fall equinoxes is slightly more than the number between the fall and spring equinoxes—or more simply, why February is the shortest month, while July and August are the only two adjacent months with 31 days.)

Since tilt, perihelion date and eccentricity are the variables that can change in a multi-body system (and if fact do change slightly over a few thousand to hundreds of thousands of years on Earth, just due to the other planets in our system) the seasons in a multiple star system are likely to be rather variable and not really conducive to earthly life.  Even on Earth, the slight variations in orbital elements produced by the other planets are thought to be enough to account for the alternation between ice ages and interglacials.