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for Michael A. Seeds, "Foundations of Astronomy" Fourth Edition (1997) Copyright © 1996 Heather L. Preston |
This chapter is mostly based on things we observe, and compare to theories.HST Deep Field image (courtesy NASA)
A. The Universe is dark ("Olbers'" Paradox) for two reasons:
Farthest (youngest) galaxies known (image courtesy NASA)
Caption: A giant HII star-forming region (image courtesy NASA )
A. Making the Solar System
As we discussed in cosmology, the matter making up everything in the universe came into existence in the first few minutes of the Big Bang. Most of this material (75% by mass) was hydrogen, and most of the universe is still hydrogen. Elements heavier than lithium and beryllium were all made in the interiors of stars, either (for those less massive than iron) during fusion reactions or (for those more massive than iron) during supernova explosions.
After the universe was about a billion years old, it had settled into agglomerations that we call galaxies, mostly composed of stars. Star formation has been a continuous process since that time.
Recall our earlier discussion of star formation: stars form in large gas clouds, in reaction to a compression or shock wave, generally either from a nearby hot star or a nearby supernova. The gas cloud then contracts, heats up, and fragments, and new stars emerge from these fragments. As the stars emerge from the contracting clouds, they are surrounded by cocoons of dust and gas, which (by the conservation of angular momentum) revolve around the central protostar and are flattened into disks by their rotation.
In this model, planets form by contraction of density enhancements within the disk, and formation continues until the young central star has become hot enough and luminous to blow the disk material into interstellar space, leaving the young planets orbiting the star.
Observational tests of the theory are difficult, although some include:
The solar system, except for the Sun, is mostly empty. That is, the planets are extremely small compared to the spaces between them. Consider the earth, with a diameter of about 12,000 km. The distance to the nearest planet (Venus) at its closest point (conjunction) is 41.4 million km -- over 3,000 times the earth's diameter!
Most of the planets circle the Sun in orbits that lie close to the ecliptic place. The only exceptions to this rule are Mercury, with a tilt of 7 degrees, and Pluto, with a tilt of 17 degrees. Of course, Pluto is a frequent exception to generalizations about the solar system! The Asteroids and cometary bodies will be dealt with in later sections. All of the planets orbit the Sun in the same directions (counter-clockwise as seen from above the north pole), and most revolve around their axes the same way (except Venus and Uranus)
Broadly speaking, planets come in two varieties:
Other constituents of the solar system include asteroids, comets, and meteoroids. To summarize:
The most accurate way of finding the age of a rock is by radioactive dating, which compares the amount of radioactive material left in the rock to the amount that the rock started with. Using this method tells us that the oldest earth rocks are about 3.9 billion years old, somewhat younger than the earth itself. (Plate tectonics means that the surface of the earth is continually remaking itself) Lunar rocks recovered by the Apollo program are over 4 billion years old, and meteorites have been found with ages up to 4.6 billion years. This gives us a date for the end of planet-building. The beginning is estimated to have been about 5 billion years ago, when the Sun was contracting towards the main sequence.
The early solar nebula at that time was mostly hydrogen and helium, with small fractions of heavier elements. The Jovian planets (since they lay outside the ice line) were effective at trapping rock and ice particles from the nebula. As their masses grew, some were able to trap large amounts of gases as well, and they grew to be the low-density, hydrogen-rich planets that we see today. The terrestrial planets, on the other hand, have low enough surface gravities that they could not hold on to hydrogen or helium, and thus they ended up as concentrations of the heavier elements in the nebula.
Looking closely at the inner solar sytem reveals an interesting pattern. If we correct for the increase in density caused by the immense gravitational pressures in the planetary interiors, we find that the so-called uncompressed density of the planets drops as we move away from the Sun. This is because, early on, the temperature in the inner parts of the nebula was higher than that farther out. This meant that only elements with very high melting points (such as metals) could condense out of the nebula to make planets -- so the innermost planets (such as Mercury) are mostly metallic, and thus denser. Farther out, it was cool enough for silicates (rocks) to condense, and thus the earth and Mars are mostly silicate rock. In the outermost regions, it was cold enough for water ice, methane, and ammonia to condense, and so the outer planets are mostly made of these materials. This sequence, which describes how the different materials condense from the gas as we decrease the temperature (usually by moving farther from the Sun), is called the condensation sequence.
Planet formation actually takes place in three steps. In the first, dust grains grow by condensation onto small grains of material (one atom at a time). Once they are larger, these grains in turn stick together in a process called accretion to make larger bodies. Small-scale accretion is helped along by stickiness due to static electricity on the grains, or by organic stickiness (carbon compounds). As accretion continues, the new larger bodies (little rocks) tend to fall into the plane of the solar nebula, which in turn increases the local density of particles and makes them grow even faster. By the end of stage two, we have a flattened disk of bodies (called planetesimals) about 100 km in diameter. Accretion can now get a boost from gravity.
Stage three is the coalescence of the planetesimals into protoplanets. At this stage, both gravity and adhesion are important in keeping bodies that collide stuck together. Of course, since everything was generally moving in the same direction, most collisions were relatively low-velocity, so "sticking" was easier. The largest planetesimals, naturally, grew the fastest, since they had the largest gravitational fields.
The original material of the planets varied somewhat depending on location in the solar system. Planets forming close to the Sun started to form from metal oxides because it was too hot for silicates to condense (Mercury is an example). Farther out, silicates could condense and planets like the earth were formed, with both silicate and metal oxide composition. In the outer solar system, volatile ices could condense, and nitrogen, hydrogen, etc. became important planetary building blocks. Whatever they were made of, as these protoplanets grew, though, the energy released by the infalling material (the heat of formation) heated up the new planets and made them molten. When this happened, the process known as differentiation occurred, and denser material (such as iron) sank to the center of the planet, while less dense material (oxygen, silicon) floated to the top. In addition, in the inner solar system, the heat baked gas out of the rocks and formed the original planetary atmospheres, which were mostly hydrogen and helium. In the outer solar system, planets rapidly grew massive enough to trap hydrogen and helium from the solar nebula, making their original (and still!) atmospheres. There is an alternate accretion theory that differs in how fast the nebula cooled off.
D. Clearing the Nebula
Planetary formation was halted when the Sun began to approach the main sequence, and thus to blow away its cocoon. Two effects play a role here:
The planets themselves contributed to clearing the nebula. First of all, they tended to sweep the solar system clear of material simply by colliding with it. Even if they didn't collide with other bodies, though, they frequently threw material from the solar system by graviational interaction (the "slingshot" effect). Jupiter probably was responsible for more of this "cleaning" than any other planet.
E. The Titius-Bode Rule
One of the interesting weird facts about the planets around our Sun is that the semi-major axes of their orbits form a mathematical progression of sorts. It works well in the inner Solar System, and less well farther out... in a column, write down 0,3,6, and keep doubling. Add 4 each time, and divide the sum by ten each time. Neptune does not fit the pattern, but the asteroid belt, between Mars and Jupiter, does.
Chapter 21. The Earth
Color image of Earth (courtesy NASA)
1. Early History - Four Stages of Planetary Development
Cratering as shown in Seeds' text (courtesy Wadsworth Publ.)
We know more about the interior of the earth than about that of any other world, and our knowledge comes from seismology, the study of vibrations travelling through the earth. Earthquakes produce seismic waves, which come in two varieties:
These waves don't travel through the earth in straight lines. Density changes or gradients, in particular, cause them to bend, or refract. Looking at where waves from a specific earthquake emerge, and where they don't, allows mapping of the terrestrial interior.
The general picture is as follows. At the bottom is the central core of the earth. It is mostly liquid, and about half the radius of the entire earth. It is hot (4000 K) and composed of iron and nickel. Near the center of the core, the pressure rises to such high levels that the core is no longer molten but solid iron and nickel. Above the core lies the mantle, a layer of dense rock (3.5-4 g/cc) and metal oxides. The mantle is a plastic material, neither solid not liquid: if struck, it breaks, but under steady pressure, it flows. Finally, on top of the mantle is the crust, made up of brittle and light (2.5-3.5 g/cc) silicate rocks floating on top of the mantle. The crust is quite thin, about 70 km deep under the continents and 10 km deep under the oceans.
3. Crustal Features
The earth has an active crust, which moves around, and is continually remade in a process called plate tectonics. In essence, the crust is broken into plates, and these plates float on top of the semi-molten mantle. Since the mantle is heated from below (via residual heat and radioactive decay of elements deep inside the earth) it shows convection, and these convection currents slowly (a few centimeters per year at most) move the crustal plates around -- at about the rate at which your fingernails grow!
The main features of plate tectonics are most visible on the ocean floor. A good example is in the Atlantic Ocean, which shows a central mountain range (called the midocean rise) with a deep cleft in the middle (the midocean rift). Measuring the ages of the rocks shows that new rock is being made in the midocean rift and continually being pushed out in both directions -- the Atlantic Ocean is expanding!
Of course, the surface area of the earth is fixed, so if crust is being made at one point, it is being destroyed somewhere else. The somewhere else is a subduction zone, where one plate rides over the top of another, forcing it down into the mantle, where it melts. The heat generated by this process frequently manifests as volcanic activity -- the volcanoes on the West coast of the Americas are due to this process, as the Pacific Ocean plate sinks beneath the North and South American plates.
Sometimes colliding plates are of nearly the same density, and so neither one sinks beneath the other. In this case, the plates slowly accordion-fold against one another, and build mountain ranges. The Himalayas are due to the collision of the Indian subcontinental plate with Asia.
Plates also split apart to form new valleys and seas, called rift valleys. The clearest example of this is in Africa, where the Red Sea and the Great Rift Valley represent one continuous example.
4. Magnetic Field
The magnetic field of the earth is generated by the dynamo effect, due to the interaction of the rotation of the earth with convection currents in the molten iron-nickel core. The main effect of this magnetic field is to protect us from the solar wind. Since the solar wind is composed of charged particles, it cannot cross magnetic field lines (at least not without a lot of trouble!), so it is deflected at the point where it meets the earth's magnetic field (the bow shock). The cavity formed by the earth's magnetic field is called the magnetosphere, and the main detailed features of the magnetosphere that you should be aware of are the Van Allen belts, formed by the slow leakage of particles from the solar wind into the inner magnetosphere, where they remain stable (and dangerous to spacecraft and astronauts) for a long time.
5. The Atmosphere
The primeval atmosphere of the earth probably formed from the strong early heating of the rocks, which would have driven any volatiles into the atmosphere. Since the earth's surface gravity is not strong enough to hold on to hydrogen and helium, they would be lost (except for that hydrogen which was able to chemically combine with other elements to make things like water), and the remaining atmosphere is mostly carbon dioxide, water, and nitrogen.
Changes in the atmosphere began with the advent of the oceans, which dissolved much of the carbon dioxide, which in turn reacted with other dissolved compounds in the water to make mineral sediments (Calcium carbonate, or limestone). Life, when it appeared, accelerated this process, particularly through the building of limestone reefs, which also remove carbon dioxide from the atmosphere. Photosynthesis, when it began to occur, accelerated the process, and also began to add significant amounts of oxygen to the atmosphere.
Lacking carbon dioxide for protection, any methane or ammonia in the atmosphere would have been broken up by the solar ultraviolet radiation. Fortunately, in our current atmosphere, we are mostly protected from solar UV by the ozone layer (O3), located about 25 km above the earth's surface. However, ozone can be attacked and destroyed by chlorine, a constituent of the CFCs found in aerosols and refrigerants, so we need to be careful about how we treat our atmosphere.
Actually, the earth's surface is warmer than it would be if we lacked an atmosphere. This is because even the small amounts of carbon dioxide in the atmosphere give rise to a greenhouse effect, which warms the earth. The greenhouse effect is due to the fact that carbon dioxide is transparent to ultraviolet radiation but opaque to infrared. This means that solar UV radiation can reach the earth's surface to heat it, but the infrared wavelengths radiated by the warm ground have difficulty leaving the earth. Of course, adding too much carbon dioxide to the atmosphere (mostly by burning things) can increase this effect far beyond what is useful. A rise in the average terrestrial temperature by only a few degrees would melt the polar caps and flood much of the earth.
6. Cratering
Earth undergoes a constant destruction and renewal of its crust in geologically active areas near plate boundaries (and in shield volcano regions over hot spots). In addition, weathering due to rain and wind break down the features of craters. Finally, some of the larger impact craters are masked as features in large bodies of water, such as the Sudbury Basin in Canada, or a curve of the Gulf of Mexico near Yucatan. Large, dry impact sites such as Meteor Crater in Arizona are rare.
Chapter 22. The Moon and Mercury
Composite Image of Mercury (courtesy NASA)
1. The Moon from the Earth
A. The most obvious features of the Moon as seen from the earth are:
The craters are mostly quite old, dating from the cratering period of the formation of the solar system. However, some are much younger, such as Copernicus and Tycho. These are usually quite bright, and have bright rays (sprays of debris, or eject, from the impact) stretching out from them.
The crater density on the lunar surface provides a way to estimate the relative ages of different parts of the surface. Areas which have accumulated many craters must be older than those which are still pristine. Doing this tells us that the mares are relatively young, while the lunar highlands (the brighter areas of the surface) are quite ancient.
There are a few traces of ancient volcanism on the lunar surface, such as small volcanic vents and sinuous rilles -- channels cut by flowing lava. In addition, there are graben rilles, where crustal sections have dropped, creating a steep-sided canyon. However, the Moon is no longer geologically active. What moonquakes there are take place deep in the mantle, near the core, as it shrinks and cools.
B. Visiting the Moon
Starting on July 20, 1969, 6 Apollo missions landed on the Moon. The first (Apollo 11) and second landed on mares -- relatively smooth surfaces judged to be safe to land on, while later missions were more ambitious. The missions returned rock samples, which could be dated directly. Samples from the maria were basalts (from lava) with ages of 3.1 to 3.8 billion years, about as old as the oldest rocks on the earth. The last (Apollo 17) landed on the lunar highlands, and returned the oldest rocks found on the Moon -- up to 4.6 billion years old. These are absolute, rather than relative, ages.
As well as being older, lunar rocks were compositionally different than terrestrial rocks (isotope ratios are different than on Earth). None are sedimentary of course, since the Moon has no water. Many highland samples are a low-density, light-colored rock called anorthosite, a granitic rock. Some from maria are vesicular basalts, which contain holes where gas bubbled up through the lava that they formed from. Another comon lunar rock type is breccias, or conglomerates, cemented masses of many fragments -- evidence that the lunar surface has been heavily pounded by meteoritic impacts.
C. Lunar formation
Lunar rock is much less dense on average than terrestrial (3.4 g/cc vs. 5.5). In addition, the lunar magnetic field is nearly nonexistent, so there is no large molten core on the moon. Seismographs left by the Apollo astronauts show some low-level seismic activity which has enable some mapping of the interior, and has led to the theory that there may be a semi-molten core which is structurally much like the earth's mantle.
Essentially, lunar formation proceeded much like the formation of the earth, except that stage 3, the flooding of the basins, was the last stage reached. Only lava flooded the basins, of course, to make the maria, since the moon has no water. Stage 4, gradual surface evolution, only occurs on the Moon as a result of metor impact, so it is quite slow, leading to the production of a deep regolith (dust layer) over the lunar surface.
The real issue with the Moon is: Where did it come from? There are three traditional models:
If the impact occurred after the earth had already differentiated, then the moon would be made of crustal and mantle material, which seems to fit.
2. Mercury --
A. General
Mercury is hot at "noon" since it is close to the Sun, and as cold as space where it faces away from the Sun. It is difficult to observe from the earth, and until the mid-1960s many astronomers thought that it had a period equal to its year. It turns out that Mercury is indeed tidally coupled to the Sun in a 3:2 resonance, which means that its year is 1.5 times its day. It has a diameter of about 5000 km, about 40% that of the earth. However, its density is slightly higher than that of the earth, implying that it has a very large iron core (it formed at the highest presolar nebula temperatures), probably 80-90% of the radius of the planet!
Mercury has only been visited by one spacecraft, Mariner 10, which flew by the planet three times during 1973-1975. During the flyby, the spacecraft looked for signs of a magnetic field. None was expected, since Mercury should have cooled too rapidly (since it is so small) to still have a molten iron core. However, a weak magnetic field was found. This field is probably due to residual magnetism in the rocks of the planet. In addition, Mercury has a very tenuous atmosphere made up of solar wind particles temporarily trapped by the planet, or captured in the rock surfaces and outgassed: Some polar frost!
B. Mercury -- Surface Features
The surface of Mercury looks like the surface of the Moon. No dark maria are visible, but this is only because the lava inside Mercury is lighter in color than that of the Moon. The major features of Mercury that are not visible on the Moon are the lobate scarps, large curved cliffs. These seem to be wrinkles, due to shrinkage of the crust as Mercury cooled. Some are as high as 3 km, and they can be hundreds of kilometers long. Some other, straighter, fault lines are visible as well -- these are probably due to tidal stresses from the Sun.
Most of the surface is old, cratered plains, but intercrater plains exist as well, and these are less heavily cratered, suggesting that they were produced by lava flows that buried older terrain. Smaller regions called smooth plains are even younger than the intercrater plains, and seem to be more recent lava flows. The craters themselves are somewhat different than those on the Moon. Due to Mercury's higher surface gravity, ejecta were only thrown about 65% as far on Mercury as on the Moon, so ray systems are less extensive.
The history of Mercury parallels that of the Moon, effectively ending with stage 3. The only real surprise on the surface of this very hot world is the apparent presence of ice at the poles. This is probably due to water or other volatiles trapped in perpetually shaded crater floors.
Chapter 23. Venus and Mars
UV image of Venus' clouds (courtesy NASA)
A. Venus' most distinctive characteristics - Rotation and Atmosphere
Sometimes Venus is referred to as earth's "twin" because it is so similar in size and composition. However, a closer look shows that the surface conditions are dramatically different. In particular, rather than the nitrogen-oxygen atmosphere we have on the earth, Venus has an atmosphere dominated by carbon dioxide. This situation has led to a runaway greenhouse effect, and Venus has a surface temperature exceeding 700K(melts lead)! In addition, Venus shows a strange rotation -- rather than rotating from west to east (prograde) like nearly all other bodies in the solar system, Venus rotates from east to west (retrograde). And it rotates very slowly, with a "day" equal to 243 days. This slow retrograde rotation is probably due to a large late impact on Venus, which gave it a 177deg axial tilt, or which had the effect of reversing the planetary rotation (take your pick: the result is the same and the impact is awesome).
The atmosphere on Venus is mostly (96%) carbon dioxide, with significant amounts of nitrogen, water vapor, sulfur, and various acids. It's 90x as dense as Earth's atmosphere at ground level. Cloud cover is 100% all of the time. Probes have revealed that the clouds are arranged in layers, or decks, ranging down to about 48 km above the surface. Below that is 15 km of haze, and then generally clear conditions. The clouds are so stable because the slow rotation of the planet leads to a much more regular atmospheric circulation than occurs on earth. The heated atmosphere at the subsolar point (the point on the planet where the Sun is directly overhead) rises and moves towards the dark side of the planet and the poles, where it cools and sinks. This motion produces 300 km/hour jet streams in the upper atmosphere, which are visible in the ultraviolet.
When Venus was formed, since it was closer to the Sun than the earth (mean distance from the Sun is about 0.7 AU), it was hotter than the earth, so the liquid water did not form oceans. This meant that carbon dioxide could not be removed from the atmosphere, so instead, as the volcanoes outgassed, CO2 accumulated, leading to a runaway greenhouse effect. The resulting heating baked sulfur, chlorine, and fluorine out of the surface rocks, and thus added sulfuric, hydrochloric, and hydrofluoric acids to the atmosphere. Not a very friendly place!
The lack of liquid water does not mean that Venus never had any water, though. The presence of large quantities of deuterium on Venus indicates that at some time in the past, large quantities of water were present, although never as much as on the earth. The lack of an ozone layer meant that water was broken up by UV light -- the hydrogen escaped, and the oxygen combined with the surface rocks to form various oxides and silicates.
B. The Surface of Venus
Color image of Venus' surface (courtesy NASA)
Venus has been mapped extensive using radar, both from the earth and from orbiting satellites, 18 of which have orbited or flown past the planet. We now have maps of most of the surface to a resolution of 100 meters. These maps show that the surface is divided into 70% rolling plains, 20% lowlands, and only 10% highlands.
The highlands look a lot like continents on the earth and the largest, Ishtar Terra, is larger than the United States. It is bounded on one side by a huge shield volcano, Maxwell Montes, and a large plateau, Lakshmi Planum, which seems to have been formed by lava flows. Stunning lightning storms have been detected over Ishtar Terra. Volcanism is obviously important on Venus, and its importance is underlined by the coronae, large circular features up to 1000 km in diameter, and the arachnoids, their smaller cousins. Both are domed plains caused by rising molten rock beneath them.
There are more craters on Venus than on the earth, but not nearly so many as on Mercury or the Moon. While Venus lacks plate tectonics, it does have erosion, so surface features do change with time. There's a theory out currently that has the low number of craters ascribed to a periodic melting of Venus' crust due to trapped heat from radioactive decay. The jury's still out on that one. The lack of plate tectonics probably has three main causes:
Surprisingly, Venus lacks any significant magnetic field, and no one yet knows why (you heard my theory!) Venus and Mercury also lack moons.
C. Mars and Its "Canals"
Mars from Space Telescope: note large ice cap ( image courtesy NASA)
Mars is a planet intermediate in size between Mercury and the Earth -- not in position (it is about 1.5 AU from the Sun). It is about 53% the diameter of the earth, and has a rotational period of about a day, and an orbital period a bit less than two years.
Starting in the 1870s and 1880s, astronomers observing Mars reported the presence of canals on the surface. This led to the belief that there was intelligent life on Mars, and that the canals were intended to bring water from the polar caps to the equator, where a dying race struggled to maintain its civilization. The seasonal shrinking and increasing of the polar caps, and the periodic darkening of the northern hemisphere (interpreted as vegetation) strengthened this belief.
We now know this to be untrue, and the canals to be optical illusions. Mariner 4, which ws the first spacecraft to visit Mars (in 1965) sent back images which showed a fascinating planet, but one without intelligent life. Mars has since been visited by many space probes, some of which (the Vikings) spent a significant amount of time on the surface. Try telling this to Weekly World News...
D. Surface Features
The two hemispheres of the planet look significantly different. The southern hemisphere is old and heavily cratered, and thus looks like the surface of the Moon or Mercury. The northern hemisphere, on the other hand, has few craters, and shows signs of repeated lava flows. In addition, it shows recently active volcanoes and deformed crustal sections that suggest significant geological activity in the not-too-distant past.
The Martian volcanos are shield volcanos, with extremely broad bases that indicate that the lava flowed freely. The largest shield volcano on the earth is Mauna Loa in Hawaii, which is 10 km high and 225 km broad at the base. The tallest shield volcano on Mars (and that we know of in the Solar System) is Olympus Mons, which is 25 km high and over 600 km in diameter at the base. Interestingly, although Mauna Loa bends the crust around it and thus appear to sit in a depression, Olympus Mons does not, indicating that the Martian crust is thicker (and/or stiffer) than that of the earth.
However, deep valleys on Mars such as the nearly 8-km (5 mi) deep Valles Marineris indicate that the crust cannot be too thick, or it would not fracture in this way. Thus the Martian crust must be thinner than that of the Moon, where we see no such valleys.
The reason that volcanoes on Mars can grow to such vast sizes has to do with the lack of plate tectonics on Mars. Mars is essentially a one-plate planet. On the earth, volcanoes periodically punch through the overlying crust to make their cones. However, the crust is always moving, so the result is generally a volcanic arc, or an island chain like the Hawaiian islands. On Mars, the static crust leads to continuous building up of volcanoes, resulting in Olympus Mons and similar mountains.
Valles Marineris is over 4000 km long, and begins in an area of volcanism called the Tharsis bulge, which also contains Olympus Mons. The bulge is a region of intense volcanic activity, perhaps as recently as a few hundred million years ago. There is no evidence that Martian volcanoes are still active, although that is by no means impossible. One theory of Mars' history goes that Mars used to be more inclined on its axis than it currently is, but the Tharsis bulge pulled it more upright (into a more stable configuration, from a axial-rotation point of view), causing the poles to get colder and part of the atmosphere to freeze out into polar caps.
E. Atmosphere and Surface Water
Dry streambed "islands" on Mars (courtesy LPI)
Dry streambeds, much like the arroyos of the American desert, are visible on Mars. Some look like runoff channels, such as those made by rivers and streams on the earth, but others look more like the channels made by sudden flooding (outflow channels).
The pink color of the Martian atmosphere, like the red color of the surface rocks, comes from iron oxides (rust). The air itself is mostly carbon dioxide, nitrogen, and argon, but at a pressure only 1% of that on earth. The lower density of the Martian atmosphere compared to its state in the past is due to the low escape velocity of Mars (only a little more than 10% Earth's mass!), which meant that it could not hold on to the lighter molecules and atoms. The "high" proportion of argon in the current atmosphere is a relic of the earlier atmosphere (argon is too heavy to escape, and so nonreactive that it does not get tied up in the crust), and tells us that the early Martian atmosphere was 10-20 times denser than it is now.
Substantial water has survived on Mars only in the polar caps, where it provides a small layer of water ice under the larger overlying layer of frozen carbon dioxide, and in the soil, which Viking landers found contained about 1% water. The variation in size of the Southern Polar Cap with season is a change in the amount of frozen CO2, rather than water ice. In the summer, when the CO2 cap shrinks, there's more of it in the atmosphere. The thicker atmosphere makes giant dust storms possible!
F. Moons
The two moons of Mars, Phobos and Deimos, are really large rocks. In fact, they are probably captured asteroids whose irregular shape is due to their low masses (their gravity is not strong enough to pull them into spheres). In fact, Phobos's orbit is so close to Mars (thus so high-velocity) that it orbits West-to-East across Mars' sky; it's unstable and it will either be tidally disrupted or crash into Mars within about a hundred million years. See the text for a more detailed discussion of the surface features of the two moons (if interested!) -- the Internet also has loads of planetary information available!
Chapter 24 - Jupiter and Saturn
Jupiter (image courtesy NASA)
A. Interior
Jupiter is the most massive planet in the solar system, and in fact contains over 70% of the mass in the solar system outside the Sun. It is about 11 times the radius and 330 times the mass of the earth, and is the first representative of the outer solar system, which is very different from the inner.
Unlike the inner planets, Jupiter is not a solid body, but instead is a ball of gas and liquid (mostly hydrogen and helium). From the earth, there are two obvious clues to this:
The chemical composition of Jupiter is very like that of the Sun: mostly hydrogen and helium, with traces of carbon, nitrogen, oxygen, etc. However, unlike the Sun, Jupiter is cool enough to have large numbers of molecules in its atmosphere. These include substances like ammonia, methane, and water. In some sense, Jupiter is a sample of the material in the original solar nebula, since it is so massive that it has lost little of its original material. (Forming outside the "ice line" helped in this regard!)
Jupiter actually radiates about twice as much energy as it receives from the Sun. This "extra" energy arises from continuing contraction of the planet -- in a very real sense, the formation of Jupiter is ongoing.
One immediately noticeable feature of Jupiter is its strong magnetic field of 10-20 Gauss (ten or more times stronger than that of the earth). This leads to a large magnetosphere (which is somewhat flattened due to the rapid rotation of the planet) which is capable of trapping quite energetic particles from the solar wind. We know that the field (and thus the region in the planet that produces it) rotates every 9 hours 55 minutes because it emits decimeter (10s of cm) radiation that varies with that period. This radiation arises from synchrotron emission from relativistic electrons spiralling around the magnetic field lines. Decameter (10 of m) radiation also is seen from Jupiter, more sporadically. This radiation seems to arise from lightning, and its strength and frequency seem to be correlated with the position of the moon, Io. Volcanoes on Io contribute lots of sulfur to the Jupiter system, and these sulfur ions contribute to the charged particles resident in the Jovian magnetic field. They also produce dramatic aurorae on Jupiter.
Obviously the presence of the magnetic field implies that there must be some conducting material inside Jupiter to give rise to the dynamo effect. In the inner planets, this has been a molten iron core, but on Jupiter it is something quite different. At the high temperatures and pressures characteristic of the Jovian interior, ordinary hydrogen has its nuclei forced into a rigid structure, while its electrons become free to move. This strange state is called liquid metallic hydrogen, and it is an excellent conductor. (FYI, metallic hydrogen was made in the lab for the first time only last month!)
Inside the region of metallic hydrogen may be a small rocky core, perhaps the size of the earth. Owing to the strong pressure inside Jupiter, the rock will be in some strange liquid or semi-liquid state, but it contains the same elements as comprise the earth. Outside the metallic hydrogen layer, the clouds start.
B. Clouds
Voyager 1 image of Jupiter: note band structure (image courtesy Nasa)
The apparent surface of Jupiter is just the highest cloud deck. These clouds are composed of white ammonia crystals. The temperature here is about 150 K and the pressure about the same as that on the surface of the earth. Going down, things get hotter and denser. The next layer is composed of ammonium hydrosulfide crystals (the orange layer), and has a temperature of 200 K. Deeper lies a layer of liquid ammonia and (perhaps) some water crystals, and below that is liquid hydrogen.
The clouds themselves are arranged in belts and zones visible in even a small telescope. The belts are generally brown or red and the zones yellow or white. The colors must arise from impurities in the clouds, but Galileo results won't be available for two more months, so we can only speculate on the nature of these impurities -- probably ammonia and sulfur compounds broken down by sunlight and lightning.
The zones are higher in altitude than the belts and represent high-pressure areas of rising gas, while the belts are low-pressure areas. At the boundaries between the two are high-speed jet streams. The zones and belts themselves are semi-permanent features, lasting at least centuries. They may be linked to internal circulation in Jupiter, or they may be driven by heat input from the Sun.
Jupiter's Great Red Spot (courtesy NASA)
There are some long-lived spots along with the belts and zones, the most well-known being the Great Red Spot. The Red Spot seems to be somewhat cooler and higher than the zone in which is resides. In addition, it rotates in such a way as to suggest that it is a high-pressure area in which the gases are rising and cooling. Fast winds surprised the Galileo probe below the cloudtops -- 500 m/sec, as on Saturn!
C. "Smoke" Rings
Voyager 1, during its Jupiter flyby, detected rings around Jupiter, but rings far more tenuous than Saturn's. Jupiter's rings are no more than 30 km thick. Since the rings scatter light forward (see Figure 23-9), the particles that comprise them must be about the same size as the wavelength of the light in which Voyager observed them, about 10 microns at most, or about the size of the particles in cigarette smoke. The material is dark and reddish, so must be silicate powder rather than ice.
The ring material could have come from a satellite of Jupiter than came too close and was destroyed by tidal forces. A test of this is to compare the distance of the rings from Jupiter to the Roche limit of Jupiter. The Roche limit is the distance from a planet (or other body) within which a moon cannot hold itself together by its own gravity. If both the moon and the planet have similar densities, the Roche limit is 2.44 planetary radii, and the Jovian rings lie within this distance (as do those of Saturn and Uranus).
The ring material cannot be left over from the early system, because the solar wind acts to disintegrate particles and blow them out of the solar system within about 100 years. The ring system is therefore a work continuously in the making, perhaps from micrometeorites chipping at the surface of small Jovian moons.
D. Moons: Io, Europa, Ganymede, Callisto, and their buddies...
The Galilean Moons of Jupiter (courtesy NASA)
The four largest moons are called the Galilean moons, because Galileo discovered them. In order inwards towards the planet, they are:
Saturn with rings (courtesy NASA)
A. The Planet: Yet another gas giant. Strong surface winds (500 m/sec).Washed-out appearance but still has belts and zones. The "fading" is due to the methane haze that lies above Saturns deeper cloud decks, due to its lower temperature (although it, too, is overluminous, it's farther from the Sun than Jupiter). Saturn is less dense than water. We'd never find a tub big enough to try the experiment, but it would definitely float... its magnetic field is 20x less than Jupiter's, but its core rotation period (10.5 hours) is similar.
B. The Rings: Extensive, big division (Cassini's) due to resonance with moon Mimas. F ring is kept in braids by shepherd moonlets. Thousands of smaller divisions. Thickness of 10 m, but folded with a wave amplitude of 1 - 2 km. Flash -- Mimas collided with part of the outermost ring and is changing its orbit faster than expected -- may be the next "food source" for Saturn's ring system.
Hubble Space Telescope enhanced image of Titan (including IR) showing "continental mass" (courtesy NASA)
C. Moons: Titan is the big one, larger than planet Mercury! It is rich in carbon compounds, and probably has organic gooey "rains" of hydrocarbon smog through its ammonia-rich Nitrogen-argon and methane atmosphere, which, due to the cold temperatures, is about 1.6x as dense as Earth's. At that pressure and temperature, there's probably an ocean of liquid methane on the surface, with islands of hydrocarbon goo like beach tar floating atop it, or covering up any rocky shores there may be. It would not smell good to us! Mimas, which we've already mentioned, looks like the Death Star (or AT&T logo) due to a huge crater. Epimetheus and Janus, just inside the orbit of Mimas, are continually exchanging orbits with one another in a "waltz" -- they are called the coorbital satellites. There are small moonlets trapped at the Lagrangian points (L4 and L5) 60deg ahead of and behind Dione and Tethys in their orbits, which are farther out than Mimas'. Saturn has at least 18 satellites, and probably more, as yet unnamed.
Chapter 25: Uranus, Neptune, and Pluto
I. Uranus
Hubble Space Telescope enhanced image of Uranus and its rings (including IR)(courtesy NASA)
A. Discovery and Orbit
Uranus was discovered in 1781 by Herschel. This caused a sensation, because everyone had assumed they knew all the planets that there were. Its orbit is very slightly elliptical, its year is 84 Earth years long (because -- remember P2=a3 -- it's at an average distance "a" of 19.18 A.U.). Its weirdest feature is that it's turned on its side: its axial tilt is 97deg to the plane of its orbit around the Sun (the ecliptic). Like the other planets, its own equator dominates the dynamics of its ring-and-moons system (not the plane of its orbit around the Sun), so the moons' orbits are really easy to measure right now, as the planet's south pole is pointing toward the inner solar system.
B. Atmosphere and Interior
Uranus is, like Jupiter, Saturn, and Neptune, mostly made up of hydrogen, helium, and hydrogen-rich compounds like methane. Its blue color comes from the methane tainting its outer thick layer of mostly-pure hydrogen. There are clouds, but they are so far down in the atmosphere (due to the low temperatures) that we don't see them. Below them the atmosphere just gets denser until it is a liquid consisting of water with hydrogen, helium, and methane and ammonia mixed in. Despite the fact that the Sun heats one pole for 42 years and the other pole for the next 42 years, the rapid rotation rate (11 hours) combined with the weakness of the sunlight at Uranus' distance from the Sun causes the atmosphere to be well-mixed in a subdued belt-zone pattern reminiscent of Jupiter's and Saturn's (but much harder to see, due to the great atmospheric depth at which clouds form). Another weirdness about Uranus (and Neptune) is that its magnetic field is off-center by a considerable amount (from the center of the planet). It may be produced by a dynamo effect that uses water, ammonia, and methane in the mantle rather than liquid metallic hydrogen (as in Jupiter; Uranus and Neptune don't develop liquid metallic H due to their lower masses and central pressures).
C. Rings and Moons
Uranus has very dark, thin, low-mass rings, kept thin by shepherd moonlets. They are within the magnetosphere and radiation belt of the planet, which may account for the sooty color of the ring particles, as methane ices darken under radiation of most kinds. Again, these rings lie within the Roche limit of Uranus. The weirdest of the five large moons of Uranus is Miranda. It has features that look like auto race tracks on its surface -- these are called ovoids, and it is believed that they formed because Miranda was shattered by a large impact, then settled back together in a jumble of rock and ice, and gradually the larger chunks of rock sank through the ice, leaving ovoids.
II. Neptune
Hubble Space Telescope enhanced image of Neptune(courtesy NASA)
A. Discovery and orbit
Predicted in 1845 by English astronomer John Couch Adams, but nobody in England really believed him, so it was discovered the following year by European astronomers. Fortunately, because both sets of astronomers were 'way off on the distance (thinking Neptune would obey the Titius-Bode Rule, which it doesn't), it was in the right part of its orbit for them to be looking in the proper direction for it no matter how far out it was... it was found because of its gravitational effects on the orbit of Uranus. Apparently it had been seen by Galileo more than 200 years before, but he had thought it was a star. Its mean distance is 30 A.U. from the Sun, the orbit is only slightly eccentric, and it takes almost 165 years to go around the Sun once (so you see, it was lucky that we discovered it when and where it was!)
B. Atmosphere and Interior
Almost the same thing. Again, the blue tinge comes from the methane contaminating the hydrogen. Unlike Uranus, Neptune is only tilted 28deg to the ecliptic, but like Uranus, Neptune's weaker magnetic field is off-center from the planet's core, implying a lower-atmosphere-based generating mechanism. It has belt-zone circulation, and a "Jovian" dark spot that is probably a long-standing storm system. Again, like Jupiter, Saturn and Uranus, it rotates fast, taking 16.05 hours to go around on its axis (its "day") once. It is presumed that the interior contains a rocky core with an icy mantle and a deep layer of liquid hydrogen.
C. Moons and Rings
Neptune appears to have ring segments that are denser spots in thin, full-circle rings. It is guessed that these are rocky, not icy.Triton is the largest moon, frozen methane, water, ammonia ices and a thin nitrogen-methane atmosphere (it's not that massive, but the low temperature means the gas molecules are really slow, so they don't achieve escape velocity). Also, it is tilted on its axis so that one pole is currently facing the sun for a few decades. It may have nitrogen volcanoes, but this has not yet been proven.
III. Pluto
Hubble Space Telescope enhanced image of Pluto(courtesy NASA)
A. Discovery and Orbit
Percival Lowell, from a wealthy old Massachusetts family, built (and funded) an observatory to study Mars. He hired a "natural," Clyde Tombaugh, a young man from a farm in Kansas who had a wonderful eye for astronomy, and Tombaugh found Pluto. The designation used for Pluto today is a "PL" rebus that could stand for PLuto, or for Percival Lowell. Pluto is very small as planets go, only 0.002 Earth masses. Its orbit is also pretty elliptical for a planet: it varies from 29 to 49 A.U. from the Sun, crossing inside of Neptune's orbit. Finally, Pluto's orbit is inclined 17deg to the ecliptic, so it goes farther above and below the plane in which the other planets formed (the plane of the Sun's equator) than any other planet.
B. Structure of Pluto
Pluto is made up of rock and ice, but the ices range from nitrogen and carbon monoxide ice through water ice through methane and ammonia ices, down to the extremely low-temperature ices of neon and argon. The Sun just looks like a fairly bright star as seen from Pluto's orbit. There is a seasonally-enriched, mostly-nitrogen (and CO) atmosphere as Pluto gets nearer to and farther from the Sun on its elliptical orbit; the enrichment is probably methane, which freezes out again in "winter" to form methane frost caps at the poles.
C. Moon: Charon
The weirdest thing about Pluto, in addition to its elliptical orbit, is that it is almost a double planet -- its moon, Charon, is 1/2 the diameter of Pluto! Of course, this is only about 1/12 the mass (it doesn't compress as much due to self-gravity, as the more massive Pluto does). It's tidally locked, forever facing Pluto, and completing its lunar orbit (month) every 6.4 days! Unlike Pluto, Charon has more ice than rock, which (along with axial tilts that suggest it: 122deg) means it's probable that Pluto and Charon were planetesimals that collided, or nearly did, and the more massive one wound up being the planet, while the lass massive one would be the moon.
Chapter 26: Meteorites, Asteroids, and Comets
I. Meteorites
Chunk of asteroid Vesta that arrived as a meteorite (image courtesy NASA)
A. Origins
The Earth gains about 10,000 TONS of mass per year in the form of meteorites -- space debris! Like stars, the massive ones are the rarest, and the smaller they get, the more common they are, right down to dust-grain-sized meteorites (see page 4 for a summary). Meteors (the glowing trails called "shooting stars") happen when a meteoroid hits Earth's atmosphere at 10 - 30 km/sec (36,000 to 100,000 km/hr). The periodic meteor showers originate from debris thrown off by comets as they heat up and lost the ice holding them together: small hunks of rock fall off. The sporadic meteors can come from either cometary debris, or asteroid belt collisions. You are probably also aware of the really rare occurrences, meteorites that originated on the Moon or Mars. These were ejected from the bodies in question as the result of impacts -- when a large meteorite or late-blooming planetesimal smashed into Mars, there would be debris thrown up by the impact directly, but a stranger effect also occurs: the shock wave from the impact travels around the outside of the planet's crust in an expanding ring, which then shrinks back down as it reaches the opposite side of the planet, and focuses at the point directly opposite the impact zone. This can blow off chunks of planetary crust at velocities greater than escape velocity, and a few of these Mars-originated rocks floated through the inner solar system for millions of years before encountering the Earth. At which point, early in this century, one of them entered our atmosphere, survived the burning off of its outside surface, and struck and killed a dog on the street in Cairo, Egypt. Dog Killed By Meteorite From Mars: Truth, if not stranger than fiction, can at least sound a lot like a tabloid headline sometimes.
B. Composition
Meteorites that are seen arriving at Earth, or found in smoking craters, are called falls, which means their extraterrestrial origin is pretty much assured. Looking first at falls, we get a complete sampling of what's actually falling into our atmosphere (and making it to the surface in large-enough chunks). 92% of falls are of the compositional type called stony. 6% are iron meteorites, and the remaining 2% are stony-irons.
When we speak of meteorites, we speak of several different types of space debris at once. The big hunks of rock that make really large craters come from collisions in the asteroid belt. Nowhere else do we get really big hunks of rock. Occasionally, a cometary body will enter earth's atmosphere. This happened, for example, in Tunguska, Siberia, in 1908. The ices tend to make the body explode rather dramatically, but there's not much left in the way of a crater. Rocky remains would be the small chunks of rock usual with cometary debris. Most small chunks of rock from normal cometary orbits, though, burn up enough on entering our atmosphere that what reaches the surface is not very big! This is one of the reasons that meteorite collecting on ice caps is such a good idea -- even small rocks get noticed. Also, the airborne collection of micrometeorites (flypaper on the research airplane) samples a good deal of cometary debris.
So, most of the big space rocks we study on Earth came from the asteroid belt. And that's another story...
II. Asteroids!
High-resolution image of asteroid Gaspra (image courtesy NASA)
A. Orbits
Between Mars and Jupiter lies the asteroid belt, predicted by the Titius-Bode Rule and discovered in 1801 and thereafter. Asteroids started out with stately names like Ceres, Pallas, Vesta, but eventually we got over 3000 or so and decided to let the discoverers go nuts, resulting in names like Chicago, Zulu, and Beer (not to mention the recent addition of Garcia, which was discovered by some old school buddies of mine from UWashington shortly after the death of Jerry Garcia, the Grateful Dead guitarist.) Orbital resonances with Jupiter tend to clear certain areas out, as you discovered in your recent HW, leaving Kirkwood's gaps in the asteroid belt. People are discovering more every year, and this is something amateurs, as well as professionals, can do (so are discovering comets, and monitoring variable stars).
High-resolution image of asteroid Ida (first asteroid discovered to have a natural satellite, Dactyl) (image courtesy NASA)
High-resolution image of asteroid Dactyl, the natural satellite of asteroid Ida (image courtesy NASA)
In other news related to Jupiter and asteroids, there are two breakaway groups located 60deg ahead and 60deg behind Jupiter in its orbit (at the L4 and L5 points, see page 15): the Trojan asteroid groups. As with Jupiter, any planet has stable Lagrangian points at those places in its orbit (which is why Earth's L5 Society wants to build a space colony there), and at least one "Trojan"-style asteroid has been found trailing Mars in its orbit.
High-resolution images of asteroid Toutatis, taken during its most recent close approach -- eight views in composite (image courtesy Goldstone radar installation)
Dangerous NEOs
The Apollo asteroid group is bad news for us. They are asteroids that have gravitationally interacted with Jupiter to the extent that: a) their orbits cross Earth's orbit, and b) Jupiter's continuing influence on some of them precesses their orbits, so eventually they smash into Earth -- at the rate of about one every 250,000 years. As you know if you saw the meteorites tape in class, a 1-km object could end civilization, and possibly mammalian life on the planet, in an event similar to the K-T boundary event that probably put sufficient stress on food sources to force the larger dinosaurs into extinction. As of 1996 we had only found about 30 Apollo asteroids, but we suspected there were many more, as yet undiscovered, such as 1991BA, a 9-meter rock that brushed past us inside of the Moon's orbit in January of 1991. It would have messed us up a lot (tidal waves, etc.), but would probably not have completely killed our food supply. One interesting example of a future close encounter is NEO (Near-Earth Object) asteroid Toutatis. On September 29, 2004, Toutatis will pass by Earth at a range of four times the distance between the Earth and the Moon, the closest approach of any known asteroid or comet between now and 2060. One consequence of the asteroid's frequent close approaches to Earth is that its trajectory more than several centuries from now cannot be predicted accurately. In fact, of all the Earth-crossing asteroids, the orbit of Toutatis is thought to be one of the most chaotic.
High-resolution images from NEAR satellite of asteroid Eros with JPL wireframe computer models (images courtesy NASA/JPL/NEAR mission)
Perils to mars - the Amors
Amor asteroids are like the Apollos, but they cross Mars' orbit. Since the deflection needed is not as great, there are probably more of them than of the Apollos, but we don't have as urgent a need to know about them. Since they cross the asteroid belt at right angles to the flow of traffic, the Apollo-Amor asteroids are probably responsible for most of the collisions that knock meteoroids into our path.
B. Composition
S-type are stony, M-type are metallic, C-type are carbonaceous, and they give rise to (but, except for the irons, are not exclusive sources of) equivalent types of meteorites found on Earth. The largest asteroids may have differentiated due to melting produced by the heat given off by radioactive Aluminum-26, but there's also a good chance that a large planetesimal was forming in the region and got creamed by accelerated rocky bodies from Jupiter's feeding zone as Jupiter was accreting planetesimals. M-type asteroids could be remnants of the differentiated core, and the low-pressure cooling patterns we see could simply be what happened to that core after it was fragmented. We just don't have all the data yet.
Comet Hale-Bopp, 1998(courtesy NASA)
Comet West, 1981(courtesy NASA)
A. Orbits and Origins
Comets are made of the same stuff as Pluto and Charon, so it should be no big surprise that outside of the orbit of Neptune there is a belt of cometary bodies ("dirty iceballs") called the Kuiper Belt. These appear to be the source of the short-period comets (like Halley's with its incredibly brief 76-year orbit), although a gravitational encounter with Jupiter can further modify cometary orbits and make them really short-period. Cometary bodies consist of water and CO2 ice, and probably some methane and ammonia ices, and before they start their inward plunge, they may have more exotic ices as well. The total number and extent of cometary bodies in the outer solar system is not known with great precision, but is estimated at about 3 Earth masses of material spread into around 200 billion (to 2 trillion) medium-sized (10-or-so km) iceballs in a relatively spherical arrangement called the Oort cloud, after the Dutch astronomer who first proposed its existence. The Oort cloud would extend almost 1/3 of the way to Proxima Centauri! These bodies would have gotten out there by being gravitationally ejected from the feeding zones of Saturn and Uranus. Once in a great while, close encounters in the Oort cloud or gravitational perturbations due to stars passing through the area would send a cometary body plunging inward toward the Sun. Those are the long-period comets, but they may also become the comets with hyperbolic orbits (one-time-only!) and the ones that plunge inward so accurately that they hit the Sun and are destroyed. These perturbations and encounters can happen anywhere in the sphere of cometary bodies, which explains why the long-period comets can have such highly-inclined orbits to the plane of the solar system.
B. Structure
The nucleus of a young comet is like a dirty snowball, or iceball, around 10-15 km in diameter (maybe larger). When the comet approaches the inner solar system it crosses the ice line, and formerly-frozen gases such as water vapor and carbon dioxide start to surround the nucleus in a Jupiter-diameter cloud called the coma (actually, the very tenuous outer edge of the cloud can be as large as a solar diameter). The gases and dust being spewed out as the comet is heated by the Sun are pushed away by the solar wind (for the gas tail) and by "sunlight pressure" (the dust tail). The dust tail curves, the gas tail is usually straight, except that its shape is determined by what the magnetic field embedded in the solar wind happens to be doing -- sometimes streamers in the gas tail look almost braided, and sometimes the comet passes through a break in the magnetic field and the tail "detaches" while gas builds up in the coma until the next encounter with the streaming solar wind and its field. Of course, in addition to gas and in with the dust, there are the small rocks we were talking about. Approaching spacecraft must beware this hazard! The tail can be up to an A.U. in length! No wonder comets can be such impressive sights -- they can take up a big angle in the night sky. Old comets (in the sense of many solar encounters; comets are all old!!!) develop crusts of silicates, what used to be the dust and rocks mixed with ices that have "boiled off." These look very black, and are chemically similar to the carbonaceous chondrites. Very old comets, it is theorized, may be hard to distinguish from asteroids, and some of the Apollo asteroids might be dead comets trapped in tiny orbits by an encounter with Jupiter.
You can check out some sample questions on these topics. Use them as flash cards, or tests of your reading comprehension. The multiple choice questions on the exam are mostly variants of these questions (but with a lot of plausible-sounding distractors -- read carefully!)
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