THE SOLAR SYSTEM

• The origin and evolution of the solar system. We've already discussed overall layout of the solar system, and a simple model for its formation that explains the motions of planets around the sun.

• Review: formation of solar system from an interstellar gas cloud. We've used this model to understand:
  • how the planets get the transverse velocity needed to keep them orbiting around Sun
  • why all the planets orbit in the same direction (and in roughly circular orbits)
  • why all the planets orbit in the same plane

• However, we've also learned that the planets are not all identical to each other. In particular, there seem to be two or three main groups of planets: the inner rocky planets, the outer gaseous planets, plus Pluto (and maybe Kuiper belt objects). Now we'd like to consider:
  • How do we know that planets fall into these different groups?
  • Can our model for the formation of the Solar System explain why the different groups exist?

• How do we know that there are different groups of planets?
  • We found that planets do not all have the same mass, but come in a wide range of masses, by studying orbits of moons. We can start by asking the question: how do we know that the more massive planets are not simply bigger versions of less massive ones, and that they are made of something different? We then want to understand why this might be.
  • We can see that the outer compositions of the planets are different from each other. What about the interior compositions?
  • Consider a picture: what do you think?

• These questions can be approached approximately by measuring the densities of the different planets. The density of an object is defined as:
  density = mass/volume
  Densities are relatively easy to measure; we can measure masses using the motions of satellites and our understanding of gravity, and we can measure volumes by observing that planets are roughly spherical and by measuring their diameters.
  • When we measure mean planetary densities, we find that planets do not all have the same density. The inner four planets have densities of about 5gm/cm³, while the outer planets have densities of about 1gm/cm³. Pluto doesn't quite fit with the other; it has a density of about 2gm/cm³. To get a feel for what these numbers mean, note that water has a density of 1gm/cm³. The units, gm/cm³, can be understood by noting that the density in gm/cm³ just gives the mass of a small cube of the material which is 1cm on each side.

  • We see that planets are split into 3 classes: the inner planets (terrestrial) which are more dense, the outer planets (Jovian) which are less dense, and Pluto, which is intermediate.

  • Remember that these measurements just refer to mean densities of planets. In fact, planets are not made of uniform density material; they are more dense in their inner regions, and less dense in their outer regions.

  • Since the densities of the planets are different, we infer that the compositions of planets differ. The inner planets are made of rocky material, which has higher density; these are often known as terrestrial (earth-like) planets. The outer planets are made mostly of low density gases; these are often known as Jovian (Jupiter-like) planets. Pluto may be made of some combination of rocks and ice, and may represent the largest of a third category of objects, called Kuiper belt objects, that we discussed a bit in the first unit of the class.

• Why are the compositions of planets different from each other? What determines the composition of planets?

• The reason for different composition of planets has to do with how the solar system formed. In the early solar system, there was a disk of material rotating around the Sun, from which the planets eventually formed. However, to form planets required some kind of initial clump in the protoplanetary disk. These clumps were probably formed from collisions of solid particles. Different elements become solid at different temperatures: the most common elements, hydrogen and helium, never became solid, but hydrogen compounds (like water) can solidfy at colder temperatures, like those in the outer solar system. In the inner solar system, it was too hot for these compounds to solify; only rocks and metals can solidify at these temperatures. Hence, only small planetessimals formed in the inner solar system.

• In the atmospheres of planets, atoms are held to planets by gravity. However, if an atom can move fast enough, it can escape the gravitational pull of the planet, in the same way that we can launch spacecraft which can escape the gravitational pull of the Earth by shooting them off fast enough.

• In fact, atoms do move around. The amount they move is related to their temperature. In a hotter environment, atoms move faster. However, the speed that atoms move also depend on the type of element; at the same temperature, heavier atoms move slower than lighter atoms.

• These properties can be used to generally understand why the compositions of the inner planets differ from those of the outer planets.
  • The outer planets are sufficiently massive and sufficiently cool that no elements can escape their gravitational attraction, so their composition is similar now as to what it was when they originally formed. This is also true of the Sun. This primordial composition is the same as that of most of the Universe: about 90% hydrogen, 9% helium, and only 1% for everything else combined. The normal matter in the Universe is mostly hydrogen.

  • The inner planets are less massive and hotter than the outer planets, so many elements can escape their gravity. On Earth, hydrogen and helium escape so they are not an important constituent of our atmosphere. Instead, our atmosphere is mostly nitrogen and oxygen.

  • On the smallest objects, e.g., Mercury and the moons of most planets, gravity isn't sufficiently strong to retain any atoms - so these objects don't have any atmospheres at all.

• Although these principles are the most important in determining the composition of planetary atmospheres, other effects can also be important, such as the presence of volcanism, life, or manmade pollution.

• Although the inner planets are roughly similar to each other, and the outer planets are similar to each other, there are some important differences, especially, for example, if one wanted to consider a visit to other planets. The details of the nature of each planet is different. Also, planets are not totally uniform (they're not the same in the center as on the outside), and they change as time passes. So we can talk about the evolution of planets, we will discuss physical processes that are important on each planet.

• First let us consider the terrestrial planets. The planet we know most about is our own planet, Earth.
  • The surface of Earth is mostly composed of water, which has a density of 1gm/cm³. Even if one considers just the surface rocks, one only finds typical densities of 2.7gm/cm³. Since the global density of earth is $\sim$ 5gm/cm³, we immediately know that the inner parts of earth are different from the surface. The central regions of Earth are significantly denser. We believe that the central regions of Earth are composed primarily of iron-like elements, while the surface rocks are composed of lighter elements.

  • We can learn about the internal structure of Earth using earthquakes. These send vibrations through the Earth, and by measuring how long it takes these vibrations to pass through various parts of the Earth, one can learn a lot about the internal structure of the planet. We find that the Earth has three main layers: a core, a mantle, and a crust. The core is probably hot and molten in the inner parts. The mantle is a semisolid plastic-like consistency, and the crust is rocky. We know that the inner parts of the Earth are hot because of the presence of volcanoes.

  • Earthquakes also provide important clues about the structure of the crust because of their location; earthquakes occurs only along specific fault zones on Earth. Associated with the locations of earthquakes, we find long surface features such as trenches and mountains. In fact the earth's crust is broken up into several dozen plates. These observations lead to the theory of plate tectonics: each plate is moving around. Plate tectonics is driven by motions in the mantle, which are probably driven because the inner part of the Earth is hot and molten. Plate tectonics accounts for the appearance of many geological features, e.g., mountain ranges, some volcanoes. Note that there are some volcanoes which don't appear on plate boundaries: these are caused by hot spots under the crust.

  • Volcanoes (volcanism) change the appearance of the surface of the Earth because they are constantly creating new crustal material. Volcanoes can be seen from space.

  • Another driving force on the surface of the earth is erosion: we find some sharp mountains and older, more rounded mountains. Erosion arises both from the action of water and wind. Erosion by water leaves distinctive markings on Earth as seen from space

  • There is another important thing that changes the surfaces of planets, though it not so obviously important on Earth: cratering. The solar system, in addition to containing planets and moons, contains a lot of much smaller objects ranging from small dust particles to rocky objects miles in diameter; these are known as meteors. Occasionally, these objects collide with the planets. When they do, they can make craters. On Earth, cratering is not so important because most of these objects burn up as they pass through the atmosphere; this creates the phenomenon known as shooting stars. However, the largest objects will make it to Earth's surface and make craters. In fact, it is possible that the history of life on Earth has been profoundly affected by a few large meteoric impacts in the past. Craters on Earth rapidly disappear because they are covered up by new material (volcanism), destroyed as a plate moves into the mantle when it collides with another plate, or are eroded away.

  • Not only the surface is changing, but also the atmosphere changes with time. If you looked at Earth as a planet from space, you'd clearly see the presence of an atmosphere with cloud patterns that change over time. However, atmosphere is just a tiny layer of Earth.

• To summarize, we have discussed 4 potentially important physical processes that affect the surface of a rocky planet: plate tectonics, volcanism, erosion, and cratering. What do the presence or absence of these physical processes tell us about the nature of planets?
  • The presence of atmosphere has an important effect: if a planet does not have an atmosphere, it won't have significant erosion, and as a result, cratering may become more important (although the amount of cratering can also be affected by the presence of volcanism which can erase craters). What determines whether a planet will have an atmosphere? As previously discussed, it depends on the distance of the planet from the Sun (which affects its temperature) and the strength of the planet's gravity.

  • The internal temperature of a planet is important: if the internal temperature isn't hot enough for it to be in a liquid-like form, then it won't be possible to have volcanism and, likely, plate tectonics. What determines how hot the inside of a planet will be? We think that all planets started out hot when they formed, because the energy of gravitational collapse acts to heat up an object formed by gravity; it is likely that bigger planets generate more internal heat during their initial formation, because the stronger gravity leads to more heating. In addition, bigger planets will take longer to cool off than smaller ones, so one would expect that bigger planets will have hotter cores for a longer time than smaller planets.

• Now we can consider the other rocky inner solar system ojbects, with the question: how similar are they to Earth? Of the dominant forces which occur on Earth (plate tectonics, volcanism, erosion, cratering), which occur on the other planets, and in what order of importance? What do we infer about the nature of these planets? Is it consistent with what we expect?
  • Moon. Not a planet, but useful as a comparison to Earth. Apparent features are totally different from Earth. Two distinct types of regions: highlands and maria. Dominant surface feature is craters. Note difference in number of craters between highlands and maria; this can be used to infer relative ages of the two regions. Because of prevalance of craters, there is clearly a lack of volcanism, erosion - understood given lack of atmosphere and smaller size of Moon compared with Earth. However, maria suggest that there was sort of volcanic processes in the past.

  • Mercury. Learned about from Mariner spacecraft in 74-75. The most prominent feature is craters - Mercury looks a lot like the Moon. One interesting geological feature are scarps, long cliff like regions. We do not see any volcanoes or clouds. We infer that Mercury has no atmosphere and a cool interior. However, we suspect that the interior of Mercury was once hot (we see cliff-like features, called scarps, which plausibly formed from shrinking crust as planet cools).

  • Venus. Although very similar in size to Earth, it's actually quite a different place. Venus appears totally covered by clouds when viewed from Earth; it has thick atmosphere, mostly CO₂. Although clouds shroud Venus, the surface was mapped by the Magellan spacecraft in the early 90's using radar; from this, it is possible to reconstruct a 3D map of Venus' surface and as a side result to simulate a flyby. The main geological features are rolling plains, highlands, and lowlands. There are not many craters, but there are a few big ones; the lack of small craters is probably because of the thick atmosphere. There does not seem to be any evidence that the surface of Venus is broken up into plates. However, Venus does have volcanoes (although we have not seen any active ones), and it has some peculiar circular bulges, called coronae, which appear to have a volcanic origin. The implications are that Venus has a hot core, though the lack of plate tectonics suggests that the structure of the mantle/crust may be very different from that of Earth. Venus has a very high surface temperature (472 C); this arises because of an effect called the greenhouse effect.
    • What determines the temperature of a planet? To a large extent, it is determined by the distance of the planet from the Sun. How does this work? Sunlight is absorbed by the planet causing it to heat up. As the planet gets warmer it starts to glow with its own blackbody radiation. The warmer it gets, the more it glows. The process stops when the amount of glowing from the planet balances the amount of heating from the Sun. When the details are worked it, one finds that planets farther from the Sun should be cooler than those closer to the Sun.

      However, note that the kind of light that the planet emits is not the same kind of light that is incident on it from the Sun. Since the planet is cooler than the Sun, it glows predominantly at longer wavelengths, in the infrared part of the spectrum. So incoming sunlight is predominantly visible light, but outgoing radiation is predominantly infrared light.

    • The greenhouse effect can occur when light (energy) is incident on a planet with an atmosphere. The atmosphere of some planets transmits some wavelengths of light, but is opaque to other wavelengths because of the chemical composition of the atmosphere, clouds, etc. On Venus, sunlight, which is composed largely of visible light, passes through the atmosphere and warms the planet. The planet, as a solid, dense object, shines light back into space, but since it is much cooler than the Sun, this radiation is predominantly infrared light. However, the infrared light cannot penetrate the dense clouds of carbon dioxide in Venus' atmosphere, so the energy cannot escape. As a result, Venus is heated. It keeps getting hotter until the re-radiation of the Sun's energy is able to escape - but this doesn't occur until Venus is several hundred degrees hotter than it would have been without an atmosphere!

    • The greenhouse effect also occurs on Earth - Earth is warmer than it should be based on its distance from the Sun, although it is not nearly so dramatic as for Venus, because our atmosphere has a composition that is much more transparent to infrared light. There is a serious concern, however, that the amplitude of the greenhouse effect on Earth is increasing because human activity (in particular the burning of fossil fuels, as well as deforestation) is releasing significant amounts of carbon dioxide into the Earth's atmosphere. This has the potential of raising the Earth's temperature by a few degrees, which could have some very serious consequences including partial melting of the polar caps and global climate change. There is already strong scientific consensus that global temperatures are increasing; there is also evidence that the abundance of greenhouse gasses are increasing; there is general (although not complete) consensus that human production of carbon dioxide is the cause of this effect. Although there is not complete agreement on this, the potential consquences of global warming are sufficiently serious that it seems prudent to avoid behavior which is likely to increase the greenhouse effect. We definitely know that the greenhouse effect is real - all one needs to do is learn about Venus! Given that fossil fuel resources are finite in any case, it seems hard to justify not decreasing our consumption of them. Technology already exists to improve efficiency of burning of fossil fuels (in particular, in automobiles), and there are several promising technologies for alternative energy production; we need to invest in these.

  • Mars. In many ways Mars may be the planet most similar to Earth. It is smaller and further from the Sun, but still has enough gravity to have a thin atmosphere. The atmosphere is mostly carbon dioxide like Venus, but it is thin enough that it doesn't have a runaway greenhouse effect, so the temperature is not so hot; combined with the fact that its further from the Sun than Earth, the temperature is typically a few tens of degrees cooler than Earth. Mars also has similarities to Earth in that it has a rotation period close to that of Earth (similar length days), and a tilt of rotation axis similar to Earth (so there are seasons, although it's more complicated on Mars because Mars also has a more eccentric orbit).
    • View from Earth. Mars has polar caps which are made of some combination of dry ice and water ice. There are some large volcanoes. There are surface markings. However, occasionally there are large, and even global, dust storms.
    • More detailed view from orbiting satellites: the global topography is a bit unusual, with one hemisphere significantly lower than the other(Mars globe. There are several very large volcanoes on the surface, although they do not appear to be active; there is also a large impact basin (Hellas) on the opposite side of the planet which may be related. There is a gigantic rift valley, comparable in depth to the Grand Canyon but several thousand miles long! There are many craters, but the surface also shows features that suggest that water may have flowed there in the past. Flyby.

    • Overall, the picture is consisent with our expectations: we expect that Mars had a hot core and volcanism at some time in the past. Hard to say if there is evidence for plate tectonics.

    • Because of the possible evidence for water in the past on Mars, it is an interesting place to look to see whether life might have developed there. NASA has plans for fairly extensive visits to Mars over the next decade; the first remote landing was made in 1997 by the Pathfinder spacecraft, with its rover, Sojourner, which went roving around the surface to look at specific rocks. Two rovers were sent to Mars in January 2004 to roam around two different locations on the planet (Spirit landing site). There's lots of images and animations available about Mars; see, e.g. NASA site with lots of images/animataion).

    • We are in an active period of Mars exploration, with several missions currently at Mars and several more planned in the next decade or so; two landers are there right now!

    • Much of the interest in Mars stems from the possibility that water has existed in liquid form at some point in the past on Mars. The existence of liquid water is thought to be critical for the possible development of life. The Viking landers on Mars in the 70s looked for evidence of life, but didn't find anything conclusive.

    • Additional interest in the possibility of life on Mars was spurred by investigations in the 90s of a meteorite (ALH84001 found in Antarctica in the 80s. This meteorite has been determined to have originally come from Mars. The composition of some of the compounds in this meteorite, as well as microscopic features in the meteorite, suggest to some people that there may have been life on Mars at some point in the past.

  • Brief general discussion of missions to planets, manned and unmanned: scientific benefits, human motivations, cost, etc.

• The outer planets are much larger than the inner planets. Because of their large mass and cooler temperature, they have a very different composition; they are mostly made of light elements in a gaseous form. The outer planets are mostly gigantic balls of gas.
  • Outer planets exist in much bigger systems than inner planets. All outer planets have ring systems and moons.
    • In some respects, moons are similar to terrestrial planets. However, they have somewhat different compositions: more icy. There is a great diversity of geologic features on different moons of the outer planets. Also, some of these moons have an important new driving force: tidal heating. The moons of the giant planets, especially those closest to the planet, are constantly being pulled by gravity by an amount which differs from one side of the planet to the other, leading to heating of the internal parts of the moons. This is most dramatically seen in Io, the inner moon of Jupiter; it has active volcanoes on its surface, even though it is a relatively small moon. Europa is of particular interest right now, as the surface suggests the appearance of a frozen liquid. Titan, a moon of Saturn, is also of great interest; as the largest moon in the Solar System, it has an atmosphere!

    • Rings are made of many small particles. They may be the result of previous collisions. We don't know how long rings live around a planet. The dynamics of rings are very complex; they are influenced strongly by the gravitational force of moons nearby the rings.

  • Since they are made of gas, the important phenomena which we observe in the outer planets are winds and clouds. When we look at these planets, we see clouds in the outer layers. We observe different colors when we observe to different depths, because the clouds that form at different heights in the atmosphere are made of different compositions.

  • All outer planets rotate very fast, so winds are very important in determining what they look like; also, storm systems.

  • Our understanding of the interiors of the outer planets comes mostly from our physics models of how gas balls work. We expect that the pressure increases as one moves in towards the centers of these planets. Eventually, you reach a layer far inside of the planet where the pressure is high enough that the hydrogen takes a liquid form. There may be a small rocky core in the centers of the outer planets, but we are not sure.

  • The large mass of the outer planets makes the interiors hot when they form, even more so than for the inner planets, because the larger gravity speed up the atoms more as the planet is formed. The hot cores of the outer planets lead to significant heating from the interior. Since heat escapes from the inside of these planets by the motion of hot material upward, we get belts (dark material falling down) and zones (bright material coming up).

  • Magnetic fields may be important in these systems.

  • In detail, all of the outer planets differ from each other. Jupiter is the most colorful, with lots of atmospheric structure. Saturn has less atmospheric features, and has the largest ring system of all of the outer planets. Uranus is mostly featureless, but Neptune has cloud features.

  • Much of what we know about the outer planets was learned from the Voyager missions, which flew by all of the outer planets between the late 70's and late 80's. The Galileo spacecraft recently orbited Jupiter for several years, studying it and its moons in more detail. The Cassini spacecraft is on its way to study Saturn and its moons.

• What is the age of the Solar System?
  • We can determine ages of certain elements because many elements have an intrinsic property called radioactivity, which causes an element to change to another type of element as time passes. Most elements are very stable, but some elements change faster. The powerful thing about radioactive decay is that the time which it takes for a certain amount of an element to change to another element is very repeatable. Consequently, by measuring the fraction of an element in some specimen which has decayed, we can accurately tell how long the specimen has been around.

  • When we do this for Earth rocks, we find there are some rocks which are as old as 3.2 billion years. When we do it for Moon rocks, we find the oldest rocks are about 4.2 billion years old. When we do it for asteroids, we find some rocks as old as 4.6 billion years old.

  • We thus infer that the the Solar System is at least 4.6 billion years old.

• We are in an exciting era because in the past few years, we have actually started to discover other stars which have planets around them. This is exciting for several reasons: 1) until now, we didn't know for sure whether other planets existed, although we expected that they did, 2) the presence of other planetary systems allows us to test our theories about how our Solar System formed. Do other planetary systems have similar characteristic to ours, or are they different?
  • Unfortunately, it is very difficult to discover planets around other stars. Seeing them directly is very difficult because they are much fainter than the stars around which they are orbiting, and especially because we cannot get sharp enough images to distinguish a planet from a star; images are blurred out because of the effects of the Earth's atmosphere, and even for telescopes above the atmosphere, by basic limitations of a telescope and the nature of light.

  • Despite this, several hundred other planetary systems have been discovered in the last decade or so. The planets in these systems have not been seen directly, instead, their presence has been inferred from their gravitational effect on the star around which they orbit. When planets orbit the Sun, they also cause the Sun to move slowly in a very small orbit itself; the orbit of the Sun gets larger for planets with more mass. It is possible to detect the motion of the parent star through the Doppler effect. The Doppler effect causes the light from a star to shift to a slightly different wavelength as the star alternately moves towards and away from us in its tiny orbit. By measuring the motion of the star, we can estimate the mass of planets around the star, and the radius of the orbits in which they are located.

  • Interestingly, it appears that other planetary systems are quite different from our own (see also http://exoplanets.org/massradiiframe.html. Several of these massive planets are found very close to the parent star, unlike the situation in our Solar System. Also, several of the planetary orbits are very eccentric ellipses, unlike the nearly circular orbits found in our Solar System

  • However, before jumping to the conclusion that our Solar System is an unusal place, one has to realize that this technique for detecting planets is more sensitive to higher mass planets than to lower mass ones, and more sensitive to planets that are closer to the stars than to ones that are further (because the more massive and closer planets cause a larger motion in the star). Consequently, so although only planets with masses comparable or larger than Jupiter's mass have been discovered, this does not mean that there are not Earth-like planets out there!

  • Even still, according to our model of Solar System formation, we would have expected it to be difficult to form massive planets close to a parent star, since it should be too hot there for light elements to be bound to a planet when it is forming. As a result, it would be very nice to know whether these massive, close-in extrasolar planets are gaseous or not. Amazingly, we can get an estimate of this for a handful of the objects. We do this for extrasolar planets that happen to pass in front of their parent star, leading to a slight dimming of the starlight. This dimming can be used to determine the size of the planet, which, combined with the mass estimate from the orbit, can be used to calculate the mean density of the object. For the few objects for which this has been done, we find that they are comparable to the Jovian, gaseous, planets in the solar system; the general feeling is that all of the very massive planets such as the ones being discovered must be gaseous objects, as it's unlikely to get such large concentrations of the relatively rare heavier elements.

  • These discoveries have led astronomers to reconsider the applicability of the nebular model to all Solar Systems. Before we actually found other planetary systems, we would have predicted that massive planets could not exist close to the parent star; but recent discoveries show that they do! Consequently, our models may need to be revised. This is a good example of how science works - we develop models for how things work, and these often turn out to be wrong once we collect new information! Science is an ongoing process...

  • People are actively working to devise systems to detect larger numbers of planets, and planets of significantly lower mass towards the Earth range. There also exists the possibility of getting more detailed measurements of atmospheric abundances in these planets through transit spectroscopy. These studies are opening up a whole new field...