A Note of Caution. . . .
Remember, this study guide is only an aid: a guide to studying your textbook and notes. I will try to mention all of the important themes, but I surely won't be able to recap every important detail we discussed in class. That doesn't mean you don't need to know any such details for the exam.
On the other hand, in any astronomy course there are many details that are not important: the radius of this planet, the precise name of that mineral, the launch date of some spacecraft mission. These you shouldn't worry about.
While Venus is our closest relative when it comes to geology, Mars has the most Earth-like climate. It's sort of like Antarctica, mind you -- a frigid desert -- but Antarctica is part of Earth. In fact, there are microbes living in Antarctic ice, so we'd love to know if Martian microbes have ever existed.
It's Mars' thin atmosphere, as much as its greater distance from the Sun, which keeps it so cold. We can imagine a "reverse greenhouse effect" (or "runaway refrigerator") taking place in the past, whereby water vapor gradually condensed onto the surface, allowing more IR light to escape, cooling the planet, causing more condensation, etc. Today it's so cold that even the carbon dioxide is largely in solid form (at the polar caps). As we discussed, liquid water can't exist in such a thin atmosphere. There's ice at the poles, and in 2002 Mars Odyssey gamma ray and neutron spectroscopy provided strong evidence for an icy subsurface layer below a thin, fairly dry rock layer; you should be able to discuss the reasoning in some detail.
But this theoretical "reverse greenhouse" argument implies that the atmosphere was once much warmer and thicker than it is today, so that there would have been liquid water. Many data support this view. We see valley networks which appear to have been cut out by intermittently running water (think flash floods), and catastrophic outflow channels (not canals!) which were scoured by huge one-time flows of liquid. Lots of layering has turned up in Mars Global Surveyor images, and layers suggest sedimentation suggest water. There may even have been an ocean in the northern hemisphere.
You should be able to discuss the instruments on the two Mars rovers, and you should understand why we now think that Opportunity has been probing rocks that formed under water which was at least a few centimeters deep.
Mars has some impressive geologic features, including the solar system's largest mountain (Olympus Mons) and largest crack (Valles Marineris). These features occur on or near the Tharsis bulge, a region of geological uplift produced by a mantle hotspot. There's no plate tectonics, however -- which means no recycling of CO2 into the atmosphere. This keeps things too cold for Goldilocks to be happy.
Mars's Earth-like rotation rate and axis tilt lead to day/night variations and seasons -- in short, to weather. There's no rain, of course, but there is frost and fog and (thin) cloud. There are global dust storms that kick up and obscure the entire planet for months at a time. The polar caps grow and shrink with the seasons, and may give evidence for past ice ages. (One residual [summer] cap is mostly carbon dioxide, while the other is mostly water.)
So, is there or was there ever life on Mars? Although the canals of last century were figments of overactive imaginations, there could be life, and there certainly could be fossilized microbes from long ago when the climate was warm and wet.
Jupiter, the largest planet in the solar system, is also the innermost of the four giant gas planets. (These four -- Jupiter, Saturn, Uranus, and Neptune -- are often referred to as the Jovian planets, since Jove was an alternative name for Jupiter, the chief Roman god.) The planet's low density tells us that it has to be mostly gas. It's mostly hydrogen, and what's not hydrogen is mostly helium; this makes it chemically similar to the Sun. In fact, the Galileo probe which did a kamikaze dive into Jupiter in 1995 showed that the helium fraction (11%) is nearly the same as in the Sun's atmosphere (10%). Jupiter has only 0.1% of the Sun's mass, but had it been 80 times more massive than that, its core would have become hot enough (roughly ten million K) for nuclear fusion to take place: it would have been a star. Instead it's a "failed star." Incidentally, astronomers think they've found a few objects in our galaxy that are a lot bigger than Jupiter, but which still aren't big enough to be stars; these are called "brown dwarfs."
Neither Jupiter nor a brown dwarf is brown, of course, but they are warm inside and so they do emit infrared light. (Remember, the light we see when we look up in the evening isn't emitted by Jupiter -- it's reflected visible sunlight!) Jupiter emits infrared light energy at twice the rate it absorbs visible light energy, so we know that it has a significant internal energy source, although this source is far weaker than what nuclear fusion would provide.
Between this energy source and the planet's rapid rotation, Jupiter has extremely interesting weather. The rotation is important due to the same phenomenon that we encounter on Earth: the Coriolis effect. Essentially, Isaac Newton said that all matter tends to move in straight lines unless pushed or pulled to the side, but winds don't seem to move straight because the planet itself is spinning. Instead, they seem to curve rightward in the northern hemisphere, leftward in the southern hemisphere. This means that when gas tries to move from a high pressure zone to "fill in" a low pressure zone, it curves and misses. Furthermore, things are different at ground level vs. aloft because of friction. The end result is that highs and lows can persist, so we have weather. Mind you, this condenses about a month's worth of a meteorology course into a paragraph, but you get the gist....
There are enormous storms on Jupiter; the largest and longest-lived is the famous Great Red Spot. Storms are long-lived because there's no land for them to cross over. High-speed winds are another notable feature. The atmosphere, warmed from below, is in a state of convection, much like a pot of boiling water heated from below. Hot material expands and rises in "zones," then cools and sinks in "belts." In other words, gas circulates in convection cells. (Belts are darker than zones because chemical changes occur as the gas cools, and the resulting molecules are better at absorbing visible light.) Each convection cell involves north/south as well as up/down motion, and on Earth the Coriolis effect bends the north/south winds into the low-latitude "trade winds" and high-latitude "polar easterlies" (both of which blow from the east) and the mid-latitude "prevailing westerlies" (which blow from the west). Similarly, Jupiter has an alternating wind pattern: "zonal flow."
The interior of Jupiter is also interesting. The core is probably a rock/ice mixture, perhaps 10-15 times more massive than Earth. (That sounds like a lot, until you realize that the entire planet is 300 times more massive than Earth!) The gas surrounding that core is under such great pressure -- due to the weight of all of the outlying gas -- that electrons are free to move about. This dense, electrically conducting fluid is known as "metallic hydrogen." Rapid rotation causes turbulence in this fluid, which through the complex "turbulent dynamo" mechanism produces Jupiter's strong magnetic field.
OK, what about the moons? Of the 60-odd satellites found to date, the most interesting are the four Galilean moons. The largest moon in the solar system is Ganymede, which has a mixture of dark, cratered terrain, younger icy "maria," and odd grooves and ridges that may result from early plate tectonic activity. Callisto is entirely heavily cratered, with the biggest impact basin (Valhalla) about 3000 km across. (The only larger basin we know of is near the south pole of our own Moon, on the far side where it's largely hidden from our view.) But the really spectacular satellites are Io and Europa. In brief, Io's volcanoes result from (a) tidal distortion by Jupiter; (b) a 2:1 "orbit-orbit resonance" with Europa, whereby Europa orbits Jupiter exactly once for every two of Io's orbits, preventing Io from settling into a circular orbit with synchronous rotation; hence (c) Io flexes back and forth and so remains incredibly hot inside.
(Remember that tidal forces are simply the differences in gravitational force exerted on opposite sides of extended bodies. Tides distort objects and, as we showed in class, can result in torques which influence rotation rates. This is why our Moon always faces the same side toward us: "synchronous rotation." Tidal distortion raises ocean tides on Earth, slows our rotation [i.e., lengthens the day], and forces the Moon to move into an ever-larger orbit around us [i.e., lengthens the month].)
Europa in turn is in a 2:1 orbit-orbit resonance with Ganymede, leading to tidal flexing and heating (although not so much as Io experiences). The satellite is dense enough that it's mostly rock, but the outer layer (crust) is ice. Europa's surface is only 50-100 million years old (how do we know?), so some process is continually "resurfacing" Europa. We see strong circumstantial evidence (such as?) that the outer ice crust is floating atop a global ocean. Some of that evidence could also be explained by soft ice, or by an ocean which used to exist but recently froze; magnetic field measurements, however, seem to require a salty ocean to exist right now. You should go through the various Web links to remind yourself of all these lines of evidence.
There might even be life in that ocean, despite the inability of sunlight to get through the ice above. (We're wondering if the same can be said for Lake Vostok, the recently discovered body of water under the ice of Antarctica.) Life could exist at the ocean floor, or at lesser depths, or perhaps even near the surface within cracks in the ice. Where might the necessary nutrients come from?
Why are the gas giants so large, and why do they have so many moons? To understand this we must talk about the origin of the solar system. The "solar nebula" was the molecular hydrogen cloud from which the solar system formed. As it contracted, most of the mass condensed and heated at the center until nuclear fusion became possible; this is the Sun. Yet a small fraction of the gas was forced (via "conservation of angular momentum") to form a rotating disk instead.
Infrared emission to space enabled the gas to cool off, so solid material (a.k.a. dust) began to condense in the inner solar system. But only the most refractory materials -- metals -- could condense in the innermost regions, while silicates (rock) could also condense in the vicinity of the future Earth. The outer solar system was able to make solids from volatile materials: ices. These dust grains began sticking together via collisions, until reasonably large "planetesimals" had formed.
Further coalescence (or "accretion") resulted in eight very large objects -- the major planets -- which continued to sweep up / get battered by the remaining planetesimals near their orbits. Some planetesimals disappeared this way; others smashed each other into bits. The Jovian planets became large enough (10-15 times more massive than Earth) that they were able to pull in much of the surrounding hydrogen gas, producing huge atmospheres and disk-shaped "mini solar systems" (moon/ring systems). Finally, the Sun went through an adolescent phase in which the solar wind became extremely strong, thus "blowing" the remaining hydrogen and helium gas out of the solar system.
Titan, the second largest moon in the solar system, interests us because of its thick atmosphere -- about 60% thicker than our own! Although the main constituent is nitrogen, with the rest largely argon, there's also some methane, the simplest hydrocarbon. In fact, there's trace amounts of ethane, propane, and carbon monoxide, and sunlight provides the energy to convert simple orgainic molecules to complex "photochemical smog." This reddish haze hides the surface from our gaze, although we've recently been clever enough to use microwave radar beams and reflected infrared sunlight that can penetrate the haze.
The atmospheric methane implies that there's some reservoir of methane and other simple hydrocarbons, because UV sunlight would quickly destroy (dissociate) methane in the upper atmosphere unless it were being continually replenished from below. This reservoir was postulated to be a global hydrocarbon ocean. But even before Cassini reached Saturn, our IR images and radar data obtained from here on Earth indicated that the surface isn't uniform and smooth, ruling out global oceans produced by methane/ethane rain. Yet there still might have been hydrocarbon lakes or seas....
Cassini dropped off the Huygens probe, which landed on Titan in January 2005; it measured atmospheric conditions on the way down and determined what kind of surface it landed on (how?). (Huygens relayed data for about four hours, since its batteries wouldn't work any longer in the extreme cold.) What does this part of Titan look like up close, and what process(es) appears to be taking place?
(Incidentally, how did Cassini manage to get out to the Saturn system in the first place?)
The probe mission having been accomplished, the Cassini orbiter will spend at least four years studying the Saturn system, with Titan receiving frequent close-range scrutiny. (Why?)
Titan is important, despite its probable lack of life (no liquid water!), because its organic content may make it similar to the early Earth; the low temperature has prevented chemical reactions (or life!) from altering things much over the last 4.5 billion years. So we can understand something about the origin of life on Earth by going to Titan.
Finally, a few words about the trademark ring system. Why did some of the solid stuff that condensed near Saturn form moons, and some form ring particles? Because close to Saturn (closer than the so-called "Roche limit" for all you jargon-lovers out there) the tidal distortion induced by Saturn is too great for a moon to be held together by its own gravity. More precisely, the small bits of solid stuff within the Roche limit were never able to stick together (coalesce) to form moons in the first place, while further from Saturn this wasn't a problem. The same thing goes for Jupiter, Uranus, and Neptune.
Saturn's rings are so bright because they reflect visible sunlight very well, and this in turn is because the individual ring particles are made of ice and (perhaps) ice-coated rock. The Voyager 1 and 2 missions revealed ringlets, spokes, and a braided ring, leading to theoretical explanations involving (for example) density waves and shepherd moons. The Voyagers also turned up a few really faint rings. But we already knew about the A, B, and C rings, plus the major gaps such as the Cassini division. You'll want to know something about how the narrow gaps are produced by moonlets, and (especially) about how the large gaps are produced by orbit-orbit resonances with some of Saturn's moons.