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.
Earth's atmosphere, liquid water, and plate tectonics keep the surface fairly new, but we can still find old, eroded impact craters if we look hard enough. Our cratered neighbor the Moon provides clear evidence that we get hit reasonably often. In fact, we know that a major impact occurred 65 Ma ago, just when the dinosaurs died out; it seems highly likely that this meteoroid caused or contributed to their demise.
We discussed various kinds of evidence that let us distinguish an impact crater from, say, the caldera of an extinct volcano. You should be able to list and briefly discuss these items.
What are mass extinctions? What lines of evidence do we have that the K/T extinction was caused by a massive impact?
Earth initially was hot inside due to the impact energy of the small solid objects ("planetesimals") that collided and stuck together to form our planet; since then it has stayed hot due to radioactivity. Energy released as our liquid outer core slowly solidifies is a second ongoing heat source. The planet ultimately gives up this energy as IR radiation from the surface and atmosphere.
On Earth (but not on Venus!) the primary way in which heat passes from the hot interior to the cooler surface is plate tectonics, which involves convection within the weaker rock of the asthenosphere near the top of the mantle: Hot rock under high pressure slowly flows upward, gives up its heat energy, and sinks back downward. The crust and uppermost mantle -- the lithosphere -- is broken into rigid plates whose slow horizontal and downward motions represent another portion of these convection cells. I'm not talking much about plate tectonics this semester, but in case you're ENTIRELY unfamiliar with the concept, your textbook summarizes it, or you can optionally look through this online book to learn everything you ever wanted to know on the subject.
The Moon is significantly smaller than Earth, and we saw that this means that its interior should cool faster. In other words, it's been geologically dead for a long time. Furthermore, it's too small and has too hot a surface (on the daytime side!) to have any atmosphere to speak of. Liquid can't exist unless the pressure is high enough, so no air (zero pressure) means no liquid water. No air or water means no wind or water erosion. No erosion and no plate tectonics means that the Moon has an ancient, heavily cratered surface, quite unlike the situation here on Earth.
With no air or water to moderate the climate, the Moon is incredibly hot on the sunlit side and incredibly cold on the nighttime side. It's also bombarded with UV sunlight and with cosmic rays, either one of which would be lethal to any surface organisms.
Bottom line: It's not a place where life as we know it could exist.
The rotation of both Earth and the Moon has been influenced by tidal forces, which 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 the 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). We'll see more of tides later in the course, so be sure that you understand them.
The near side of the Moon contains both bright, cratered highlands and dark, smooth, circular maria. About 3.9 Ga ago, enormous impacts dug out round basins and cracked the cooling lunar crust; a few hundred Ma later, dense magma welled up and filled these basins. That age comes from radiometric dating of lunar rocks, which demonstrated that the maria are a lot older than their smoothness would have implied. (You should know the basic principles of radiometric dating.) Why are the maria so old yet so smooth? Evidently because impacts were a lot more common during the first billion years of the solar system than during the time since. This is important support for the idea that the planets formed from "planetesimals," smaller chunks of solid material. Earth and its Moon spent their early years sweeping up planetesimals as they orbited, resulting in constant bombardment (and the delay of life's taking hold on Earth).
Impacts also account for the Moon's origin, which was quite a mystery until recently. We know that it contains almost no metal in its core, because it has too low a mass for its volume (i.e., too low a density) to have much metal. Hence it couldn't be our "sister" planet that formed next to Earth from similar materials. But similarities between its composition and ours -- more specifically, similarities in the ratios of oxygen isotopes -- show that the Moon is unlikely to have formed from different material in a different part of the solar system. Finally, the Moon is probably too large to have been gravitationally captured during a close pass by Earth. Our current best idea involves a Mars-sized object, far bigger than a mere planetesimal, hitting Earth at an oblique angle and ripping out part of our metal-poor mantle. This material, plus the mantle of the impactor, eventually coalesced to form the Moon.
(Trivia fans will want to know that the Mars-sized object has been given a name: Theia, the Greek goddess who gave birth to the moon goddess Selene.)
The online articles about reflectance spectroscopy and lunar ice should tell you how we can learn about the chemistry of the Moon's surface without actually collecting material from the surface. If we can confirm that ice exists at the Moon's poles then, over and above the inherent scientific interest, this would be useful to provide water and oxygen for human colonists.
(We discussed the physics of light as preparation for that article, and you should understand this stuff: blackbody emission, absorption bands, etc.)
Venus was once thought to be Earth's slightly warmer "sister." In fact, given that the thick clouds reflect a large fraction of incoming sunlight back out to space, you might have expected little difference in surface temperatures. But it ain't so. In the 1950s we measured the radio portion of Venus's blackbody radiation, and thereby found that the planet must be quite hot -- literally hot enough to melt lead! By the early 1960s it was clear that this is due to an overwhelming greenhouse effect produced by the thick CO2 atmosphere. Furthermore, spectroscopy shows that the clouds at the top of this atmosphere are made of sulfuric acid, and that there's no water vapor to speak of. Not a nice place for a vacation.
(You understand the basic physics of the greenhouse effect, right?)
The ratio of deuterium ("heavy hydrogen") to ordinary hydrogen is much larger than we'd expected, indicating that Venus used to have water vapor in its atmosphere. This, in fact, would have contributed to a "runaway greenhouse effect" that would have completely evaporated any surface water that once existed. But once the temperature grew to its present high value, the atmosphere was heated so strongly from below (via IR radiation) that there would be no temperature "inversion" whereby temperature starts to increase rather than decrease with increasing altitude. In other words, the temperature on Venus continually drops with increasing altitude, unlike the situation in Earth's stratosphere. Meteorology fans will recognize that without an inversion, humid air could rise indefinitely, moving high enough that solar UV radiation could rip apart ("dissociate") the H2O molecules. The hydrogen (mostly) escapes to space, although deuterium is heavier and therefore not so good at escaping. (This is why we see an excess of deuterium relative to ordinary hydrogen today.) The oxygen quickly combines with other atoms to form compounds. So there's not even much water vapor left on Venus.
Earth's atmosphere was once nearly as thick as Venus's, and it was chemically similar as well. Yet we turned out very different. This is because we were able to maintain a low enough temperature to have bodies of liquid water on the surface. Carbon dioxide dissolved into these primitive oceans, where it reacted with rock or was used by microorganisms to form their exoskeletons; this all ended up as sedimentary rock such as limestone. CO2 also dissolved into rain droplets, and the resulting acid rain weathered surface rocks, producing more sediment.
(Cyanobacteria [formerly known as "blue-green algae"], and later plants, used up more carbon dioxide via photosynthesis, producing all of our atmospheric oxygen in the process. So life has had an important effect on our atmosphere.)
OK, what about Venus's geology? Except for a few short-lived Soviet landing craft, we've learned what we know via radar, first from Earth, later from orbiting spacecraft. You should be able to discuss how radar works.
The latest and most successful mission was Magellan. There may be tectonics going on -- the Venusian crust is in motion -- but not plate tectonics. Magma wells up at hot spots, producing various strange landforms called "pancake domes" and "ticks" and so on.
There are also impact craters -- no small ones, since the thick atmosphere prevents small objects from reaching the ground intact, but medium and large ones. What's surprising about the craters is that they're uniformly spread across Venus: unlike the Moon, and unlike every other large object in the solar system, the Venusian surface has no "old" vs. "young" regions. In fact, the entire surface appears to be fairly young, perhaps 600 million years old. (How do we know?) These facts suggest that the entire planet was resurfaced a relatively short time ago. Planetary geologists are good at thinking up ways to resurface some small part of a planet or moon (such as the lunar maria), but it's a lot harder to come up with a plausible way of resurfacing an entire planet in a short time. The film we saw discussed two competing ideas on how this could happen on Venus, and you should know something about them, including which idea is currently in better favor and why.
In particular, you should understand why we could decide between these theories if we could measure how thick the Venusian lithosphere is. Venus generates thermal energy in its mantle via radioactive decay; this energy must "get out" to the surface so that it can be radiated to space. How does the mantle cool itself? Earth does it mostly through plate tectonics, but that's not happening (now) on Venus. So if the mantle is getting rid of thermal energy at the same rate it's generated, heat must be rapidly passing through the lithosphere to the surface via conduction (easier with a thin lithosphere), or via lots of small volcanoes (more likely if the hot interior reaches closer to the surface), or via regional deformation of the lithosphere (folding and faulting -- easier if it's thin). In short, Venus can make up for the lack of plate tectonics if its lithosphere is thin.
If the lithosphere is thick, however, thermal energy is generated faster than it's transferred to the surface: The mantle gradually heats up. After 500 million years or so, it gets so hot that the lithosphere catastrophically melts, and the molten surface radiates huge amounts of energy to space. Then, having cooled, the surface solidifies and begins gradually thickening again. If this idea of half-billion-year geologic boom-and-bust cycles is correct, we'd expect a thick lithosphere today, given that it's been about a half-billion years since the last global resurfacing (see above).
Venus has fewer "continents" (large elevated regions) than does Earth. The highest mountains on the planet -- the Maxwell Montes -- are found on one of these continents (Ishtar Terra). These are about as high as the highest mountains on Earth. In addition, Venusian mountains can be quite steep; the film we saw discussed why this is possible.
What's the Goldilocks effect? What factor makes Earth "just right"?