I’m going to ask you to do a little thought experiment here. Suppose that the Earth’s orbit were suddenly to become much more eccentric. Let’s keep our orbital semimajor axis as is, fixed at 1 AU, but let’s imagine shifting both the Earth’s perihelion and aphelion points by 0.9 AU, so that at perihelion we were only 0.1 AU from the Sun (much closer than Mercury), but at aphelion we were way out at 1.9 AU from the Sun (considerably farther than Mars). Make sure you can picture this new orbit!

(a) What would happen to the length of our planet’s year – would it get longer, shorter, or stay the same? Explain your reasoning!  (5 points)

(b) When, in reality (that is, with our actual, real, low-eccentricity orbit), does Earth pass through perihelion each year? During what season, and specifically what month? (Look it up if you don’t know!) How, if at all, does this connect to the variation in the seasons on our planet? Explain!  (5 points)

(c) In the new, highly-eccentric orbit, at which point in the orbit would you expect our planet to experience the hottest temperatures, and why? Would you expect a different answer depending on whether you live in the northern or southern hemisphere? Why or why not?  (5 points)

(d) In the new, highly-eccentric orbit, would you expect the hot seasons and cold seasons to be about equal in duration? If so, why? If not, which season should be longer, and why?  (5 points)

(e) Do a bit of looking around on line, and answer briefly the following question: is our solar system typical in having all of its planets in very low eccentricity orbits? Cite your source(s), and if you find any examples of star systems with highly eccentric planetary orbits, provide us with a link so we can check it out!  (5 points)

Section 3 (25 points total)

We have seen how Venus has become unbearably hot as a result of a runaway greenhouse effect  – that is, Venus may have had oceans at one point… but eventually the planet got just a little too warm, which caused the oceans to evaporate just a little too rapidly, which began to increase the concentration of greenhouse gases in its atmosphere, which caused even more warming, which caused the oceans to evaporate even faster… can you see why it’s called a runaway process?

These sorts of processes are sometimes called positive feedback loops, meaning that you have two or more processes that reinforce each other. In the case of Venus, the two processes were

1. The evaporation of the oceans

2. The increase in the concentration of greenhouse gases in the atmosphere

When you start thinking about it, you come to realize that positive feedback loops are everywhere, not just in science but in everyday life. Think of what happens when you’re feeling a bit depressed. You’re feeling down, so naturally you start thinking about how much work you have to do that isn’t getting done, which gets you feeling even more down, which starts you thinking about how unsatisfied you are in your love life, which… you get the picture. In this case the two processes in the positive feedback loop are 

1. Your depressed state of mind

2. The thoughts you keep having

Each one reinforces the other until things get out of hand!

OK, so what’s the actual assignment here? 

(a) Try and think of another situation outside of astronomy where a positive feedback loop is at work. Briefly explain what the loop is, and how the two (or) more processes in the loop reinforce one another – kind of like what I did above for the runaway greenhouse effect on Venus and for depression.  (8 points)

(b) Look at the first part of the Module 2 readings, the part about the origins of the solar system. If you think about it a bit, you’ll see that the growth of planets through the accumulation of planetesimals also involves a positive feedback loop. See if you can figure out what it is, and explain what two things are going on which reinforce one another.  (8 points)

(c) Here on the Earth, there is something called the ice-albedo feedback loop – another example of positive feedback which contributes to global warming. The word albedo refers to the reflectivity of a planetary surface. Do a little bit of research on this, and explain what two things are going on here which reinforce one another and are causing the warming of our planet to accelerate. (9 points)

Section 4 (25 points total)

Some questions to do with the planets. In each case, give a brief but complete answer.

(a) It turns out that Mercury and Mars have the same gravity as one another – that is, you would weigh the same on the surface of Mercury as you would on the surface of Mars, despite the fact that Mercury is smaller. Why is this the case?  (5 points)

(b) I mentioned in the section on Venus that even if Venus used to have oceans (and maybe even life), there is no way for us to tell today whether this was ever the case. Thinking about what we discussed about Venus’s strange geological history, why do we not expect to find any clues to Venus’s ancient past?  (8 points)

(c) In 2006, Pluto was de-classified as a planet and re-classified as a dwarf planet. What is a dwarf planet exactly, and how does it differ from a planet? What other bodies have been classified as dwarf planets? (5 points)

(d) Mars does not have a large moon like the Earth does. Thinking back to the documentary you watched in Module 2 (“If We Had No Moon”), what do you think the lack of a large Moon will mean in terms of the stability of Mars’s climate in the future?  (7 points)

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