One of the most common negative comments I get on my evaluations is that the students do not feel they know what types of answers I am looking for on the exams. I feel that this is a fair criticism, so this little tutorial is part of my attempt to address this issue. Here are a handful of classic Astro 150 exam questions with answers of various quality. The questions are boxed in bold, the various answers follow in italics, and my comments follow each answer in red. Use these answers and my comments as a guide to calibrate how you should answer questions on the exams.

tl;dr: A good answer shows that you understand the concepts behind the questions and are not just good at memorizing.

(2 pts) TA's Name and Section: _____________________________

That person with the hair who teaches in the morning.

0 point answer - You have been in this class for at least 4 weeks now. Know your TA's name. Note that some TA's are very sensitive to how you spell their names! Spell it correctly or face their wrath (or at least 2 points worth of their wrath).

(4 pts) A world had a density of 3.7 g/cm3. What is the most likely composition of this world?

Rock

1 point answer - Rocks have a density of about 3 g/cm3 so at 3.7 g/cm3, this world has something besides just rock.

Rock and Iron

2 point answer - Yes, but you say nothing about the relative amounts. A planet made of rock and iron can have a density anywhere between about 3 and 8 g/cm3

Mostly rock with a little Iron.

4 point answer - Shows you know that the density can give you an idea about both the type and relative amounts of materials that make-up a world.

(6 pts) How do we determine the age of the lava flow on the floor of the Tsiolkovsky crater on the *FAR* side of the Moon?

Get a sample from the surface and determine its age.

1 point answer - Yes, we would like to do this, but the far side of the Moon has not been sampled. Notice that the term "Far" has been emphasized - that means it is probably important. The questions asks how we determine the age, not how we would LIKE to determine the age.

Count craters / Use crater counting / determine the crater density

2 point answer - Yes, you would count craters, but then what? This is just collecting the data, what do you do with it?

Determine the crater density and compare it to another part of the Moon with a similar crater density.

4 point answer - Much better, collect data and do something with it. Only thing missing in this answer is being specific about what you are comparing your data to.

Determine the crater density and compare it to another part of the Moon with a similar crater density whose age has been determined by returned samples.

6 point answer - Best. This shows that you know that you know that determining the age of an unsampled surface depends on comparing it to a surface of know age. It also shows that you know you need a sample from the surface to determine the absolute age.

(8 pts) Most of the Earth's surface from 3.8 billion years ago is long gone. We have almost no rocks or features that existed 3.8 billion years ago. How do we know that the Earth was being bombarded by large objects at this time?

Late Heavy Bombardment.

2 point answer - Congratulations, you have memorized the term "Late Heavy Bombardment"! How does it apply here?

Because Late Heavy Bombardment happened 3.8 billion years ago.

3 point answer - A little better. You memorized the term "Late Heavy Bombardment" along with its age and realized the age matched what I asked in the question. But, why do we think LHB happened on the Earth?

Since we know LHB happened on the Moon 3.8 billion years ago, we assume the same thing was happening to the Earth.

6 point answer - Good answer, you showed you know that knowledge of the early history of the solar system the comes from studying the Moon, and you linked the histories of the Moon and the Earth.

We have returned samples from the large impact basins on the Moon and found them all to be about 3.8 billion years old. This shows the Moon was hit by large objects 3.8 billion years ago. We assume that all worlds in the inner solar system went through a similar early history as the Moon. Therefore, the Earth was hit by large objects 3.8 billion year ago.

8 point answer - Shows you clearly understand the whole chain of evidence for LHB on the Earth and have not just memorized the list of the 5 stages of planetary evolution. Notice that this answer never even uses the words "Late Heavy Bombardment". Knowing the keywords is nice, understanding the concepts behind them gets you the points.

(3 pts) Which would be older? A mare surface with N(10) = 10, or a highland surface with N(10) = 5?

The highland surface is older since highlands are older than mare.

0 point answer - Look at the data in the question. In general highland surface are older than mare surfaces, but NOT in this case.

A surface with N(10) = 10, has more craters than a surface with N(10) = 5. Therefore, this mare surface would be older than the highland surface.

3 point answer - You answered the specific question I asked, using the data in the question.

(6 pts) A world has a size of 1/4 the size of the Earth. Would you expect this world to be geologically active today? Explain your answer.

No

0 point answer - You guessed correctly. No explanation - no points.

No. The world would not be geologically active today because it is small.

3 point answer - Better, but you need to explain the connection between size and activity.

No. Geological activity scales with size. A world 1/4 the size of the Earth would not likely be active today because it would have lost all of its internal heat long ago because small worlds have less fuel, and loose heat at a faster rate than large worlds.

6 point answer - You show that you understand why geological activity scales with size.

No. A world 1/4 the size of the Earth would be smaller than Mars, Since geological activity scales with size, and Mars is not active today, I would not expect this world to be active.

6 point answer - Another fine answer. Compare an unknown world with one that we know about.

Note: If you finish an exam in this class and did not use the phrase "geological activity scales with size" at least once, you may want to look over your answers!