# astronomy assignment 8

(1) CRATERING

When a meteorite hits one of the terrestrial planets it makes a crater with a diameter of about 20 times the diameter of the impactor (the book says x10 but x20 is more typical). Estimate the size of the impactor that made the following four craters:

(1 pt.) (a) During the Apollo program the astronauts found lots of craters about 500 meters in diameter.

(1 pt.) (b) Crater C.Bach on Mercury is a double ringed crater about 100 km across.

(1 pt.) (c) Mercury’s largest craters, other than the Caloris Basin, are about 625 km across..

(1 pt.) (d) The Barranger Meteor Crater in Arizona is 1.2 kilometers in diameter.

(1 pt.) (e) The largest craters on the moon, called maria, look like “seas”. What material flooded these craters?

(2 pts.) (f) Your textbook discusses the differences between the northern hemisphere and the southern hemisphere of Mars. Explain: (1) Which surface is older? (2) What evidence, what observations, allows us to determine which surface is older?

(2 pts.) (g) How do we know that the rate of cratering on the Moon changed dramatically about 3.8-3.9 billion years ago? In other words, what is the evidence which leads us to this conclusion? (Give a brief but clear explanation in about 3-5 sentences).

(2) TECTONICS

(6 pts.) In another solar system, we discover two terrestrial planets with very different levels of tectonic activity. One planet has lots of tectonic activity, while the other has almost no tectonics at all. Propose two hypotheses and explain exactly how each hypothesis could account for this difference.

(3) EROSION

Compare the two images of a runoff channel on Mars and the red river in Louisiana:

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(2 pts.) (a) Describe the evidence of erosional processes in each figure.

(2 pts.) (b) In each case, what can you conclude about the planet’s atmosphere, past and present?

(4) INTERIORS

Say we discover two new planets — Sram and Noom. They are both believed to have a core of rock surrounded by a mantle of ice. They are the same size but Sram has a mean density of 1500 kg/m^3 and Noom has a density of 2600 kg/m^3.

(3 pts.) (a) Which has the thinner mantle of ice? Explain why.

(3 pts.) (b) Which is expected to have greater radioactivity heating the interior? Explain why.

(3 pts.) (c) Yrucrem has the same mean density as Sram but Yrucrem has a radius twice that of Sram. Which planet will have cooled faster?

(4 pts.) (d) What would be the evidence that you would look for on the surfaces of Sram and Yrucrem to confirm your hypothesis of which planet had cooled first?

(5) CRATERS

(5 pts.) In the distant future, a space probe exploring another solar system takes pictures of the surfaces of two terrestrial planets which are very close to each other. One terrestrial planet is densely covered with craters, while the other has only a few craters over its entire surface. Propose four different hypotheses to account for this difference, and in each case explain why your hypothesis would lead to a difference in the quantity of craters.

(6) Stars

(3 pts.) How many star are in our solar system?