Download ASTR1010_HW07

ASTR 1010 – Spring 2017 – Prof. Magnani
Answer Key – Homework 7
Question 1 – Ch.8; # 22
By observing that s-waves are shadowed by a core region, we deduce that the outer
part of that core region must be liquid (since p-waves can’t pass through liquid).
Question 2 – Ch. 8; #25
Plate tectonics is the fractured pieces of the planet’s upper crust floating on the
denser mantle. Convection in the mantle moves these pieces or plates, generating
new seafloor through volcanic activity as plates spread apart. Where they come
together (and one of the plates subducts under another), crust is recycled as it gets
dragged down into the magma of the upper mantle
The planets Mercury and Mars have no tectonic activity. Neither does the Moon. On
Earth, the thin, brittle crust breaks up into tectonic plates (see Figure 8.17) which
are dragged around by convective motion in the mantle (see Figure 8.15). This both
creates new crust (as plates move apart), and destroys old crust (as they come
together) – see Figure 8.16. The Earth basically resurfaces itself in about 250
million years. Venus, being roughly the same size of the Earth should have similar
stuff going on in its surface, but it does not. What exactly is happening from the
tectonic point of view is not clear. One idea is that hot spots called “pancake domes”
serve as outlets for heat transfer from the mantle, others say that basically the heat
builds up under the crust until large chunks of the lithosphere melt and basically flip
around destroying the old crust (which is now at the bottom) and exposing new
crust. The surface of the planet is about 750 million years old, so it is older than the
Earth, but young enough that we know some tectonic activity must be at work.
Question 3 – Ch. 8; #28
Counting the crater density on these surfaces tells you which is younger.
Earth and Venus are the youngest because they still have tectonic activity. Mars
had it up to 2-3 billion years ago. Mercury and the Moon “died” geologically about 4
billion years ago.
Question 4 – Ch. 8; #29
Look at the gullies in Figure 8.27. Look at the streambeds in Figure 8.28. Finally,
look at what was exposed by a robot arm from the Phoenix lander just a couple of
centimeters below the surface (see Figure 8.29).
Question 5 – Ch. 8; #35
Surface area is 4 pi r2
So for Earth, that is 5.10 x 108 km2
for Mars, this would be 1.44 x 108 km2
(1 - 0.72 = 0.28), so 0.28 of 5.10 x 108 km2 is about the same area as the whole
surface of Mars (just a coincidence, there’s no deeper significance here).
Question 6 – Ch.8; #39
Density = mass / volume = 6.0 x 1024 kg / [(4/3) pi (6.371 x 106 m)3] = 5540 kg/m3
If the crust is below the average density, the interior must be above the average
density.
Do the same for the Moon and get 3280 kg/m3
The Moon must be mostly rocky materal as opposed to metal.
Question 7 – Ch. 8; # 41
½ to the 6th power is 1/64, so 6 half lives will have to pass to reduce the radioactive
parent to 1/64 of its initial abundance.
5700 yr x 6 = 34,200 years
You would have overestimated the age of the sample.
Question 8 – Ch. 8; #43
Use d=vt and use the same units. Let’s convert 4500 km to 4.5 x 108 cm.
Then, 4.5 x 108 cm = (3.75 cm/yr)(time)
Solve for time and get 1.2 x 108 yr which is 120 million years.