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Assignment 22-4

Comparative Planetology:

To help us understand other planets, astronomers study the Earth. Why do you think we would want to study the Earth to understand other planets?

We cannot visit other planets ourselves yet, but by doing experiments on Earth we learn how things probably work in the rest of the Universe. In each astronomy packet we will study a few worlds. As we do so, we will also use comparative planetology We will look at worlds which are similar in many ways and compare them to one another.


In this astronomy packet we are going to talk about Luna and Mercury. Mercury is the closest world to the Sun, while Luna is the closest world to us. Luna, by the way, is the name of our moon.

Besides being closest to their parent, the Moon and Mercury have many other things in common. First of all, they look a lot alike. In fact, it would be very difficult to tell them apart just by looking at a picture. Both worlds have a gray, sullen, crater-covered surface.

Both worlds are tidally coupled with their parents:

What does tidally coupled mean?

Everything has gravity. Even you have gravity. The larger something is the stronger its gravity is. If two very large objects such as two worlds are close enough, their powerful gravity can cause them to spin with the same side always facing each other.

This is why you only see one side of the Moon. The Moon rotates at just the right speed so that as it circles the Earth it keeps the same side towards us. Eventually the Earth will become tidally locked to the Moon, meaning that the Earth will also keep its same face towards the Moon. In other words, someday only the people who live on one side of the Earth will ever get to see the Moon.

Similarly, Mercury is tidally coupled with the Sun, but not as much as the Moon is with the Earth. Mercury hasn’t slowed its spinning down enough yet to keep one side towards the Sun, but it is very close. Because its spinning has been slowed down so much, one day on Mercury lasts two Mercury years.

The Moon and Mercury have two major differences:

Mercury is both slightly larger and more dense. Mercury is about 1400 km (870 miles) larger from one side to the other. Mercury also has a very large iron core. This iron is very heavy. The Moon has almost no iron at all. The moon is made up almost entirely of lightweight rocks called silicates.

As iron cools over time it shrinks. This shrinking has caused the surface of Mercury to get wrinkly. These wrinkles are called Lobate Scarps. Because the Moon has little iron, it does not have any Lobate Scarps.


Assignment # 4.

What are the similarities and differences of Mercury and the Moon?

What is the Moon’s name?


Click Here to find answers to the following questions:

How big is Mercury?

How far from the Sun is Mercury?

Does Mercury have an Atmosphere?

What does the word Mercury mean?

How long does it take for Mercury to circle the Sun one time?


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5% of the Earth

36 Million Miles

Yes, a very thin one

Mercury was the messenger for the other gods.

87.969 Days