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# More or Less: How Do the Masses of Other Planets Compare to the Mass of Earth?

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Author: Janice VanCleave

### PROBLEM

How do the masses of other planets compare to the mass of Earth?

### Materials

• pen
• ruler
• sheet of typing paper
• calculator

### Procedure

1. Use the pen, ruler, and paper to make a table like the one shown here.
2. Complete the table by calculating each planet's mass ratio, which is the ratio of a planet's mass to the mass of Earth. NOTE: The mass of planets is measured in trillion trillion kg. For example, the mass ratio of Mercury is:
mass ratio = planet's mass + Earth's mass
= 0.33 + 5.986
= 0.06/1
3. Mercury's mass ratio is 0.06/1, which means that Mercury's mass is 0.06 times that of Earth.

### Results

The mass ratio of each planet in the solar system is calculated.

### Why?

The term mass ratio, as used in this book, is a number indicating how many times as massive as Earth a celestial body is. The mass ratio is calculated by dividing the mass of a planet by the mass of Earth.

### LET'S EXPLORE

1.
1. A planet's gravity (the force of attraction between two bodies) increases with its mass. If only mass is considered, how would the gravity of planets compare to Earth's? Prepare a list of the planets' mass ratios in order, from greatest to least.
2. The gravity multiple for planets is a number indicating how many times Earth's surface gravity another planet's surface gravity is. Make a list of the planets' gravity multiples in order from greatest to least. (For gravity multiples, see the Weight on Different Planets table in "Show Time!") How does this list compare with the list of mass ratios?
3. Surface gravity decreases as the diameter of a planet increases. Use the gravity multiples, mass, and the diameters of planets (see the appendix) to explain why:
• Venus and Uranus have about the same surface gravity
• Neptune has more surface gravity than Pluto

### SHOW TIME!

Objects with a given mass would not weigh the same on other planets as on Earth. Make a table to compare your weight on Earth to what it would be on other planets. First, weigh yourself on a bathroom scale (weight on Earth). Then multiply your weight by the gravity multiple. For example, if you weigh 90 pounds (41 kg) on Earth, your weight on Mercury would be:

90 pounds (41 kg) × 0.38 = 34.2
pounds (15.58 kg)

### CHECK IT OUT!

Earth's crust has a density of about 2.8 g/ml, while Earth as a whole has an average density of about 5.5 g/ml. (For comparison purposes, water has a density of 1 g/ml.) This indicates that the density of Earth's interior is greater than the density of the crust. Therefore, the interior must be made of denser material than the crust. What clues do the densities of other planets give to their composition? See the appendix in this book for the average density of each planet. For information, see pages 149–151 in Isaac Asimov's Guide to Earth and Space (New York: Fawcett Crest, 1991).