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Luna and Io - A Comparison Study
by Pam Eastlick (, Planetarium Coordinator, University of Guam
American Astronomical Society Teacher Resource Agent


Key Question

How do these two moons differ and how are they alike?

Possible Preconceptions

Students might be familiar with Io, and will realize that Io and Luna are very different. Students, however, might not realize that Io and Luna are amazingly similar.

Key Concepts

Io and Luna are very similar worlds in some respects; however, gravitational stresses have caused them to be dissimilar in other ways.


Students will be presented fact sheets about Io and Luna. They will compare and contrast the two worlds. They will construct paper scale models of Io, Luna, Earth and Jupiter and use these models to explain the differences between the two moons.



Assemble the materials and practice making circles with string and a pencil. For extensions, have a source of planetary satellite data available for the students.

General Information

  • Target Grades: Upper elementary to high school (4-12)
  • Participant Size: Whole class (as a demonstration), individuals and groups of three to four
  • Length of Activity: 30-45 minutes
  • Where: Inside
  • When: Any time
  • Method: Teacher-guided discovery, modeling, discussion,
  • Focus: Gravity, orbital speed,
  • Skills: Questioning, interpreting data, comparing and contrasting, using logic, inferring, predicting

Materials List

  • Per Student:
    • Io and Luna fact sheet handouts

  • Per Group:
    • Unlabeled moon-phase diagram from a textbook or other source (a slide or viewgraph of such a diagram would be ideal)
    • Large paper, at least 40 centimeters wide (or smaller sheets pasted or taped together)
    • String and pencils or pens
    • Meter stick

Doing the Activity

Show the students the picture of the moon-phase diagram and ask them what it represents. They will probably answer that it shows the phases of the moon. Then ask them to identify the central figure. They will likely respond that it is the Earth.

Tell the students that it is not Earth and Luna, and ask them to guess again. Depending on the age group, they may guess that it is another planet and one of its moons. Explain to the students that the picture is not to scale, but that it represents Jupiter and one of its moons. Lead them to generalize that all moons appear to go through phases as viewed from the primary.

Tell them that the moon in the picture is Io. Explain to the students that Io and Luna are very similar worlds. Pass out the Io and Luna fact sheets, and have the students read them. Lead a general compare and contrast discussion of the similarities of the two moons. The students should discover that Io and Luna are very similar, but differ in two major ways. First, Io is an active world while Luna is dead. Second, Io travels 17 times faster than Luna.

Have the students divide into groups of three or four and construct scale models of the Earth/Luna system, and the Jupiter/Io system, using the facts on the data sheets. A scale of 1 centimeter for the diameters of Io and Luna works well. At that scale, Earth is roughly 4 centimeters in diameter and Jupiter is 40 centimeters in diameter. The circles for each body can be made using a regular compass for the smaller worlds, and a string compass for Jupiter. Have the students place Luna and Io at the proper distances from Earth and Jupiter. Using the above-mentioned scale, Luna should be placed about 110 centimeters from the center of Earth, and Io should be placed about 120 centimeters from the center of Jupiter.

After the students have constructed their models, tell them that they should answer two questions. Why does Io travel so fast? Why is Io volcanically active?


Have the students answer the questions and generally discuss the effects of gravity on orbital speed and tidal pull.


Most people are aware of the general axiom, "The closer you are to a large body, the faster you must go." However, many people do not realize that the mass of the parent body also makes a difference in orbital speed. This activity provides a graphic demonstration of that fact. Io and Luna are essentially similar bodies orbiting at essentially similar distances from their primaries, but Jupiter's enormous mass causes Io to orbit at 17 times the speed of Luna. The enormous gravitational tides on Io have made it a geographically active world, in great contrast to the dead Luna.

(This activity was developed during the 1994 AASTRA training workshop in Flagstaff, Arizona)

Io Fact Sheet

  • Parent Planet: Jupiter
  • Size (Diameter): 3,630 km (How big it is)
  • Mass: 8.94 X 1022 kg (How much it weighs)
  • Orbital Distance: 421,600 km (How far it is from Jupiter)
  • Orbital Period: 42 hours (How long it takes to make one orbit)
  • Rotational Period: 42 hours (How long it takes to rotate once on its axis)
  • Orbital Velocity: 17.33 km/s (How fast it travels)
  • Atmosphere: None
In the year 1610, Galileo became the first person, as far as is known, to observe Jupiter through a telescope. Galileo saw four, small star-like dots near the planet that changed position from night to night. The moon Io was one of those dots. Because Jupiter is so far from the sun, it is difficult to tell anything about its moons from Earth. Before automated spacecraft (radio-controlled robots) were sent out to explore the solar system, about all that was known concerning Io was its size, orbital speed, and that it appeared to be a pale orange.

During the 1970's, the Voyager spacecrafts were sent out to the outer planets to take pictures and gather scientific data. Scientists were at first puzzled by the pictures of Io taken by the two Voyager spacecraft. It had no impact craters, and almost resembled a giant pizza! They soon found out why. Io has at least 200 volcanoes on its surface, and the Voyagers caught nine of them erupting! The brilliant colors of red, orange, and yellow are colors of sulfur compounds. If a breathable atmosphere was present on Io, it would smell like rotten eggs!

The gravitational pull of Jupiter, and neighboring moons Europa and Ganymede, raises tides in Io's solid surface that are 100 meters high. The friction of this pushing and pulling causes the interior of Io to be heated enough to liquefy rock.

Luna Fact Sheet

  • Parent Planet: Earth
  • Size (Diameter): 3475 km (How big it is)
  • Mass: 7.349 x 1022 kg (How much it weighs)
  • Orbital Distance: 384,400 km (How far it is from Earth)
  • Orbital Period: 28 days (How long it takes to make one orbit)
  • Rotational Period: 28 days (How long it takes to rotate once on its axis)
  • Orbital Velocity: 1.02 km/s (How fast it travels)
  • Atmosphere: None
Luna is the only natural satellite of Earth and is one of the darker objects in the solar system. Its rocks are mostly dark gray and it reflects less than 15% of the sunlight that falls on it. If Luna was as reflective as Earth, night would be much brighter than it is, when Luna is in the sky.

Humans first visited Luna in 1969 and found it to be a barren and sterile world. Luna has many craters, some of which are billions of years old. The space probe Clementine recently took pictures of the largest impact crater known in the solar system. The impact crater is known as the Aitken Basin, and is located on Luna's side that is permanently turned away from Earth. It is 2,250 kilometers across and 12 kilometers deep.

Processes such as volcanoes and other land movement that would have erased the craters stopped billions of years ago. Scientists have discovered that Luna is completely cold and does not have a hot interior like that of Earth's.

Luna always has one side facing Earth similar to Io and Jupiter. However, it is not perturbed by neighboring moons like Io. Luna's center of mass has been permanently offset towards the Earth by 2 kilometers. Luna is near enough to Earth that Luna's gravity raises tides in Earth's oceans that average 1 to 1.5 meters high.

Views of Jupiter, Io, Earth and Luna

Jupiter, Io, Earth, and Luna
This image shows the relative scale of Jupiter, Io, Earth, and Luna. Jupiter is the largest planet to the left; Earth is to the right. Io is at the lower right of Jupiter and Luna is at the lower right of Earth. The scale is approximately 191 kilometers per pixel. (Credit: Calvin J. Hamilton)

Io/Luna Relationship
This image shows the relative scale of Io and Luna. Io is at the left and Luna is at the right. The scale is approximately 7.3 kilometers per pixel. (Credit: Calvin J. Hamilton)


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