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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?
(This activity was developed during the 1994 AASTRA training workshop in Flagstaff, Arizona)
Io Fact Sheet |
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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 |
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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 |
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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)