- Overview of Jupiter
- Introduction to Galilean Moons
- Kepler's Third Law
- Download the CLEA program to study the Galilean Moons' orbits

[SLIDE 1]

Jupiter is the fifth planet from our sun and the largest in our solar system. 

It can reach an apparent magnitude of minus 2.94, which makes it, on average, the third brightest object in the night sky, after the Moon and Venus. 

It is a gas giant, meaning it is mostly composed of hydrogen and helium, although it may have a rocky core or at least a core composed of heavier elements. 

Its gas composition means it has no solid surface. It also has a rapid rotation around its axis. This means Jupiter bulges at the equator to form an oblate spheroid. 

The atmosphere of Jupiter is heavily segregated into different layers. At the boundaries of these layers, turbulence and storms are known to occur.


[SLIDE 2]

One such storm is referred to as the Giant Red Spot. This storm has been ongoing since at least 1830. 

The Giant red spot is a region of high pressure in Jupiter's atmosphere produced by a storm just south of the planet's equator. 

In the images on the screen, the Giant Red Spot is highlighted by the white box on Jupiter's surface. The image to the left of the screen is a close up of the giant red spot. This false color image was taken by the Voyager 1 mission. The blue oval below the white spot has the approximate diameter of Earth, providing some context on just how massive this storm is. 


[SLIDE 3]

Jupiter has 79 known moons including the four Galilean Moons. The Galilean Moons are the four largest moons of Jupiter and are Io, Europa, Ganymede, and Callisto. 

The term Galilean moon is coined after Galileo, who not only first discovered these objects in 1610 but later in the same year, identified them as moons of Jupiter. 

The moons are all named after lovers of Zeus and comprise the largest bodies in our solar system after the 8 planets and the sun.


[SLIDE 4]

The image above is an illustration of the relative sizes of the Galilean moons compared to Jupiter.

The moon to the left is Io, the closest moon to Jupiter and the most volcanically active body in the solar system.

After that is Europa, which is a similar size to our own Moon. The surface of Europa is ice and it has been hypothesized that below this ice is liquid water. It is thought that if life exists on other bodies in our solar system, Europa would be one of the most likely candidates.

Next is Ganymede, the largest moon in our solar system. Ganymede has a silicate composition similar to Earth but with an icy surface. It also has a magnetosphere which means the core of Ganymede must be made of metal and be undergoing convection. Convection of planetary cores is not widespread throughout our solar system.

Callisto is the fourth Galilean moon and the third largest moon in our solar system. It is just a fraction smaller than the planet Mercury. Callisto is half rock and half ice making it the least dense Galilean moon.
It is also thought to have liquid water below the surface that could harbor life.


[SLIDE 5]

In astronomy, there are three laws of planetary motion referred to as Kepler's laws. Here, we are only interested in his third law. This law states that the square of the orbital period of a planet is proportional to the cube of the semi-major axis of its orbit. This law is shown as an equation above, where M is the mass of a planet (in this case, Jupiter), "a" is the semi-major axis of the moon's orbit, and "p" is the orbital period of the moon.

An orbital period is simply the time it takes a moon to do a full rotation around a planet, as shown in the diagram above.

The major axis would be the largest distance from a planet to a moon. The semi-major axis is half of this. Assuming a moon's orbit around a planet is circular, then the semi-major axis refers to the radius of its orbit.

This law is important because it allows us to deduce properties of celestial bodies based purely on their motions, negating the need for difficult, if not impossible, measurements. 


[SLIDE 6]

In the accompanying exercises, we will use Kepler's third law to calculate the mass of Jupiter by observing the orbits of the Galilean moons. 

Before beginning the exercises for this lesson, be sure to download the CLEA software and instruction manual. 

Pages three through nine of the manual will provide further background and instruction to the exercises.