Jupiter



Statistics:
*This means that Jupiter is mostly atmosphere; that is, no solid surface has yet been detected for this and other gas giant planets.

 Early Observations of Jupiter
 The Jovian Atmosphere
 The Moons of Jupiter
 The Galileo Mission


Early Observations:

Some of the earliest observations of Jupiter began in 1610 when the Italian astronomer and scientist Galileo Galilei turned his telescope towards this planet.  What Galileo discovered astonished him... he noticed that orbiting the planet Jupiter were four bright points of light.  Later, Galileo concluded that these objects were moons of the giant world, and they were subsequently named in his honor.  They are now known as the Galilean moons of Jupiter.  Further observations with better telescopes showed that Jupiter also had a very interesting atmosphere in addition to numerous satellites.

Jupiter's Atmosphere:

Jupiter is by far the largest planet in our Solar System, capable of holding almost 1500 Earth's within its volume.  However, Jupiter has some very interesting properties:

  1. The average density of Jupiter is only about 1.3 grams per cubic centimeter, which means that heavier elements (such as iron and other metals) do not make up a substantial amount of the planet's composition.  In comparison, the inner planets (Mercury, Venus, Earth, & Mars) have average densities that range from 3.94 to 5.51 grams per cubic centimeter.
  2. The result of this analysis has lead planetary scientists to conclude that Jupiter is composed of mostly lighter elements, such as hydrogen and helium.; any heavier elements exist only in very small quantities.  Thus, the planet Jupiter is composed mostly of gas, and this is where the name gas giant originates.
As for the chemical composition of the Jovian atmosphere, spectrographic analysis has revealed that it is composed of about 86% hydrogen, 13.8% helium, and the rest is a mix of ammonia and methane molecules.  Since there exists no real solid surface on Jupiter, it is speculated that as one penetrates deeper and deeper into the atmosphere the gasses will become denser due to higher atmospheric pressure.  Eventually, a point will be reached where the gasses will be squeezed into a liquid state, making an ocean of liquid hydrogen and helium.

Observation of Jupiter's atmosphere has revealed that it is extremely active and turbulent in nature.  Jupiter's rapid rotation and internal heat engine churn up the atmosphere into a series of cloud bands and storms.  The most prominent weather feature on Jupiter is The Great Red Spot.  The Great Red Spot has been visible on Jupiter since the planet has been observed with powerful telescopes (about 350 years).  Essentially, the Great Red Spot is a gargantuan hurricane that is about twice the width of Earth.  Other storms exist on Jupiter, but none compare to this monster hurricane.
 

The Great Red Spot
 
Another interesting feature of Jupiter that is related to it's atmosphere is the fact that Jupiter is the most oblate of all of the planets.  That is, due to Jupiter's rapid rotation and the fact that it's atmosphere is so extensive and fluid, Jupiter tends to bulge out at the equator.  This makes Jupiter appear like a squashed sphere in shape.

The Galilean Moons:

In reality, Jupiter actually has 16 moons and a small ring system, but we shall only focus upon the major satellites here.  As stated before, Galileo discovered the four main moons of Jupiter.  However, these satellites were also discovered independently by a German astronomer named Simon Marius in 1611.  Marius named the four moons Io, Europa, Callisto, and Ganymede, which represent four of the lovers of the Greek god Zeus (also known to the Romans as Jupiter).  Thus, even though Marius' names for the four satellites stuck, Galileo gets credit for the original discovery, and so the four moons are now collectively known as the Galilean moons.  Most of the early information about these moons was gathered by the Voyager space probes in 1979.
 

   
The Galilean Moons:  from left to right -- Io, Europa, Callisto, & Ganymede
 
Io has the Solar System's most active volcanism; in the time that Voyager 1 was observing Io for the first time it actually witnessed eight volcanic eruptions.  These same volcanoes were still erupting when Voyager 2 flew by Io a few months later.  Due to such violent eruptions occurring so frequently, the surface of Io is constantly being renewed and destroyed.  The surface of Io is composed mainly of sulfur and sulfur compounds, including frozen sulfur dioxide; Io even has a small sulfur dioxide atmosphere, a result of the common volcanic eruptions.  It is also speculated by some planetary scientists that Io may even have an underground ocean of liquid sulfur!

Europa is the brightest of all of the Galilean satellites.  The surface of this moon is covered in a smooth, flat layer of ice that is permeated with a series of streaks and cracks.  Infrared spectroscopy has confirmed that the material comprising the surface of Europa is indeed water ice.  Since the visits by the Voyager probes, the surface of Europa has appeared to have changed or shifted in some manner; these observations were made by the  Galileo probe during its current mission.  This evidence of a sort of plate tectonic activity on Europa suggests to some planetary scientists that there could exist a vast ocean of liquid water underneath the surface of the moon.  It is thought that this ocean of liquid water is kept above freezing temperatures by some internal heat source:  either radioactivity or gravitational heating effects.  These factors make  Europa a prime candidate for the possibility of life, and scientists plan to study Europa in greater detail.

Callisto is thought to be the oldest of the four Galilean moons.  This is due to the fact that Callisto has the most heavily cratered surface of any of the Jovian satellites.  Spectral analysis of Callisto has revealed that it too has a surface comprised mostly of water ice; however, unlike Europa, Callisto appears to have a more solid subterranean structure.  No large craters are found on this body, except for a massive impact site named Vahalla.  Vahalla seems to form a huge bulls-eye on the side of Callisto, and it is surrounded by ripple-like features in the icy surface of Callisto.

Ganymede is the largest of the four main moons of Jupiter.  In fact, Ganymede is actually larger than the planet Mercury!  The surface of Ganymede contains large amounts of water ice, and it is believed that this body has a solid, rocky core at its center.  Ganymede appears to have some kind of plate tectonic-like activity as well; portions of the surface show lateral displacements where some groove-like features on the surface have slid.  It is thought that these features are much like the fault-lines here on Earth.

The Galileo Project:

In 1989, a space probe named for the italian astronomer Galileo Galilei was launched from the Space Shuttle in Low Earth Orbit (LEO).  Six years later, in December of 1995, the Galileo probe entered the Jovian system to begin more detailed investigations of both Jupiter and its vast system of moons.  The Galileo probe had two primary missions:
 

The Launch of the Galileo Probe to Jupiter
 
  1. A section of the robotic probe would break apart from the main body and be released into the atmosphere of Jupiter.  It is hoped that by having had this atmospheric probe enter the upper layers of Jupiter that we can learn more about the structure and composition of this gas giant planet.
  2. After having released the atmospheric probe in December of 1995, the rest of the Galileo orbiter went on a two-year tour of the moons of Jupiter.  Officially, this mission ended in December of 1997, but since that time the mission has been given an extension for another two years.
There has been a great deal of information gathered and return to Earth by the Galileo probe, and it is hoped that there are still many more discoveries to be made!  For more information and updates on the progress of the Galileo probe to Jupiter, check out the Jet Propulsion Laboratory's (JPL)  Galileo website.
 
 
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M. Lowry -- 12-16-97