Catch A Star- Io

By Melissa Magee and Ruth Jess

Glenlola Collegiate School Bangor

Facts about Io

One of the ways scientists have found out details about Io is using spacecraft. The most recent is NASA’s Galileo and Cassini spacecraft, which found two tall volcanic plumes and the rings of red material they have deposited onto the surrounding surface areas of Io. This was in late December 2000 and early January 2001. A plume near Io's equator comes from the volcano Pele. It has been active for at least four years, and has been far larger than any other plume seen on Io, until now. The other, nearer to Io's North Pole, is a Pele-sized plume that had never been seen before, a fresh eruption from the Tvashtar Catena volcanic area.

. The observations were made during joint studies of the Jupiter system while Cassini was passing Jupiter on its way to Saturn. The two craft offered complementary advantages for observing Io, the most volcanically active body in the solar system. Galileo passed closer to Io for higher-resolution images, and Cassini acquired images at ultraviolet wavelengths, better for detecting active volcanic plumes

The Cassini ultraviolet images reveal two gigantic, actively erupting plumes of gas and dust. Near the equator, just the top

The Cassini ultraviolet images reveal two gigantic, actively erupting plumes of gas and dust. Near the equator, just the top would be illuminated if it were less than 240 kilometers (150 miles) high. These images indicate a total height for Pele of 390 kilometers (242 miles). The Cassini image at far right shows a bright spot over Pele's vent. Although the Pele hot spot has a high temperature, silicate lava cannot be hot enough to explain a bright spot in the ultraviolet, so the origin of this bright spot is a mystery, but it may indicate that Pele was unusually active.

Facts about Io

In spite of the large quantities of gas ejected by its many volcanoes, Io does not have a significant atmosphere. Io's average surface temperature is so low, (about 100 to 110 K (-280 to -290 degrees Fahrenheit), that much of the released gas condenses back onto the surface as frost deposits. The thin atmosphere that does exist is composed primarily of sulphur dioxide gas.

Some molecules of gas do escape, however, and a cloud of sodium, potassium, and oxygen atoms surrounds Io. The sodium cloud is the most easily observed. The source of the sodium remains a mystery to scientists because it has not yet been detected anywhere on Io's surface. Recently, the element chlorine was also discovered. This finding leads scientists to believe that sodium chloride, or common table salt, may exist on Io and influence its violent volcanic activity. Prior to this discovery, only sulphur, oxygen, sodium, and potassium atoms were observed escaping Io's atmosphere.

Is there water on Io?

Some of the first images from Io showed material being thrown up from the surface into space. At first scientists thought these might be similar to explosive volcanic eruptions on Earth such as the eruption of Mt. St. Helens. After further study, however, they decided that the eruptions are more closely related to geysers on Earth. Geysers are found in volcanically active regions on Earth such as in Iceland, New Zealand, and Yellowstone National Park in Wyoming. Geysers occur when water located in open spaces beneath the Earth's surface comes into contact with hot rocks. As the water heats up, pressure builds up within the confined space. Eventually the water expands and is forced upward through cracks in the overlying rock this results in an eruption of steam and water from the crack

  Io Statistics

Discovered by

Simon Marius & Galileo Galilei 

Date of discovery

January 7, 1610 

Mass (kg)


Mass (Earth = 1)


Equatorial radius (km)


Reference ellipsoid (km)


Mean density (gm/cm^3)


Mean distance from Jupiter (km)


Rotational period (days)


Rotational Period (days, hours, minutes)

1 day, 18 hours, 27 minutes

Orbital period (days)


Mean orbital velocity (km/sec)


Orbital eccentricity


Orbital inclination (degrees)


Escape velocity (km/sec)


Visual geometric albedo


Mean surface temperature


Magnitude (Vo)


Highest point (name, height in km)

Boosaule Mons (south peak), 15.8 km

Lowest point (name, height in km)

Kanehekili or Culann

Surface Gravity (Earth=1)


Heat Flow

2.5-13.5 W/m2

Io in Relation With Our Moon

In the Longman Dictionary of the English Language the moon is described as many different things, some of them being:

·         The Earths natural satellite that shines by reflecting the suns light

·         One complete moon cycle, consisting of four phases

·         A satellite of a planet

Io is one of the largest out of the sixteen moons of Jupiter along with Europa, Ganymede and Callisto. These four moons are known as the Galilean satellites, as Galileo discovered them in 1610. The other twelve moons were discovered in 1979 by the Voyager space probes, as was a faint ring of dust around Jupiter’s equator. Io is the third largest moon of Jupiter-3,630 Km/2,260 mi in diameter, orbiting in 1.77 days at a distance of 422,000Km/262,000 mi. It is the most volcanically active body in the solar system, covered by hundreds of vents that erupt not lava, but sulphur, giving Io an orange coloured surface.

Our moon goes through many phases (New moon, Crescent moon, Full moon and Gibbous moon) And Io does aswell! It has been proven that the planets whose orbits lie within that of the earth can also go through a full cycle of phases, as can an asteroid going through the earths orbit.

I have listed below a few similarities between Io and our moon:

·         Both go through a full cycle of phases

·         Both have volcanoes on them

·         Both in relation with planets

·         Both around the same size

Io and our moon do not have many similarities. For example, you could live on our moon if you had suitable oxygen supply; but it has not yet been attempted to walk on Io, as living conditions there would be unbearable! Io is a unique moon and very different from the earths.


Demonstration of Kepler’s Third Law

For this task we will use a chart showing the position of Jupiter’s satellites Io, Europa, Ganymede and Callisto over the course of one month.  Pupils will also need a ruler to measure distances.

The task will be to calculate the orbital periods of Io and Ganymede (using the diagram) and then to use the ratio of distances and periods to demonstrate Kepler’s Third Law, that is, the relationship between the size of each satellite’s orbit and it’s period.

We will produce a worksheet for the class, asking the following questions:

  1. (a) Where is Io, relative to the planet Jupiter, at midnight on 3 rd of the month? 

Give the approximate time and date when Io is next eclipsed by Jupiter.

What is the orbital period of Io?

(b)    Determine the orbital period of Ganymede in the same way you have just done for Io.

  1. Measure on the diagram, to the nearest millimetre, the maximum distances from Jupiter of both Io and Ganymede.

Io distance …………………………………

Ganymede distance ………………………..

From your results for questions (1) and (2), work out the following ratios:

                     Ganymede radius    

Ganymede period

Answer …………………………………

        Io radius

Io period

Answer ……………………………….

How does this demonstrate Kepler’s third law?

Note also that the further away a satellite is from Jupiter, the longer its orbital period.


We have enjoyed finding out about the moon, Io. We both found Io very interesting and would possibly, in the future find out more about it. I think we would like to live there because of its beautiful coloured surface and the fact we could learn more about this special moon.


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