IO , Moon of Jupiter

We have chosen to study the Jovian moon, Io.  Io is one of the four moons of Jupiter discovered by Galileo Galilei.  It is a strange world, with very low surface temperatures but with active volcanoes dominating the surface landscape.  A truly alien landscape, but one which may hold secrets about the history of our own planet Earth.

All about Io

The innermost of Jupiter's four planet-sized moons, Io is also one of the most volcanically active bodies in the solar system.

The Vital statistics of Io

Discovered by
Galileo Galilei

Date of Discovery

Distance from Jupiter
422,000 km

1830x1818.7x1815.3 km

8.9316 × 10 25 g

Orbital Eccentricity

Mean surface temperature

-143 o C

Orbital Inclination
0.04 degree

Orbital Period
1.769137786 days

Mean orbital velocity km/s


Rotational Period

Major Atmospheric Constitutent
Sulphur dioxide

The Landscape of Io

Io is about the same size as our own Moon, but is very different. In the 1980’s, observed by the Voyager probes, more about the nature of Io was discovered.  Io’s surface also has numerous lakes of molten sulphur with extensive flows hundreds of kilometres long of low viscosity fluid (possibly some form of molten sulphur or silicate).

Io was found to have to have volcanoes, which eject sulphur in plumes hundreds of kilometres high.  Some of the particles from these plumes escape from the surface of Io, and orbit Jupiter on their own.  This ring of sulphur particles can be detected from the Earth!

Io was found to have to have volcanoes, which eject sulphur in plumes hundreds of kilometres high.  Some of the particles from these plumes escape from the surface of Io, and orbit Jupiter on their own.  This ring of sulphur particles can be detected from the Earth!

Two schools of thought exist as to the composition of the volcanoes of Io. Are they silicate lavas similar to those seen on Earth, such as in Hawaii or in Sicily ? Or are they composed of liquid sulphur (which might account for Io's yellowish colour) or a combination? 

Sulphur and its compounds produce a wide range of colours, which could be responsible for Io's surface appearance.

More recent photographs of Io detail, as well as yellows, reds and browns, certain blue areas.  These areas interested us very much and we decided to investigate further. 

We decided to investigate the surface colours of Io by doing some *experiments in the lab using sulphur.  We compared out observations with the images taken of Io by the Galileo probe.

* detailed in our experiment 1 later.

The Volcanoes of Io

‘Scientists believe the bright blue patch may be clouds of gas issuing from volcanic vents. The gas clouds may condense to form extremely fine particles that appear blue. Since Voyager 1's infrared spectrometer has discovered sulphur dioxide on Io, it is possible that sulphur dioxide is the main component of the clouds. Sulphur dioxide clouds would rapidly freeze and snow back to the surface.’  ( Taken from the NASA website

We return to looking at the Ionian snow later.

Previous analysis of the Voyager images have prompted scientists to believe that the many lava flows on the surface of Io may be composed mostly of various compounds of molten sulphur.  There also appears to be flows of dark rock lava, as in volcanoes on Earth.  These dark areas would point to elements other than sulphur, such as our silicate volcanoes here on Earth.

However, newer ground-based infrared studies indicate that the volcanoes may be too hot to be liquid sulphur; some of the hottest spots on Io may reach temperatures as high as 2000 K (though the average is much lower, about 130 K).   One current idea is that Io's lavas are molten silicate rock.

Recent Hubble space telescope observations indicate that the material may be rich in sodium. There may be a variety of different materials in different locations.  We looked at information on silicate based volcanoes on earth and found that several of these volcanic sites are sources rich in sulphur,  used as  ‘sulphur mines’ to produce the Earth’s sulphur resources.   We concluded that it may indeed be possible for Io to be comprised of silicate molten rock as well as rich in sulphur.  This would account for both its infra-red profile and the surface colours observed by the Voyager and Galileo probes.

Our experiment 1 – Testing sulphur

We  experimented in the laboratory with burning sulphur.  Initially we noted that the colour of sulphur was indeed similar to the more recent photographs of Io with its vivid yellow areas.                       


                           Experiment 1 – burning  sulphur

When we burnt the sulphur we observed its colour change, ranging from yellow to orange to brown also closely matched the colour profile of many areas of Io.  Having noted these changes we then proceeded to burnt h sulphur in  oxygen.  When the light level was lowered we could clearly see that, when burnt in pure oxygen, the sulphur flame turned bright blue (although it was difficult to show this in the photograph).  This result closely resembled the blue areas seen on Io in the volcanic regions.  This presence of the element oxygen in the atmosphere of Io would substantiate the belief that Io has a thin atmosphere of sulphur dioxide (SO 2 ).

Experiment 1 – burning sulphur in oxygen (O 2 )

Why is Io volcanic?

The energy for all this activity probably derives from tidal interactions between Io, Europa, Ganymede, and Jupiter. The three moons are locked into Laplace-resonant orbits such that Io orbits twice for each orbit of Europa which in turn orbits twice for each orbit of Ganymede. Though Io always faces the same side toward its planet, the effects of Europa and Ganymede cause it to wobble a bit. This wobbling stretches and bends Io by as much as 100 meters and generates heat through internal friction. Although Io always points the same side toward Jupiter in its orbit around the giant planet, the large moons Europa and Ganymede stretch Io's orbit into an irregularly elliptical one. Thus, in its widely varying distances from Jupiter, Io is subjected to tremendous tidal forces. These forces cause Io's surface to bulge up and down (or in and out) by as much as 100 meters (330 feet)! Compare these tides on Io's solid surface to the tides on Earth's oceans. On Earth, in the place where tides are highest, the difference between low and high tides is only 18 meters (60 feet), and this is for water, not solid ground!

This tidal pumping generates a tremendous amount of heat within Io, keeping much of its subsurface crust in liquid form, seeking any available escape route to the surface to relieve the pressure. Thus, the surface of Io is constantly renewing itself, filling in any impact craters with molten lava lakes and spreading smooth new floodplains of liquid rock.

(information taken from:


Our experiment 2

We decided to do a brief test on whether movement of a ‘pliable’ substance would cause a temperature rise, as is believed to happen on Io. After taking the temperature of the core of a ball of putty-based material, we pushed and pulled it to try to cause a temperature rise.  We only used a very small amount of the putty to try to record a rise.  Otherwise the heat generated by be too small to note. Using a sensitive


Experiment 2 – measuring temperature rise

thermometer we saw a slight temperature rise.  Although it would appear that the frictional movements of the material caused a temperature rise, we found it difficult to do this without any ‘contamination’ from the heat of our hands!  However, other, well-documented evidence of frictional effects would suggest though that that movement back and forth of material would create a rise in temperature, as that seen on Io.  We would probably need more advanced laboratory equipment to test this theory satisfactorily.

The craters of Io

Io's surface is almost completely lacking in craters, indicating that it could be very young. In addition to volcanoes, the surface includes non-volcanic mountains, calderas (the crater rings like those found on Earth) up to several kilometres deep.

Scientists have found that sulphur dioxide is the primary constituent of a thin atmosphere on Io. It has no water to speak of unlike the other, colder Galilean moons. Data from the Galileo spacecraft indicates that an iron core may form Io's centre, thus giving Io its own magnetic field.

Voyager observed eight active volcanic plumes in March and July 1979 and some of these have been monitored since using infrared telescopes on Earth.  Lava flows and irregularly shaped mountains are also observed, but no craters (consistent with ongoing volcanic activity, which would erase impact craters not long after they form).

The Sulphur Snow of Io

We have already looked at the burning of sulphur and how it is likely that this element exists in abundance in Io.  Sulphur (S) is a solid on earth with a low melting point. At high temperatures, it could also be turned into a vapour.   So the volcano plume could be gaseous sulphur.   It is usually yellow, but can be black or red.

Sulphur dioxide (SO 2 )  is a gas on Earth so we can study it here.    If this was discharged in a plume from an Io volcano, it would crystallise in the cold and make a white ‘snow’  - white deposits – which have been observed by Galileo.

‘It's not frozen water like Earth's snow, but a sulphur-rich material that looks like white snow, said Dr. Alfred McEwen, a planetary scientist at the University of Arizona, Tucson.  In some of Io's active volcanic plumes, the volatile material apparently even falls to the ground as frozen particles or crystals, like snowflakes.  "We see this volatile material everywhere on Io where we've had a close-up look," McEwen said. It includes sulphur dioxide, and probably other sulphur-rich substances, he said.

On Earth, sulphur dioxide is a gas. On Io, it can be either a solid or a gas at the surface, or a subsurface liquid.

Other aspects of the atmosphere and environs of Io

Io is composed primarily of rocky material with very little iron. Io is located within an intense radiation belt of electrons and ions trapped in Jupiter's magnetic field. As the magnetosphere rotates with Jupiter, it sweeps past Io and strips away about 1,000 kilograms (1 ton) of material per second. The material forms a torus, a doughnut shaped cloud of ions that glow in the ultraviolet. The torus's heavy ions migrate outward, and their pressure inflates the Jovian magnetosphere to more than twice its expected size. Some of the more energetic sulphur and oxygen ions fall along the magnetic field into the planet's atmosphere, resulting in auroras.

Io acts as an electrical generator as it moves through Jupiter's magnetic field, developing 400,000 volts across its diameter and generating an electric current of 3 million amperes that flows along the magnetic field to the planet's ionosphere.

Our conclusions

From our research and experiments we believe the following ideas are likely regarding Io:

·        That Io is likely to have large amount of sulphur in its composition. 

·        That the infra-red profile and recent photography also indicates the presence of silicates in the composition

·        That the eruptive material on Io probably largely comprises of compounds of sulphur together with sulphur dioxide gas.

·        That much of the material being lost  to the atmosphere  in the gravitational winds is probably returned to Io as frozen crystals of sulphur dioxide

·        That much of the heat radiated by Io is likely to be due to the frictional effects of gravitational forces of Jupiter and its other moons.

·        That some of the heat radiating by Io may be due to energy given off by its role as an electrical generator.

·        That the magnetic field of Io is likely to be due to an iron-rich silicate crust.  The silicate materials would be more likely to exist in closer, denser moons that those found further away from the centre of orbit.

Website references used in compiling this report: