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Io
Io Overview
• Innermost Galilean Moon
• 4th largest moon in the
solar system with a
diameter of 2,263 miles
• Most geologically active
object in the solar system
“Looking like a giant pizza
covered with melted cheese
and splotches of tomato and
ripe olives, Io is the most
volcanically active body in the
solar system.”
Solarsystem.nasa.gov
Io: Technical Information
• Orbital period: 1.769 days, rotation is
synchronous with orbit
• Surface gravity is 1.80 m/s2
• Albedo: 0.62
• Surface temperature: 85K daytime, 140K
nighttime (away from hotspots)
• Atmospheric Pressure: <10-9 bar
Encyclopedia of the solar system: Io the Volcanic Moon
Discovery of Io
• Discovered on January 8, 1610 by Galileo.
• Simon Marius claims he discovered the
satellites of Jupiter before Galileo, but Galileo
published first so he is credited.
• Io was one of Jupiter’s lovers in mythology.
1. http://planetarynames.wr.usgs.gov/Page/Planets#JovianSystem
2. http://galileo.rice.edu/sci/marius.html
Discovery of Io
Effects on Science
• The discovery of moons in orbit around
Jupiter became a key piece of evidence for the
Copernican view of the universe.
• A Danish astronomer, Olaus Romer, used
observations of the timing of ellipses of
Jupiter’s moons to calculate the speed of light.
Encyclopedia of the solar system: Io the Volcanic Moon
Voyager I
• Just a few weeks before
Voyager I passed by Io, several
scientists made the prediction
that Io would have volcanic
activity. Voyager I found
evidence of this activity as it
passed.
• Voyager I also measured much
higher levels of sulfur in the
magnetosphere which had not
been detected by the earlier
Pioneer 10 and 11 which
passed Io in December 1973
and December 1974.
Encyclopedia of the solar system: Io the Volcanic Moon
Voyager II
• Voyager II did not pass as
closely to Io as Voyager I.
• In the 4 months since Voyager
I, a change in volcanic activity
was noted.
• 7 of the 9 volcanoes imaged by
Voyager I were shown to still
be active.
• Pele, an active volcano imaged
by Voyager I was seen to be
inactive at the time that
Voyager II passed.
http://voyager.jpl.nasa.gov/gallery/images/jupiter/io.gif
Tidal Heating
• Laplace discovered a relationship between the orbits of Io, Europa, and
Ganymede in 1805. This relationship is called Laplace Resonance.
• The satellites’ orbits have a 4:2:1 ratio. Meaning for each time Io orbits
Jupiter, Europa only orbits ½ way and Ganymede orbits only ¼ way.
http://en.wikipedia.org/wiki/File:Galilean_moon_Laplace_resonance_animation.gif
• This forces Io’s orbit into an oval shape: Io is closer to Jupiter during some
parts of the orbit than others.
• This, combined with the gravitational pull from Europa and Ganymede,
causes Io to deform and relax multiple times during its orbit.
• As it gets closer to Jupiter it deforms toward Jupiter and relaxes as it
moves further away and the gravitation pull of its neighboring moons gets
stronger.
• This pattern causes Io’s interior to heat up in the same way that repetitive
bending of a wire hanger would eventually cause the wire to heat up.
Encyclopedia of the solar system: Io the Volcanic Moon
http://nasa.gov/topics/solarsystem/features/io-volcanoes-displaced.html
Tidal Heating
• Tidal heating is a much stronger heating mechanism than internal
heating (by 200x) and is responsible for Io’s hyperactivity.
• The interior temperature is due to an equilibrium between tidal
heating and heat transfer from Io to the surrounding space.
• The heat transfer on Io is driven by melt segregation. This is the
process of molten rock flowing to the surface and burying older,
solidified flows. This leads to a constantly renewed surface.
http://www.nasa.gov/topics/solarsystem/features/io-volcanoes-displaced.html
Encyclopedia of the solar system: Io the Volcanic Moon
http://photojournal.jpl.nasa.gov/jpegMod/PIA02501_modest.jpg
Williams, David A.; Howell, Robert R.; Active Volcanism: Effusive eruptions
Surface Characteristics
• Io has a metallic core and a silicate mantle.
• There is some sort of differentiated crust, but its composition is
unknown.
• Io has at least 166 active volcanic landforms interspersed among
sulfurous frost deposits.
• The many colors of the surface are due to sulfur deposits in many
stages of cooling.
• There are mountains of volcanic and tectonic origin.
• Scarps cross the moon as well.
– Some are parallel and appear to be tectonic in nature.
– Others are jagged and seem to be the result of erosion.
– There is not enough atmosphere or liquid on the surface of Io for traditional
erosion. Sulfur and sulfur dioxide escaping from subterranean reservoirs has
been suggested as the source of the erosive material.
Encyclopedia of the solar system: Io the Volcanic Moon
Volcanoes On Io
• Two volcanoes represent the different types of
activity seen on Io:
– Pele: eruption times on the scale of days or weeks,
plume heights of approximately 300km, temperatures
of 650 K and dark deposits
– Prometheus: year long eruptions, plume heights 60100km, ring deposits
http://www.nasa.gov/topics/solarsystem/fe
atures/io-volcanoes-displaced.html
Williams, David A.; Howell, Robert R.; Active volcanism: Effusive eruptions
Other Recognizable Features
• Loki Patera is a huge lava lake that is very recognizable. It is
sometimes the brightest hot spot on Io.
• There have been recurring “brightenings”
observed in the area around Loki.
• This phenomena has not been explained
but was observed by Galileo and
confirmed by ground based
observations.
• In many images of Io, Loki
appears as a dark horseshoe.
Encyclopedia of the solar system: Io the Volcanic Moon
Atmosphere
• Io’s atmosphere has very low density which consists mainly of sulfur
dioxide.
• Sulfur dioxide is largely emitted by volcanic plumes.
• Some is also evaporated from the sulfurous frost deposits.
• Io’s low gravity permits this sulfur dioxide to escape and it dissociates into
sulfur and oxygen neutrals.
• These neutral particles become a part of the aurora and further afield, the
plasma torus.
• In the aurora, green is sodium atoms, red is oxygen and
blue is sulfur. This is the only evidence thus far for the
presence of sodium on Io.
• The blue regions of the aurora could also be correlated to the flux tube
between Io and Jupiter. The flux tube allows the exchange of charged
particles between the two bodies.
Encyclopedia of the solar system: Io the Volcanic Moon
Plasma Torus
• Donut shaped trail of charged sulfur and oxygen particles that
follows Io in its orbit.
• Formed from the sulfurous emissions from the moons volcanic
activity.
• UV radiation from the sun bombards ions of the gases such as
sulfur dioxide resulting in a cloud of ions and electrons.
• These ions and electrons then move with Jupiter’s magnetic field
in the same orbit as Io.
• There is evidence that the plasma torus has large impacts on
Jupiter’s magnetic field.
• This effect could be correlated with levels of volcanic activity
which may change the density and composition of the plasma
torus periodically.
Encyclopedia of the solar system: Io the Volcanic Moon
http//:vega.lpl.arizona.edu/iotorus/
Questions that Remain
• What is the composition of the crust? What type of magma is
emitted from the volcanic landforms? Is sulfur dioxide the only
volatile present in the magma?
• What is the exact nature of the tidal heating?
– Volcanic eruptions take place away from where scientific models would
predict.
– This could be due to faster than calculated rotation of Io, the ability of
magma to travel far distances beneath the surface, or inadequacies in
the tidal heating model which neglect a crucial unknown component.
• Galileo only managed to study the side of Io facing away from
Jupiter. What is the behavior of the Jupiter facing side? Are there
active landforms that could provide more insight into the nature of
Io’s interior?
• What is the exact nature of the flux tube? What is the composition
of the particles and where do they com from?
Encyclopedia of the solar system: Io the Volcanic Moon
http://nasa.gov/topics/solarsystem/features/io-volcanoes-displaced.html
Future Missions?
• There are currently no missions planned to Io.
• A mission to Jupiter named Juno is currently
planned and this could provide information
about the interaction between the two bodies
but Juno will not carry technology useful for
observing Io itself.
• Planning of future missions is on hold,
awaiting development of more radiation
robust technology.
Marchis, Franck; Spencer, John R.; Lopes, Rosaly M. C.; Outstanding Questions and future
explorations
Comparison of Voyager and Galileo Images
http://nssdc.gsfc.nasa.gov/image/planetary/jupiter/gal_io2_47972.jpg