EIU Astro

Io, the closest moon to Jupiter, has an orange-brown
surface containing sulfur, the light areas are an icy
mix of sulfur and the dark areas highlight volcanic areas. Composite picture from NASA

As we have studied the Universe, one of the main ways that we have learned in the past is by using the Earth as a comparison, using all that we know about our planet as a reference for the other galactic bodies that we explore.  What is amazing now is the shift that is taking place, where we are beginning to use what we learn about other celestial bodies and apply that information to our own planet.  We learn even more about Earth as advances are being made in the exploration of our universe.  One example of how we are applying our knowledge of other bodies is Io.  The new information that is being gained from Io gives clues to the processes that occurred on Earth when it was young.

Io is an icy satellite of Jupiter 628,866,000 km from Earth, far enough from the sun that its surface temperature is 175 K (-143°C or -230°F) and is covered in sulfur dioxide frost.  Io’s yellow tinged crust is not fractured, therefore, it is not thought to have tectonic activity.  Despite these two factors, Io has the most volcanic activity in our solar system, spewing out over 100 times as much lava as all Earth’s volcanoes combined and may have as many as 300 active volcanoes.

Image from New Horizons, highlighting the Tvashtar volcanic plume reaching 300 km above the horizon.
Credit: NASA

The surface of Io is much different than previous expectations had dictated, and contains potential clues to the history of Earth.  When the Voyager spacecraft missions took images of Io in1979, NASA was surprised to see that Io was not full of craters, as had previously been thought.  It was assumed that Io would be cratered much like our moon.  Yet Io hardly had any craters at all, instead it had irregular pits and blotches of color.  When the images were carefully examined, volcanic plumes and lava flows were discovered.  Infrared spectrometry also detected abundant sulfur and sulfur dioxide in the volcanic plumes.

The sulfur on Io’s cold crust is solid, though when heated inside the crust, it explodes much like steam in a geyser on Earth.  The sulfur cools as it is ejected and may fall back down as “snow” on Io’s surface.  The lava flows on Io can range in color from orange to red to black and are found around the active vents.  The Galileo spacecraft monitored volcanic areas in the late 90s and found that the active lava flows of Io were between 1700 to 2000 K (around 1450 to 1750°C, or 2600 to 3150°F).  Earth’s lava temperatures are around 1300 to 1450 K.  The lava on Io is probably ultramafic, containing magnesium and iron that have higher melting points.  Ultramafic lava is found on Earth, but was formed when the Earth was young and the interior was much hotter than today.

A blue-tinged volcanic eruption forcing out rock and sulfurous gas,
taken from the NASA Galileo spacecraft.

The volcanoes on Io are mostly caldera-like, containing  large pools of lava, though some are fissures or cracks where the molten material can flow over the surface.  Loki Patera is a caldera with a diameter of 200 km, which makes it the largest in the solar system.  Some of the volcanoes form fountains, umbrella-shaped flows that spread up and out over large distances.  The Prometheus plume is a volcanic region that has been seen in almost every image taken of Io from 1979 to 1997, suggesting that it has been continually erupting for years.

Jupiter’s four largest moons (from left to right: Io, Europa, Ganymede, Callisto)
with sizes to scale. Credit: NASA

Aside from the similarities, we can also learn from the stark differences between Earth and Io.  While the heat in Earth’s core is mainly due to the radioactive decay of uranium, thorium and potassium, Io’s extreme internal temperature is caused by gravity.  Io is the closest natural satellite to Jupiter and is one of Jupiter’s four largest moons.  Due to the close proximity to Jupiter and the slightly elliptical orbit of Io, the gravitational pull on Io ebbs and flows, creating contraction and expansion on Io’s crust.  The next two moons closest to Io, Europa and Ganymede, also interact gravitationally with Io and increase the forces on Io as they routinely pass by.  The speed of the moon’s orbits are not the same; during the same period of time Ganymede orbits once, Europa orbits twice and Io orbits four times around Jupiter.  The differences in the orbits cause the moons to line up often.  This increases the gravitational pull on Io from both Europa and Ganymede as well as from Jupiter.  This continual pull creates tidal heating causing temperatures that melt the rock within Io and fuels the intense volcanic activity.  The process of squeezing and flexing is similar to how a ball of clay will soften and warm as a person kneads it.  However, the heating of Io is unlike what we have ever experienced on Earth.  The tidal heating on Io adds as much energy as 24 tons of TNT exploding every second.  Io’s surface receives 2.5 watts of power to each square meter, compared to 0.06 watts per square meter on the Earth’s crust from global heating.  The only areas on Earth that are comparable to Io’s average are in Earth’s volcanic areas.