When Hell Freezes Over

And death and hell were cast into the lake of fire. This is the second death. Revelations 20:14.

Rivers and lakes of lava, geysers spewing toxic sulphurous plumes up to 500 kilometres into the sky and collapsed tracts of land covering a frozen multicoloured landscape. It sounds a lot like the vision of Hell described in the Bible or Dante’s epic poem The Divine Comedy. With an average surface temperature of -135 degrees Celsius, such a place gives a whole new meaning to the phrase “when Hell freezes over.” This frozen Hell actually exists right on our cosmic doorstep on one of the moons of Jupiter.

Io, along with Europa, Ganymede and Callisto, is a Galilean moon - the fourth largest moon in the solar system - that orbits the planet Jupiter. It is the closest moon to its parent planet, orbiting at a distance of 350,000 kilometres above the cloud tops. It is named after a Greek mythological character, Io, a priestess of Hera who became a lover of the king of the pantheon of Greek gods, Zeus.

The moon was originally discovered on January 7, 1610 by Galileo Galilei using his refracting telescope at the same time that he discovered the other moons. Initially, he saw Io and her sister moon Europa as one moon, which he differentiated on his next night of observation. The moon is officially in the record books as having been discovered on January 8, 1610.

It took around two and a half centuries before Io became more than a pinpoint of light, after improved telescope technology and later spacecraft allowed astronomers and planetary scientists to view large surface features on the moon. However, the intervening years were not a total waste. Observations of the orbital movements of the Galilean moons including Io helped in validating Kepler’s Third Law of Planetary Motion, which states that “the square of the orbital period of a planet is proportional to the cube of the semi-major axis of its orbit.”

As telescope technology improved in the late 1800s, American astronomer Edward Barnard was able to observe variations in Io’s brightness between the yellow-white equatorial and the reddish-brown polar regions.

Further improvements in telescopes in the mid-20th century produced the first spectroscopic observations of the moon. They showed that the moon was not only devoid of water ice, unlike the other Galilean moons, but was covered instead with sodium salts and sulphur.

In 1973 and 1974, Pioneer 10 and 11 provided an improved estimate of Io’s mass and size. The moon was found to have the highest density of all of the Jovian moons. The probes also revealed a thin atmosphere and intense radiation belts near the orbit of the moon. Pioneer 11 did take the first good close-up image of the moon but no clear surface features were identified. Pioneer 10 was supposed to do the same, but its images were lost due to the high radiation environment discovered by its successor.

In 1979, the two Voyager spacecrafts, with their more advanced imaging systems, gave us the first close-ups of the moon and its unusual surface features. They revealed a strange multicoloured surface devoid of impact craters, with oddly shaped depressions, high mountains and what appeared to be lava flows.

The multicoloured black, yellow, red, white and even green lava plains of Io are the result of sulphur compounds. There are over one hundred mountains on Io, some of which are taller than the tallest mountain on Earth, Mount Everest. Most mountains on Io have a height of around six kilometres, with some reaching up to seventeen and a half kilometres .

Based on what we have found on surfaces of other rocky objects in the solar system, one would expect a pockmarked surface due to impact craters (and so did the scientists) but none were found. There was a reason for this and it turns out that Io is the most geologically active body in the solar system, which effectively wipes out any possible impact craters. When Voyager’s navigation engineer Linda Morabito noticed a plume emanating from the surface of the moon on one of the spacecraft’s images, it gave scientists the first evidence of volcanic activity on a world other than the Earth. Later analysis would show nine such plumes across the surface.

It has been since found that Io is actually covered with four hundred active volcanoes which send plumes of sulphur-laden gases up to five hundred kilometres into the sky. The main source of heat for the active volcanism on Io is not radioactive decay but tidal dissipation, the result of Io’s orbital resonance with its neighbours, Europa and Ganymede. It has been calculated that the amount of heat created is probably around two hundred times greater than if the heat source was radioactive decay, as happens on Earth.

By looking at the distribution of the mountains versus that of the volcanoes on the moon, we find that they are opposite. This supports the theory that it is compressive forces, rather than plate tectonics as we have here on Earth, that are responsible for mountain formation.

Io is also dotted with depressions with flat floors and steep walls called paterae. They resemble terrestrial calderas, which here on Earth are often created through the collapse of an emptied lava chamber below ground. Further research is required to come to some sort of consensus as to their origin on Io.

When the Galileo spacecraft on its Jupiter mission made several close flybys of the moon in the 1990s and early 2000s, we found out even more about the surface and interior of Io. In addition to the moon’s geological features, the craft showed a relationship between Io and Jupiter’s magnetosphere, creating a belt of radiation centered on Io’s orbit; Io receives 3600 rem of radiation per day, which is about nine times the amount of radiation that Earth receives in a year from the Sun. We certainly would not last long unprotected on the surface of Io. That has not stopped science fiction from exploring the possibility of colonies on the moon. Kim Stanley Robinson in his novels Galileo’s Dream and 2312 describes human colonies on Io.

The close flyby of Galileo did also yield, coupled with data from the earlier Voyager spacecraft, significant results such as an outer, silicate-rich crust and mantle with an inner iron or iron sulphide core much like the planets of the inner solar system. Galileo’s magnetometer failed to find an intrinsic magnetic field, but it did find an induced field caused by a liquid silicate mantle fifty kilometres below the surface of the moon.

It is hard to imagine that the moon Io, with a diameter of 3642 kilometres, can have such a profound effect on the magnetosphere of its parent planet, the giant planet Jupiter with a diameter of 139,822 kilometres, but it does. The magnetosphere of Jupiter has a number of similarities to the magnetosphere of Earth, but there are also significant differences that are a direct result from the effects of its nearest moon, Io. Io, being so volcanically active, loads the magnetosphere of Jupiter with about one thousand kilograms of material per second, mostly made up of huge amounts of sulphur dioxide which is broken down by solar ultraviolet light into sulphur and oxygen ions. These ions react with the magnetosphere of Jupiter, forming a torus, a thick and relatively cool ring of plasma surrounding Jupiter located near the moon’s orbit. Tori play an important role in Larry Niven’s novel The Integral Trees and its sequel, The Smoke Ring. In these tales, a fictional gas giant orbiting a neutron star creates a torus with enough density to support life.

On its way to explore Pluto and the Kuiper Belt at the outer reaches of the solar system, the New Horizons spacecraft detected particles from Io as far as the magnetotail. The magnetotail is an elongated section of Jupiter’s magnetosphere that extends in a direction away from the Sun almost to the orbit of Saturn, 4.3 astronomical units (AU) long; one AU is the distance of the Earth to the Sun.

As Io moves in its orbit through the torus and the magnetic field of Jupiter, a large current is created that links the parent planet and moon electrically. In fact, a flux tube, a tubelike region of space containing a magnetic field, is created. A flux tube occurs when a superconductor material is cooled to extremely low temperatures, thus allowing electrical conduction with very little resistance. It is this electrical reaction that produces the auroral glow at Jupiter’s polar regions, and because of its relationship to the moon Io, it is known as the “Io footprint.” Dan Simmons in his novel Ilium utilizes this flux tube to hyperaccelerate spacecraft through the solar system.

Currently there is one planned European Space Agency mission to study the moons of Jupiter, the Jupiter Icy Moons Explorer or JUICE mission in 2022. The mission is intended to end up in Ganymede orbit and will not fly by Io but will use its instruments to monitor volcanic activity and measure surface composition during the two-year phase of the mission prior to Ganymede orbit insertion. There is also NASA’s Io Volcano Observer that is still in its conceptual stage with no funding being directed to the project.

It may not be a priority to land on a world that for all intents and purposes appears to be a world-sized depiction of Hell. There would be some enormous technical problems associated with it as well. However, the information that would be garnered from a world such as Io in the field of geology alone would be worth such a mission. Who knows, maybe somewhere in that Hell, there may also be life. We do, in fact, have analogues here on Earth. At the Kilauea Volcano Microbial Observatory in Hawaii, some microbes have been detected, mainly sulphur metabolizing bacteria. Scientists have also found carbon monoxide oxidizers and methanotrophs, bacteria that metabolize methane, in ash or lava shortly after cooling.

Science fiction has also speculated at the possibility of life on Io. One early science fiction author, Stanley G. Weinbaum, in 1935, wrote the short story “The Mad Moon.” Without the current scientific knowledge of Io, it speculated of a world that was home to two native races including balloon-headed loonies and ratlike slinkers. In Michael Swanwick’s more scientifically accurate 1999 Hugo Award-winning story, “The Very Pulse of the Machine,” the main character is an explorer who might have found a global form of life that is powered by the extreme electrical forces. The story also contains some very vivid descriptions of the moon. Whatever our reason for going to Io, geology or the possibility of life, we should always be sure that the advancement of science should never stand still.

When Hell Freezes Over

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