Io owes its perpetual, noxious-looking colors to the deposits left by volcanoes. We know sulfur dioxide can dramatically change colors when it cools—red, orange, yellow, or blue, and white in its frozen state. There are also green areas that are jokingly referred to as ‘golf courses.’ Here the color is either provided by sulfur or a substance called olivine.
The volcanoes might be impressive and gigantic, but have you heard about the walking mountains of Io? They really exist. They might not be as tall as the lava curtains or the tectonic fracture lines. But they are unique within the solar system—at least in this aspect. Once more, Jupiter is behind it all. The giant planet on one side and the three other Galilean moons on the other pull so strongly on Io that there is a high and a low tide—on its firm crust! This generates a tidal mountain up to 100 meters high that walks around the moon according to Io’s orbit. If you were to stand still at a specific location near the equator, the ground below you would rise and fall by up to 100 meters during one day on Io. The exact height depends on the position of the other three large moons. In Earth’s oceans, the tides reach a maximum height of 18 meters, and in the firm crust of the Earth no more than 20 centimeters.
Io also has ‘real’ mountains, but not very many—astronomers count up to 150. The highest, Boösaule Montes, rises 17,500 meters above its surroundings, but average heights are only 6,000 meters. All of the mountains are tectonic structures. At fault lines, parts of the crust are pushed up, because at other spots volcanic material sinks into the interior. You can often see how the mountains were created, since they have steep escarpments on one side and rise gradually on the other. Some mountains were pushed out of the ground like individual pistons, and each consists of a flat plateau with steep cliffs on all sides. In rare cases, lava streams formed low-profile mountains resembling Earth’s shield volcanoes, but these are usually only 1,000 to 2,000 meters high.
The Interior: Hot and Liquid
Io is not an icy moon like many of its siblings in the outer solar system, but instead it is a rocky moon. Its density is even slightly above that of Earth’s moon and it is the densest of all known moons in the solar system. This is probably related to the way it came into being, losing 99 percent of its water early in its existence. Its interior therefore consists mostly of silicate rocks and iron.
While this ‘apple’ might look rotten from afar, it contains a core that is probably rich in iron and perhaps iron sulfide. It accounts for a fifth of the mass of Io and has a radius of 350 to 900 kilometers—depending on what sulfur content the core is assumed to have. The more sulfur, the larger it has to be. The Galileo probe was not able to measure a magnetic field belonging to Io so the core is probably solid, because otherwise its movement would generate a magnetic field.
The mantle and the crust are located above the core, and both consist primarily of silicates. It is suspected 75 percent of the mantle is made of the mineral Forsterite, a silicate containing magnesium. The iron content in the mantle is higher than on Earth or its moon, but lower than on the ‘Red Planet’ Mars. Up to 20 percent of the mantle must exist in molten form. Galileo measured an induced magnetic field that could only be explained by the existence of an ocean of molten lava below the crust. While Enceladus and Europa have their oceans, Io possesses a lava ocean that starts 50 kilometers below the surface, is 50 kilometers thick, and accounts for up to 10 percent of the mantle rock. The lava here might reach a temperature of 1,200 degrees. The crust, which begins above it, differs in thickness according to region, between 12 and 50 kilometers. It mostly consists of basaltic rock and sulfur.
Why does the crust contain so much sulfur? Researchers assume the solubility of sulfur decreases with depth, so the existing deposits are concentrated near the surface.
And where does the heat come from that keeps the mantle partially molten? On Earth it is mostly a remnant of the early period of the solar system. The core not only releases heat to the mantle, but even more energy is released when previously liquid matter crystallizes. This is called ‘heat of crystallization.’ In addition, as the gradually solidifying inner core also slowly shrinks, a compression of matter sets in, which releases additional energy. Scientists used to assign an important role to the decay of radioactive matter, but this was eventually proved to be wrong.
On Io, though, the slightly eccentric orbit around Jupiter is the culprit, with help from the three other large moons. This exposes Io to forces from different directions. Just like a lump of clay heats up when you knead it vigorously, so does poor Io. On the other hand, without this effect Io would be as boring as Earth’s moon and would certainly not have become the subject of this novel. Other moons—for example, Enceladus or Europa—also generate energy through tidal forces and thus maintain liquid oceans, but on Io this process happens to a much larger extent.
The Birth: It Could Have Been Worse
The creation of Io is closely linked to the birth of its father planet Jupiter. The gas giant developed in the early period of the solar system, about 4.5 billion years ago, after the sun ignited inside the planetary nebula. Jupiter probably captured most of its current moons later. But the four Galilean moons were born at about the same time as the planet. They developed from the gas disk that at the time surrounded Jupiter. Inside it, pressure differences arose that finally became the cores of some moons.
If you run a computer simulation, you will notice there is something missing, though—the combined mass of the moons should be about ten percent of the mass of Jupiter. In reality, they add up to only two percent. The reason for this might be that Jupiter repeatedly ate the children next to him. These newborn moons obviously were slowed down by the gas disk in which they moved. They moved closer and closer to their father Jupiter and were eventually swallowed. This repeated itself until the gas in the interior section of the disk had been depleted. Then, new moons were no longer slowed down and could keep their orbits. Io, then the innermost moon, was lucky not to be eaten. On the other hand, it was unfortunately so close to the planet—which was much hotter then—that all of its water evaporated and the vapor disappeared into space.
Life in a Sulfur Lake?
Could there be life on Io? In this novel, the crew discovers a sulfur-based life form. Sulfur is a very chemically active element that exists in numerous configurations and compounds. In contact with water, though, sulfur will create extremely caustic sulfuric acid. Since there is almost no water on Io, that should not be a problem—though it is hard to say whether such life forms could protect themselves sufficiently if water was present or introduced. The kind of life described in the novel is highly speculative, though not completely imaginary.
One thing Io offers more than enough of is energy differences—an important prerequisite for the development of life. One could think of this moon as a giant chemistry lab. If there is a possibility for creating life with the materials available on Io, then this lab has probably tried it out several times during the four billion years of its existence. Of course we do not know yet if it succeeded.
On Earth there are actually life forms using sulfur to generate energy. For 138 days, between October 2011 and March 2012, the submarine volcano named Tagoro, near El Hierro in the Canary Islands, turned its surroundings into an inhospitable ocean-floor desert. Molten lava and poisonous gases destroyed any marine life within a radius of several kilometers. The eruption created a volcanic cone rising from the sea floor at a depth of 363 meters to a depth of 89 meters. The water temperature increased, as did the percentage of carbon dioxide and hydrogen sulfide, while the oxygen content decreased.
Tagoro was far away from other submarine volcanoes, so there were no nearby life forms adapted to these conditions. Researchers were thoroughly surprised when they discovered a thick mat covering the top of the volcano 32 months after the eruptions. This mat consisted of white strands, up to three centimeters long, of a hitherto unknown bacterium. Scientists gave it the common name Venus’ hair, due to its appearance, and the scienti
fic name Thiolava veneris. This bacterium has some unusual abilities that predestine it for survival in such inhospitable environments. It can gain energy by oxidizing sulfur or sulfur compounds. In addition it possesses three ways of breathing carbon dioxide, and it can dissolve organic material.
The researchers were amazed at how quickly the bacterium reached the volcano, and then how rapidly a new ecosystem formed, as there were also unusual higher species living on and in the bacterial mats. Life, it seems, always succeeds surprisingly well, settling in even the most inhospitable locations.
Another possibility for life developing on Io could be related to the plentiful supply of silicon. Like carbon, silicon is ‘tetravalent,’ something that allows for numerous compounds. The reactions might be much slower, but who says life always has to be in a hurry? Maybe someday visitors to Io will encounter silicon-based life forms flourishing in a hot lava environment, moving only a few millimeters a day, but living for 10,000 years. Hypothetically speaking, due to such a near-stationary existence, these lifeforms might perceive time so slowly as to miss a human walking quickly past them, maybe similar to how you sometimes think you saw a shadow or a ghost from the corner of your eye.
Scientists have also discussed a very different scenario—during the first ten million years of its existence, Io might have still had enough water for carbon-based life to develop. These life forms, in theory, could have gradually retreated into the warmer interior, which would have protected them from radiation, and eventually they could have adapted to using other chemical compounds. The sulfur bacteria mentioned above show what these researchers might imagine.
The Exploration of Io
Since Io was discovered in 1610 by Galileo Galilei, and almost simultaneously by the German astronomer Simon Marius, the Latinized form of his name, Simon Mayr. Io and its three siblings have played an important role in shaping humans’ view of the cosmos. After all, the four Galilean moons show that the model of bodies orbiting around a central object is not uncommon in the universe. Galileo built himself a telescope with a magnification of 20x after reading a description of telescopes built in the Netherlands. In January 1610 he aimed it at Jupiter and discovered the four large moons.
For a long time, Io was called Jupiter I, because it was the first moon of Jupiter, meaning it is closest to the planet. It was only in the middle of the 20th century that the name already suggested by Simon Marius came into usage.
In the late 19th and early 20th centuries, details on the surface were first seen through telescopes, but Io was still considered a common moon full of craters. The big surprise came with the visits by the two Voyager probes: Io was a very active world, unique in our solar system.
Pioneer 10 and Pioneer 11, which explored Jupiter in 1973 and 1974 respectively, revealed that Io resembled our moon more than the three large icy moons did, and that its density was relatively high. Both of these probes were supposed to take many photos, but almost all of the photos were lost due to the unexpectedly-intense radiation.
Researchers were even more amazed when the two Voyager probes entered the Jupiter system starting in 1979. Io, they realized, might have a density similar to Earth’s moon, but otherwise it hardly resembled it, due to the strong volcanic activity that is completely absent on our moon. The Voyager probes photographed the first volcanic eruptions, mapped mountain ranges, and found lava lakes, whose exact nature was only determined later. They also helped explain the unusual heat output discovered in the 1970s using infrared imaging through telescopes.
Voyager 1 approached Io as close as 20,600 kilometers and managed to take photos with a resolution of 500 meters per pixel. Some of the photos, however, were blurry due to the strong radiation. The more detailed photos revealed lava rivers, volcanic craters, and mountains higher than Mount Everest. On March 8, 1979, Voyager 1 also discovered the first plume created by a volcanic eruption.
Today’s knowledge about Io is based mostly on the Galileo probe. This probe had a problematic beginning, as its launch had to be postponed several times due to the Challenger disaster in 1986, and then its powerful high-gain antenna failed. Therefore, all the images had to be sent with a lower data rate by the low-gain antenna. At the end of the 1990s Galileo performed several fly-bys of Io. Among other things, it measured the magnetic field and the gravitational field that indicated an iron core and a differentiated interior, and took numerous pictures of the surface, including the classic true-color images. Scientists were particularly excited about the moon’s changes since the visit by Voyager 1. The Prometheus plume, for instance, had moved 75 kilometers westward. In the photos taken during the ninth orbit they discovered a new explosion in Pillan Patera. An occultation of the sun by Jupiter allowed Galileo to photograph the aurora in the sky of Io.
In 2000, Cassini came by on its way to Saturn and managed to photograph the aurora. The next visitor to pass by was the probe New Horizons in 2007, on its way to Pluto and beyond. It captured, among other things, images of the plumes above Tvashtar.
Currently, Juno is in the Jupiter system. This probe will mainly explore the planet itself, but it will keep its infrared spectrometer trained on Io’s volcanic activity.
Starting in 2030, things could get really exciting. That’s when JUICE—the ‘Jupiter Icy Moons Explorer’—is supposed to reach the system, ending in an orbit around Io’s brother Ganymede. As its name indicates, this ESA probe will focus on the icy moons Ganymede and, to a lesser extent, Callisto and Europa, but it certainly will take a few intensive glances at Io.
Two U.S. institutions—the University of Arizona and Johns Hopkins University’s Applied Physics Laboratory—proposed a mission called Io Volcano Explorer (IVO) to NASA, which would approach the moon as close as 200 kilometers and investigate its volcanic activity. IVO twice reached the shortlist for NASA’s Discovery Program. During the last vote in January 2017, though, the missions Lucy and Psyche were instead chosen. Their destinations are different classes of asteroids.
It might take 100 years or more before a human actually sets foot on Io. ILSE only explored this moon after the crew received a message about it. No matter how fascinating this lover of Zeus may be, she is kind of tough on her fans, and mankind will probably start out with friendlier destinations.
Glossary of Acronyms
AI – Artificial Intelligence
API –Application Program Interface; Acoustic Properties Instrument
ASCAN – AStronaut CANdidate
BIOS – Basic Input/Output System
AU – Astronomical Unit (the distance from the Earth to the sun)
C&DH – Command & Data Handling
CapCom – Capsule Communicator
Cas – CRISPR-associated system
CELSS – Closed Ecological Life Support System
CIA – (U.S.) Central Intelligence Agency
COAS – Crewman Optical Alignment Site
Comms – Communiques
CRISPR – Clustered Regularly Interspaced Short Palindromic Repeats
DEC PDP-11 – Digital Equipment Corporation Programmable Data Processor-11
DFD – Direct Fusion Drive
DISR – Descent Imager / Spectral Radiometer
DNA – DeoxriboNeucleic Acid
DoD – (U.S.) Department of Defense
DPS – Data Processing Systems specialist (known as Dipsy)
DSN – Deep Space Network
ECDA – Enhanced Cosmic Dust Analyzer
EECOM – Electrical, Environmental, COnsumables, and Mechanical
EGIL – Electrical, General Instrumentation, and Lighting
EJSM – Europa Jupiter System Mission
ELF – Enceladus Life Finder
EMU – Extravehicular Mobility Unit
ESA – European Space Agency
EVA – ExtraVehicular Activity
F1 – Function 1 (Help function on computer keyboards)
FAST – (Chinese) Five-hundred-meter Aperture Spherical Telescope
FAO – Flight
Activities Office
FCR – Flight Control Room
FD – Flight Director
FIDO – FlIght Dynamics Officer
Fortran – FORmula TRANslation
g – g-force (gravitational force)
GBI – Green Bank Interferometer
GNC – Guidance, Navigation, and Control system
HAI – High-Altitude Indoctrination device
HASI – Huygens Atmospheric Structure Instrument
HP – HorsePower
HUT – Hard Upper Torso
IAU – International Astronomical Union
ILSE – International Life Search Expedition
INCO – INstrumentation and Communication Officer
IR – InfraRed
ISS-NG – International Space Station-Next Generation
IT – Information Technology
IVO – Io Volcano Explorer
JAXA – Japan Aerospace eXploration Agency
JET – Journey to Enceladus and Titan
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