What Lies Beneath
The world’s as ugly as sin. And almost as delightful.
Callisto, named after one of Zeus’ many lovers in Greek mythology, was discovered in January 1610 by Galileo Galilei through his telescope along with the other Jovian moons that would bear his name, the “Galilean”moons, Io, Europa and Ganymede.
The name Callisto was suggested by Simon Marius, a rival of Galileo who claims to have discovered the Galilean moons before Galileo. He attributed his suggestion to Johannes Kepler, the founder of the laws of planetary motion. The name, Callisto, and the names that Marius gave the other Galilean moons were not overly popular at the time, though. Callisto went by the name of Jupiter IV up until the early twentieth century, a system introduced by Marius's rival, Galileo himself.
Most of the Galilean moons have held interest by scientists for different reasons. Io, the closest to Jupiter, is a world of extreme volcanic activity driven by tidal forces from nearby Jupiter. Europa is a world that shows surface features that remind one of ice fractures, leading to speculation of an ocean underneath. Ganymede, the solar system’s largest moon, has a rich surface fragmented by plates, indicating a rich tectonic history. Callisto is the furthest of the Galilean moons and is the ugly sister of the bunch, orbiting at a distance of approximately 1.8 million kilometers from Jupiter. It is the third largest moon in the solar system, after its sister Ganymede and Saturn’s moon, Titan. It is by far the most heavily cratered body in the solar system and has shown little activity in its geologic past.
To add insult to its ugly exterior, for years it held relatively little interest to scientists. It was geologically dead, change only being attributed to impact by heavenly bodies.
For many years, we relied solely on ground-based observations of the moon. When Pioneers 10 and 11 flew by Callisto in the 1970’s they did little to enhance our knowledge of the world. In 1979 and 1980, with the flybys of Voyager 1 and 2, the real probe of Callisto began. The probes imaged almost half of the surface with a resolution of 1 to 2 kilometers. In 2000, the Cassini spacecraft on its way to Saturn produced high quality infrared spectra of all of the Galilean moons. New Horizons, the craft on its lengthy 2007 flight to Pluto, obtained further imaging of Callisto.
It was, however, Galileo, a NASA spacecraft launched to study Jupiter and its moon system, which between 1995 and 2003 moved Callisto from the boring, non-interesting world to the class of something worth exploring.
One of the first things detected by Galileo with its Near Infrared Mapping Spectrometer was that Callisto actually had an atmosphere, albeit tenuous. The atmosphere was found to contain mostly carbon dioxide at a temperature of about -125 degrees Celsius. Calculations of the atmospheric pressure would indicate that the atmosphere is being replenished; otherwise it would dissipate into the vacuum of surrounding space within four days. Where the carbon dioxide is coming from is somewhat of a mystery, but one mechanism that has been theorized is the slow sublimation of carbon dioxide ice from the surface of the satellite. Interestingly enough, in one of the first science fiction stories ever set on Callisto, The Callistan Menace written by Isaac Asimov in 1940, there is the description of an atmosphere of carbon dioxide.
Galileo also discovered that Callisto had an ionosphere that only existed in the hemisphere illuminated by the Sun. In spite of this relationship to the Sun, the electron density could not be explained by the photoionization of the carbon dioxide in the atmosphere alone. There had to be another mechanism at work. One hypothesis is that the atmosphere is actually dominated by molecular oxygen; however, this is only theoretical at this time as no direct observations of the gas have been made, but calculations show that in spite of our inability to detect it due to limited technology, it could exist in abundance. Adding fuel to the theory, observations through the Hubble Space Telescope have revealed the presence of condensed oxygen on the surface of the moon.
Galileo did not restrict its observations to the atmosphere. The landscape of Callisto holds many surprises as well. As one would expect from the most heavily scarred world in the solar system, Callisto’s surface is dominated by features such as multi-ring basins, catenae (crater chains) and their associated scarps and ridges. Galileo revealed the makeup of these features. The surface of Callisto is dominated by water ice, but other non-ice materials such as iron and magnesium rocks, carbon dioxide, sulphur dioxide, ammonia and possibly organic compounds are present as well.
The largest impact features of Callisto are the multi-ring basins with Valhalla and Asgard being the largest. Valhalla has a diameter of about 3000 kilometers and Asgard 1600 kilometers. They appear as two bulls eyes on the face of the moon. Just north of Asgard, there is another multi-ring basin of about 500 kilometers in diameter. Multi-ring structures are probably the result of post-impact concentric fracturing of the lithosphere.
Catenae, notably the Gomul and Gipul catenae, 350 kilometers and 620 kilometers long respectively, are a series of impact craters arranged in a chain. They probably resulted from a spray of space debris that was tidally broken up by the gravity well of Jupiter. We only have to remember Comet Shoemaker Levy 9 as it was sucked into Jupiter in July 1994 to recall the awesome gravity well of the giant planet at work when it broke the comet into many smaller pieces.
On a smaller resolution, Callisto’s surface is very degraded with, in spite of it being heavily cratered, a deficit of craters less than one kilometer in diameter. Instead, the surface is covered in knobs. The knobs are thought to be the remnants of crater rims that have been degraded by an unknown mechanism. A hypothesized mechanism is the sublimation of water which is possible in the temperatures that exist on Callisto. Such sublimation of water from dirty ice that makes up the lithosphere of Callisto would explain the apparent erosion of the crater rims resulting in the knob-like structures. The non-ice remnants form debris avalanches into the sloping crater walls. In turn, the darker surface allows the further absorption of solar energy allowing the process to continue.
As one moves towards the south pole, the smoothness of the surface increases. Theories for this range from bombardment by micrometeorites to energetic ions that flatten out any protuberances.
What lies beneath the surface of Callisto is perhaps even more interesting and revolutionary than the surprises we have noted on its surface. There may be water under the moon's skin and where there is water, there is the potential for life.
Galileo’s magnetometer detected an induced fluctuating magnetic field which appeared to fluctuate in time with Jupiter’s rotation. Jupiter’s powerful magnetic field must be creating electrical currents within Callisto, which in turn create the moon's fluctuating magnetic field. Since ice is a poor conductor and the atmosphere is very thin, they are unlikely sources. However, if there were a salty ocean beneath the ice, it could create such a field. Further analysis showed that the electrical currents flowed in seeming synchronicity with the effects of Jupiter’s rotation. This would be consistent with the presence of a salty ocean. This speculation allows for estimates of the thickness of the ice layer over the theorized ocean. For Callisto the ice would be from eighty to one hundred and fifty kilometers thick.
How would a subsurface ocean be possible in such a frozen environment? Callisto’s distance from Jupiter limits the effects of internal heating due to tidal effects as is theorized to occur on a sister moon, Europa. There are no evident plate tectonics as we find on Ganymede to explain any subsurface melting. There may be some radioactive decay and further accretion of the rocky interior of Callisto, but not enough to provide the heat necessary for a subsurface ocean on a frozen world.
The presence of the ocean can be explained by the properties of water. Water will freeze at a temperature below zero degrees Celsius when the pressure is above one atmosphere (the air pressure on earth at sea level). As the pressure increases, the melting point decreases.
The decreasing melting point with increasing pressures is one of the theories behind why glaciers move. The enormous pressure of the overlying ice actually melts a layer at the base, thus lubricating the solid ice on the bedrock below allowing movement.
A ten kilometer deep ocean is possible with pure water and would fit the data of the fluctuating magnetic field. If we add a component of ammonia, the ocean’s melting point is lowered further allowing for an ocean up to 300 kilometers thick.
Other indirect evidence of a subsurface ocean is the impact crater that created Valhalla. Computer modeling showed that if Callisto had a water layer in its interior, then the impact that created Valhalla would have dispersed the seismic shock waves. The absence of a grooved or hilly antipode from the Valhalla impact suggests a layer of liquid water.
Where there is liquid water there is potential for life. Could there be life in Callisto’s ocean? What would it be like?
Science fiction has speculated on the nature of life on Callisto. H. P. Lovecraft in his 1919 story Behind the Wall of Sleepreferenced “insect-philosophers that crawl over the fourth moon of Jupiter.” Lin Carter’s Callisto series takes place on a habitable Callisto.
Science has taken us beyond Lovecraft and Carter, however. Even with the presence of water, the conditions on Callisto would still be very extreme. Could life thrive in such extreme conditions?
On Earth, where we have our only examples of life to draw on, there is a class of microorganisms that can live under extreme conditions ranging from vents deep within ocean trenches; ice more than hundreds of thousands of years old; water above boiling such as is found in geysers; and oil wells. Others microorganisms prefer highly acidic or alkaline water and many live in the digestive tracts of animals. These microbes are called archaeobacteria, a name that has fallen out a favour recently since they are not bacteria. They are the third group of organisms on Earth, the others being bacteria and eukaryotes (plants and animals).
On Earth, we have an opportunity to potentially explore such a subsurface body of water. Lake Vostok, about the size of Lake Ontario, was discovered in 1996 under the Russian Antarctic base, Vostok Station, by radio . It is overlaid by almost 4000 meters of ice. No one has penetrated the water but drills have come within 100 meters of the water layer.
Even if we do not discover an ocean or life on Callisto, all is not lost. Callisto will never go back to being the dead, ugly world that it was originally thought to be. It is a world of the future.
Kim Stanley Robinson’s Blue Mars describes a colony on Callisto and Anne McCaffery’s The Rowan, has a protagonist who lives in a terraformed dome on Callisto. Perhaps most interesting is that aliens may have seen the benefits of a Callisto base. Ian Stewart and Jack Cohen’s Wheelers begins with the discovery of an alien artifact on the surface of Callisto.
In 2003, NASA commissioned a study called “Human Outer Planets Exploration (HOPE) regarding the future of exploration of the outer solar system. The ugly sister was hands down the winner of being the body of choice to set up a base for future human exploration.
The NASA study cited that Callisto would make a great base for human expansion into space. Benefits include: Its relatively low radiation levels due to the distance from Jupiter; its geological stability; and the proximity of Jupiter to allow for a gravity assist for future spacecraft venturing further into the solar system and beyond. The ugly sister has certainly grown out of her awkward stage.
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