A Groovy World

Let the great world spin for ever down the ringing grooves of change.

Alfred Lord Tennyson

Ganymede, one of the Galilean moons, seems to take a back seat to some of its astronomical siblings. Io is known for its extreme volcanism and Europa for its subsurface ocean that may harbour life. Even Callisto is known for its “ugliness,” having the oldest and most cratered landscape in the solar system. However, if we are to dig deeper, both figuratively and literally, we find that the third Galilean moon, Ganymede, is not just another interesting world, but may be the most intriguing of the Galilean moons. Not only is it the solar system’s largest satellite (1.5 times as large as our Moon), orbiting its parent planet Jupiter at a distance of just over a million kilometers in an almost circular orbit, it is also the only moon that has an ionosphere in the upper reaches of its very thin atmosphere. Like its neighbours, Europa and to a lesser extent Callisto, Ganymede most likely also has a subsurface ocean, which means a possibility of life.

Ganymede is documented as having been discovered by Galileo on January 7, 1610 when he looked up into the night sky through his homemade telescope; however, there are some ancient Chinese astronomical records that indicate that it was Chinese astronomer Gan De, who in 365 BC, actually discovered the moon with naked eye viewing, describing it as small reddish star next to the planet Jupiter. This early naked eye observation is easy to imagine  since Ganymede is very bright in the heavens with an albedo, which is a ratio of the light reflected to the light received from external sources, of 0.43, probably due to the predominately large water ice surface. Whether or not Gan De or Galileo was the true discoverer of the moon is a matter for historians to debate. It is the world that they discovered that is of interest.

Up until recently, Ganymede remained just another mystery of the night sky and science fiction writers were able to let their imaginations run wild. In 1940, Isaac Asimov wrote the short story, Christmas on Ganymede in which native beings on the moon are introduced to the holiday. Leigh Brackett wrote another short story, The Dancing Girl of Ganymede in 1950 which describes Ganymede as a volcanic, jungle covered world. Another short story, by famous editor (Astounding 1937 to 1971-now Analog Science Fiction and Fact) John W. Campbell Jr., is more in keeping with what we know of Ganymede today; Marooned, which was published posthumously in 1976, is about a base camp of a Jovian mission set on a frozen surface of Ganymede.

The surface of Ganymede, generally covered with water ice, also displays darker and lighter regions indicating a complex geological history. The darker regions, heavily scarred with impact craters, cover about a third of the satellite and are relatively old when compared with the lighter regions; it is believed that the dark areas are the original crust of Ganymede. Whereas the craters of our Moon have their high ridges and deep depressions, the craters of Ganymede, though large, are relatively flat. One theory to explain this is that there is a slow adjustment to soft ice surface. Such flat craters are known as ‘palimpsests.’ Palimpsests is an ideal word to describe these faded craters as it was originally used to describe reused writing media such as parchment or leather, on which older writing may still be visible reused ancient writing materials on which older writing was still visible below. The lighter parts of Ganymede, on the other hand, are relatively young and covered with sulci or grooves that can be as wide as five to ten kilometres and can stretch for thousands of kilometres across the surface. The ridges that run along the grooves can reach heights of up to seven hundred meters.

The formation of the grooved terrain remains an unresolved geological mystery. There are several ideas, though. One is that cryovolcanism may play a role, but it is insufficient to explain the terrain entirely; cryovolcanism is a process whereby water, methane or ammonia are ejected from an ice volcano rather than molten rock. Another more likely explanation for the grooves is tidal flexing that happens when the moon passes through unstable orbital resonances with its satellite neighbours, Europa and Io. Orbital resonance is a phenomenon that happens when two or more orbiting bodies exert regular, but periodic gravitational drag on each other. This tidal heating may have heated the interior and strained the lithosphere leading to the development of grooves.

An expansion of the tidal flexing theory to explain the origin of the Ganymede grooves is that it was residual heat due to radioactive decay from early core formation of the moon coupled with subsequent tidal heating that resulted in the formation of the grooves but also to an expansion of the moon of up to six percent of its original size.

In one of the earliest stories set on Ganymede, Stanley Weinbaum, in his 1938 short story Tidal Moon, exploits the tidal heating to explain a quarterly flooding of the surface of the moon, due to tidal forces from Jupiter rather than the neighbouring moons.

In addition to grooves, Ganymede also has polar caps, more than likely made up of water ice. They were first observed by the Voyager 1 and 2 spacecraft when they flew by the moon in 1979. There are some theories on the origin of the caps. One theory on the origin of the polar caps is that they may be the result of water vapour migration to higher latitudes. Another more likely theory and based on data gathered by the Galileo spacecraft, during one of its six flybys between 1996 and 2000, is that the caps are due to plasma bombardment of the moon’s surface by charged particles from its parent planet Jupiter and the Sun. The moon’s strong intrinsic magnetic field, first discovered by Galileo, in 1996, is thinnest at the polar caps. This actually creates a funnel for the charged particles onto the surface at the poles resulting in more intense particle bombardment. This, in turn, leads to the redistribution of water molecules, with frost migrating to colder areas of the polar terrain. 

The intrinsic magnetic field of Ganymede, the only known moon to possess one, is possibly created by convection through a liquid iron core.  The problem with this theory is that in spite of the presence of the iron core, similar objects lack a magnetic field. In other words, given the relatively small size of the moon, it should have lost the heat so that the core would have cooled to a point where it is no longer liquid and thereby incapable of magnetic field generation through convection. 

Here again, the orbital resonances with its closest neighbour Galilean moons may provide an explanation. The orbital resonances that have so disrupted the surface of the moon, may also have allowed the magnetic field to persist through tidal heating, allowing the core to remain liquid. Another theory is that the intrinsic magnetization of Ganymede is a fossil of magnetization of the silicate rocks in the mantle made possible if it had a dynamo-generated field in the past.

In addition to the intrinsic magnetic field, Ganymede also has a detectable induced magnetic field. The presence of this induced magnetic field, much like that found on Europa and Callisto, suggests something fascinating, the possibility of a saltwater layer under the surface of Ganymede. Two hundred kilometers beneath the surface, there is hypothesized to be a saltwater ocean, sandwiched between layers of water ice.

Other evidence of a subsurface ocean comes ironically from the surface itself. Though water ice covers much of the surface, Galileo also did find in its close flybys, other chemicals on the surface including carbon dioxide, sulphur dioxide, sulphate compounds and some organics. The chemicals of particular interest are magnesium sulphate and sodium sulphate, both salts which may originate in a subsurface ocean.

Wherever there is water, there is the possibility of life as well. We have found life analogues here on Earth at the deep sea vents on the oceanic ridges at the bottom of the sea, so life in Ganymede’s oceans is certainly not out of the question. Philip K. Dick, together with Ray Nelson, in their novels, went beyond deep sea vent life and created sentient life forms in their novels set on Ganymede. In Clans of the Alphane Moon they introduce us to intelligent slime mould on Ganymede and in their The Ganymede Takeover, we find the Earth confronting sentient wormlike aliens from Ganymede in an invasion.

Back in 1972, Indian, British and American astronomers claimed to have detected a thin atmosphere around Ganymede when Jupiter and its moon passed in front of a star. Voyager’s observation contradicted their hypothesis in 1979 when it revealed that there was no atmosphere at all. Despite the Voyager data, the astronomers were vindicated once again when an oxygen-rich atmosphere, albeit thin, was found on Ganymede in 1995 by the Hubble Space Telescope.

Hubble Space Telescope discovered the atmosphere in a particularly interesting fashion. It was through the airglow of atomic oxygen which results from electron impact of molecular oxygen; in simple terms, it suggests that the atmosphere of Ganymede is predominately an oxygen-rich one with a tenuous pressure. Airglow, also known as nightglow, is an optical phenomenon whereby the night sky is never truly dark even after we remove the effects of starlight and sunlight that diffuses from the sun-exposed side of a moon or planet. Though the oxygen is not evidence of life, it does suggest that there is water ice on Ganymede’s surface. The ice is split into hydrogen and oxygen when bombarded by radiation with the lighter hydrogen being quickly lost to space. The airglow is also not uniform over the entire surface though. The brighter areas of the northern and southern hemisphere are theorized to be actually the result of polar auroras, a phenomenon that results from charged particles interacting with the magnetosphere of a planet or moon.

There is also an ionosphere (upper atmosphere of a planet or moon filled with ionized particles created by interaction of the atmosphere with solar wind) around Ganymede but its nature is somewhat fleeting. When Galileo did its flybys of the moon, measurements indicated the presence of elevated electron activity (indicator of an ionosphere) in some areas and in others, not at all. It would appear that the ionosphere is really not well contained. Why this should be continues to confound astronomers and space scientists.

Our exploration of Ganymede like the other Galilean moons is still in its infancy but there are plans for further exploration beyond Galileo. Recently, NASA’s New Horizons on its way to Pluto, made topography and composition maps of the moon in 2007. There are also plans of another mission to Ganymede in 2022 when the European Space Agency’s Jupiter Icy Moon Explorer dubbed JUICE launches. After flybys of the three icy moons of Jupiter, Europa and Callisto, the probe is going to enter Ganymede orbit. Who knows what wonders we will discover? The Russians may be looking at a 2024 launch of a Ganymede Lander as a partner to JUICE, with an emphasis on astrobiology.

We are still a long way from colonizing the Ganymede, but that has not hindered the imagination of science fiction authors. Poul Anderson wrote The Snows of Ganymede, a novella about a group of explorers set on terraforming the moon. Upon visiting they find a settlement that had been established there two centuries before by religious fanatics from the United States.

Lester del Rey set several of his novels on Ganymede. In Outpost of Jupiter, a plague strikes a settlement on the moon. In Space Jockey, Ganymede is actually a former penal colony of Earth. He also wrote The Runaway Robot, whose main character a robot, that lives in a colony on Ganymede. Hard science fiction writer, the late Charles Sheffield wrote The Ganymede Club, in which the characters unravel a murder mystery set on the moon.

Robert Heinlein wrote a number of science fiction novels in the 1940’s and 50’s geared towards a younger audience. One, entitled Farmer in the Sky, is about an Eagle Scout boy who wants to join his father who plans on joining a colony on Ganymede. The Ganymede of Robert Heinlein has been fully terraformed with machines retaining heat and creating a breathable atmosphere. 

James Hogan’s Giant series is about an alien race that lived on the world of Minerva .which through a nuclear war between inhabitants of Minerva and its moon Cerios, was destroyed to become the asteroid belt and the planet Pluto. The moon, Cerios, became Earth’s satellite partner, the Moon. Before the nuclear catastrophe that according to the series timeline occurred around fifty thousand years ago, there was a time  from approximately twenty-five and four million years ago that carbon dioxide levels on the planet Minerva began to rise, and the Minervans were forced to seek another home.  Though most of the Minervan population escapes to another star system one of their ships had crashed on Ganymede which is now the focus of scientific exploration by human explorers in the series.

In his novel, Bloom, after destroying the Earth with a grey goo which annihilates all life on Earth, Wil McCarthy has the human race settle and survive on the Jovian moons including Ganymede and in the asteroid belt. In Paul McCauley’s The Quiet War, Ganymede is one of the earliest sites for human colonization who live in colonies several kilometers below the icy cover.  

In another spin on colonizing Ganymede, Arthur C. Clarke does not see humans doing the work of terraforming the moon but only benefiting from the work of others. In Arthur C. Clarke’s 2061: Odyssey Three and 3001: The Final Odyssey, Ganymede has been warmed by the newly created sun, Lucifer, created by a superintelligent race that was introduced in previous Odyssey books. People live on the moon on a large equatorial lake called Anubis City.

Perhaps the most interesting human character to inhabit Ganymede can be found in Bradley Denton’s Buddy Holly Is Alive and Well on Ganymede. In the story, television sets begin to broadcast a concert from Ganymede, with a singer who alleges he is Buddy Holly.

Ganymede continues to fire up the imagination of scientists and science fiction writers. The unusual geology and the possibility of alien biology continues to make us wonder, to drive us forward, to hopefully further explore this strange grooved world. 

Further Reading:

Barr, A. et al. 2001. Rise of Deep Melt into Ganymede’s Ocean and Implications for                               Astrobiology. 32nd Lunar and Planetary Science Conference. Abstract1781.

Barr, C. and Canup, R. 2010. Origin of the Ganymede-Callisto dichotomy by impacts during the late heavy bombardment. Nature Geoscience. 3:164-167.

Barth, C. et al. 1997. Galileo ultraviolet spectrometer of atomic hydrogen in the atmosphere of Ganymede. Geophysical Research Letters. 24(17):2147-2150.

Bills, B. 2005. Free and forced obliquities of the Galilean satellites of Jupiter. Icarus. 175(1):233-247.

Bland, M. et al. 2009. The orbital-thermal evolution and global expansion of Ganymede. Icarus. 200(1):207-221.

Broadfoot, A. et al. 1981. Overview of the Voyager Ultraviolet Spectrometry Results through Jupiter Encounter. Journal of Geophysical Research: Space Physics. 86(A10):8259-8284.

Brown, M. 1997. A Search for a Sodium Atmosphere around Ganymede. Icarus. 126(1):236-238.

Calvin, W. and Spencer, J. 1997. Latitudinal Distribution of O2 on Ganymede: Observations with the Hubble Space Telescope. Icarus. 130(2):505-516.

Canup, R. and Ward, W. 2002. Formation of the Galilean  Satellites: Conditions of Accretion. The Astrophysical Journal. 124(6):3404-3423.

Carlson, R. et al. 1973. Atmosphere of Ganymede from its occultation of SAO186800 on 7 June 1972. Science. 182(4107):53-55.

Casacchia, R. and Strom, R. 1984. Geologic evolution of Galileo Regio, Ganymede.  Journal of Geophysical Research: Solid Earth. 89(S02):B419-B428.

Delitsky, M. and Lane, A. 1998. Ice chemistry of Galilean satellites. Journal of Geophysical Research. 103(E13):31391-31403.

Dickinson, Terence. 2012. Hubble’s Universe: Greatest Discoveries and Images. Firefly Books.

Eviatar, A. et al. 2001. The ionosphere of Ganymede. Planetary and Space Science. 49(3-4):327-336.

Feldman, P. et al. 2000. HST/STIS Ultraviolet Imaging of Polar Aurora on Ganymede. The Astrophysical Journal. 535(2):1085-1090.

Freeman, J. 2006. Non-Newtonian stagnant lid convection and the thermal evolution of Ganymede and Callisto. Planetary and Space Science. 54(1):2-14.

Grundy, W. et al. 2007. New Horizons Mapping of Europa and Ganymede. Science. 318(5848):234-237.

Hall, D. et al. 1998. The Far-Ultraviolet Oxygen Airglow of Europa and Ganymede. The Astrophysical Journal. 489(1):475-481.

        Harland, D. M. 2000. Jupiter Odyssey, Springer Praxis.

Hauck, S. et al. 2002. Internal structure and mechanism of core convection on Ganymede. 33rd Lunar and Planetary Science.  Abstract No. 1380.

Ip, W-H. et al. 1997. Energetic ion sputtering effects of Ganymede. Geophysical Research Letters. 24(21):2631-2634.

Johnson, R. 1997. “Polar Caps” on Ganymede and Io Revisited. Icarus. 128(2):469-471.

Kivelson, M. et al. 1996. Discovery of Ganymede’s magnetic field by the Galileo spacecraft. Nature. 384:537-541.

Kivelson, M. et al. 1997. The magnetic field and magnetosphere of Ganymede. Geophysical Research Letters. 24(17):2155-2158.

Kivelson, M. et al. 1998. Ganymede’s magnetosphere:  Magnetometer overview. Journal of Geophysical Research: Planets. 103(E9):19963-19972.

Kivelson, M. et al. 2002. The Permanent and Inductive Magnetic Moments of Ganymede. Icarus. 157(2):507-522.

Khurana, K. et al. The origin of Ganymede’s polar caps. Icarus. 191(1):193-202.

Leutwyler, Kristin and Casani, John. 2003. The Moons of Jupiter. W. W. Norton.

Malhotra, R. 1991. Tidal origin of the Laplace resonances and resurfacing of Ganymede. Icarus. 34(2):399-412./

McCord, T. et al. 1998. Non-water ice constituents in the surface material of the icy Galilean satellites from Galileo near-infrared mapping spectrometer investigation. Journal of Geophysical Research. 103(E4):8603-8626.

McCord, T. et al. 2001. Hydrated Salt Minerals on Ganymede’s Surface: Evidence of an Ocean Below. Science. 292(5521):1523-1525.

Musotto, S. et al. 2002. Numerical Simulations of the Orbits of the Galilean Satellites. Icarus. 159(2):500-504.

Noll, K. et a. 1996. Detection of Ozone on Ganymede. Science. 273(5273):341-343.  

Orton, G. et al. 1996. Galileo Photopolarimeter-radiometer observations of Jupiter and the Galilean Satellites. Science. 274(5286):389-391.

Pappalaardo, R. 2001. The Grandeur of Ganymede: Suggested Goals for an Orbiter Mission. Forum on Innovative Approaches to Outer Planetary Exploration, 2001-2020. p. 62.

Paranicas, C. et al. 1999. Energetic particle observations near Ganymede. Journal of Geophysical Research: Space Physics. 104(A8):17459-17469.  

Patterson, W. et al. 2009. A Global Geologic Map of Ganymede. American Geophysical Union, Fall Meeting. Abstract No. P15E-1169.

Peale, S. and Lee, M. 2002. A Primordial Origin of the Laplace Relation Among the Galilean Satellites. Science. 298(5593):593-597.

Schenk, Paul. 2010. Atlas of the Galilean Satellites. Cambridge University Press.

Showman, Al and Molhotra, R. 1997. Tidal Evolution into the Laplace Resonance and the Resurfacing of Ganymede. Icarus. 127(1):93-111.

Showman, A. and Molhotra, R. 1999. The Galilean Satellites. Science. 286(5437):77-84.

Showman, A. et al. 1997. Coupled Orbital and Thermal Evolution of Ganymede. Icarus. 129(2):367-383.

Sohl, F. et al. 2002. Implications from Galileo Observations on the Interior Structure and Chemistry of the Galilean Satellites. Icarus. 157(1):104-119.

Spohn, T. and Schubert, G. 2003. Oceans in the icy Galilean satellites of Jupiter? Icarus. 161(2):456-467.

Vidal, R. et al. 1997. Oxygen on Ganymede: Laboratory Studies. Science. 276(5320):1839-1842.

Volverk, M. et al. 1999. Probing Ganymede’s magnetosphere with field line resonances. Journal of Geophysical Research: Space Physics. 104(A7):14729-14738. 

Zahnle, K. et al. 1998. Cratering Rates on the Galilean Satellites. Icarus. 136(2):202-222.


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