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Callisto is a battered, ancient, and unchanging world - a world of pulverized ice ground into chaotic coffee-cake-like terrain, with little in the way of distinguishing geography beyond a huge number of overlapping craters.  It is a world without the Wagnerian drama of its sibling Io, the stately dynamism and exotic potential of Europa, or the stark contrasts of Ganymede.  In other words, to a geologist, a biologist, or a cartographer, Callisto would be the most boring place in a million miles.  But for those with an eye to humanity's spacefaring future, this melancholy museum piece of a moon may turn out to be the most important place in the solar system.

The progress of our adventure so far (current in bold):

1.  The Sun
2.  Mercury
3.  Venus
4.  Earth (Vol. 1)
5.  Earth (Vol. 2)
6.  Earth (Vol. 3)
7.  Earth (Vol. 4)
8.  Earth (Vol. 5)
9.  Earth (Vol. 6)
10.  Luna
11.  Mars (Vol. 1)
12.  Mars (Vol. 2)
13.  Mars (Vol. 3)
14.  Phobos & Deimos
15.  Asteroids (Vol. 1)
16.  Asteroids (Vol. 2)
17.  Asteroids (Vol. 3)
18.  Ceres
19.  Jupiter (Vol. 1)
20.  Jupiter (Vol. 2)
21.  Io
22.  Europa (Vol. 1)
23.  Europa (Vol. 2)
24.  Ganymede
25.  Callisto
26.  Saturn
27.  Rings of Saturn
28.  Mimas
29.  Enceladus
30.  Tethys
31.  Dione
32.  Rhea
33.  Titan
34.  Iapetus
35.  Minor Moons of Saturn
36.  Uranus
37.  Miranda
38.  Ariel
39.  Umbriel
40.  Titania
41.  Oberon
42.  Neptune
43.  Triton
44.  The Kuiper Belt & Scattered Disk
45.  Comets
46.  The Interstellar Neighborhood
Gloomy old Callisto in true-color, as we would see it with our own eyes:


I.  Context

Callisto is the eighth moon of Jupiter, and the fourth and final Galilean moon.  Its orbit around the planet is roughly 5 times the size of the Earth-Moon system, and occupies the shallowest portion of Jupiter's gravity well of any Galilean moon - a fact that makes it the most accessible of the four.  Diagrams of the Jovian system and gravity well:

Callisto Orbital Diagram 1

Callisto Gravity Well Comparison

As noted in the Ganymede diary, you can see a leveling-off in the steepness of Jupiter's gravity well with each successive Galilean moon, even though (as seen in the orbital diagram) the distances from the planet grow by increasing increments.  This is a normal feature of natural orbital systems, and is based on how gravitational force declines with distance.  In other words, gravity wells are not straight-sided funnels, but rather curve from a shallow rim (that extends infinitely outward) toward a steep center.  To better visualize this relationship, here is a graphic representation of a gravity well:


Because the well begins at the center of mass, which is below the surface of an object, it actually increases in steepness as you approach the surface before beginning to level off again.  As a result, the first few rungs outward from the "surface" - which we can identify as somewhere in the first wide band - show considerable change along the vertical axis, but the further you get from the center, the more modest the upward climb.  What this means in practice is that less fuel/energy is needed to move a given radial distance away from the central mass once in orbit around it, so even though (for instance) the orbits of Io and Europa are much closer to each other in terms of distance than those of Ganymede and Callisto, it would involve much less fuel to move between the latter two than between the former.  

Now for some contextual images.  Here we see a visual alignment of Callisto with Europa and Jupiter:  

Callisto, Europa, and Jupiter

Some low-resolution global views:



It should be noted that there are much fewer images of Callisto than its brethren due to the lesser degree of scientific interest in it, which is ironically due to the same stability and relative environmental benevolence that will make it attractive to future human development.  So the place around Jupiter we may come to know the best in the future is the one we have the least experience with today due to scientific boredom.  But for the sake of imagination, here is what Jupiter would look like from Callisto's orbit, as superimposed into an Apollo image of the lunar sky:

Imagining Jupiter in the Lunar Sky As Seen from Callisto

Jupiter's angular size from Callisto is 4.4°, which is close to nine times the size of the Moon from the terrestrial surface.  As with Ganymede and Europa, the main difference in reality with the conceptual image above is that the surface is an ice/rock slush rather than solid rock, and has considerably different landforms as a result - which we get into later.


II.  History

The formation of Callisto is unique among the Galilean moons in that it is thought to have accreted relatively slowly (100,000 to 10 million years after Jupiter formed), without building up much internal heat.  Leftovers from the original nebula that coalesced into Jupiter and the other moons were left to agglomerate in relatively low-energy impacts spread over a considerable period of time, and as a result there was insufficient heat to fully differentiate its interior.  So instead of having a number of distinct layers, its internal structure is thought to mostly just be the same kind of slush as the surface, but with increased proportions of heavier elements due to partial melting from long-term radioactive decay.  In other words, Callisto is a dirty snowball through and through - a real slacker of a moon that took its sweet time forming and then just couldn't get off the couch to do anything more.

However, because it formed at such a leisurely pace and has so little internal dynamism, the vast majority of the surface is covered in layer upon layer of primordial impact craters and their associated ejecta blankets - very little has occurred to erase them except more of the same, and some degree of sublimation (when surface water ice transitions directly to gas and is lost into space).  Due to this highly unusual history, Callisto would be a prime target for scientific exploration if it weren't in the same system as three vastly more exotic and intriguing worlds.  But as it stands, it has to compete with the other Galilean moons for the attention of Jupiter probes, and thus usually ends up getting short shrift.  


III.  Properties

1.  Orbital and Rotational Features

The Callistoan month is 16.7 Earth days, which is the same as its day because it's tidally locked to Jupiter - i.e., presents the same face to the planet at all times.  This means that, like the other moons, there is no "Jupiter-rise" on Callisto, and the planet is only ever visible from the Near Side.  Additionally, wherever the planet happens to be in the sky at a given location on the Near Side is more or less where it will always be, although Jupiter would go through phases.  Callisto experiences a total solar eclipse with every orbit, which is also the case with the other three Galilean moons.

2.  Size and Mass Characteristics

Callisto is the twelfth largest object in the solar system, the third largest moon (after Ganymede and Titan), and is effectively the same size as the planet Mercury.  It is about 38% the size of Earth, and 38% larger than the Moon.  However, due to its icy composition, its density is much less than that of Mercury and has only about 1/3 the mass.  This causes its surface gravity to be 0.126 g - much lower than Mercury (0.38 g), and somewhat lower than Io (0.183 g), Europa (0.134 g), Ganymede (0.146 g), and the Moon (0.165 g).  If you weigh 150 lbs on Earth, you weigh 18.9 lbs on Callisto and would fall a bit more slowly than on the Moon.  

Size comparisons to comparable solid bodies in the solar system - mouse over to see the title if you don't recognize something:



















3.  Temperatures

Temperatures on Callisto are significantly warmer across the board than on Ganymede and Europa, probably due to its darker surface absorbing more energy.  Conditions range from 80 K (-193 °C / -316 °F) to 165 K (-108 °C / -163 °F) with an average of 134 K (-139 °C / -218 °F).  This is by far the highest mean temperature of any Galilean moon (Ganymede and Io share second place at 110 K), and also the highest high temperature (excluding local volcanic activity, in the case of Io).  Meanwhile, the low is 30 K higher than that of Europa and 10 K higher than Ganymede (though 10 K lower than Io).  These double-digit Kelvin/Celsius temperature advantages over the other two icy moons will be important to future activities on Callisto, as energy requirements may be a bit lower.  A proportional comparison of low, average, and high Kelvin temperatures:


4.  Internal Structure

Due to its formation, Callisto does not have much in the way of distinct internal layers: Rather, most of its bulk is thought to be a rock and ice mixture that does not greatly change with depth, or if it does, does so gradually with no hard and fast boundaries.  However, magnetic readings have indicated there may be a liquid water layer at some depth, although what kind of heating process could be responsible for it is not clear: It never trapped very much heat during formation, nor is it undergoing tidal heating to any major degree by Jupiter or its fellow Galilean moons.  So if there is indeed a liquid layer, it would probably have to result from radioactive decay of heavy metal compounds distributed throughout the body (since they had never differentiated into a core).  A theoretical internal structure for Callisto put out by NASA:


What this shows is a crust of asteroid rubble and comet ice built up over billions of years above a primordial interior composed of largely undifferentiated rock/ice slush.  If there is an ocean layer, it would occupy a thin shell between these two regions and perhaps stay liquid due to the presence of compounds like ammonia (NH3) that reduce the freezing point of water.  Life as we know it, however, seems highly unlikely in such a low-energy domain.  To the best of current knowledge, Callisto is geologically dead, and the only changes that occur to it result from impacts or very slow processes like sublimation or radioactive decay.

5.  Surface Features

Although Callisto looks dark, and is considerably darker than its fellow Galileans, it's actually 62% more reflective than the Moon due to all the ice.  Below is a simulated rotational view of the surface created from actual images, though the stars and Jupiter background are entirely simulated to provide an accurately moving perspective:  

By far the most prominent Callistoan feature - which is also obvious in the signature image at the top and in the size comparisons - is a giant impact region called Valhalla, which is surrounded by multiple concentric rings:



Multi-ringed impact structures like this are created because a surface briefly turned fluid sloshes inward after the initial shockwave, causing the surrounding regions to slump and collapse toward the central basin.  The central region of Valhalla is upwards of 360 km wide, and the ring halo is 1,500-1,900 km in radius.  A closer image gives a sense of surface details within Valhalla and its ring halo:


The only other feature on Callisto that can compete with Valhalla is another large, multi-ringed impact structure - Asgard (noticing a theme with these names?):


Asgard is also very prominent by association with two large, bright-white impact craters that occurred later, Burr (top) and Tornarsuk (right).  A smaller multi-ring structure, Utgard, is associated with Burr, but it may be coincidental.  A couple of Asgard close-ups - the second is largely manipulated to highlight features, but is normal black and white along an obvious vertical strip:



Below a certain threshold of resolution, it stops mattering where you are on Callisto - with few exceptions, the terrain increasingly resembles churned-up coffee cake (lower right) before resolving into hills and ridges (lower left).  Here is a progressive zoom-in of Valhalla (the top left is enhanced color):


More examples of this coffee-cake terrain (which is just my term for it - nothing official):





Higher-resolution coffee-cake terrain:



Interesting fact about the image directly above: You can see landslides that have occurred down the slopes of the two biggest craters.  Around this scale, we discover a very exotic feature: Slender, pointy ice peaks protruding from the ground in large fields:


These ice pillars, which are hundreds of meters tall, are thought to be the result of sublimation - i.e., the water ice surrounding them slowly blew away into space for whatever reason over a long period of time, so they were basically carved "by default" by the loss of the ice that used to be around them.  I think those features would by far be the most fascinating on Callisto to personally experience and explore.  As we zoom in further, the landscape resolves into dirty valleys between jagged ice ridges and hills:




And now for the highest-resolution image ever taken of Callisto, with the smallest details being just 46 meters (150 feet) across - a cliff-side scene we can relate to intuitively due to the clear dimensionality:

Resolution 46m

There are currently 153 officially named features on Callisto, with the nomenclature going according to the following themes:

Multi-ringed impact structures: Homes of the Norse gods.

Craters: Heroes and heroines from Nordic myths.

Catenae (crater chains): Mythological Northern places.

The official map of Callisto, as maintained by the US Geological Survey, can be accessed here in PDF form:


IV.  Radiation

Daily unshielded radiation dosage on Callisto is about 0.1 mSv or (0.0001 Sv).  This is 800 times lower than the dosage on Ganymede, 54,000 times lower than the dosage on Europa, and a staggering 360,000 times lower than what you would get every single day on Io.  The reason is that Callisto is outside the most active parts of Jupiter's magnetic field while still being protected by it from solar and cosmic particles.  To include Callisto in our running chart comparing radiation levels among the Galilean moons, we now have to use a logarithmic (i.e., factor of 10) vertical axis:


Though 0.1 mSv per day is not a trivial amount of radiation, it's actually better than what astronauts orbiting Earth experience - eight times better - and only about 13 times the average daily background radiation dosage on Earth.  NASA has standard career radiation exposure limits for its astronauts, limiting how often they can fly: A 55-year-old male astronaut can accumulate a maximum of 4 Sv (4000 mSv) over the course of his career, and a female astronaut of the same age can safely accumulate 3 Sv (3000 mSv).  To receive the lower amount (3 Sv) on Callisto, you would have to remain unshielded all day, every day for 82 years straight.  In other words, in terms of radiation risk, colonizing Callisto would be safer than what NASA astronauts are doing right now.

However, thirteen times Earth background is nothing to sneeze at either - human colonists would want their habitats at least modestly shielded, and would have somewhat more stringent safety measures for growing children and pregnant women.  But this would be the case pretty much anywhere in the solar system humans would colonize, so it's just a standard environmental factor rather than an outright hazard that would prevent settlement.  Actually, it's a lot more benign than Earth's Moon would be, since the Moon is so much closer to the Sun and is completely outside Earth's protective magnetic field, and the same could be said about Mars since it has no magnetosphere of its own.  As far as radiation is concerned, Callisto is one of the most attractive environments in the solar system.


V.  Modern Relevance to Humanity

There has been some recognition of Callisto's potential in science fiction literature, although it is not a very frequent setting.  The most prominent occurrences are in Kim Stanley Robinson's Blue Mars - the third novel in his epic Mars trilogy.  As you might guess from the title, Callisto is not the main setting of action: It's just shown as a flourishing colony that one of the protagonists who had been part of colonizing Mars ends up visiting in a relatively brief part of the novel.  Callisto is somewhat overlooked in fiction as well as actual science, and for the same reasons already mentioned: It lacks the flare of the exotic that Io and Europa have, the immediacy of Mars, or the shocking alienness of Titan - it's just a dead, dirty rock-and-snowball.  But ironically, that's precisely why it holds such promise.

NASA did a study in 2003 on the feasibility of a manned mission to Callisto in the 2040s using nuclear-electric propulsion - a theoretical rocket system not yet developed, but at least reasonably understood in principle - and decided that it was at least physically achievable.  That isn't saying much, especially when it comes to actual federal budgets, but it showed that NASA is aware of the potential, and willing to at least put pen to paper in order to think about how it could be done.  

For the time being, however, exploration of Callisto will continue to be the exclusive domain of unmanned probes that have to divide their time between the four Galilean moons and Jupiter itself - a fact that will likely continue to mean the most promising and portentous body in the Jovian system will continue to slip through the cracks of both public and scientific attention.


VI.  Future Relevance to Humanity

A laundry list of reasons why a thriving human civilization will take root on Callisto:

1.  Inexhaustible supply of water ice.

2.  The ice is randomly mixed with other useful, easily accessible raw materials in large proportions, including metals, so that's a big bonus.

3.  There's enough gravity to live and work on the surface with ease and comfort compared to being in free space.  Granted, we don't yet know the effect on human health of living in "mini-gravity" for extended times as opposed to pure weightlessness, but Callisto is attractive enough that I'm sure a whole lot of people are ultimately going to find out.

4.  Half the surface has views of Jupiter.

5.  Can serve as a safe, stable, economically plentiful staging area for scientific exploration and/or economic exploitation of the other Galilean moons and Jupiter itself.

6.  Gateway to the rest of the outer solar system.  Over the longest term, I could see it being both economically and strategically crucial to whatever political states arise among a spacefaring humanity.

7.  Relatively benign radiation environment because it's protected by Jupiter's magnetic field without being inside its radiation belts.  I.e., a lot safer than the Moon or Mars.

8.  Warmest surface temperatures of any moon in the outer solar system.

9.  Furthest place in the solar system that solar panels are not a complete waste of space, and yet environmentally suited to benefit from radical advances in energy and propulsion.  A good "middle-case" environment in the context of the entire solar system.

10.  Ready access to the Jupiter Trojan asteroids (see the Asteroids parts of this series for an in-depth discussion), as well as outer Main Belt objects that migrate near Jupiter.

11.  Because of all the rock (roughly 44% of surface material), wouldn't have to be as careful about expelling heat into space (so as not to melt the ground beneath your feet) as you would on Ganymede.

The main obstacle is energy, since solar power verges on inadequacy at Jupiter and supplying energy from Earth - such as scarce material for fission reactors and RTGs - would be expensive.  However, it may be possible to harness some fraction of the 2 terawatts of power that pass through the Io flux tube (see the Io page for details) due to Jupiter's magnetic field.  

This would still represent imported energy, since - as we saw from the orbital diagram near the top of this page - the Jovian system is gigantic; and as the gravity well diagram showed, it would take a lot of fuel to get from Io to Callisto, let alone there and back again.  But it might be enough to drive a pioneering civilization in its early years before fusion power is invented - if it hasn't already been invented by the time people are in the Jovian system.  Which leads me to reason 12 why Callisto is so important:

12.  Furthest potentially viable human outpost until fusion energy technology is achieved.

You can't go to Saturn without fusion, but Jupiter is doable: Imported fissionables and radio isotopes, perhaps some much smaller amount mined from Callisto itself, some large-scale solar arrays as auxiliaries, and as mentioned, the possibility of harnessing the Io flux tube all mean that self-sustaining settlement on Callisto is achievable without any Star Trek technology.  Just barely.  But the moment humanity does have fusion energy, Callisto goes from marginal to full-on bonanza because its one and only critical shortcoming - lack of energy - is removed.  

The math of space colonization is simple: Water + raw material + energy = fully human habitable.  If only two are abundant, then it's only habitable under hazardous frontier conditions, and would have to build up quite a lot of trade to become self-sustaining.  And if only one of those three is abundant, then it's not suitable for habitation at all, and will only ever be utilized as a source of some commodity for populations living elsewhere.  A distant rock that has only materials but poor energy and water supplies, or lots of water but little material or energy, would just be mined or ignored completely.  Bodies that have two of the three - energy and material with little water (like the Moon), or water and material with little energy (like Callisto) - can be settled with difficulty.  With fusion power, Callisto would have all three and become what Earth is and Mars probably will be - a catalyst of accelerated human expansion.

That said, the timelines involved would almost certainly play out over centuries rather than decades, so we are not discussing NASA policy when we talk about what may become of Callisto as a platform for human civilization.  As discussed at the end of the Ganymede page, I think the most likely scenario would be nth-generation Martian colonists going on to explore and settle the Jovian system over some span of time in the 2nd quarter of the current millennium.  This makes more sense both economically and socially than imagining a continual exodus from Earth, which would likely slow down considerably when the inner solar system reached a certain point of development.  Only frontiers breed more frontiers - it's not the stolid Old World that gives birth to the New.  

That's why the European civilizations that deliberately migrated to the Americas were only a thousand years old, and not the 4,000-year-old peoples of China and the Middle East despite both having strong maritime traditions.  It's also why the Midwest and West of what would become the United States were overwhelmingly colonized by people born on the East Coast of the same continent rather than half a world away (wherever it was their ancestors had come from).  So, to me this indicates that most likely the people who settle the Jovian system will be Martians and people who've made a life among asteroids - folks whose grandparents and great-grandparents left Earth.  But who knows - this could all happen a lot sooner than that.  No one ever understands the life-accelerating potency of a New World until it becomes a present fact, and the possibilities of the solar system at large are beyond the imagining of what we experience today.


VII.  Future of Callisto

Barring an extinction-level catastrophe before we have the chance to colonize it, the future of Callisto is almost certain to be written by humans one way or another: Both by what we do to it, and what we choose not to do to it.  It will not look like it does now five centuries in the future - that is extremely unlikely.  Its face will change, and perhaps not entirely in bad ways.  On longer horizons - five millennia, fifty millennia, five hundred millennia - it may not even be there: Mined to nothing to feed to unimaginable civilizations, or used as a planet-sized spaceship on some absurd voyage to who-knows-where for who-knows-why.  Or it could be returned to some facsimile of a natural state for sentimental reasons.

But if left to its own devices, Callisto would go through considerable changes as the Sun expands - losing much of its water, but also perhaps having significant atmospheres or even seas for a limited time (millions of years) before they too dissipate into space.  As Jupiter's mass is gradually blown away by the growing Sun, its gravity would weaken and a moon like Callisto would migrate outward, maybe at some point departing entirely.  If so, its "personality" as a world would be well-suited to wandering the cold dark, quiet and unassuming.


VIII.  Catalog of Exploration

1.  Past & current probes:

Pioneer 10 (USA - 1973 flyby)
Pioneer 11 (USA - 1974 flyby)
Voyager 1 (USA - 1979 flyby)
Voyager 2 (USA - 1979 flyby)
Galileo (USA - 1995 to 2003, Jupiter orbiter / Galilean moon flybys)
Cassini-Huygens (USA and Europe - 2000 flyby)
New Horizons (USA - 2007 flyby)

2.  Future probes:

Juno (USA - en route, scheduled Jupiter orbiter to reach system in 2016, will flyby Galilean moons)
Jupiter Icy Moon Explorer (JUICE) (Europe - 2022 launch, 2030 enter Jovian system, all Galilean moon flybys then Ganymede orbit)

Originally posted to Troubadour on Sat Feb 23, 2013 at 06:08 AM PST.

Also republished by SciTech and Science Matters.

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