Beneath the web of cracks in its bright global ice shell, Jupiter's moon Europa is thought to have a liquid water ocean heated by internal geological activity - an ocean potentially capable of supporting microbial life, shielded from the lethal radiation of Jupiter's magnetic field by many kilometers of ice. Scientists as yet have few answers about this ocean beyond a general consensus of its existence - conclusive results about its depth, thickness, heating, and movements remain elusive - but the questions continue to drive research into the moon, and dominate interest in the Jovian system overall.
The progress of our adventure so far (current in bold):
1. The Sun
4. Earth (Vol. 1)
5. Earth (Vol. 2)
6. Earth (Vol. 3)
7. Earth (Vol. 4)
8. Earth (Vol. 5)
9. Earth (Vol. 6)
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)
19. Jupiter (Vol. 1)
20. Jupiter (Vol. 2)
22. Europa (Vol. 1)
23. Europa (Vol. 2)
27. Rings of Saturn
30. Tethys, Dione, and Rhea
33. Minor Moons of Saturn
35. Moons of Uranus
38. The Kuiper Belt & Scattered Disk
40. The Interstellar Neighborhood
Due to the radically low temperatures of the outer solar system, water ice is not the soggy or brittle material we experience in terrestrial climates, but is more a form of rock. On Europa in particular, temperatures vary from about 110 K (−160 °C / −260 °F) at the equator to 50 K (−220 °C / −370 °F) at the poles, so there is no analogous Earth-bound environment - the glaciers we can visit on this planet have very little in common with the Europan ice. As with Io, this range of temperatures represents a huge thermal spread across latitudes, which is particularly significant given the great difference in radiation exposure across latitudes that we discuss later, so not all regions of Europa would be as accessible to human exploration as others.
The great range of thermal environments is largely due to the difference in the angle of sunlight impacting the surface, since the surface itself is highly uniform in composition and experiences the same tidal heating from beneath. As such, the ideal landing environments on Europa would be in the equatorial and mid-latitudes where the temperature profile would be relatively manageable. Furthermore, a 3.5-day rotational period corresponding to its month around Jupiter would also be workable for the sake of ground-based solar power sources, at least to the diminished extent that solar is practical at all in the Jovian system.
4. Internal Features
The most profound unsolved questions about Europa are the size, layering, and viscosity (i.e., resistance of fluid motion) of subsurface water regions - in other words, whether and to what extent these regions are heated from tidal friction with Jupiter and the other Galilean moons to the point of becoming liquid, how thick and persistent such layers are, how fluidly they move, and most importantly, how accessible they are from the surface. As such, Europa is one of the most promising known candidates for extraterrestrial life because of the potential presence of liquid water oceans in a radiation-protected environment with a sufficient heat source to power a biosphere. But even if there such a layer, the potential is moot if it exists too far beneath a solid ice shell, and would thus be impervious to practical means of investigation.
Two competing theories of Europan internal structure have developed to explain the results gathered by various probe missions. One, which is less encouraging, holds that the solid ice layer gives way to a sticky, "gooey" region of ice that flows slowly but is not in fact liquid, and that this region absorbs the internal heat transferred directly from the rocky mantle. This scenario would not be conducive to life as we know it, except in terms of very marginal chemistry. The transfer of material, heat, and gases throughout such a region would likely be too slow and unreliable to sustain organic processes on any considerable level.
The second, and more promising scenario, is that the hard ice gives way to a thick liquid layer where convecting currents carry heat and nutrients from the higher-temperature regions below toward the ice ceiling, where they then flow and interact in ways that may promote biological processes. Of course, there are also intermediate possibilities where fluid flows occur in limited places that interrupt solid or sticky ice, but at this point there is simply not enough information to delve into that level of complexity. However, there is substantial magnetic evidence of an electrically conducting layer beneath the surface, and a global salt-water ocean beneath the ice would be consistent with that. We will not know until we get there and begin exploring in depth. An illustration of the two interior models:
In the event that a global ocean exists, it may be oxygenated by the subduction of O2 bubbles freed from water ice by radiation impacting the surface, but this is largely speculative since (a) the existence of the ocean is unproven, and (b) it is unknown to what extent surface chemistry influences layers deep beneath the ice. An oxygenated ocean would be more promising for biology, but it is not necessary - anaerobes are more basic, and in our own planet's history evolved earlier than oxygen-breathing organisms.
If indeed Europan life does exist, it would very likely be simple, microbial, and not have evolved any appreciable level of complexity since it would have depended on a limited, unchanging, and isolated source of energy for the entire history of its existence. Such life would be equivalent to the very basic microbes that first evolved in seafloor vents on the primordial Earth, and are unlikely to have experienced either the pressures that drove terrestrial evolution or to have ever had access to the environmental "rewards" that would have made such evolution adaptively advantageous. As such, Europa is less interesting as a potential abode for life than Mars - which is so similar to Earth compared to the rest of the solar system - or Saturn's moon Titan, which is so utterly bizarre and dynamic.
5. Surface features
By far the most prominent features of Europa are the lineae, or surface cracks and folds in the global ice shell. Apart from these features, the face of this world is remarkably smooth and untarnished by impact craters or other geological features, which indicates a continuously replenished surface via upwelling from water beneath. This does not necessarily mean that there is a liquid ocean - ice may be partially melted at certain points due to tensions in the shell and move fluidly enough to smooth over most features without there being a contiguous body of liquid below, so the shape of the surface is not proof of anything about the internal environment. Examples of lineae can be seen from a high-altitude context in Vol. 1 on Europa, but here are some more detailed shots - Agenor Linea:
Astypalaea Linea - a strike-slip fault in the ice, in the same way that California's San Andreas fault occurs in terrestrial rock (i.e., the two sides of the crack move in opposite directions along the fault rather than outward or against each other).
A 3D topographic zoom of a double-ridge:
There are also a few rare impact craters, which means they are relatively recent - although how recent would depend on unresolved questions about Europa's interior and the rate of fluid resurfacing. Shown clockwise from top left are Pwyll, Cilix, Tyre, and Mannann'an:
As you can see, these particular craters don't look much like those on more rigid surfaces like Luna or Mars - the impacts melt the ice, and most of it flows back to equilibrium before freezing again, leaving behind only relatively vague impressions of outward flows. And, of course, since ejecta would just be vaporized ice, there are no rays of material as in an impact on a rocky body in most Europan craters (although there are some) - it would just refreeze and fall back to the surface as frost, looking little different from the surroundings except for a light sprinkling of whatever the impactor was made of. A wider shot of Tyre, showing how some linea interrupt the crater features, indicating the ice shell in the region has undergone changes since the impact:
So-called "rugged terrain" southeast of Tyre:
Conamara Chaos - terrain so thoroughly criss-crossed by fractures that "islands" of flat land are isolated from each other:
An enhanced-color view of some lineae as well as spotty features called "lenticulae" that may be caused by bubbles of liquid water penetrating through to the surface:
The reddish coloration seen in many features in true-color images could be material from the rocky mantle beneath the ice carried to the surface via liquid upwelling - possibly sulfurous compounds. These colors are exaggerated in the false color images, but they are still present in reality, as the image at the very top and those from the previous volume show. Some relatively young terrain, indicated by its relative smoothness and darkness (ice pulverized by tension over time tends to be brighter than upwelled fluid that subsequently froze):
Some ridges seen in relief perspective with a resolution of a few kilometers:
Extreme close-up of the surface, with a resolution of only a few meters - you can really get a sense of what being there might look like, especially with the ridgeline seen from an angled perspective:
An ultra-high-resolution view of a part of Conamara Chaos, with a resolution of only 9 meters:
A so-called "tripleband" - a linear feature comprised of three parallel ridges:
The highest-resolution view of the Europan surface ever taken, seen from an angled perspective showing rolling white hills and ridges of ice with darker material in the ditches between - too bad it's not in color:
Various additional examples of features identified above:
The International Astronomical Union's full list of named features on Europa can be found here. Features are generally named after figures in Celtic mythology, or else related to the mythic figure of Europa (Ευρώπη) - a legendary Phoenician woman associated with a number of Greek myths, and after whom the continent of Europe was named.
Also, the official map of Europa can be found here - there are not a lot of named features relative to the resolution of the map, and it's not visually compelling compared to the images above, so I will save myself the drudgery of converting the pdf file into jpeg and then sectioning it for presentation as I've done in other entries in this series. The map is in black and white, and the contrast isn't as clear as it should be, but I suppose the USGS deserves credit for making and keeping such maps at all given how relatively few people would access such material at any given time.
Like Io, Europa has a plasma torus - a cloud of ionized particles blasted off its surface by radiation that occupies its orbit around Jupiter - and a flux tube: An electrically active ring-shaped region that carries charged particles from its poles to the corresponding poles of Jupiter, where they impact the Jovian atmosphere and create aurorae. However, the Europan versions of these features are far less powerful than Io's, although still significant in terms of creating hazards for spacecraft and future human exploration.
In general, the environs of Europa delivers an unshielded spacecraft or human with a radiation dosage of 5.4 sieverts (Sv) per day - less than a sixth of the dosage at Io, but still lethal. This is about 108,000 dental X-rays per day, or about 180 full-body CT scans per day. Radiation dosage chart for the Galilean moons, which I will add to with each new entry in the Jovian system:
V. Modern Relevance to Humanity
Europa had been little more than a point of light in telescopes until the 20th century, when interplanetary space probes showed a world of cracked ice potentially covering a liquid water ocean. It was at this point that it became a major subject of interest to planetary scientists and science fiction authors, which had ironically been foreshadowed by a fortuitous coincidence of filmmaking: Arthur C. Clarke's classic 1968 novel 2001: A Space Odyssey had established Saturn's moon Iapetus as the target of an alien monolith's radio transmissions, but in adapting the book for the big screen, Stanley Kubrick wasn't satisfied with the ability of contemporary visual effects to convincingly render Saturn. So, instead, the film version had the monolith targeting Jupiter and the "stargate" into which Dave Bowman enters is placed in an unstated Jovian orbit rather than being on the surface of Iapetus.
However, once the Pioneer and Voyager probes returned information about Europa and began generating a buzz about potential life, the next novel in the series - 2010: Odyssey Two - as well as its film adaptation retconned Europa as the focus of the monolith and the location of a nascent alien intelligence being shepherded by the godlike beings who had created the monolith. In the novel, these creatures are slow-moving, seaweed-like blobs existing beneath the ice, but close enough to the surface to emerge and swallow a spacecraft - preposterous, of course, but dramatically effective and portrayed in a narratively convincing way. The film version of the sequel (subtitled "The Year We Make Contact") starring Roy Scheider, Helen Mirren, and John Lithgow, doesn't get that specific, but does show Europa pretty much as it appears in some of the close-up images above - as an icy, chaotic place - and has the monolith reacting violently to human attempts to investigate the moon.
Clarke continued the series for two more books - 2061: Odyssey Three, and 3001: The Final Odyssey - although the former was more focused on Europa than the latter. In Odyssey Two, the monolith had chemically altered Jupiter until it underwent nuclear fusion and became its own star, melting the Europan ice so that the life beneath it could flourish and evolve, and Odyssey Three follows the accelerated program of evolution of this life as directed by the monolith-builders. By this point, of course, the series has nothing whatsoever to do with the real Europa, but it's very inspiring nonetheless.
In more recent times, Europa has been emblematic of the collapse in financial support for even robotic exploration of space, when the planned Jupiter Icy Moons Orbiter (JIMO) that would have conducted detailed orbital exploration of Europa was canceled in 2005 in favor of the Constellation manned space program that has itself been subsequently canceled without the original funding priorities being restored.
VI. Future Relevance to Humanity
It is unclear exactly when dedicated robotic exploration of Europa will resume, although the Juno mission primarily targeted at investigating Jupiter in 2016 may conduct some distant, tangential observations of the Galilean moons. However, the ramp-up in general space activity that will hopefully be made possible by the advance of SpaceX launch vehicles and associated ventures (particularly Planetary Resources) could bring about a Renaissance in robotic exploration of the solar system over the next 10-15 years, in addition to whatever gains are made in human spaceflight nearer to Earth.
Due to the growing profusion of satellites in Earth orbit, radiation-hardened electronics are already becoming commoditized and highly efficient, which will be very beneficial for unmanned exploration of the Jovian system once long-distance propulsion systems become similarly economical. I would not, however, bet on human exploration of Europa or any other Galilean moon in this century - precisely because I would bet on a huge amount of human exploration of the inner solar system in this time frame. Ironically, I think millions of people flitting around between Earth orbit, the Moon, Mars, and various asteroids would make it less likely anyone would devote the time and resources to reaching Jupiter until things had become relatively well-established in this region of space - or if they did, it would be some really impractical stunt, like a 21st century Apollo program, with similarly limited applicability.
Still, human exploration of Europa would be far more practical than Io, since shielding requirements - and thus the amount of mass that has to be lugged around or harvested from somewhere to make the radiation environment survivable would be far less. In fact, since water ice is one of the ideal shielding materials - being useful for multiple purposes including fuel when it's not absorbing radiation - Europa could be a very hospitable place once people had settled in and bored some tunnels into the ice. But, of course, you first have to get there, and you have to keep going back and forth until you build something self-sustaining, so the radiation would remain a huge barrier. And since solar power is so marginal at Jupiter, fusion technology would be enormously helpful - but I'm stating the obvious.
However, aside from the abstract theoretical possibility of microbial life beneath the ice - and the ice itself - Europa doesn't have a lot to offer in terms of prima facie attractions. Callisto and Ganymede have as much ice if not more to meet the needs of fuel or shielding, far less radiation, and at a much shallower location in Jupiter's gravity well, so it's a lot easier to both reach and leave them than Europa. It's not clear what economic basis would ever make this moon attractive, but I'm sure it will happen eventually just by sheer osmosis - after all, humans occupy even the most ridiculous niches of planet Earth, so it would make sense if we ever find ourselves in the Jovian system at all that we would eventually make our way to Europa, and someone would end up living there permanently. After all, it does seem like the view from the surface - particularly with Jupiter in the sky - could be enormously beautiful, although one's views of it would have to be brief to keep down the radiation exposure, and would have to be through thick, leaded glass rather than in a spacesuit.
What I don't buy are the notions, popular in science fiction, of undersea cities or vast biological ecosystems beneath the ice - that's just not likely, and not ever going to be practical for Europa. I would not bet on the presence of macrobiota of any kind, at any depth, and it will probably be a lot easier - and happen a lot sooner - that we develop fusion power technology than it would ever become advantageous to seek geothermal power from dozens to hundreds of kilometers beneath the ice. So I don't see any clear developmental path for humanity with respect to Europa - I think for the foreseeable future (and that means centuries in the case of this blog series), it will just be a weird place that scientific expeditions periodically visit, but that has no compelling hook for human civilization to take root. But it will eventually, because that's just how life works.
VII. Future of Europa
Barring "post-human" technological scenarios where its mass is consumed in unimaginable, solar-system-scale engineering projects, Europa left to its own devices will probably share the fate of Io, whatever it is. One possibility is that as the Sun expands and heats Jupiter's atmosphere, the planet itself will expand and its closer moons will experience increased orbital drag from the tenuous gases blowing past them until they spiral inward while the farther moons spiral outward because of the lost mass and thus weaker gravity. I would of course defer to any actual scientist who knows the answer from solar system modeling.
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 / flybys of Io)
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)