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 SunThe best existing image we have of Europa in true-color, as our eyes would see it if we were there (except for minor discoloration at the Northern polar region), thanks to processing work done by Ted Stryk:
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
Europa is the sixth moon of Jupiter, and the second innermost of the four Galilean moons that are the planet's largest and most interesting satellites. The size of its orbit varies between 67% and 83% larger than the Earth-Moon system, although it is still a lot deeper in the Jovian gravity well than Luna is in Earth's - though not as deep as Io. Diagrams of these relationships:
The second illustration above shows that Europa is about 14 times deeper in Jupiter's gravity well than the Moon is in Earth's well, which means that you would have to add roughly 14 times more change in velocity (Δv) to escape the Jovian system from Europa's orbit than to escape Earth's gravity well from the Moon's orbit. This is about 26% lower than the energy requirements of leaving the system from the location of Io, but still an enormous force of gravity to overcome relative to the Earth-Moon system.
Just in case you're curious, you can roughly compute the depth of a gravity well by first using this web-based tool to calculate escape velocity, inputting a planet's mass in the "mass of body" field (in this case, Jupiter being 317.8 Earth masses), and the average radius of a moon's orbit in the "distance from center" field. When you click "Calculate!" the tool will give you a speed in km/s that would represent escape velocity from the system at that distance. However, while this is good enough to give you a sense of the proportional scale of a gravity well, you have to subtract the orbital speed of the moon from the calculated escape velocity to get a meaningful numerical figure, because the moon itself already contributes some amount of velocity toward this figure.
So, for instance, if you were stationary relative to Jupiter at the location of Europa's orbit, you would have to add 19.4 km/s of Δv along a tangent to the orbit to escape from the Jovian system - the figure that the tool gives as the escape velocity. But if you had just escaped Europa, and were thus traveling around Jupiter at the same speed as the moon (13.7 km/s), you would only need to add 5.7 km/s. This does not include the Δv needed to escape the gravity of a moon itself (which, like on Earth, would be a different figure depending on the latitude of your launch point), and real spacecraft can take advantage of complicated gravitational alignments to reduce fuel expenditures, but it does show the relative difficulties of moving to and from various locations in a gravity well on a rough conceptual level. Europa in relation to Jupiter:
Using NASA's solar system simulator - which can show you both the angular size and lighting phase of any major body in the solar system from any other on any date and time - Jupiter seen from the surface of Europa covers 12.3° of sky: More than six times the size of Earth from the Moon, and almost 23 times the size of the full Moon from Earth. As we did in the Io diary, let's imagine such a view by superimposing Jupiter as it would be seen from Europa into the sky of Earth's Moon:
The surface of Europa will of course look very different from the Moon - the Europan surface is overwhelmingly dominated by water ice with some reddish-brown sulfuric dust here and there, while our Moon is gray-to-black silicate rock and similar regolith. But the above perspective of Jupiter is probably not too far away from being realized with future probes in coming decades, although there are no Europa landers currently planned.
As with the other Galilean moons, Europa is thought to be a product of the final generation of moon-formation in the early Jovian system: A process under which new moons were continually forming from available material, migrating inward, and being devoured by Jupiter. This migration was largely the result of the intervening gases in the region being thick enough that moving through them gradually slowed down the moons, and caused their orbits to spiral inward. Eventually the gas thinned enough for this to stop happening, and thus we have the current four Galileans. However, without the current orbital resonances of Io, Europa, and Ganymede in particular, the migration would have reversed and these moons would now be much farther out: Instead, they keep each other relatively close to Jupiter without decaying inward, and the tidal forces that act on Io keep it volcanic while those acting on Europa may continue to heat a subsurface ocean and keep it liquid. See the Io diary for a more detailed discussion of tidal heating.
1. Orbital and Rotational Features
The Europan month is about 3.5 Earth days, which it manages despite having an orbital radius nearly double that of Earth's Moon because it travels 13 times faster. However, the moon is tidally locked to Jupiter, so it always presents the same face to the planet and there can thus never be a "Jupiter-rise" from the surface of Europa. On the side facing the planet, Jupiter always hangs basically in the same place in the sky (with only slight oscillations), and changes phases as the moon proceeds around its orbit. Just as a random thought, when people are colonizing moons like this centuries from now that are tidally locked, the side facing the planet - i.e., with the most profound views - would likely be prime real estate, although I wouldn't hazard to guess how high in the sky people would prefer to have such a view: Probably most pleasant to have it somewhat above the horizon, but not so high in the sky that you have to crane your neck to see it.
Europa has a very circular orbit with little eccentricity or inclination - this is typical of moons that formed around a massive planet - but even the slight deviations that occur are enough to cause some amount of tidal flexing within Europa, and inflict even greater stresses on Io. The continuous trauma inflicted on Io, in addition to its profoundly radioactive environment due to the Jovian magnetic field, have made it so energetic and unstable that its surface is virtually devoid of water, but the other three Galilean moons are stable enough to have built up thick water-ice mantles and crusts. Europa, however, may be hot enough inside to keep a thick global ocean of liquid water at a modest depth beneath the ice, whereas the other two icy moons - Ganymede and Callisto - are thought to be frozen to great depths, with slush and liquid layers occurring far too deep to practically access.
As a tidally-locked moon, Europa's day is the same as its month, so a full day/night cycle lasts 3.5 Earth days. But if there is a liquid ocean beneath the ice it would rotate at a different rate than the outer shell and leave distinct signatures detectable by a gravity-mapping mission (like the GRAIL mission sent to the Moon) once humanity gets around to sending one.
2. Size and Mass Characteristics
Europa is slightly less than a quarter the size of Earth and has about 0.8% the mass - a bit smaller and less massive than the Moon. It is the smallest and least massive Galilean moon, only 59% the size of Ganymede, 65% the size of Callisto, and 91% the size of Io, with a similar spread of mass percentages: 32%, 44%, and 53%, respectively. Surface gravity on Europa is about 13.4% of g, about 3 percentage points lower than on the Moon - so if you weigh 150 lbs on Earth and about 25 lbs on the Moon, you weigh about 20 lbs on Europa. Rough size comparisons of comparable objects - mouse over the image if you don't recognize an object:
I've noticed that people don't necessarily appreciate having long, extensive diaries dumped in their laps, even if crammed with awesome imagery - they just bookmark it for later and then forget to come back to it, not getting the full benefit of the experience. I too would prefer to break it up into smaller pieces, so I'll be dividing the Europa diary into two parts as well, with this one focused on the "getting to know you" parts of the science without getting too deep. I'll include a poll at the bottom so people can tell me whether they appreciate having multi-part diaries for each subject, or if they would rather have each subject covered comprehensively in one huge, long diary that they may miss if it gets posted at an odd hour.
Anyway, for the remainder of Part 1, we'll just take it easy and show some close-ups from the high-definition true color image above to get a sense of the surface. In Part 2 we'll explore different models of the Europan interior, including parameters concerning the size and habitable potential of subsurface oceans, describe surface features, shows maps illustrating the given names of regions and features, discuss possible mechanisms of forming these features, the radiation environment on the surface, and of course the future possibilities with respect to searching for life and/or human colonization. For now, enjoy these images of an icy, cracked other world that may have life beneath its surface - they are true color, so it is like you're seeing them with your own eyes: