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Module 4 - Follow the Water

Voyager mission

Artist's impression of the Voyager space probes (1977—) with the outer worlds they passed. Artist's description: "This painting was commissioned by JPL to commemorate the outer planets mission of the successful pair of Voyager spacecraft. Although done in traditional media, computer drawings were generated as an aid to creating perspective rings of the proper scale for each world, which also has one Moon each highlighted. Only distant shots of Neptune were available at the time the work was done, and the hypothetical 'ring arcs' are included as modeled from earth based star occultation data. Acrylic on board for NASA, JPL."

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The Icy satellites The icy satellites of the outer planets are a great example of exploration leading to unexpected discoveries that completely change the way we view things – in other words – Science! Most scientists’ expectations in 1977 were that the satellites of the outer planets would be dead little worlds. (Note: if you read this section about Io, you’ll see that there was already discussion about a more active surface).

That changed forever when the 2 Voyager spacecrafts were launched towards the outer portions of the solar system in the summer of 1977. From January 1977 – August 1977 Voyagers 1 and 2 took pictures of Jupiter and its larger moons. Possibly the most surprising discovery from those encounters was that we first got a glimpse of active volcanism on Jupiter's moon Io. These moons were not simply dead lumps. For the purposes of discussing water, Io is not a good candidate because it is a dense, rocky hot world. However Voyager 2 also took images of Europa, an icy world, and it was immediately clear that this was a very active world as well. The image of Europa showed a fairly smooth surface covered with large grooves and very few craters.

As we know by now, this suggests a young surface. The density of Europa is ~3g/cm3 which suggests that ices must form a large percentage of its mass. It was thought that radioactive decay of their internal components was not sufficient to keep these worlds alive, so what could account for their activity?

The answer, for Io, Europa and pretty much every other outer satellite that shows signs of activity turns out to be tidal heating. Before we examine the icy satellites in more detail, we will have a brief look at the tidal heating mechanism.

But before we move on ......
A Final Farewell to Voyager
Please note that the Voyagers also made many other exciting discoveries during their trip through the outer parts of the solar system. But the images of Jupiter’s moons taken earlier changed the way we think about these satellites and made them some of the most exciting exploration targets in the solar system.

At present both Voyagers continue along their paths. Recently NASA announced that on August 25, 2012, Voyager 1 entered interstellar space making it the first spacecraft to do so.
Voyager 1 update -"Firing Thrusters" - December 2, 2017

After 37 years, the Yoyager 1 team successfully fired up thrusters that had been dormant since 1980. It took 19 hours and 35 minutes for the confirmation signal to reach an antenna in California. As the the "why did the do it?", check here.
Finally, a Star Trek comment

If you’re wondering, it was Voyager 6, which sadly has not been designed yet, that returns to Earth in the Star Trek movie.