Sen—Over the last eleven years, NASA’s Cassini spacecraft in orbit around Saturn has taken some truly eye-catching, jaw-dropping images: waves in the delicate rings thrown up by tiny Daphnis; a storm larger than Earth shattering the planet’s tranquil atmosphere; the rays of the Sun filtering through the enormous rings. But perhaps nothing so took my breath away as the image, seen above, of jets of water shooting out into space above the icy surface of Enceladus.
That something strange would be found on the surface of Enceladus was not altogether surprising. After all, the small moon’s location within Saturn’s tenuous E ring prompted speculation since the days of Voyager that perhaps the moon was actually the source of the ring. Seeing streams of nearly-pure water pouring forth into space, though, that was something truly amazing.
It has now been more than a decade since the plumes were first sighted and still many fundamental aspects of the process remain a mystery. But what we have learned has revealed Enceladus as one of the most intriguing objects in the Solar System. Unfortunately, as Cassini begins to make its final observations in preparation for the mission’s end in 2017, it’s likely we won’t learn too much more about this remarkable phenomenon.
That fact masks what could be the real legacy of Cassini at Enceladus, though. Over the last ten years, we’ve begun to develop a blueprint of where and how to look for similar features across the Solar System and we’re about to have some opportunities to apply it. Let’s take a look at how that might happen.
Follow the heat
The Solar System is a breathtakingly cold place; temperatures at Enceladus can dip more than 240 °C below freezing—that’s cold enough to freeze all but three of the elements! So how could the water in the moon’s ocean ever be liquid? Observations by Cassini’s Composite Infrared Spectrometer, basically a fancy thermal camera, have revealed that small patches of the surface can be more than a hundred degrees warmer; since the heat must be coming from within the moon, the interior is likely to be far hotter still. And the hottest locations correspond directly to the vents through which Enceladus’ plumes emanate. The source of all that heat is one of the most intriguing mysteries in planetary science but the connection is clear: hot spots are the oases of the frigid cosmic desert.
The Dawn spacecraft, now in orbit about the dwarf planet Ceres, has already begun to put this principle into action. As its visible-light camera began to detect a series of strange white spots on the surface of this giant asteroid, mission controllers directed the thermal imager to observe the same regions. They noticed something remarkable: while one of the spots disappeared (suggesting it matched the background surface temperature), another became a dark splotch, indicating that it was cooler than the surrounding terrain. What could cause some spots to be warm and others cool? That's just one of the mysteries Dawn will have to sort out.
In this series of images captured by the Dawn spacecraft, unusual spots on Ceres are seen in three different kinds of light, from visible on the left to thermal infrared on the right. Spots that appear quite similar in the visible can be remarkably different in the infrared. Image credit:NASA/JPL-Caltech/UCLA/ASI/INAF
Bigger today, smaller tomorrow
Although scientists can’t fully account for all the heat within Enceladus, the lion’s share likely comes from what’s known as tidal heating. Just like the gravitational effect of our Moon can literally lift the ocean (causing high tide), the immense gravity of the outer planets can cause their moons to substantially stretch and compress. This back-and-forth action causes the ice and rock within moons like Enceladus to rub and scrape, generating large amounts of heat through friction. It’s the same basic effect that warms your hands as you rub them together on a cold winter morning.
Tidal heating relies on the fact that the orbits of virtually all the moons within the Solar System aren’t perfect circles. Enceladus, for example, deviates from its average distance by up to about half a percent. While this motion drives the tides which create the heat, it also means that heat production isn’t constant. And, if the plumes are driven by the heat, then they, too, will vary. This exact effect has been observed by Cassini’s Visual and Infrared Mapping Spectrometer (VIMS); over the course of one orbit, the brightness (and thus likely the volume) of the plumes could be three times higher than the minimum at certain times. That’s quite the difference!
At Jupiter’s moon Europa—where an ocean of water larger than all the oceans on Earth is believed to lurk beneath the surface—any plumes are likely to be even more variable. Europa’s eccentricity, or deviation from a circular orbit, is nearly twice as high as that of Enceladus. In order to have a better chance of finding a plume, NASA’s forthcoming Europa Clipper mission will need to ensure that it observes the same area of Europa at a variety of points within its orbit. This will no doubt complicate mission planning, but Cassini at Enceladus has shown that it can make a major difference in how easy these features are to detect.
They might not be jets after all
Our study of the plume at Enceladus is so recent that one would be hard-pressed to identify any sort of accepted explanation. If any idea has received more support, though, it’s that a number (perhaps a hundred or so) of individual vents near the south pole emitted isolated jets that merged above the surface to form the plume as observed. How many vents were active and the activity of each accounted for the variability observed by VIMS. The latest research, however, suggests that such a view might be overly simplistic. Instead of just a few individual sources, particles might be emanating from within all parts of the four giant cracks, known as tiger stripes, on the surface of Enceladus.
In this false-color mosaic of Cassini images, the four large cracks in the sourth polar region of Enceladus can be seen. Known as the "tiger stripes," they are the source of Cassini's plume. Image credit: NASA/JPL/Space Science Institute
Europa, too, is home to enormous surface fissures. So, even if the plumes initially appear to be localized to certain areas, researchers shouldn’t discount the notion that they exist across long stretches. This could be especially important in the future as we begin to evaluate possible plans for landing on the surface of this icy world. It would be pretty bad news to find out halfway through a descent that our landing site was actually a cryovolcano!
Just the start
It seems increasingly likely that our Solar System is actually chock full of plumes. In addition to evidence at Ceres and possibly at Europa, New Horizons will be keeping a sharp eye out for jets of material on the dwarf planets Pluto and Charon as it flies by later this summer. It might seem like something so volatile could never exist so far from the heat of the Sun, but Voyager 2 actually discovered clouds of material drifting across the atmosphere of Neptune’s largest moon Triton, one of the coldest places in the Solar System. Some evidence suggests that Pluto may well be the same.
Whatever New Horizons finds and whatever may be discovered after, it will for now always be in the context of Enceladus that these incredible phenomena will be considered.