The more the craft Cassini flies past these two moons, the more tantalizing they become as targets for follow-up missions. "We've got to find a way to get back there," says Hunter Waite Jr., a space physicist with the Southwest Research Institute (SwRI) in San Antonio.
In Cassini's latest encounter with Enceladus, the orbiter zipped through a plume of dust, ice, and gas erupting from its south pole. Findings released Wednesday show this plume contains far larger quantities of water vapor than researchers had expected. US and European scientists also found in the plume a smorgasbord of organic chemicals, including methane, acetylene, formaldeyhde, and hydrogen cynanide.
Indeed, the plume's chemistry more closely resembles the primitive materials found in a comet than the more heavily processed chemicals one might expect to see in a mature moon, researchers say.
"To have primordial material coming out from inside a Saturn moon raises many questions on the formation of the Saturn system," says Dr. Waite, lead scientist for one of the Cassini orbiter's instruments.
From a biological standpoint, "the organics are there," he says. Whether liquid water – another key ingredient for organic life – is present beneath the surface remains an open question, he says.
Temperatures at the south pole, the plume's source region, suggest that liquid water may be present close to the moon's icy surface, adds John Spencer, another Cassini scientist with SwRI.
The geysers that are feeding the plume at supersonic speeds originate from four long fissures, dubbed the tiger stripes. These rifts are warmer than the surrounding moonscape, though the temperatures are frigid by Earth standards. The region surrounding these rifts is chilled to minus 343 degrees F., compared with temperatures of up to minus 126 degrees F. along the fissures, Dr. Spencer says.
But with the 217-degree difference, "it starts to look like there's liquid water not too far down in order to get temperatures up as high as this."
In addition, the team discovered more hot fissures near the tiger stripes. The heat source: largely friction within the moon as Saturn's gravity tugs on Enceladus as it travels in its slightly elliptical orbit around the planet.
Titan's hidden ocean?
On Tuesday, Cassini was making its 42nd science flyby Titan, Saturn's largest moon. There, Cassini has uncovered evidence that the moon's crust is blowing in the wind – literally. The implication: The crust may be riding on a global layer of liquid roughly 62 miles below the surface.
Some models had suggested that this might be the case. Still, finding evidence for it "is an amazing result," says Spencer.
Using Cassini's radar to map Titan's surface, a team led by Ralph Lorenz at the Johns Hopkins Applied Physics Laboratory plotted prominent surface features – dunes of organic "sand," lakes, river channels, and mountains. Surprisingly, during 19 passes between October 2005 and May 2007, they found that some 50 surface features had shifted from their initial positions by up to 19 miles.
The only way to explain the shift at the moment is through the presence of a liquid ocean between the crust and the core, with the crust's drift driven by winds in Titan's dense atmosphere.
If further flybys confirm the results, Titan would join Jupiter's moons Europa, Ganymede, and Callisto on the small, but apparently growing, list of moons in the solar system hosting oceans deep beneath their surfaces. The results appear in the current issue of the journal Science.
Why Titan would have a sustained ocean that hasn't frozen solid over the eons, given the icy temperatures, is another intriguing question, notes Jeffrey Kargel, a planetary scientist at the University of Arizona in Tucson.
He notes that Titan's heat source is different, and, because of the moon's size, much weaker, than Enceladus's. One explanation: Titan's ocean is laced with ammonia, which acts as a natural antifreeze.
Indeed, says Dr. Kargel, one of the features he was hoping to see in the data from Enceladus's plume was some sign of ammonia. It could imply that ammonia was a feature of the chunks of primitive planetary material that formed the moons.
Yet no ammonia has appeared in the Cassini data from Enceladus so far.
Others caution that it's still a bit early to attempt to extrapolate conclusions from one moon to another. "We're seeing so little of Enceladus," says Bruce Jakosky, a researcher at the University of Colorado at Boulder.
Returning with robots
Seven more flybys are planned through the end of 2010, including three more this year. Even so, he says, it will still be a long, hard slog to figure out what the moon is really revealing about itself, let alone what it may or may not reveal about any of its Saturnian siblings.
Which is why some researchers are keen to return with robots that can explore these moons' surfaces. Even without them, researchers anticipate more eyepopping moments during Cassini's remaining tour. "Surprises are coming fast and furious in this business," says Kargel.