As Cassini prepares for a death dive into Saturn next year, planetary scientists met in Boulder, Colorado, last week to discuss its possible successors.

Enceladus has a massive global ocean under its frozen surface, and cracks in its exterior spew plumes of water into space. The plumes continually add icy material to one of Saturn’s rings, and offer a tantalising taste of the water within. But Cassini can’t test them. Its instruments aren’t detailed enough to analyse the water, because when it was built, no one knew the plumes were there.

“That is a very fine example of why it’s so hard to design space missions,” says Alexis Bouquet, a PhD student at the Southwest Research Institute in San Antonio, Texas. “By definition, we are going to an object that we don’t know much about. So we always get surprises.”

As Cassini flew through Enceladus’s plumes a handful of times in the past 11 years, its instruments were flooded with hydrogen molecules, which are a possible smoking gun for hydrothermal vents in the oceans. If confirmed, those vents would have major implications for life beneath the ice.

Bugs on a windshield

But it’s unclear whether the hydrogen molecules came from Enceladus or from Cassini itself. That’s because when ice grains in the plumes smack into Cassini’s instruments they break apart, like insects on a car windshield. “They are smashing so fast that they can actually chip the windshield and form tiny craters,” says Bouquet. This releases titanium into Cassini’s instruments, which steals oxygen from the icy water to release hydrogen molecules.

At the meeting in Boulder, Bouquet presented computer simulations he is using to figure out how much water is really there and how much is the instrument’s confusion – although he hasn’t come to a conclusion yet.

To improve matters, a future Enceladus plume sampler could use gold sensors, which wouldn’t react in the same way as the titanium ones. Or it could use a soft, spongy net, similar to the capture devices developed for the Stardust mission, which grabbed a few specks of cosmic dust from interstellar space in 2006.

A net about 12 square centimetres in area would be big enough to capture a few micrograms of plume spray, says Richard Mathies, a chemist at the University of California at Berkeley. While that’s not a lot, the proposed lab-on-a-chip Enceladus Organic Analyzer — new details of which Mathies’s collaborators presented in Boulder — can sniff out one organic molecule in a billion others, Mathies says.

Subsurface sea

Landers and drills would be able to get an even closer look at the subsurface sea. But to enter they would have to crash with immense force or melt the ice, disturbing anything living there even as they tried to detect it. Tests on the EOA’s instruments suggest it could still do its job after an impact with an energy 50,000 times greater than Earth’s gravitational pull, which is a greater g-force than that felt by an artillery shell.

At the meeting, Amanda Stockton at the Georgia Institute of Technology presented design concepts with optical instruments in the centre of a lander, which would make them more likely to survive impact.

One other robot concept could break more than just ice grains. A proposed Enceladus Explorer mission could set up a robotic base station near the moon’s southern pole, where the plumes are thought to originate. A robot drill called the IceMole would both melt ice and ram through it, reaching down about 100 to 200 metres to the ocean below the surface.

Researchers at Aachen University of Applied Sciences in Germany told the meeting of plans to test a smaller model of the probe in a vacuum chamber under simulated space conditions.

Even as they plan future missions, planetary scientists will continue analysing data from Cassini long after it makes its final measurements. Cassini has not only fulfilled its mission, but opened the door to an armada of probes destined for oceans in the outer solar system, says Angela Stickle at the Applied Physics Laboratory at Johns Hopkins University in Baltimore, Maryland.

“Cassini is fantastic and marvellous,” she says. “But, as with any good spacecraft mission, it leaves us with more questions than answers. Having more missions to these planets will only help answer our questions.”

Source: New Scientist