The Cassini spacecraft has coasted to its closest encounter yet - skimming just 175 kilometres above Saturn's icy moon Enceladus. But astronomers are at a loss to explain its observations.
On 14 July, Cassini swooped in for an unprecedented close-up view of the wrinkled moon. Its Imaging Science Subsystem (ISS) camera has since returned pictures of a boulder-strewn landscape that is currently beyond explanation. The "boulders" appear to range between 10 and 20 metres in diameter in the highest-resolution images, which can resolve features just 4 m across.
“That’s a surface texture I have never seen anywhere else in the solar system,” says David Rothery, a planetary geologist at the Open University in Milton Keynes, UK.
Cracks crisscross Enceladus's surface - possibly as a result of the moon being repeatedly squeezed and stretched by the gravity of Saturn and other moons nearby. But Rothery points out the boulders avoid - rather than fill - the cracks. This might indicate that the fracturing took place after the boulders had already formed.
John Spencer, a Cassini team member at the Southwest Research Institute in Boulder, Colorado, US, agrees that the images are puzzling. “You would expect to see small craters or a smooth, snow-covered landscape at this resolution," he told New Scientist. "This is just strange. In fact, I have a really hard time understanding what I’m seeing.”
NASA scientists have been locked in discussions since 15 July and are expected to pass judgment on what they think this peculiar surface might be later on Tuesday.
But Elizabeth Turtle, a Cassini imaging team member at the University of Arizona in Tucson, US, warns there will be no quick answers. “Trying to figure out what is going on is going to take a lot longer than a weekend of swapped emails,” she says.
These images - like those from previous flybys - reveal a surface clawed with fractures and swollen with ridges. It could point to a substantial heat source within the moon, driving the internal convection of ice. And this raises the possibility that Enceladus could possess a sub-surface ocean similar to that on Jupiter's moon Europa.
That could be a problem, according to Spencer. Superficially, the two worlds bear a passing resemblance, but Enceladus is six times smaller than Europa. “Enceladus seems too small to have enough internal heat to create a sub-surface ocean," he says. "But, since we don’t understand the surface, we might not understand the interior either,” he says. Turtle, however, is sceptical of the ocean hypothesis and says "we see no evidence of liquid flows on the surface”.
Key information in this debate may come from Cassini’s Dual Technique Magnetometer. It was fluctuations in Europa’s magnetic field that finally convinced scientists that it harboured a subsurface ocean. Perhaps the same will be true of Enceladus. At present, the data is being analysed by scientists at Imperial College in London, UK.
Regardless of the outcome, NASA has already decided that Enceladus is worth an even closer look. They have scheduled another grazing flyby of the moon in 2008, when Cassini will skim even closer than ever - to within 100 km of the boulder-strewn surface.