Something downright weird has been sighted twirling over Saturn's north pole: a long-lived double hexagon formed in the clouds. The two six-sided features, one inside the other, are in stark contrast to the hurricane-like vortex that has been observed at the ringed planet's south pole. NASA's orbiting Cassini spacecraft has imaged both poles. "We haven't seen a [geometric] feature like this anywhere else on any other planet," says Cassini scientist Dr Kevin Baines of the NASA's Jet Propulsion Laboratory. "It's unbelievable." One of the unexplained hexagons was glimpsed obliquely before, by the Voyager 1 and 2 spacecrafts more than 20 years ago, which is how scientists know it's a durable feature.
Now, Cassini's infrared mapping instrument has provided the first whole, irrefutable images of the feature from a higher-latitude orbit. The 25,000-kilometre-wide feature appears to be some sort of deep-seated standing wave, through which other things move without changing the wave pattern, Baines observes. It also appears to be in sync with the planet's quick 10-and-a-half-hour rotation. Beyond that, nobody is sure what to make of it. "It's perplexing," says Baines. "It's a bizarre pattern."
Cassini's recent fly over Saturn's southern hemisphere gave scientists a chance to verify that no such feature appears there.
Underneath, the planet appears to be as pocked and turbulent as giant Jupiter, he says. The reason for the difference is that Jupiter is warmer and has stronger gravity, which makes it hard for an ammonia haze to survive there, he explains.
The next good view of the hexagons is expected to be in about a year and a half, says Baines. That's when Cassini is scheduled to venture even further north.
The Sun will be shining on it then, which will allow scientists to see how the hexagon influences higher clouds that were invisible in the infrared images.
"We will have an even better view," Baines says. It's even possible that by that time someone will have come up with an explanation for the alien hexagons.