This week, data from NASA’s Cassini probe has shed light on a couple of the stranger features of Saturn, providing answers to questions that were raised years ago. In terms of the planet itself, Cassini has spotted strange hexagonal patterns in the clouds near the north pole that were observed for the first and only time 30 years ago, when the Voyager probes swung by the planet. And researchers have used Cassini data to produce what they think are satisfying models for the strange coloration of Saturn’s moon Iapetus, something that was first noted over three centuries ago—by Cassini himself.
First, Iapetus, which would look like a fairly typical moon if it weren’t for a rather distinctive feature: the face that leads it around its orbit is about 10 times darker than its trailing face. This difference is so pronounced that Cassini himself reported it all the way back in 1677—both of today’s papers about the moon cite his paper in the the Philosophical Transactions of the Royal Society (volume 12, page 831, for those inclined to look it up). In Arthur C. Clarke’s book 2001: A Space Odyssey, the moon’s bright side was created by an alien civilization as a way of drawing the intention of any intelligent life.
Two papers that are being released by Science today offer a more mundane (but still rather exciting) explanation for the differences between the leading and trailing sides: like our own moon, Iapetus has a water cycle.
Previous explanations for the discrepancy have focused on the fact that the dark portion of the moon is on its leading edge, suggesting that its orbit brought it in contact with debris knocked loose by impacts on some of Saturn’s outer moons. The next moon inward is Titan, with a large gravity and atmosphere that should be sufficient to ensure that no other moons in the system get a similar paint job. The problem with this explanation is the slow rate of accumulation of this debris; objects hit Iapetus (referred to as “impact gardening”) frequently enough that the surface should be regularly mixed.
The new papers modify this model significantly. Dark debris landing on Iapetus’ leading edge is important, but only because it creates a temperature difference that triggers the real cause of the brightness difference: a water cycle. Because Iapetus keeps one face directed towards Saturn, its rotational period—a Iapetian day—takes nearly 80 Earth days. That means the dark material remains in sunlight long enough to create a temperature difference. Cassini registered temperatures of 129K on the dark side, and 113K on the bright side.
That’s enough to drive the sublimation of water at much higher rates on the dark side. Over time, that will gradually drive the water ice to the bright side, enhancing and reinforcing the stabilizing the brightness, and hence the temperature difference. The authors point to craters on the dark side with bright patches on their rims as evidence that exposure to sunlight is a key factor. A mathematical model of the system suggests that Iapetus only has to accumulate 0.3cm of dark material per billion years to drive this sort of behavior, and that a dramatic leading/trailing difference becomes visible after 1.2 billion years.
The authors of the modeling paper actually discovered that they weren’t the first to suggest this. Someone more or less got it right back in 1974, and successfully predicted observations that weren’t reported until 2008. Unfortunately, the paper had been overlooked.
Saturn also held another mystery that has confounded for decades rather than centuries. The Voyager probes went past the planet during its northern hemisphere’s spring, which allowed them to image the planet’s north pole. They apparently observed a strange hexagonal pattern in the pole’s clouds. Shortly after, spring ended, and nobody was sure if the pattern is a long-term feature of the atmosphere, or simply a one-off oddity that we were lucky to catch.
It’s springtime again, 30 years later, and Cassini was in orbit around Saturn, ready for further observations. And, as sunlight crept northwards, the hexagonal patterns were still present. The NASA press release quotes Ars contributor Kunio Sayanagi as saying, “The longevity of the hexagon makes this something special, given that weather on Earth lasts on the order of weeks. It’s a mystery on par with the strange weather conditions that give rise to the long-lived Great Red Spot of Jupiter.”
Fortunately, Cassini has instruments that are significantly improved from the ones that shot past on the Voyagers, and is remaining in orbit around the planet, which would enable its path to be tweaked in order to get a better look at the feature. Hopefully, this will mean that we won’t have to wait 300 years in order to produce a decent model of the processes that generate these hexagonal cloud patterns.