One of the most remarkable advantages of having an orbiter at Saturn is that we are able to record and study changes in it over time. Back during the Pioneer and Voyager programs, it was all too tempting to think that these fleeting glimpses we got of the gas giants were representative of them always. But of course it wasn’t. Even a barren world like Mars changes over human timescales; a gas giant is essentially *made* of weather and so should change constantly. Perhaps it’s because Saturn looks like such a smooth set of bands from Earth, or perhaps we have misled ourselves by thinking extremely long-term storms like the Great Red Spot are common. But just as Galileo and the Hubble Space Telescope and increasingly acute ground-based observatories have shown dramatic change in the GRS and the rest of Jupiter’s storms, Cassini is now doing the same for Saturn. Saturn has an axial tilt more like Earth, so it experiences significant seasonal variation, and this has created intriguing variety over time. One of the things only Cassini can tell us about is the Hexagon, a vast structure across Saturn’s north pole. From Earth, we cannot see structures at high latitudes; we just don’t have the right viewing angle. But Voyager 1 saw the hexagon, and now Cassini has been watching it. And although it’s clearly stable over long timescales, it isn’t static. In fact, it’s color has changed rather dramatically:
No one yet knows why, although going theories involve a shift in the distribution of methane due to changing levels of sunlight. (Sunlight accelerates the breakdown of methane.) We will not have Cassini around to watch the Hexagon for much longer, unfortunately, so scientists are working to gather whatever they can during the remainder of Cassini’s mission.
The next interesting thing to look for will be Juno: it’s the first mission ever to have a good viewing angle on Jupiter’s polar regions. What will it find? So far, we know Jupiter does not have a hexagon, and that the polar regions look very different from the mid-latitudes. Less stripey, for one thing. But that’s all we know so far.