Chilling Effect

In terms of just sublime beauty, it's hard to beat Saturn. Not just for the amazing ring system, but for the wide range of pastel colors that the globe exhibits. While I don't have data to support it, I'd be willing to bet that in terms of NASA missions resulting in wallpaper images, Cassini has to be near the top.

Over the last few apparitions the axial tilt of Saturn towards our home planet has been decreasing, moving from its Northern Hemisphere summer towards a 2025 equinox. When wide open, the expansive rings hide the winter hemisphere, not only keeping it from our view but likely increasing the frigid temps by blocking what diminished sunlight Saturn receives at a distance of nearly 1½ billion km from our home star. 

As the rings retreated over the southern hemisphere and the Southern Hemisphere began to peek out from under them, I was struck by the distinct blue coloration we were seeing. While more apparent in SPLICE photography of the planet, I also could readily discern a difference visually last year with my 10" Cyrus Newtonian under good seeing conditions. The Southern Hemisphere with a Baader Contrast Booster filter at 190x was darker, with a greenish-gray sort of appearance to my eyes. And I am not exceptional in this - others on Cloudy Nights have reported seeing similar differences between the hemispheres.

But what is causing this? One hypothesis is that a reduction in the UV radiation has a corresponding decrease in haze that it generates when encountering methane. (Consider Saturn's moon Titan, which has 5% methane in its atmosphere, is always shrouded in amber fog). Coupled with the coldness making the heavier elements sink, we end up with a "clearer" portion of the atmosphere and increased Rayleigh scattering (the same effect that makes our sky blue).

As the Southern Hemisphere begins to gain more and more solar radiation during this year's apparition it will be an interesting exercise to examine the globe to see the pale tan coloration returning to it. At the same time we should begin to see the NPR of the planet beginning to exhibit those blue tones as it begins to chill.

The rings may be less prominent, but use the opportunity to examine the globe to your advantage. How "cool" is it to be able to notice the changing of seasons on a planet close to a billion miles away from you?