One of the latest Cassini press releases states: "The gas planet's subtle northward gradation from gold to azure is a striking visual effect that scientists don't fully understand. Current thinking says that it may be related to seasonal influences, tied to the cold temperatures in the northern (winter) hemisphere. Despite Cassini's revelations, Saturn remains a world of mystery."

I've been trying to get more information on this effect ever since one of the first Cassini color images showed it. I asked some folks who were doing a poster on Saturn's atmosphere at the LPSC last year, and they appeared to know nothing about it.

It seems to me as a layman that the cause is pretty clear: reduced insolation caused by the combination of Northern winter and the shadows of the rings lowers the temperature, which causes the cloud tops to be deeper in the atmosphere. This in turn means more Rayleigh scattering, which makes it blue.

So what's the mystery?

Michael

I think you're right, although we should fill in two blanks:

Is the temperature really lowered? If thermal transport were efficient, and/or enough of Saturn's heat came from within as opposed to solar radiation, then temperature wouldn't be lowered that much from the baseline. When does the logic become circuitous? We can use the blue-ing as a measure of temperature, or the temperature as an explanation for the blue-ing, but we need some independent measurements at some point.

The reason why the clouds would lower as a function of temperature. I guess the explanation is that giant planet clouds form at about the altitude where temperatures reach the melting point of the component compounds. But is this a law, a generality, or not a law at all??

Of course, if those things were given, then the blue-ing would be from greater methane absorption, turning winter Saturn into a kind of local Uranus!

I would really like to hear an authority talk about these issues. To me, blue-Saturn is a major change in the "look and feel" of the solar system, and I can't understand why it hasn't elicited more scientific interest. I'm not being critical of the Cassini team, I'm just curious.

Michael

You can bet that it has, Michael. There's no way something that important would be overlooked or "lost in the shuffle". It probably just hasn't gotten through peer review yet -- either that, or whoever's researching it is still working on the paper.

Keep an eye on the American Geophysical Union meeting abstracts. It'll most likely pop up in the planetary section there, sooner or later.

Could it have something to do with the simple fact that water is bluish? Even ice absorbs more at the red end of the spectrum due to the overtone of an OH bond stretch. The (predominantly) water ice particles in the rings are certainly small enough to be transparent...