Celestia Forums

Just something I was wondering about: how sharp are the edges of the ring shadows on Saturn? If we pretend that Saturn has a surface, would the shadow edge be a sharp line or would it gradually fade out over many kilometres? It would definitely make for an odd and spectacular effect if the former is the case.

chaos syndrome wrote:Just something I was wondering about: how sharp are the edges of the ring shadows on Saturn? If we pretend that Saturn has a surface, would the shadow edge be a sharp line or would it gradually fade out over many kilometres? It would definitely make for an odd and spectacular effect if the former is the case.

Grant and I contemplated about this interesting issue some time ago. Clearly diffraction effects would typically be the mechanism to soften the shadow rims. We estimated, however, that from the roughly known composition of the sizes of the ring particles, diffraction effects would be rather negligible in visible light.

Bye Fridger

t00fri wrote:Grant and I contemplated about this interesting issue some time ago. Clearly diffraction effects would typically be the mechanism to soften the shadow rims. We estimated, however, that from the roughly known composition of the sizes of the ring particles, diffraction effects would be rather negligible in visible light.

Bye Fridger

Thanks.

So the main factor in the sharpness of the shadow is how quickly the particle density falls off at the edge of the ring? Are there any values for this available?

chaos syndrome wrote:

t00fri wrote:Grant and I contemplated about this interesting issue some time ago. Clearly diffraction effects would typically be the mechanism to soften the shadow rims. We estimated, however, that from the roughly known composition of the sizes of the ring particles, diffraction effects would be rather negligible in visible light.

Bye Fridger

Thanks.

So the main factor in the sharpness of the shadow is how quickly the particle density falls off at the edge of the ring? Are there any values for this available?

Chaos syndrome,

the point is different: diffraction phenomena concern light scattering regimes where the wave nature of light becomes evident (unlike the usual geometric ray approximation!). This happens when the wavelength of the light and the size of the scattering particles are becoming comparable. Whenever this happens, completely new phenomena occur. Notably light can penetrate into geometrical shadow regions that are /forbidden/ for classical light rays! This way shadow boundaries become strongly softened. So it's less a matter of the density of the scattering particles, but rather of their sizes as compared to the wavelength of the light under consideration.

Conversely: if a softening of the shadow boundaries were observed in case of Saturn, we could draw some interesting conclusions about the sizes of the ring constituents!

Bye Fridger

t00fri wrote:Conversely: if a softening of the shadow boundaries were observed in case of Saturn, we could draw some interesting conclusions about the sizes of the ring constituents!

I thought the original question of this thread was about the effect of the rings obscuring only part of the solar disc, like the Moon does during a partial solar eclipse on Earth. On Saturn, the solar disc is only some 3 arc minutes in diameter, but I would expect this to translate into several kilometers of diffuse shadow on Saturn's visual "surface" (given its distance from the rings), just as Chaos Syndrome suggests.

If you use Celestia and go to Saturn, 60 degrees N latitude, 0 degrees longitude, distance 1 km (above the "surface"), track the Sun, and set the time to 2005-02-09 22:07:26 UT, you should see approximately half the solar disc be obscured by one of the outer rings. Let time pass, and you'll find that any point on the ring takes around a minute to move from one edge of the solar disc to the other. At 60 degrees latitude, one minute means moving some 300 km, so that is apperarantly the approximate "diffuseness" of this shadow (I'm here disregarding the fact that the direction of motion of the observer doesn't form a 90-degree angle with the shadow rim).

Another effect of interest is the interaction between a shadow and a semi-transparent object, such as the upper layers of Saturn's atmosphere (or fog on Earth). If, say, Cassini is located in sunlight well above the rings and takes pictures of the ring shadow as it penetrates the atmosphere, I would expect the shadow to become less pronounced as the reflected light from the shadow rim had to pass through thicker layers of clouds and haze, and perhaps also due to atmospheric dispersion of the incoming light.

Both of these effects are obviously unrelated to diffraction caused by very small ring particle sizes (though atmospheric dispersion may be related to the wavelength of the light, depending on the composition of the atmosphere).

I wonder: Could refraction in clouds of other than water droplets, say methane, also form a rainbow? If it could, would the rainbow lead to a pot of gold?

Probably the best way to answer this question is to look at the pictures. It appears to me that there's some fuzz there, just caused by the angular size of the Sun as seen from Saturn.

andersa wrote:I wonder: Could refraction in clouds of other than water droplets, say methane, also form a rainbow?

Probably. Also, this site has some interesting speculations about ice-crystal halos from substances other than water, as might conceivably exist on Mars, the upper atmosphere of Titan and the giant planets. As far as I know, these halos have never been seen; there was an experiment to look for them on the failed Mars Polar Lander, and it would be nice if the upcoming Phoenix lander picked up the torch.

Speaking of shadows in Saturn's atmosphere, Cassini has photographed shadows cast by the cloudtops themselves near the terminator (where the Sun is barely above the horizon), indicating the threedimensional structure of the clouds.