According to Colwell

The picture below, a unique mosaic taken by Cassini last month, is visually stunning (see here for the full resolution images), packed with new information about Saturn’s ring system, and a bit of an optical illusion. At the DPS meeting two weeks ago and the Cassini Project Science Group meeting last week we got a chance to see some of the early analysis being done on these images by the Cassini imaging team (ISS, visible light cameras) and the near-infrared camera team (VIMS). While those teams continue their scientific analysis, we can take a look at why this unusual picture looks so, well, unusual.

Image Credit: NASA/JPL/Space Science Institute

What are we looking at? Cassini was in Saturn’s shadow at a distance of 2.2 million kilometers, and was above the rings. The Sun is shining on the opposite side of Saturn and on the underside of Saturn’s rings. The disk of Saturn is visible because of light scattering off the rings onto the planet. Where the rings pass in front of the planet they block this reflected light. Also there is no reflected ring light along Saturn’s equator (see the dark line above the rings along the planet) because almost no light is reflected by the edge of the rings. The very bright outer edge of the rings is the F ring. This normally faint narrow ring stands out as the brightest feature in this geometry because of the large amount of small particles in the ring. The larger particles in the main rings block light, so only a diffuse glow passes through the gaps between the large particles. In front of the planet, however, the F ring is not visible because the planet blocks the direct sunlight, and the reflected planet glow (light bouncing off the rings, onto the planet, and then back onto the rings) is too faint in this geometry to make the F ring appear. The other rings are visible in front of the planet only because they block our view of the dimly lit planet behind them. The missing F ring in front of Saturn produces the optical illusion in this mosaic of Saturn appearing to be in front of the rings.

This entry was posted on Saturday, October 21st, 2006 at 11:04 am and is filed under Astronomy. You can follow any responses to this entry through the RSS 2.0 feed. You can leave a response, or trackback from your own site.