Space Topics: Saturn
The Year in Pictures: 2009
Saturn at Equinox
Saturn at equinox
Credit: NASA / JPL / Space Science Institute
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Just a day after the equinox, Cassini turned to capture
the 75 images necessary to compose this striking panorama across Saturn and
its rings. It's a rare and bizarre view of Saturn with the Sun poised almost
exactly over the equator. With this lighting geometry, the rings are nearly
dark, except where reflected light from Saturn strikes them, or where structures
that rise above the plane of the rings catch the sunlight. The rings' shadow
has collapsed to a skinny line falling across Saturn's waist.
Because of difficult geometric and lighting conditions, extensive processing
was required to create this view. Cassini was relatively close to Saturn
when it began taking the images for the mosaic and was continuously receding
from the planet during the eight hours it took to capture the 200 images
contained in the observation. (Photos were captured through infrared, violet,
and clear filters in addition to the red, green, and blue-filter images used
for the mosaic shown here.) So each of the images had to be reprojected into
a common viewing geometry. The photos also had to be digitally processed
to remove lens flares (artifacts that result from light scattering within
the camera's optics) and to deemphasize seams between images.
Additional processing was necessary to make the rings visible on your computer
screen. Without enhancement, the rings would have been so dark as to be invisible.
The dark half of the rings (right side of the mosaic) has been brightened
relative to the bright half, (left side of the mosaic) by a factor of three,
and then the entire ring system brightened by a factor of 20. Spokes are
visible in the B ring on the right side of the image.
Similar brightening was required to make the moons visible. Janus (179 kilometers
in diameter) is on the lower left of this image. Epimetheus (113 kilometers)
appears near the middle bottom. Pandora (81 kilometers) orbits outside the
rings on the right of the image. The small moon Atlas (30 kilometers) orbits
inside the thin F ring on the right of the image. Other bright specks are
background stars.
Cassini had passed through its orbital periapsis at 11:37 UTC on August 11
and almost immediately crossed to the northern face of the rings, at 12:59.
This crossing was only 10 hours after the Sun had precisely balanced over
the plane (at 02:44). Cassini began capturing the images for this mosaic
at about 06:00 on August 12, about 1.25 days after the moment of equinox,
and finished taking data at about 14:00.
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