Emily LakdawallaFeb 28, 2008

Pretty pictures from Cassini: Rhea, Saturn, and Janus

I check JPL's Planetary Photojournal every day to see what's new from their various spacecraft, and there have been some particularly pretty images from Cassini lately. This one really caught my eye. In a rare view, we see one of Saturn's icy moons (this one is Rhea) nearly fully lit, what scientists refer to as "low phase." But the background is odd, grading from dark in the upper left to less dark at lower right. Turns out that's Saturn in the background. Because Rhea and Saturn are lit at the same angle -- nearly full phase -- that means Rhea must be perched near the edge of Saturn's disk, where the terminator (or day-night boundary) barely peeks over the edge. You can see the geometry with help from JPL's Solar System Simulator.

Rhea and Saturn

NASA / JPL / SSI

Rhea and Saturn
Cassini captured a nearly fully lit Rhea floating in front of Saturn on January 17, 2008. Rhea floats just above Saturn's terminator, the boundary between day and night. Rhea was 576,000 kilometers from Cassini at the time; Saturn is more than twice as far away.
Rhea and Saturn

NASA / JPL / SSI / color composite by Emily Lakdawalla

Rhea and Saturn
Three views from the Cassini raw images website have been merged into a color view of Rhea floating in front of Saturn, captured on January 17, 2008. The images were captured through infrared, green, and ultraviolet filters, so the color is exaggerated with respect to what the human eye can detect.
Rotating Janus

NASA / JPL / SSI / animation by Emily Lakdawalla

Rotating Janus
The seven images in this animation of Saturn's moon Janus were captured by the Cassini orbiter as it passed within 200,000 kilometers of the moon on February 21, 2008. The view is down onto Janus' south pole, and during the time that separates the seven images, Janus rotates by several degrees, sending some craters into darkness and bringing others into daylight.

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