Emily LakdawallaApr 03, 2008

Playing around with Cassini images of Janus

Today's release from the Cassini imaging team was this one of Janus, a still from the sequence that I animated back in February. Just to orient you, Janus is one of Saturn's smaller moons; it orbits just outside the most visible part of the ring system, between the F and G rings. Janus shares its orbit with a smaller moon, Epimetheus, with which it changes places from time to time. At present, Janus is closer to Saturn and Epimetheus is farther; the two will switch places early in 2010.

Janus

NASA / JPL / SSI

Janus
Cassini captured this view onto Janus' south pole on February 20, 2008. Janus is 181 kilometers across; the image scale is 1,010 meters per pixel.
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' north 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.

These are the seven best images that Cassini has taken, which show the moon at a variety of different phase angles. I've resized the images so they all show Janus at the same scale of 1 kilometer per pixel, which is why some of them are fuzzy; all but one of them was enlarged to get to that scale. I did this enlargement based upon the metadata archived with the images, so it should be pretty accurate. Therefore, if some of these images make Janus look smaller than others, that's because Janus isn't a sphere; it's significantly flattened, being roughly 194 kilometers measured across its middle but only 150 kilometers measured through its rotational axis.

Cassini's seven best views of Janus

NASA / JPL / SSI / montage by Emily Lakdawalla

Cassini's seven best views of Janus
These images represent the highest-resolution views of Janus taken by Cassini as of March, 2008. The images have all been enlarged to a scale of 1 kilometer per pixel (except the center one, which was reduced in size slightly to match this scale) and rotated so that their terminators are roughly parallel. They have been sorted by phase angle. From left to right, the phase angles are: 6, 27, 33, 62, 71, 101, and 147 degrees. Original image scales were 2.1, 5.5, 1.3, 0.95, 1.0, 5.4, and 3.2 kilometers per pixel, respectively. All but the fifth image came from the archived data on the Planetary Data System; the fifth image is from a Cassini mission public release.
Fresh craters on Phoebe

Fresh craters on Phoebe
A view of Phoebe from 13,777 kilometers away on June 11, 2004 reveals both old and fresh craters. The sharp-edged crater at the top is named Euphemus. Its walls appear to reveal layering in the upper surface of Phoebe.

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