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Emily LakdawallaJune 26, 2014

A close look at Saturn's closest moons

I have been working all day on a blog post about a couple of recent papers on some of the funny things that moonlets do when they interact with Saturn's ring system. That blog post isn't quite ready (you may see it next week), but I thought I'd share an image I put together to illustrate it, because this took quite a bit of effort on its own.

Behold: the eight moons that orbit closest to Saturn -- at least the eight ones that have formal names that we are sure aren't just temporary clumps, in images that Cassini took over a period of five years. Seven of the color composites came from Gordan Ugarkovic's Flickr page.

Saturn's innermost moons: Pan, Daphnis, Atlas, Prometheus, Pandora, Epimetheus, Janus, and Mimas, to scale

NASA / JPL-Caltech / SSI / Gordan Ugarkovic / Emily Lakdawalla

Saturn's innermost moons: Pan, Daphnis, Atlas, Prometheus, Pandora, Epimetheus, Janus, and Mimas, to scale
The eight innermost moons of Saturn, in color images collected by Cassini between 7 June 2005, and 5 July 2010. Pan and Daphnis (top left small moons) orbit within the Encke and Keeler gaps in the rings; Atlas (below Pan and Daphnis) orbits at the outer edge of the main rings. To their right are Prometheus and Pandora; Prometheus orbits just inside and Pandora just outside the F ring. Below them are Epimetheus (left) and Janus (right), which trade positions every four years, averaging out to the same distance from Saturn. Mimas orbits considerably farther away, but its gravitational effects influence the positions of gaps and waves within the rings. At full resolution, the montage has a scale of 500 meters per pixel.

I've made moon montages before, but not of these little ones. You don't have a lot of options to choose from when it comes to pictures of these smaller moons, for two reasons. One is their size: to get a decent amount of detail on a small moon, Cassini has to get a lot closer to it than it does to the big moons. Mimas is about 400 kilometers across, less than a tenth the size of Titan; Atlas is only a tenth of Mimas. So Cassini has to fly a hundred times closer to Atlas than it does to Titan to get a picture with the same number of pixels across the disk.

The other reason is that these moons are snugged up close to Saturn. Cassini's orbit rarely passes closer to Saturn than the orbit of Mimas, so its close-approach distances to the moons that lie closer to Saturn are limited; they bottom out at roughly 50,000 kilometers. When Cassini does close flybys of the more distant moons, like Enceladus, Dione, and Rhea, it can get much closer -- down to a minimum of 25 kilometers for Enceladus. So opportunities for high-resolution imaging of the small moons are quite rare, and our pics of Pan are especially lousy.

With so few image choices, it's hard to make a pleasing montage. I like to select images with similar phase angle; I prefer phase angles of around 30 or 40 degrees, because there's a nice nearly-full disk but the light comes in at enough of an angle to outline interesting topography. Anything higher and you start losing quite a lot of terrain to nighttime shadow. Anything lower and all you see is a splotch of color -- which is fine if the moon has a lot of contrast (like our Moon, or Iapetus, or Io) but not so good if the surface is relatively smooth with few color variations.

As a by-product of making this montage, I produced this list of most of the highest-resolution observations that Cassini has taken of all the moons interior to Mimas. The links take you to the Rings Node of the Planetary Data System, where you can download all the data, if you want to play with it yourself. The note in the "Color?" column refers to how many different filters were used. RGB means red, green, and blue; UVGIR means enhanced color -- ultraviolet, green, and infrared. Usually these have clear-filter images too. "Full" means they have IR, R, G, B, and UV. "Some" means they have some subset of these but not enough to make a color composite that's anything like true color. "Hyper" means they ran through every possible filter combination.

BodyColor?Image dateRangePhase
Pan full 2005-05-20 474232
Pan full 2005-06-07 580976 19°
Pan RGB 2006-12-16 780680 83°
Pan no 2006-10-27 384351 86°
Daphnis RGB 2010-07-05 75685 59°
Atlas full 2007-06-12 43608 88°
Atlas RGB 2005-06-08 162388 42°
Prometheus full 2005-06-07 436943 10°
Prometheus some 2008-05-25 538065 91°
Prometheus full 2009-12-26 56508 33°
Prometheus no 2009-12-26 79164 18°
Pandora (& Prometheus) UVGIR 2005-10-29 459384 23°
Pandora UVGIR 2005-09-05 52264 50°
Pandora UVGIR 2010-06-03 100242 29°
Epimetheus UVGIR 2005-03-30 74657 115°
Epimetheus no 2005-05-20 344454 26°
Epimetheus some 2005-07-14 87308 95°
Epimetheus hyper 2007-12-03 37116 66°
Epimetheus UVGIR 2010-04-07 86564 69°
Epimetheus full 2010-04-07 106928 62°
Epimetheus (& Janus) no 2006-03-20 490915 51°
Janus no 2005-05-20 357144
Janus (& Prometheus) full 2006-04-29 217951 33°
Janus (some with Saturn behind) UVGIR 2006-09-25 137876 62°
Janus UVGIR 2006-09-26 153707 62°
Janus hyper 2008-02-20 169606 71°
Janus UVGIR 2008-05-26 183111 83°
Janus no 2008-05-26 212942 83°
Janus UVGIR 2008-06-30 47606 133°
Janus full 2008-06-30 29688 126°
Janus UVGIR 2008-07-14 261785 78°
Janus full 2009-07-26 103586 64°
Janus full 2009-07-26 95903 58°
Janus (in eclipse) no 2009-08-27 263580 22°
Janus UVGIR 2010-04-07 74714 38°
Janus (Saturn nightside behind) full 2012-03-27 44392 112°
Janus (Saturn dayside behind) UVGIR 2012-03-27 57184 97°
Janus no 2012-03-28 86706 87°

Read more: pretty pictures, Cassini, scale comparisons, data art (was amateur image processing), Saturn's irregular moons, Saturn's moons, Saturn

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Emily Lakdawalla

Solar System Specialist for The Planetary Society
Read more articles by Emily Lakdawalla

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