Iapetus Comes Over to the Bright Side

Saturn's strange, two-faced moon will be positioned well west of Saturn — and shining its brightest — during the next two weeks.

Iapetus as seen by Cassini

Saturn's moon Iapetus, 907 miles (1,460 km) in diameter, has a dual personality. One hemisphere is covered with bright ice, the other with darker material possibly ejected by impacts on the more distant moon Phoebe.
NASA / JPL / Space Science Inst.

Being two-faced isn't considered a desirable trait, but Saturn's moon Iapetus can't help itself. One hemisphere's as black as chimney soot, the other bright as ice. Even 17th century astronomers noticed that the moon was only visible when west of Saturn. As it moved to the east side of the planet, it faded away in telescopes of the day.

This strange cycle wasn't fully understood for two centuries. Voyager 2 got glimpses of this strange moon in 1981, but the Cassini orbiter recorded much more dramatic views from closer range in September 2007. It now appears that Iapetus is basically icy overall, and its dark half is covered with a layer of fine, ice-free material whose composition is unclear.

I've got good news for you. Iapetus' bright side is returning to view for the next few weeks, meaning anyone with a small telescope can now spot the elusive moon. When farthest east of Saturn in its languid 79-day orbit, Iapetus glows at magnitude 11.9, faint enough to be easily overlooked. But all things come to the patient observer. A month later the demur satellite slowly glides to the other side of the planet and swells to magnitude 10, wresting the title of second brightest Saturnian satellite from Rhea. At magnitude 9, Titan shines brightest.

Saturn viewed in a small telescope on June 27, 2014.

Saturn and its family of bright moons viewed in a small telescope around 10 p.m. EDT on June 27, 2014. Iapetus' wide orbit takes it much farther from the planet than the bright pack of closer moons. When brightest, it's 12 ring diameters west of Saturn.
Source: SkyMap

Western elongation occurs July 3rd, but anytime from now until mid-July will be ideal for seeing dual personalities moon through a 3-inch or larger telescope. Of course you're welcome to continue observing into early August to follow Iapetus's return to the "dark side".

Orbit of Iapetus

Iapetus orbits Saturn once every 79.3. days. We see its bright, ice-covered hemisphere at western elongation, the sooty dark side at eastern elongation, and a mix of both when it's in inferior conjunction on the nearside of Saturn and superior conjunction on the farside Not to scale.
Bob King

Because Iapetus's peculiar behavior repeated orbit after orbit, Giovanni Cassini, the moon's discover, correctly concluded that it was tidally locked to Saturn, with one hemisphere forever facing the planet. Our own moon is similarly locked to Earth, the reason we're stuck seeing just one side of it.

But what's behind such a drastic change in brightness? It's thought that Iapetus sweeps up dark-toned material from the debris created by micrometeorite impacts on other moons, most likely the dark outer moon Phoebe. Once the blast ejecta dusted a small part of the surface, the additional solar radiation it absorbed darkened neighboring ice.

Iapetus west of Saturn

Iapetus swings farthest west of Saturn on July 3rd, when it will shine brightest around magnitude 10. Thereafter it fades and reaches magnitude 12 at eastern elongation on August 13th. How long can you follow the moon in your telescope? Click to enlarge.
Source: Starry Night

Caught in a positive feedback loop, a "wave of darkening" spread to eventually cover an entire hemisphere of the moon. Meanwhile, the vaporized water migrated to the other hemisphere where it re-condensed as highly reflective ice.

One of the many thrills of amateur astronomy is seeing how the universe works with our own eyes. Iapetus may be little more than a flickering point of light through the eyepiece, but being able to picture what's happening across a distance of more than 850 million miles makes this icy moon chillingly real.


Ready for some more moon hunting? You can find the locations of Saturn's bright moons for any time and date using S&T.com's free Javascript utility or S&T's Saturn's Moons app for Apple devices.

2 thoughts on “Iapetus Comes Over to the Bright Side

  1. Teamonger

    I happened to be heading out to a dark sky last night, so found timely use for the June 27 finder chart for Iapetus. Actually I made a printout from the S&T Javascript utility, and plotted Iapetus onto the page. Looking at Wikisky, I found there were no 10th mag stars in the vicinity to confuse it with. I pointed my XT8 Dob towards Saturn, and Iapetus was immediately apparent, almost four times as far from Saturn as Titan. Seemed a full magnitude brighter than Rhea, which was close to the planet on the other side. Tethys and Dione were also faintly visible, but caught no sign of Enceladus.
    So, thanks much for the useful info, Bob!
    t

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