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