June 4, 2020

Astro-Challenge: Spotting Two-Faced Iapetus.

As the majestic planet Saturn approaches opposition on March 21st, I’d like to turn your telescopic attention to one of the most bizarre moons in the solar system; Iapetus. It was way back when in the 17th century that Italian astronomer Giovanni Cassini noted that he could only see Iapetus when it was to the west of the ringed planet, but never to the east. He correctly deduced that Iapetus must not only be tidally locked, that is, holding one face towards Saturn, but must be correspondingly dark on one hemisphere and brighter on the other. In fact, Iapetus is known to vary from magnitude +10 to magnitude +12 over its 79 day orbit, a variation of 6 times in terms of brightness. the Cassini space probe has confirmed the duality of Iapetus, showing us a dark leading hemisphere with an albedo of 5% (think fresh asphalt) and a trailing hemisphere with an albedo of about 50% (think dirty snow). The third largest of the Saturnian moons, Iapetus is a “walnut shaped” world, with a large ridge running the equator of this twisted moon. Discovered by Cassini on New Year’s Eve 2004, no satisfactory explanation for the ridge is known, but the little world must have had a tumultuous history.

The good news is, you can observe these changes in Iapetus with a moderate (4” or larger) aperture instrument.  At greatest elongation, Iapetus strays about 9’ arc minutes from the disk of Saturn, the farthest of any of the moons that are within range of a backyard scope. These other moons make good comparison guides; in order out from Saturn, they are;

Mimas: Mag. +12.9

Enceladus: Mag. +11.7

Tethys: Mag. +10.2

Dione: Mag. +10.4

Rhea: Mag.+9.7

Titan: Mag. +8.3

So a good rule of thumb is; if Iapetus appears nearly as bright as the other moons (excluding Titan) you’re viewing the trailing bright side; if it’s more on the dim, Mimas-side of things, you’re looking at the dark, leading side.

If all this isn’t weird enough, Iapetus is also inclined to Saturn’s ring plane at an angle of about 16° degrees…this means that the surface of Iapetus would be the only decent vantage point from a large Saturnian moon to view the ring system. Another bit of Iapetus lore; in the original novel adaptation of 2001: A Space Odyssey, Arthur C. Clarke set the final monolith on… you guessed it… Iapetus.

As the Saturnian planetary system rises successively earlier every evening do give Iapetus a shot. A cool time lapse video could even demonstrate this brightening, catching the Saturnian moons on successive nights… why isn’t this on the web yet?

…And what, might you ask, causes this two toned appearance? That’s our astro term of the week; the Phoebe ring. Last October, the Spitzer Space Telescope spotted a tenuous ring of debris just interior to Saturn’s moon, Phoebe. At an estimated diameter of 59 to 300 Saturn radii, this wispy ring system is huge compared to the traditional rings; brightest in the infra-red, this material is thought to have been ejected by impacts on the tiny moon of Phoebe. Phoebe and the ring material are in a retrograde or backwards orbit, which spirals inward due to solar radiation remission (remember our friend, the Poyting-Robertson effect?) and causes it to slam headlong into the leading hemisphere of tidally locked Iapetus.  But careful now, that dark coating isn’t the ring material you’re seeing, but a result of the heating, ice sublimation, and eventual self-segregation that occurs. First proposed in the 70’s by Steven Soter, the Phoebe Ring was one of the exciting discoveries of 2009!


  1. [...] of Saturn in 2017. Saturn has 62 known moons in all, and Enceladus, Mimas, Tethys, Dione, Rhea and two-faced Iapetus  are all visible from a backyard telescope. The scale of the orbits of Saturn’s moons. Image [...]

  2. [...] of Saturn in 2017. Saturn has 62 known moons in all, and Enceladus, Mimas, Tethys, Dione, Rhea and two-faced Iapetus  are all visible from a backyard telescope. The scale of the orbits of Saturn’s moons. Image [...]

  3. [...] to medium-sized (8-inch) telescope, six moons are readily visible: Enceladus, Mimas, Rhea, Dione, Iapetus and Tethys. Large light bucket scopes 10” and larger might just also tease out the two faint [...]

  4. [...] range of a modest backyard telescope: Enceladus, Rhea, Dione, Mimas, Tethys and Iapetus. Two-faced Iapetus is especially interesting to follow, as it varies two full magnitudes in brightness during its 79 [...]

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