Iapetus – Wow!

Saturn’s third largest moon, Iapetus (eye-AP-eh-tuss), was discovered at the then-new Paris Observatory in 1671 by Italian-French astronomer (and observatory director) Giovanni Domenico (Jean-Dominique) Cassini (1625-1712).  Upon further observation, Cassini noted that he could only see Iapetus when it was on the west side of Saturn, never the east.  His telescope was not large enough to detect Iapetus on the east side of Saturn because it was much fainter then.  He correctly reasoned that, “it seems, that one part of his surface is not so capable of reflecting to us the light of the Sun which maketh it visible, as the other part is.”  He also must have realized that Iapetus was locked in synchronous rotation—as is our Moon—with the same side facing Saturn all the time, with its rotation period being equal to its orbital period.  Today we know these periods to be 79.3215 days.

The leading hemisphere of Iapetus has a visual albedo of only about 5%, whereas most of the trailing hemisphere is much brighter, having an albedo around 25%.  Thus, when Iapetus is on the west side of Saturn, its apparent visual magnitude is around 10.2, but on the east side of Saturn Iapetus is 1.7 magnitudes fainter at 11.9.  Without a doubt, Iapetus is one of the most outlandish places in the solar system, and the Cassini Saturn orbiter flybys certainly amplified the strangeness.

Cassini made one close targeted flyby of Iapetus on September 10, 2007, passing within 762 miles of the surface.  Here are a few of the best photos of Iapetus from Cassini.

The first high-resolution glimpse of the bright trailing hemisphere of Saturn’s moon Iapetus
This is a raw, or unprocessed, image taken by the Cassini spacecraft during its close flyby of Saturn’s moon Iapetus on Sept. 10, 2007 showing its prominent equatorial ridge—still a mystery
The “Himalayas” of Iapetus
Dark Terrain Up Close
The Transition Zone
Closest View of Iapetus
Dark material splatters the walls and floors of craters in the surreal, frozen wastelands of Iapetus
May 30, 2017 – Cassini bids farewell to Saturn’s yin-and-yang moon, Iapetus

The dark material appears to have been deposited from elsewhere in the Saturnian system, but sublimation of water ice may also play a role.  In any event, the dark material is a relatively thin veneer, significantly less than a meter thick in many places.

The warm day on Iapetus sees a surface temperature of -227° F on the dark terrain and an even colder -256° F on the bright terrain.  Inhospitable, to say the least!

Saturn at Eastern Quadrature

Wednesday evening, September 13, 2017, at 9:59 p.m. CDT, Saturn reaches eastern quadrature as Saturn, Earth, and Sun form a right triangle.  Eastern quadrature is so named as Saturn is 90° east of the Sun.  This is the time when Saturn presents to us its most gibbous phase.  Even so, Saturn will be 99.7% illuminated due to its great distance from us.

A more noticeable effect will be the shadow of Saturn on its rings, a phenomenon best seen at eastern or western quadrature.

Saturn will only be 12° above our horizon in SW Wisconsin at the exact moment of eastern quadrature Wednesday evening.  Earlier that evening, Saturn crosses the celestial meridian at 6:51 p.m.—22 minutes before sunset.  If it weren’t for daylight, that would be the best time to observe Saturn: when it is highest in the sky and we are seeing it through the least amount of atmosphere.  If you have a telescope equipped with a polarizing filter, you can significantly darken the blue sky background around Saturn since the planet will be exactly 90° away from the Sun, where the scattered sunlight is most highly polarized.  Rotate the polarizer until the sky is darkest around Saturn.

Speaking of Saturn, the Cassini mission will come to a bittersweet end on Friday, September 15 around 5:31 a.m. CDT when the storied spacecraft, which has been orbiting Saturn since June 30, 2004, will have plunged deep enough into Saturn’s atmosphere that it is no longer able to point its high gain antenna towards Earth.  Soon after that, Cassini will burn up in Saturn’s massive atmosphere.  We on Earth will not receive Cassini’s last radio transmission until 1h23m later—at around 6:54 a.m. CDT.

Emily Lakdawalla, who is arguably the best planetary science journalist in the world these days, includes the visual timeline of Cassini’s demise shown below and in her recent blog entry, “What to expect during Cassini’s final hours”.

Also, on Wednesday evening, don’t miss NOVA: Death Dive to Saturn, which will air on Wisconsin Public Television’s flagship channel at 8:00 p.m.

It may be a while before we visit ringed Saturn and its retinue of moons again.  But further exploration of Titan and Enceladus is certain to feature prominently in humankind’s next mission to Saturn.  Hopefully, that will be soon.