Impetus for Iapetus

PIA11690: Global View of Iapetus’ Dichotomy, NASA/JPL/Space Science Institute

What a strange world Iapetus is! The third largest satellite of Saturn—and the outermost of Saturn’s large satellites—is a moon of many mysteries. We’ll take a look at three of them.

Mystery #1: Iapetus appears to be an original satellite of Saturn, and yet unlike the other regular satellites, its orbit is inclined 15.5˚ relative to Saturn’s equator. The reason for this steep inclination is not well understood.

And, oh, the view! Iapetus is the perfect perch to view Saturn’s rings, as it orbits Saturn every 79.3 days in its steeply inclined orbit.

Saturn from Iapetus at the highest point of its inclined orbit

Mystery #2: Iapetus has the largest albedo dichotomy in the solar system. Why? Iapetus is locked in synchronous rotation as it orbits around Saturn, with the leading hemisphere ten times darker than its trailing hemisphere.

Iapetus has an average visual magnitude of 10.2 west of Saturn and 11.9 east of Saturn. Its albedo ranges from 0.5 to 0.05. (Diagram not to scale)
Bright and dark material on Iapetus. The 500-km-wide crater Engelier is at bottom.

It is thought that the natural state of the Iapetian surface is the bright icy part, with the dark material a thin veneer, less than a meter thick.

Mystery #3: Iapetus has a shape consistent with a body spinning every ~16 hours and yet its rotation period is 79.3 days, and it has a prominent ridge that can be followed 3/4 of the way around the equator.

Walnut-shaped Iapetus with its prominent equatorial ridge
Iapetus’ equator-girdling ridge, up to 20 km high, is heavily cratered and therefore ancient

The surface of Iapetus is heavily cratered, indicating it is very old. Could two comparable-sized objects have collided almost head-on billions of years ago to form Iapetus?

Mountainous terrain along Iapetus’ equatorial ridge imaged by the Cassini spacecraft during its closest flyby on September 10, 2007

As beautiful as spacecraft flyby and orbital images are of Iapetus and the many other interesting moons in our solar system, can you imagine what vistas await us once we start exploring their surfaces with rovers? Anticipation of these images and scientific discoveries surely is an impetus to explore the surface of Iapetus (and other moons) sooner rather than later.

Dark and light material on Iapetus was imaged up close by the Cassini spacecraft during its September 10, 2007 flyby.
Sizes of Iapetus, Earth’s moon, and Earth compared


Bonnefoy, Léa E., Jean-François Lestrade, Emmanuel Lellouch, Alice Le Gall, Cédric Leyrat, Nicolas Ponthieu, and Bilal Ladjelate. “Probing the subsurface of the two faces of Iapetus.” arXiv preprint arXiv:1911.03394 (2019).

Leleu, Adrien, Martin Jutzi, and Martin Rubin. “The peculiar shapes of Saturn’s small inner moons as evidence of mergers of similar-sized moonlets.” Nature astronomy 2, no. 7 (2018): 555-561.

Rivera-Valentin, Edgard G., Amy C. Barr, EJ Lopez Garcia, Michelle R. Kirchoff, and Paul M. Schenk. “Constraints on planetesimal disk mass from the cratering record and equatorial ridge on Iapetus.” The Astrophysical Journal 792, no. 2 (2014): 127.

Largest Satellites of Our Solar System

Here is a table of the 12 largest satellites in our solar system.  In addition to the size of each satellite, its home planet, its median distance from that planet, and discovery information, its median distance from its home planet is given in terms of the median lunar distance from the Earth.  Remarkably, Pluto’s moon Charon is just 0.05 lunar distances from Pluto, only 19,591 km.  Only one other of the largest satellites orbits closer to its home planet than the Moon orbits around the Earth, and that is Neptune’s moon Triton at 92% of the Earth-Moon distance.  At the other end of the scale, Saturn’s moon Iapetus orbits Saturn over nine times further away than the Moon orbits the Earth.

Now let’s look at the orbital eccentricity of each of the largest moons, and the orbital inclination relative to the equator of its home planet.

Our familiar Moon is really an oddball: it has the greatest orbital eccentricity of all the largest satellites, and, with the exception of Triton and Iapetus, by far the greatest orbital inclination relative to the equator of its home planet.  Triton is the oddball among oddballs as it is the only large satellite in our solar system that has a retrograde orbit: it orbits Neptune in a direction opposite the planet’s rotation.  Iapetus has an orbital inclination relative to Saturn’s equator almost as much as the Moon’s orbital inclination relative to the Earth’s equator, but this anomaly can perhaps be forgiven because Iapetus orbits so very far away from Saturn.  Its orbital period is over 79 days.

Note that the Moon’s orbital inclination relative to the equator of the Earth varies between 18.33˚ and 28.60˚.  This occurs because the intersection between the plane of the Moon’s orbit around the Earth and the plane of the Earth’s orbit around the Sun precesses westward, making an entire circuit every 18.6 years.





The Moon








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

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!