Tag Archives: Iapetus

Iapetus – the black-and-white walnut Moon

Iapetus is the third largest moon of Saturn, with a radius of about 42% that of our moon, and a mass that weighs up to only 2.5%. But Iapetus has a number of shocking features, unique throughout the entire solar system.

The Walnut Moon

walnut moon

This is the equatorial ridge that  runs along the center of Cassini Regio; in case you were wondering, the ridge has an average height of 13 km, occasionally going up to 20km, a length of 1,300 km and a width of about 20 km. It was discovered when the Cassini spacecraft imaged Iapetus on December 31, 2004, and even in 2013, it has astronomers baffled. Why did it form and why does it follow the equator so closely, no one knows for sure, though there are three competing theories:

1. The ridge could be a remnant of the oblate shape of the young Iapetus, when it was rotating more rapidly than it does today, but if this were to be true, for reasons you can read here, this would mean that the moon was formed much earlier than previously believed.

2. The ridge could be icy material that welled up from beneath the surface and then solidified – this hypothesis requires that rotational axis would have been driven to its current position by the ridge.

3. Iapetus could have had a ring system during its formation and the ridge is actually an accretion of that material. However, the ridge appears too solid to be the result of a collapsed ring, and images show tectonic faults crossing the ridge, so this seems highly unlikely.

The likelihood of any theory is not striking, and the matter is still a hot topic among some astronomic circles.

Black and white

iapetus bright

In the 17th century, Giovanni Cassini observed that he could see Iapetus only on the west side of Saturn and he could never see it on the east. So Cassini, the bright man that he was, deducted that Iapetus is locked in synchronous rotation about Saturn (much like the Moon is to the Earth) and that one side is darker than the other. Centuries passed until actual images confirmed this man’s deduction, but Cassini wasn’t able to explain why this is.

The difference is striking – the leading hemisphere and sides are dark while most of the trailing hemisphere and poles are bright. To make it even more puzzling, when the Cassini spacecraft passed at only 1640 km from Iapetus, it showed that both hemispheres are heavily cratered.

Now, NASA researchers believe the dark material is a lag (residue) from the sublimation (evaporatio

iapetus darkn) of water ice on the surface of Iapetus, possibly darkened by exposure to sunlight. Another possibility is that the front hemisphere is gathering brighter matter, much like your windshield gathers most of the dust, leaving the back of the car cleaner.


Iapetus has been imaged multiple times from moderate distances by the Cassini orbiter, but due to its distance to Saturn, close observation remains difficult; no near future flybys are planned, but a viewing opportunity will be possible 2 years from now.

A giant landslide on Iapetus reaches halfway across a 75-mile (120 kilometer) impact crater.(c) NASA/JPL/Space Science Institute

Giant landslides on Saturn’s icy moon intrigues scientists

A giant landslide on Iapetus reaches halfway across a 75-mile (120 kilometer) impact crater.(c) NASA/JPL/Space Science Institute

A giant landslide on Iapetus reaches halfway across a 75-mile (120 kilometer) impact crater.(c) NASA/JPL/Space Science Institute

Planetary scientist Kelsi Singer initially studied satellite photographs of  Saturn’s icy moon Iapetus‘ surface looking for stress fractures in the moon’s ice, what she found in process however was far more interesting. Huge landslides, stretching across tens of miles across the moon’s surface were observed, not in one, but multiple locations, hinting this is a common phenomenon on the ice covered satellite. Very broad landslides have been recorded on Earth as well, although nowhere near this magnitude, and the study at hand might serve to hint towards the mechanisms involved in these natural formations.

Iapetus is one of the oddest cosmic bodies in the solar system. Barren, cold and mostly covered in very thick ice, the satellite presents a highly rugged terrain, with ridges that can reach as much as 12 miles in height or two times the altitude of Mount Everest. Like the ubiquitous yin-yang, the moon’s surface is half covered in darkness, while the other side is much brighter. Moreover, it has the most eccentric geometry out of all the solar system’s planets or moons, made evident by a mountainous ridge that bulges out at its equator – this is why it’s commonly referred to as the “walnut”.

Iapentu's eccentric topography

Iapentu’s eccentric topography

Because of this incredibly odd topography, Iapetus  has more giant landslides than any Solar System body other than Mars. So far, evidence of 30 massive landslides have been found – 17 along crater walls and 13 along the giant equatorial ridge, however even more might be encountered if an exhaustive observation were to be performed.

“Not only is the moon out-of-round, but the giant impact basins are very deep, and there’s this great mountain ridge that’s 20km (12 miles) high, far higher than Mount Everest,” explained Prof William McKinnon, also from Washington University,.

“So there’s a lot of topography and it’s just sitting around, and then, from time to time, it gives way.”

The icy landslides are similar to long-runout landslides on Earth known as sturzstroms (German for fallstreams) – massive landslides can move up 20 to 30 times the height they fall from. Typically, on Earth, conventional landslides only travel around two times the height they fall from.

Apparently, the mechanism that governs the formation of these massive landslides, on Iapetus or here on Earth, has yet to reach an consensus from scientists. Various theories have been suggested from  riding on a cushion of trapped air, to sliding on groundwater or mud, to sliding on ice, or slipping caused by strong acoustic vibrations.

According to Singer, a graduate student in earth and planetary sciences at Washington University in St. Louis and lead author of the paper presently discussed, the massive landslides most likely  occur by frictional heating of the ice. Since it doesn’t have an atmosphere, the coefficient of friction – a measure of how much the slip-sliding of material in a landslide tends to slow it down – on Iapetus is far lower than expected for ice.

Despite the ice on Iapetus is as a solid as rock, scientists hypothesize that  tiny contact points between bits of ice debris in such a landslide may heat up considerably,leading to a thin layer of ice crystals that melts. This might cause the huge landslides on Saturn’s icy moon, but could also serve to explain how sturzstroms form on Earth.

“The landslides on Iapetus are a planet-scale experiment that we cannot do in a laboratory or observe on Earth,” Ms Singer said.

“They give us examples of giant landslides in ice, instead of rock, with a different gravity, and no atmosphere. So any theory of long-runout landslides on Earth must also work for avalanches on Iapetus.”

The findings were reported in the journal  Nature Geoscience