When the Cassini spacecraft first photographed Saturn’s moon Iapetus on 31st of December 2004, astronomers were hoping to find out more about the moon’s dramatic two tone colouring, first observed by Giovanni Cassini more than 400 years earlier.

Instead, the images sent back by the spacecraft revealed something much more astonishing. Around this moon’s equator there is a dramatic mountain range some 1300 kilometres long, 20 kilometres wide and 13 kilometres high. There is nothing like it anywhere else in the Solar System.

That raises an obvious question: how could such a ridge have formed?

There are two possibilities. The first is that volcanic or tectonic forces caused the surface to warp. In other words, that the mountains are the result of internal forces.

The problem with this is that there ought to be other signs of volcanic or tectonic activity. But planetary geologists have yet to see any evidence of this.

The second explanation is that the mountains are the result of external, or exogenic, forces. In other words, the material from which these mountains are made somehow fell from space.

If this second explanation is correct, the sides of the mountains should be roughly at the angle of repose, the steepest angle that a granular material can adopt before it avalanches. So an important question is what shape are these mountains.

Today we get an answer thanks to the work of Erika Lopez Garcia at Brown University in Providence, Rhode Island, and a few pals. These guys have used data from Cassini images to reconstruct the shape of the mountains and work out once and for all whether they could indeed have fallen from space.

Iapetus is Saturn’s third-largest satellite. It is about half the radius of our Moon, tidally locked so the same side always faces Saturn and perhaps best-known for its two-tone colouring.

But it’s the mysterious equatorial ridge that astronomers puzzle over most. Garcia and co use three-dimensional digital maps of Iapetus that have been generated using images taken by the Cassini spacecraft.

This process can be done in two ways. The first is to use two different photos of the same area to create a stereo image that provides 3D information. The second is known as photoclinometry, which uses shadows and light direction to determine the topography of the surface.

This process has generated eight 3-D maps of the mountain range that Garcia and co have then studied to work out the shape of the mountains.

They say these peaks can be classified into six different types: triangular, trapezoidal, crowned, twinned, dissimilar and saddle.

The most common are triangular and these also have the steepest slopes. Garcia and co-suggest that they are probably the mountains that have been least modified by meteor impacts, landslides and so on.

The average slope of these peaks is about 15° but can be as much as 39°. That’s significant because the angle of repose can be anything between about 25° to 40° depending on size and type of particles involved. “Slopes on triangular ridge faces are close to the angle of repose,” conclude Garcia and co.

Although this is not proof that these mountains fell from space, it is certainly evidence in favour of this idea. “This may point to an exogenic origin for the ridge,” they say.

So how might a mountain range have fallen from space? The current thinking is that early in its life, Iapetus was hit by another moon which catapulted a huge volume of rubble into orbit, rather like the collision that formed our Moon.

Some of this ejecta recombined to form a new body that escaped into space. The rest formed a ring around Iapetus that must have been unstable.

Over time, this ring spiralled in towards the moon eventually depositing the material on the surface. The result is the narrow, steep mountain range that astronomers now observe.

It now turns out that the morphology of these mountains is entirely compatible with this theory. And the collision might also account fro the moon’s lopsided orbit too. Not quite a slamdunk but certainly the best theory astronomers have for the moment.

Incidentally, the moon’s two–tone colouring has also been explained, at least in theory. The thinking here is that over time, ice on one side of Iapetus has sublimated more quickly than on the other, leaving a dark residue. This warms more quickly increasing the rate of sublimation and making it even darker.

At the same time, the ice condensed on the other side of the moon, making it lighter. The result is the two-tone marking we see today.

Cassini next encounter with Iapetus is in 2015 which will give astronomers a chance to gather more data on these mysteries. Gradually, this moon is giving up its secrets.

Ref: arxiv.org/abs/1404.2337 : Topographic Constraints on the Origin of the Equatorial Ridge on Iapetus