In this vintage piece, Patrick Moore offers fascinating information on Saturn’s peculiar black-and-white satellite.

Iapetus, the most distant of Saturn’s main satellites, has been a mystery since its discovery by Giovanni Cassini on 25 October 1671.

A portion of its surface is dazzling and frosty, yet another portion is as dark as a chalkboard.

Cassini made two conclusions, one correct and the other incorrect.

Cassini discovered that Iapetus was a straightforward telescopic target when it was in its orbit to the west of Saturn, but he was unable to see it when it migrated to the east of Saturn.

The orbital period of Iapetus is 79 days, therefore Cassini lost contact with it for several weeks.

He reasoned correctly that Iapetus must have hemispheres of differing albedo and that the rotation period must be synchronous (i.e. equal to the orbital period) such that the same hemisphere must always face Saturn; while west of Saturn, the bright hemisphere was facing us.

Cassini ultimately found Iapetus at eastern elongation in 1705 using a much more powerful telescope; the magnitude was barely around +13.0.

I can track its orbit with my 15-inch reflector, but when it is east of Saturn, it is fainter than Cassini’s other three discoveries, Rhea, Tethys, and Dione.

The average distance between Iapetus and the other main satellites is 3.5 million kilometres.

In contrast to the inner satellites, the orbital inclination is 15.5 degrees relative to Saturn’s equator.

The globe is triaxial (ellipsoid) with dimensions of 747x749x713 kilometres with an equatorial bulge; its form has been compared to a walnut.

White on black or black on white?

The first puzzle to be solved (the “zebra problem”) was quite simple.

Iapetus was either a brilliant sphere with a black stain or a dark sphere with a white deposit.

Studies of the movement revealed that the density was low, thus the globe must be composed primarily of ice and rock; nevertheless, the nature of the black deposit remained unknown.

Carl Sagan even thought it was biological; yet, was it thick or thin?

Cassini, which was launched in 1997 and orbits Saturn and sends back astoundingly comprehensive data, resolved this issue.

Iapetus’s ring

WH Pickering found Iapetus and Phoebe, a much more distant Saturnian moon, in 1898. The Voyager spacecraft, despite its success, revealed nothing about Iapetus and Phoebe.

Nonetheless, Cassini returned sufficient photos to generate a map of Iapetus.

The term given to the dark area on the leading hemisphere is Cassini Regio.

In the trailing hemisphere, the brilliant area became Roncevaux Terra north of the equator and Saragossa Terra south of the equator.

Both are rugged and cratered, with the Turgis crater being about 600 kilometres in diameter.

The probe also spotted a new ring rotating in Phoebe’s orbit; it had been overlooked previously since it mostly emits infrared radiation and is exceedingly difficult to detect at visible wavelengths.

This clarified the “zebra issue.” Black dust dispersed from the ring migrates inward and stains the leading hemisphere of Iapetus.

Keep in mind that Phoebe has retrograde velocity, and so must the particles in her ring.

Iapetus, like all other large satellites, is prograde. This indicates that the incoming ring particles collide almost directly with the leading hemisphere of the Iapetus.

At the centre of Cassini Regio, the probe observed a massive ridge 1300 kilometres long, 20 kilometres broad, and 13 kilometres tall.

There are peaks that rise 20 kilometres above the ground.

Not along the border between Cassini Regio and Roncevaux Terra, where there are solitary summits, but along the equator.

Uncertain is the origin of the ridge that extends halfway around the moon.

There have been speculations that it is material from a collapsed ring, either of Saturn or of Iapetus itself, but this is pure speculation and we must recognise that the origin of the ridge remains unknown.

Notably, Cassini Regio is 15°C warmer than Roncevaux Terra and, at its hottest, may reach a sweltering –143°C. The black covering cannot be more than 12 centimetres thick.

Exploring Iapetus

Currently, the concept of sending crewed expeditions to Iapetus is science fiction.

The radiation problem is simply one of the numerous dangers that must be faced, and we have no clue how to cope with it; our Saturnian explorers will have to spend months in space unprotected.

It is true that Saturn’s radiation belts are smaller than Jupiter’s, and that Iapetus orbits outside of the most hazardous one, but this is of no use on the outbound voyage.

It is also likely that when (or if) Iapetus is ever tried, it will fall to third position in terms of importance.

Titan, which is bigger than Mercury and has a thick atmosphere and lakes of liquid methane, must be at the top of the list.

I consider Enceladus to be the most peculiar planet in the entire Solar System.

Its diameter is about 50 kilometres, less than the greatest crater on Iapetus, yet it is highly active, with water geysers erupting from fractures in the crust.

There must be a warm subsurface ocean; no other explanation seems plausible, but we do not know how it got there or how it is continually supplied.

I am confident that Titan and Enceladus will be discovered first. Next, it will be Iapetus’s turn.

The travellers to Iapetus will have one significant benefit.

The other large satellites orbit in the plane of the rings, such that an observer from one of these satellites would always view the rings edge-on.

Iapetus is unique. Its orbit is well inclined to the ring plane, allowing our astronauts to observe the rings as they appear as a majestic arch in the Iapetan sky.

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