Idiosyncratic Iapetus

Saturnian moon puts a time stamp on the outer solar system

Iapetus, the third-largest and second-farthest-out of Saturn’s satellites, is the weirdest moon in the solar system. One half of it is as bright as snow, the other as black as charcoal. Neither spherical nor ellipsoidal, as most moons are, Iapetus looks like a walnut, with a bulging waistline and squashed poles. Accentuating its nutty appearance is a narrow, 20-kilometer-high ridge that girdles most of the moon’s equator, like the brim of a hat. No other moon in the solar system has such a ridge.

Saturn’s strange moon Iapetus, imaged by the Cassini spacecraft late in 2004, has a ridge running along its equator. This feature and others of the walnut-shaped moon suggest that Iapetus has been frozen in time, giving astronomers a glimpse into the outer solar system’s early development. SSI, JPL/NASA

INFRARED IAPETUS. Temperature map of Iapetus, both color coded and in a gray scale image. The latter is equivalent to what the eye would see if it were sensitive to radiation at an infrared wavelength of 15 micrometers. Red indicates the warmest temperature, blue the coldest. The highest temperature, 130 kelvins at noon at the equator, occurs within the dark material that covers most of this view of the moon and which may be the warmest place in the Saturnian system. JPL/GSFC/NASA

DARK SIDE. Composite portrait of Iapetus, combining near-infrared, visible light, and ultraviolet images, shows the northern part of the dark Cassini Regio region and the transition to a brighter surface at high northern latitudes. This image doesn’t show Iapetus’ north pole or the moon’s bright hemisphere. Space Science Institute, JPL/NASA

Now astronomers think that they may have cracked the mystery of this walnut moon. Iapetus’ odd shape and isolated location, they say, suggest that its evolution came to an abrupt halt just a few hundred million years after it came into being. If that’s so, Iapetus may serve as a well-preserved relic from the early days of the solar system, not long after the planets were born.

Flash-frozen as it appeared several billion years ago, this distant moon “may be the Rosetta stone of the outer solar system,” says Dennis Matson of NASA’s Jet Propulsion Laboratory (JPL) in Pasadena, Calif.

Misshapen moon

Matson, Julie Castillo-Rogez of JPL, and their colleagues were prompted to develop a new model for the formation of Iapetus after studying images from the Cassini spacecraft, which flew past the moon in 2005. Those images revealed Iapetus’ bulging waistline and equatorial ridge—a chain of mountains that wraps at least halfway around the moon’s 2,600-km-long midsection.

The odd features surprised planetary scientists. A rapidly rotating moon or planet with a sufficiently pliant crust can develop a bulge because centrifugal forces push material out around the equator and flatten the object at its poles. Astronomers have known for more than 2 decades, however, that Iapetus rotates extremely slowly, taking 79 Earth days to complete a single revolution.

To develop the bulge seen today, Iapetus in its youth must have spun much faster, initially rotating about once every 10 hours and then decreasing to a 16-hour period, Matson’s team calculates. But for Iapetus to have preserved that shape to this day, the moon’s rotation must have drastically slowed and its crust must have cooled and thickened within 100 million to 900 million years after the satellite’s birth. Those two processes, moreover, would have had to proceed in a precisely choreographed dance, in which timing was everything, says Matson. Only in that way could the moon have preserved its youthful, bulging figure, the researchers assert in the September Icarus.

“Iapetus literally stopped in its tracks,” says Castillo-Rogez.

The ridge, which may encircle the entire moon, provides additional evidence that Iapetus froze in place relatively soon after it coalesced, says Matson. The chain of mountains is peppered with craters, indicating that it’s an ancient structure that has endured bombardment by space debris for several billion years. To preserve a structure this large and long, Iapetus’ once-pliable crust must have solidified quickly. Otherwise, the ridge would have slumped away long ago, like a scoop of ice cream melting on a warm day.

The second part of the Iapetus storyline focuses on the forces that conspired to slow the moon’s rotation. Iapetus experiences tides, induced by the difference in Saturn’s gravitational tug on the moon’s near and far sides. Tidal stresses would have acted as a brake on the moon’s spin, just as the tides raised by Earth have slowed its moon’s rotation.

But the colder and more rigid a body is, the weaker those tidal forces are. To slow Iapetus to its present spin rate, Saturn’s tidal forces must have acted when the moon’s interior was nearly warm enough to melt water ice.

“The shape corresponds to that of a fluid body spinning every 16 hours, so something must have heated its interior” even while its surface became rigid, notes planetary scientist Joseph Burns of Cornell University.

Hot times

What could have heated Iapetus? Whatever provided the extra warmth had to have done so only briefly. Otherwise, the moon’s equatorial bulge would have continued to diminish, rather than to have been frozen in place. “We searched and searched” for just the right mechanism, says Matson.

The researchers propose that the geologically brief infusion of heat came from the rocks that formed Iapetus. In particular, the team suggests that the energy released by the radioactive decay of two short-lived radioisotopes—aluminum-26 and, to a lesser extent, iron-60—within those rocks kept the moon warm for some hundreds of thousands of years. Then the radioactivity petered out, the heat from the rocks declined, and the moon froze, preserving its immature figure.

Because these isotopes decay at known rates, the planetary scientists can calculate when Iapetus formed. For the decay of aluminum-26 to have kept the moon’s surface pliable enough to allow tidal slowing of its rotation, the moon must have coalesced between 2.5 million and 5 million years after the solar system’s first objects—its most primitive asteroids—started forming.

If the theory proves correct, it would be the first time that scientists could compare the early chronology of an object in the outer solar system with that of a body in the inner solar system, where asteroids were born.

“This model is pretty improbable since it requires quite special timings [for the formation of the moon and other events], but Iapetus itself is pretty improbable,” says Burns. “If the authors are correct, we’ve learned a lot about conditions in the early years in Saturn’s neighborhood.”

The odd moon is “a remarkably primordial body,” says William McKinnon of Washington University in St. Louis. “We could be looking at the [formation] epoch when we look at its surface. Most satellites have been so heavily bombarded since the very earliest times that some of the moons have even broken up or reassembled.” But in part because Iapetus formed far from Saturn, and so hasn’t been heavily pummeled, its surface appearance seems to have been preserved, McKinnon says.

All in the timing

Saturn and its moons formed simultaneously, theorists argue, because all these objects coalesced from the same region within a cloud of gas, dust, and ice that swaddled the young sun. Likewise, most theorists believe that although Jupiter arose from a part of that cloud far closer to the infant sun than did Saturn, the two planets came into existence at about the same time. Dating Iapetus’ beginnings therefore pegs Jupiter’s formation to 2.5 million to 5 million years after the birth of the solar system.

That time frame is consistent with the formation of Jupiter’s large outer moon, Callisto, says McKinnon. Callisto consists of a well-mixed amalgam of ice and rock. The relatively uniform composition implies that the moon didn’t experience heating severe enough to have melted the ice, which would have allowed water to collect and refreeze into a distinct layer above the rock. That means Callisto could not have coalesced until 2 million years after the formation of the solar system, the time it took for temperatures to fall to freezing at Jupiter’s location.

A decade ago, the leading model of planet formation would have had a hard time making a large planet in 5 million years or less, notes theorist Jack Lissauer of NASA’s Ames Research Center in Mountain View, Calif. In that model, known as core accretion, Saturn and Jupiter began as solid, rocky objects with about 10 times Earth’s mass. Those objects would have coalesced from particles in the disk that swirled around the young sun. Only later did those cores capture enough gas to create the giant planets seen today.

Researchers initially proposed that planet formation in the core-accretion model could take about 10 million years. But faced with a multitude of data indicating that the protoplanetary disks that surround young stars last only a few million years, the scientists have now fine-tuned the model to make Saturn and Jupiter faster.

A newer, competing model, in which these gas giants form all at once from the sudden fragmentation of the protoplanetary disk, could make these behemoths even quicker, notes Alan Boss of the Carnegie Institution of Washington (D.C.), who developed this model. However, with core-accretion theorists inching closer to accepting a more-rapid time frame for planet formation, “I expect that the core-accretion folks would not be terribly concerned” with Iapetus’ apparently quick creation, he says.

Ridge riddle

It’s still unclear how Iapetus’ ridge was created. Although Matson, Castillo-Rogez, and their colleagues didn’t develop their model to explain the ridge, their theory provides a possible solution. As the moon’s rotation slowed, the centrifugal force pushing material outward at the equator diminished and the still-warm interior of Iapetus took on a more spherical shape. But the surface material, which had already frozen, couldn’t adjust. Instead, the crust cracked and piled up, forming the ridge.

“The spherical shape has less area than the [bulge],” says Matson. “The extra yardage has to go somewhere,” namely, sticking out on the surface.

Last year, in Geophysical Research Letters, Wing-Huen Ip of the National Central University in Chun Li, Taiwan, outlined an alternative model. He suggested that the equatorial ridge was created when icy rings that may have initially surrounded Iapetus collapsed onto the moon.

The rings could have arisen in either of two ways, Ip says. Because of its mass, Iapetus wields substantial gravitational influence on its surroundings. As it began to coalesce from ice and dust, Iapetus might have formed its own system of rings from surrounding material that didn’t become part of the moon itself.

Alternatively, says Ip, rings could have arisen after a massive asteroid or other body struck a still-forming Iapetus, knocking off debris that temporarily lined up in a circle around the moon. He adds that a separate satellite, which has since escaped, may have accompanied the temporary ring. In a similar manner, researchers have proposed that Earth’s moon formed when a Mars-size projectile struck the young Earth.

Regardless of how the rings formed, they would have orbited Iapetus’ equator. And if they fell back onto the moon, they would naturally have fallen along that line.

Competing ridge theories may be put to the test next September, when the Cassini mission is scheduled to fly within 1,000 km of Iapetus. So far, the craft has imaged only about half of Iapetus, and researchers are uncertain whether the ridge there goes much beyond what’s been photographed so far. If Ip is correct, the ridge ought to wrap around the moon’s entire equator, since his model predicts a uniform ring all the way around, notes McKinnon. Debris from the ring would have fallen all along the planet’s midsection, he adds. A ridge that doesn’t follow the equator exactly might favor Matson’s model instead, McKinnon says.

Whatever the solution of the ridge puzzle, observations of this strange satellite may prove a gold mine for planetary scientists. “The weirdness of Iapetus could be giving us a view into the distant past, just like the Rosetta stone did,” Burns says.

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