A burst of radioactivity warmed and softened Saturn's moon Iapetus soon after it formed, allowing it to be moulded into its walnut-like shape, a new study by NASA astronomers has revealed.

Iapetus has a broad bulge around its equator capped by a narrow ridge, giving it the appearance of a walnut.

Scientists have been puzzling over how it acquired its distinctive shape since they first discovered the ridge in 2004 in images from the Cassini spacecraft.

Now, a team led by Julie Castillo of NASA's Jet Propulsion Laboratory in Pasadena, California, US, has found that the moon was warmer and more malleable in its early days.

'If it also spun more quickly back then than it does now, it would have bulged out at its equator as a result. When the moon's rotation later slowed, it would have become more spherical in shape. But by that time, it was colder and stiffer - too rigid to mould itself into a smooth shape. As a result, the process left behind a distinctive ridge of material at the equator,' the researchers wrote in their study.

'Iapetus spun fast, froze young, and left behind a body with lasting curves,' said Castillo.

Castillo and her team further found that a short burst of radioactivity provided the crucial heat needed during its formative years to warm it and consequently give it the ubiquitous walnut shape.

According to their findings, Iapetus was born with an abundance of radioactive material in the form of isotopes like aluminium-26 and iron-60.

These materials - which may have been injected into the embryonic solar system by a nearby supernova - quickly broke down into other elements, and their rapid decay could have provided a burst of heat to soften Iapetus soon after it formed, she said.

In order to get enough heat from this process to explain its early softening, Iapetus would have had to have formed during a narrow window in the solar system's early history, when these short-lived radioactive materials are thought to have been abundant, New Scientist quoted her as saying.

Castillo's team have now dated the formation of Iapetus to a 2.5 million-year period between about 4.565 and 4.562 billion years ago.