Saturn鈥檚 moon Iapetus resembles a walnut, with a ridge 20 kilometres high running around its centre. A new model suggests it could be made of rubble from the collapse of a former ring. And data from NASA鈥檚 Cassini spacecraft could confirm it.
Iapetus鈥� mountain range runs 1400 kilometres around the equator of the tiny moon. Scaled to Earth proportions, its peaks would reach over 100 kilometres high. Astronomers initially thought internal geological processes could have pushed the ridge up, but their models struggled to replicate what the Cassini spacecraft saw as it flew by in 2004.
Recent alternative theories suggest impact debris. Material kicked up by moon impacts generally falls straight back down or gets sent into orbit around the host planet. However, Iapetus鈥� orbits Saturn at a distance of more than 13 million kilometres, which means the moon鈥檚 own gravity could hold on to debris and force it into a ring. If destabilised, falling ring material could form a thin equatorial surface ridge. But there鈥檚 a problem.
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鈥淯sually impacts make a crater. Here we need to build topography.鈥� says at Johns Hopkins University in Baltimore, Maryland. Craters would certainly be the most common result of impacts from objects unbound to Iapetus鈥檚 gravity 鈥� like meteorites or bumped off disk material from Saturn 鈥� which would often come in at steep angles to the surface, causing maximum damage.
A slow spiral
Falling ring material doesn鈥檛 behave like impacting asteroids or meteorites. Trapped inside the moon鈥檚 gravity, the falling ring particles would slowly spiral downwards over several orbits. Roberts and his colleague wondered if the resulting lower angle impacts could allow ring debris to build up instead of digging out Iapetus鈥檚 surface.
Stickle simulated the impacts of icy blocks 1 metre to 1 kilometre in diameter over various shallow angles. Their results show that falling ring material would rarely make deep craters. Also, because impactors generally survive these glancing impacts, ring material piles up. 鈥淵ou get little traffic jams of debris as each bumps up against previous impacts,鈥� says Stickle.
鈥淚t isn鈥檛 a final demonstration by any means, but I can鈥檛 imagine another physically plausible scenario鈥� says at Washington University in St. Louis, Missouri, who wasn鈥檛 involved in the work.
Roberts now wants to mine Cassini data for associated geological features to seek further confirmation for their model. Such a heavy load sitting on the surface of Iapetus should cause the icy crust to bulge upwards on each side of it, which may be visible in the images Cassini took.
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