Thanks to New Horizon’s epic jaunt to the ends of the Solar System, new discoveries about Pluto are seemingly endless. A new study eschews this trend and instead shines some light on Charon, one of its five moons. As the preprint on arXiv reveals, the enormous network of canyons that mark its equatorial region may have been formed by a gargantuan double-whammy of contraction that spanned the entire moon.

Charon surprised researchers with its remarkably youthful surface, implying it was – and still may be – geologically active, just like Pluto. It’s pockmarked with unusual features, including the enigmatic, vast collection of canyons, termed “chasmata,” that stretch around its equator.

One of them, Serenity Chasma, is several kilometers deep and up to 60 kilometers (37 miles) wide. Like the other chasmata, Serenity crisscrosses through a series of high-elevation features, and it’s clear that, at some point in the past, huge faults have pushed around vast bits of Charon to form this complex network.

Ever since it was first spotted, the origin of this so-called "tectonic belt" has eluded planetary geologists. A team of researchers led by Uri Malamud from the Israel Institute of Technology has attempted to solve this conundrum by conjuring up a mathematical model describing Charon’s early evolution, and as it turns out, this mystifying feature is likely to be pretty ancient.

They began with the assumption that Charon started out as a cold icy ball containing an evenly distributed mixture of rock and ice. Radioactive elements would have existed within the rocky parts, which release heat as they decay.

^{A close-up of the tectonic belt on Charon. NASA/JHUAPL/SwRI}

This heating mechanism on Earth provides half of the heat emerging from its depths, helping to drive plate tectonics and volcanism. On Charon, the heat generated by these unstable elements would have initially raised the temperature of the ice. Warm ice is able to contract, and under the gravitational field of Charon, this is exactly what it did – the entire planet would have shrunk in size.

Within just the first few million years of its formation, this moon-wide ice shrinkage would have created vast cracks and faults at its surface, according to the researchers. This contraction accelerated when newly liberated free water was subsequently absorbed by the rocks. The upper crust of Charon remained icy throughout as it’s too far from the radioactive decay to experience any melting. Consequently, as the inner segments of the moon shrink, the surface ice resists and begins violently cracking.

Later on, ice freezing within Charon’s depths caused the moon to expand outwards, generating high-relief features at the surface. Later still, the ongoing radioactive decay heated up the moon’s core to the point where the rocks themselves became warm enough to experience contraction under the moon’s gravitational field.

This second contractive phase generated more fissures, faults, and fractures at the surface. Further along in time, the core had heated up and expanded to a point wherein Charon as a whole increased in size yet again, but only slightly this time.

All in all, this two-step contraction-expansion mechanism explains the epic features seen in Charon’s tectonic belt. However, if you’re wondering why this belt runs along the equator, as opposed to anywhere else, you’re not alone: The authors of the study aren’t certain themselves. So for now, the reasons behind this positioning remains a tantalizing mystery.