Of all the craters on Pluto’s moon Charon, this one is unlike the others. The bright green marks a unique splash of frozen ammonia at a concentration higher than any other crater examined in detail on the massive moon. But does that mean it’s the youngest?
The abnormal crater is on the Pluto-facing side of Charon, within the region imaged in detail by the New Horizons spacecraft when it soared past this summer. Although the crater—informally named Organa after Princess Leia in Star Wars—is approximately the same size as the nearby Skywalker Crater (named in tribute to her brother Luke from the same story) at roughly 5 kilometers (3 miles) across, its composition is entirely different.
In optical light, both craters are surrounded by rays of ejecta, wisps of brighter material radiating out of the central crater. The ejecta from Organa is slightly darker, but the real difference shows up when examining the craters in infrared. Unlikely Skywalker (and every other crater examined so far on Charon) which is rich in water ice, Organa is exceptionally bright in the 2.2 micron wavelength. This is indicative of frozen ammonia in concentrations higher than seen anywhere else on the moon so far. The splash of ammonia appears to extend beyond the ejecta visible in optical light.
Theorizing about possible explanations for the compositional difference between Organa and Skywalker, New Horizons Composition team lead Will Grundy mused:
“Why are these two similar-looking and similar-sized craters, so near to each other, so compositionally distinct? We have various ideas when it comes to the ammonia in Organa. The crater could be younger, or perhaps the impact that created it hit a pocket of ammonia-rich subsurface ice. Alternatively, maybe Organa’s impactor delivered its own ammonia.”
If this is indicative, it could go a long way towards explaining why the surface of Charon is weirdly smooth and young. Deputy lead for the Geology, Geophysics and Imaging team Bill McKinnon explains:
“Concentrated ammonia is a powerful antifreeze on icy worlds, and if the ammonia really is from Charon’s interior, it could help explain the formation of Charon’s surface by cryovolcanism, via the eruption of cold, ammonia-water magmas.”
If Charon’s interior is loaded down with ammonia, it could help sustain a subsurface ocean produced during a long-ago collision that would’ve otherwise frozen solid in the intervening years.
Although the New Horizons spacecraft made its closest approach of Pluto and Charon last summer, it collected so much data that it will take until autumn of next year for us to receive it all. We’re going to continuing learning about strange landforms, odd chemical distributions, and even the particle distribution of Pluto’s odd comet-like trailing atmosphere for months to come. That, and get the occasional drop-dead gorgeous photograph!
Meanwhile, the spacecraft is completing orbital corrections to reorient itself to head to a second Kuiper Belt Object to continue its exploration of the outer solar system.