Maybe it has something to do with the sweltering heat these days, but it seems that every time I turn around, someone else is talking about ice volcanoes. First it was Saturn's moon Enceladus. Then two other Saturnian moons had them: Tethys and Dione. Now Pluto's largest moon, Charon, might be spewing frozen geysers.
The latest result comes courtesy of a group of astronomers who pointed the 8-meter Frederick C. Gillett telescope at Gemini Observatory toward the famous dwarf planet's satellite. Using Gemini, Jason Cook (Southwest Research Institute, Colorado) and Steven Desch (Arizona State University) captured some of the best spectra of Charon to date.
Quick mini-lecture about ice. In the solar system, water ice comes in two flavors: crystalline and amorphous. Crystalline is fresh (like what you find in your iced coffee), and it only turns amorphous (or old) after being bombarded with cosmic rays and solar radiation. You can make old ice fresh again with heat — such as by hitting it with meteorites — but most folks would expect ice in the outer solar system to be amorphous.
And that's what made Charon so wacky. Past observations revealed that it's completely coated with crystalline ice. The whole moon somehow needed to be resurfaced with ice every 100,000 years. But the impact environment out there isn't ferocious enough to "freshen" ice that quickly. The ice needed to come from somewhere else.
Turns out that it's all about antifreeze. In the press release and a paper in Astrophysical Journal the group touts "the best evidence yet for the existence of ammonia hydrates on Kuiper Belt objects." Just like on Earth, ammonia acts as antifreeze and depresses the melting point of the water.
Putting the pieces together, heat from decaying radioactive elements deep inside Charon warm the subsurface ice. In time the ice melts (thanks to the presence of the ammonia hydrates) and it oozes out cracks in the frozen surface. If that happens under pressure, you get ice volcanoes.
Snow cones anyone?