NASA’s New Horizons mission that flew past Pluto and its moons in 2015 revealed some exciting discoveries: first, that Pluto was, in fact, geologically active and not the cold, dead world some thought it might be. Second, Pluto’s moon Charon shows evidence of being geologically active in the past. Pluto and Charon are the only large Kuiper belt objects (KBOs) that have been visited by spacecraft. This makes the data we obtained for these two bodies extremely important because not only can we learn information about Pluto and Charon but we can additionally learn about processes that may be occurring on other large KBOs we have yet to visit.
Charon is a fascinating world whose surface shows evidence of an eventful past. When you look at images of Charon, a few main features stand out: the moon has a distinct ridge across the equator, an oddly smooth region in the southern hemisphere, and perhaps the most noticeable feature: a north, dark red polar cap. Based on the data taken from the New Horizons spacecraft, we know what this polar cap of Charon is made of: processed hydrocarbons, i.e. methane that has been processed into the substance we can see (also known as tholins).
Charon, as pictured by the New Horizons spacecraft.
Charon’s red polar cap raises the question: how did this feature form? One hypothesis states that the methane comes from outside Charon: from Pluto’s escaping atmosphere. In our paper, we explored another hypothesis as to how this polar cap could have formed: with methane from the inside of Charon, released at the surface through cryovolcanism. “Cryovolcanism” can be generally thought of as volcanism, but occurring on icy bodies, with water and ice mostly composing what we think of as “lava” with some other components mixed in, instead of molten rock. Charon’s southern hemisphere (seen in the image above) is covered by a smooth region called Vulcan Planitia. At some point in Charon’s past, it experienced an episode of cryovolcanism that resurfaced much of the southern hemisphere, creating Vulcan Planitia. In our work, we predict that this episode of cryovolcanism probably released some methane gas onto Charon’s surface, similar to how large eruptions on Earth release carbon dioxide or other gases. We wanted to know how much methane could have been incorporated into this cryoflow, and the first step in estimating this number is to determine how large this flow actually was. We estimated exactly how much methane this eruption could have released through looking at geologic features on Charon’s surface and estimating the overall volume of the cryovolcanic eruption. We found that the eruption could have released as much as 1015 kg of methane, which is over 1 trillion tons of methane, onto Charon’s surface! That would fill over 400 million Olympic-sized swimming pools with methane ice.
After we found a methane estimate, we wanted to test how much, if any, of this methane would make it to Charon’s pole. To do this, we wrote a model that tracked methane particles as they ‘hopped’ across Charon’s surface to determine their end destination, i.e. if they would get to a latitude cold enough where the methane says put as ice, or if the methane escapes to space and is lost from Charon permanently. What we found was that most methane would end up migrating to Charon’s poles, where the temperature is then cold enough for this methane to stay in place as ice and get processed by radiation, creating the red-colored north pole that we observe in images.
We also wanted to know what would happen to methane supplied by cryovolcanism on other KBOs that are much, much farther away from the sun. We selected Sedna as a candidate to model surface temperatures, because this object orbits at an extreme distance from the sun (at its closest, Sedna is only at 76 AU as compared to Pluto and Charon’s 39 AU). Our models show that even at the equator of Sedna, temperatures are low enough year-round that the whole body acts as a sort of ‘cold trap’. In other words, it’s so cold that methane on Sedna’s surface will stay put in the same place it was deposited as ice. This is important because many KBOs, including Sedna, show evidence of methane or methane products on their surfaces, and many do not contain a larger nearby object like Pluto to draw methane from. Instead, it could be that methane makes it to these KBO surfaces from their interiors through cryovolcanism, like we suggest happened on Charon. This means that cryovolcanism could be a very important process in the Kuiper belt – it could be the reason why we observe so much methane and methane products on KBO surfaces!