Dissecting intermediate-sized Kuiper belt objects

Cryovolcanism, the act of bringing subsurface liquid to the surface, has been invoked to explain observations of Charon and Quaoar. Charon, with its highly reflective surface, likely represents a class of Kuiper Belt Objects (KBOs) with relatively young surfaces. The water ice on Charon is known to be crystalline. Its formation requires the ice was heated to greater than 100 K in the past. This indicates that Charon's surface undergoes changes. The same feature has been detected on KBOs, including Quaoar, Orcus, 2002 TX300. Also seen in Charon's spectrum is ammonia hydrate, an ice composed of ammonia intimately mixed with water. This feature appears to shift in wavelength, indicating different fractions of ammonia and water are present. Crystalline ice and ammonia hydrate are both removed due to cosmic ray bombardment. Ammonia hydrate is also present on Quaoar, and possibly Orcus. Studies of crystalline water ice indicate small abundances of energies can disrupt the crystalline structure, and convert it to amorphous water ice. This constrains the lifetime of crystalline water ice between 1.5 Myr for cosmic rays, and 0.03-0.05 Myrs for solar UV/visible light. Mechanisms for surface renewal are examined. It is found that cryovolcanism is the only mechanism which can accommodate such short timescales. Cryovolcanism can only take place if the body has differentiated. Time-evolutionary models show that starting from a homogeneous mixture of rock and ice, radiogenic heating from potassium-40 will allow partial differentiation to take place. Over time, the interior assembles itself into a rocky core, a liquid/ice mantle and an undifferentiated rocky-icy crust. The liquid layer at an initial ammonia to water ratio of 5% will increase in ammonia content as the water fraction is frozen out. Models show that a liquid ocean can be present on Charon, Quaoar, Orcus and 2002 TX300 to this day, suggesting cryovolcanism may be at work. If true, the Kuiper belt may harbor more liquid water than found on Earth.