Project 6

It is commonly assumed that the deep interior of a comet nucleus preserves a chemical inventory intact from the time of its formation in the
protoplanetary nebula. This is justified because when comets approach the Sun, material ablates from the nucleus much faster than the thermal wave can penetrate into its interior, so interior material remains pristinely cold until it is abruptly spewed into the comet’s coma. However, before they become Jupiter family comets, these objects can spend millions of years as Centaurs in the middle solar system, slowly raising their deep interior temperatures above what they had been during their cold storage phase in the Kuiper belt. This Centaur phase has the potential to drive internal migration and/or escape of the more volatile constituents, altering the body’s internal distribution of materials and even its bulk composition. This evolution has not been studied much owing to the challenges of modeling volatile migration in the Knudsen flow regime as well as the paucity of laboratory data for relevant materials and temperatures. This is especially true for mixtures of materials which can behave very differently from pure substances. The goal of this project is to study the fundamental properties of cometary volatile mixtures in the temperature and pressure regime relevant to evolution through the Centaur phase. This will be done by means of laboratory studies in the Astrophysical Materials Laboratory at Northern Arizona University.

The student will perform laboratory studies of sublimation into vacuum of mixtures of cometary volatiles at low temperatures and pressures relevant to objects transitioning from the Kuiper belt to short period comets. The student will do this by vapor depositing materials onto a quartz crystal microbalance housed within an ultra-high vacuum chamber and cooled to extremely low temperatures with a closed cycle helium refrigerator. They will collect spectral and sublimation flux data and process the data to obtain measurements of temperature dependent vapor pressures, enthalpy of sublimation, and solid state diffusion coefficients. These parameters are the essential inputs to models of the evolution of volatiles in the interiors of primitive bodies responding to temperature changes resulting from orbital evolution. We estimate that substantial progress can be made with 10 hours per week FTE over two semesters by a qualified undergraduate.

The student will present results at the spring 2026 SpaceGrant symposium. We also expect the results to be worthy of publication in the peer-reviewed scientific literature and the student would be invited to participate in authoring such publications at a level consistent with their interest (we have had undergraduates lead scientific papers before, but most are more comfortable in a co-author role). We are collaborating with several theoreticians who are interested in incorporating new laboratory measurements into their numerical models of volatile transport within migrating small bodies, which could open additional paths to publication and/or future research proposals.