Fresh Eyes for an Old Moon: ALMA and JWST Perspectives of Callisto

Citation

Camarca, Maria Noel (2025) Fresh Eyes for an Old Moon: ALMA and JWST Perspectives of Callisto. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/y6t5-en36. https://resolver.caltech.edu/CaltechTHESIS:05282025-033216413

Abstract

As though its surface were frozen in time, Jupiter's moon Callisto has seemingly done little more than collect and degrade impact features since its formation some ~4.5 billion years ago. One outcome of Callisto’s quiescence is that its geologic map retains only a few units, with large-scale landforms consisting of either enormous multi-ring impact basins or crater-laden plains. Despite this geologic simplicity, our knowledgebase of Callisto’s material surface properties and volatile ice distributions is limited compared to the other icy Galilean moons. Understanding how Callisto localizes its thermal properties and delicate volatile ices is essential to understanding how long-term particle bombardment, solar insolation, and extended impact damage has sculpted its aged surface. While Callisto’s much more active sibling satellites have scrubbed some or all of their surfaces free of the most ancient records, the oldest surface processes in the Galilean system remain visible today on Callisto. And with large telescope facilities such as the Atacama Large Millimeter/submillimeter Array (ALMA) and the James Web Space Telescope, these surface properties are now accessible to Earth-based observers.

In Chapters 2 and 3 of my dissertation, I complete the icy Galilean satellite ALMA catalogue with the full Callisto dataset which includes leading and trailing hemisphere images at ALMA Bands 7, 6, and 3, corresponding to 343, 223, and 97 GHz, respectively. At these frequencies, we sample the subsurface depths of order a few centimeters down to about half a meter. From these data, I demonstrate that Callisto’s’ subsurface thermal emission is much less susceptible to diurnal variation compared to the other icy satellites and that while Callisto’s largest craters are thermally consistent with much smaller ones, the warm surface anomalies tell a story of impact bombardment not recorded in current geologic maps. Moreover, I identify several cold anomalies associated with large impacts, as well as one that might be relevant to Callisto's tenuous and patchy CO₂ atmosphere.

In Chapter 4 of my dissertation, I present the results from a JWST NIRSpec 2.85–5.35 micron observing campaign that allowed us to inspect many of Callisto's volatile surface materials for the first time since the end of the Galileo mission in the early 2000s. In this work, I identify the Lofn/Heimdall impact region as Callisto's largest source of non-radiolytic CO₂. This particular crater suite may represent one of the best locations on Callisto to look for deep subsurface materials brought to the surface by the impact. Additionally, I propose Callisto's well-known radiolytic CO₂ trailing hemisphere bullseye is accompanied by a second bullseye in water ice exposure that may share a common origin.

Lastly, in Chapter 5, I offer a brief synthesis of the icy moon ALMA survey, an endeavor that fulfills scientific promises that pre-date the array itself.

Altogether, this dissertation offers the community two of the key Callisto datasets of the 2020s era of research. Now that ESA’s JUICE mission and NASA's Europa Clipper are en-route to the Jovian system, this research offers a timely complement to what is a blossoming era for Callisto and broader icy satellite exploration.

Thesis Files