When electrons from Jupiter's rapidly rotating magnetosphere collide with the thin atmospheres of the Galilean satellite\u2014Io, Europa, Ganymede and Callisto\u2014they create faint auroras like Earth's northern and southern lights. However, reflected sunlight from their icy surfaces overwhelms these faint auroral emissions at visible wavelengths. Fortunately, the satellites transit Jupiter's large shadow once per synodic orbit. When Jupiter is near elongation, we can view these transits from Earth and observe the satellites' faint auroral emissions in the absence of reflected sunlight. Over the past five years, we have used the High Resolution Echelle Spectrometer (HIRES) on the Keck I telescope at the summit of Maunakea to observe the optical wavelength auroras of these four satellites during the eclipse phase of their orbits. My analysis of the auroras and the modulation of their brightnesses over five- to ten-minute timescales has provided new insight into the composition and density of their atmospheres and how they interact with the local plasma environment.
\r\n\r\n\r\nAt Io, I found 13 auroral emissions not previously detected. I compared these detections to optical wavelength images taken through a variety of narrowband filters during the Cassini flyby to understand the morphology of the different auroral emissions. Using an auroral emission model, I showed that the oxygen emissions were consistent with electron impact on an atmosphere composed of O, SO and SO\u2082, disagreeing with prior studies which posited the oxygen emissions came from electron impact on its extended atomic oxygen corona. The brightest oxygen emissions did not vary with ambient upstream electron density, suggesting the electron flux into Io's atmosphere is more complex.
\r\n\r\n\r\nAt Europa, the oxygen emissions varied systematically with the upstream electron density, indicating a simpler electron precipitation process than that at Io. This allowed me to use the variability in the auroral emissions as a proxy to understand variability in Europa's plasma environment. I evaluated how the brightnesses changed on short timescales during single eclipses as well as systematically on longer timescales over the four years we made the observations. I validated the results by comparing to in situ measurements taken during the Galileo mission. I also considered the timescale under which a sublimated water atmosphere would need to freeze back onto the surface as Europa passed into Jupiter's shadow.
\r\n\r\n\r\nAt Ganymede, I showed the brightness had a hemispheric asymmetry that correlated with the geometry between the satellite and the centrifugal equator of Jupiter's magnetosphere. The brightness ratio between the northern and southern hemispheres suggested that its auroras are triggered by the bounce motion of electrons along Jovian magnetic field lines rather than by ram flux from the upstream plasma population. I compared the results to model simulations of Ganymede's O\u2082 and H\u2082O atmospheres at different times during the eclipse to understand the detectability of a sublimated water atmosphere which, like with Europa, would need to collapse rapidly at the onset of the eclipse.
\r\n\r\n\r\nFinally, at Callisto, detections of auroral emission have been challenging because of the very low magnetospheric electron densities at its orbital distance from Jupiter. However, through careful data processing I was able to retrieve brightnesses of the 630.0 nm [O I] emission from individual spectra, allowing for the first time-series analysis of Callisto's aurora. Like Europa, I found the brightness modulated with distance from the centrifugal equator as expected for excitation by the upstream ram electron flux. This indicated the absence of a substantial ionosphere and supported the paradigm that the ionosphere exists only over the trailing hemisphere when it is illuminated by sunlight. I did not detect enough emission lines to perform the atmospheric compositional analysis I did with the other satellites, but I forward-modeled the expected brightness of the O, O\u2082, H\u2082O and CO\u2082 atmospheres inferred or measured by other studies and found agreement with the aurora brightnesses and upper limits I observed with HIRES.
", "doi": "10.7907/8ymw-rj97", "publication_date": "2026", "thesis_type": "phd", "thesis_year": "2026" }, { "id": "thesis:18616", "collection": "thesis", "collection_id": "18616", "cite_using_url": "https://resolver.caltech.edu/CaltechTHESIS:05242026-035435474", "primary_object_url": { "basename": "marlin thesis.pdf", "content": "final", "filesize": 36379175, "license": "other", "mime_type": "application/pdf", "url": "/18616/1/marlin thesis.pdf", "version": "v5.0.0" }, "type": "thesis", "title": "From Ocean Floor to Stratosphere: Investigating Astrobiologically Relevant Processes and Molecules on Solar System Bodies", "author": [ { "family_name": "Marlin", "given_name": "Theresa", "orcid": "0009-0003-0670-5474", "clpid": "Marlin-Theresa" } ], "thesis_advisor": [ { "family_name": "de Kleer", "given_name": "Katherine R.", "orcid": "0000-0002-9068-3428", "clpid": "de-Kleer-K-R" }, { "family_name": "Cable", "given_name": "Morgan L.", "orcid": "0000-0002-3680-302X", "clpid": "Cable-Morgan-Leigh" } ], "thesis_committee": [ { "family_name": "Newman", "given_name": "Dianne K.", "orcid": "0000-0003-1647-1918", "clpid": "Newman-D-K" }, { "family_name": "Knutson", "given_name": "Heather A.", "orcid": "0000-0002-5375-4725", "clpid": "Knutson-H-A" }, { "family_name": "de Kleer", "given_name": "Katherine R.", "orcid": "0000-0002-9068-3428", "clpid": "de-Kleer-K-R" }, { "family_name": "Cable", "given_name": "Morgan L.", "orcid": "0000-0002-3680-302X", "clpid": "Cable-Morgan-Leigh" } ], "local_group": [ { "literal": "div_gps" } ], "abstract": "In astrobiology, the quest to understand the origin of life on Earth is often paired with an outward focus on whether life can be found elsewhere in the universe. The most accessible corner of the universe in which to search for extraterrestrial life is our own Solar System \u2014- the planets, their satellites, and the minor bodies present throughout. An examination of the essential elements for life on Earth returns three critical factors: liquid water, complex chemistry, and a source of energy. Searching for these three elements in bodies beyond Earth returns a suite of candidates: the ocean worlds of the outer Solar System. These moons, while too cold to harbor liquid water on their surfaces, are thought to have subsurface liquid water reservoirs which could potentially harbor life. Of particular note are two Saturnian satellites: Enceladus, with predicted hydrothermal activity and water-rock interactions, and\r\nTitan, with a dense atmosphere filled with complex organics with potential prebiotic significance. This thesis broadly addresses astrobiological questions on Earth, Enceladus, and Titan via laboratory chemistry and observational astronomy.
\r\n\r\n(I): Chemical gardens as analogs for hydrothermal vents on ocean worlds. This chapter probes laboratory-synthesized analogs of hydrothermal vents as reaction catalysts with alpha keto acids as reactants under various conditions. NMR spectroscopy results support the formation of prebiotically-relevant molecules including glycine (an amino acid) and maloyl formate (a precursor to alpha keto glutarate (AKG)).
\r\n\r\n(II): Propyne: determination of physical properties and unit cell parameters under Titan-relevant conditions. Many questions remain about the organic species that are photochemically produced in Titan\u2019s atmosphere. Propyne (CH3CCH) has been detected in gas phase and is a candidate for Titan\u2019s mixed-species ice clouds. This work presents a characterization of solid propyne using Raman spectroscopy, infrared spectroscopy, and X-ray diffraction.
\r\n\r\n(III):Zonal winds in Titan\u2019s middle atmosphere from a stellar occultation observed with Keck adaptive optics. Winds in Titan\u2019s atmosphere vary seasonally, and stellar occultations present a unique chance to probe them. This work presents spatially-resolved images of the refracted \u201clightspots\" visible around Titan\u2019s limb during an occultation, and discusses the zonal wind profile which best matches the observed data.
", "doi": "10.7907/dcap-p838", "publication_date": "2026", "thesis_type": "phd", "thesis_year": "2026" }, { "id": "thesis:17281", "collection": "thesis", "collection_id": "17281", "cite_using_url": "https://resolver.caltech.edu/CaltechTHESIS:05282025-033216413", "type": "thesis", "title": "Fresh Eyes for an Old Moon: ALMA and JWST Perspectives of Callisto", "author": [ { "family_name": "Camarca", "given_name": "Maria Noel", "orcid": "0000-0003-3887-4080", "clpid": "Camarca-Maria-Noel" } ], "thesis_advisor": [ { "family_name": "de Kleer", "given_name": "Katherine R.", "orcid": "0000-0002-9068-3428", "clpid": "de-Kleer-K-R" } ], "thesis_committee": [ { "family_name": "Blake", "given_name": "Geoffrey A.", "orcid": "0000-0003-0787-1610", "clpid": "Blake-G-A" }, { "family_name": "Ehlmann", "given_name": "Bethany L.", "orcid": "0000-0002-2745-3240", "clpid": "Ehlmann-B-L" }, { "family_name": "Hallinan", "given_name": "Gregg W.", "orcid": "0000-0002-7083-4049", "clpid": "Hallinan-G-W" }, { "family_name": "de Kleer", "given_name": "Katherine R.", "orcid": "0000-0002-9068-3428", "clpid": "de-Kleer-K-R" } ], "local_group": [ { "literal": "div_gps" } ], "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\u2019s 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\u2019s 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\u2019s 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.
\r\n \r\nIn 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\u2019s\u2019 subsurface thermal emission is much less susceptible to diurnal variation compared to the other icy satellites and that while Callisto\u2019s 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 CO\u2082 atmosphere.
\r\n \r\nIn Chapter 4 of my dissertation, I present the results from a JWST NIRSpec 2.85\u20135.35 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 CO\u2082. 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 CO\u2082 trailing hemisphere bullseye is accompanied by a second bullseye in water ice exposure that may share a common origin.
\r\n\r\nLastly, 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.
\r\n \r\nAltogether, this dissertation offers the community two of the key Callisto datasets of the 2020s era of research. Now that ESA\u2019s 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.
", "doi": "10.7907/y6t5-en36", "publication_date": "2025", "thesis_type": "phd", "thesis_year": "2025" }, { "id": "thesis:17382", "collection": "thesis", "collection_id": "17382", "cite_using_url": "https://resolver.caltech.edu/CaltechTHESIS:06022025-233903333", "primary_object_url": { "basename": "Seeger_Christina_2025_Thesis.pdf", "content": "final", "filesize": 70932931, "license": "other", "mime_type": "application/pdf", "url": "/17382/1/Seeger_Christina_2025_Thesis.pdf", "version": "v6.0.0" }, "type": "thesis", "title": "Surface Evolution on Basaltic Bodies: Tectonic, Geomorphic, and Diagenetic Modification on Io and Mars", "author": [ { "family_name": "Seeger", "given_name": "Christina Hope", "orcid": "0000-0003-4993-9724", "clpid": "Seeger-Christina-Hope" } ], "thesis_advisor": [ { "family_name": "Grotzinger", "given_name": "John P.", "orcid": "0000-0001-9324-1257", "clpid": "Grotzinger-J-P" }, { "family_name": "de Kleer", "given_name": "Katherine R.", "orcid": "0000-0002-9068-3428", "clpid": "de-Kleer-K-R" } ], "thesis_committee": [ { "family_name": "Lamb", "given_name": "Michael P.", "orcid": "0000-0002-5701-0504", "clpid": "Lamb-M-P" }, { "family_name": "Ehlmann", "given_name": "Bethany L.", "orcid": "0000-0002-2745-3240", "clpid": "Ehlmann-B-L" }, { "family_name": "Grotzinger", "given_name": "John P.", "orcid": "0000-0001-9324-1257", "clpid": "Grotzinger-J-P" }, { "family_name": "de Kleer", "given_name": "Katherine R.", "orcid": "0000-0002-9068-3428", "clpid": "de-Kleer-K-R" } ], "local_group": [ { "literal": "div_gps" } ], "abstract": "All planets and moons in the Solar System evolve over geologic timescales, though the processes affecting each body vary widely depending on gravity, atmosphere thickness and composition, volcanic activity, and perhaps most importantly, the presence of a hydrologic cycle. This dissertation investigates the surface evolution of two basaltic bodies in our Solar System: one that has barely changed in 3.5 billion years, and one that changes almost daily. Jupiter\u2019s moon Io is continually resurfaced by large-scale volcanic eruptions of low-viscosity lava and sulfur dioxide gas, driven by interior heating generated by diurnal tidal stresses. Such tidal stresses have been linked to eruptive activity and tectonic ridge formation on other moons like Titan and Europa; while they strongly influence Io, they are orders of magnitude weaker than the crustal subsidence stresses which control the expression of tectonic features on the surface (kilometers-tall mountains and caldera-like volcanic features called paterae). Chapter 2 investigates whether tidal stresses may have any influence on the formation of mountains and paterae. Though no global trends have been identified, I suggest that local correlations between patera orientations and the large volcanic center of Loki Patera may provide insight into the magma plumbing pathways of this unique volcano. As soon as tectonic mountains are uplifted on Io, they are subject to gravity- and seismicity-driven erosional processes tearing them down. In Chapter 3, I present the first regional geologic map of a trio of mountains named Cocytus Montes and identify a new geologic unit\u2014a blocky deposit composed of kilometer-scale slab-shaped blocks of crust\u2014that are visible thanks to the favorable resolution and near-terminator lighting conditions of new Junocam imagery. I explore several new erosional mechanisms for Io that could create these blocks, determining regolith creep-modified cliff collapse to be the most likely. The orders of magnitude higher resolution imagery collected by the Mars Science Laboratory Curiosity rover provides a backdrop for much closer analysis of how sediments moved, deposited, lithified, and were subsequently modified by diagenetic fluids on ancient Mars. Chapter 4 takes advantage of hand-sample scale data to categorize a diverse array of diagenetic fabrics (nodules, pits, color variations) that correlate with the stratigraphy in a region defined by a transition from clay-bearing rocks to sulfate-bearing rocks. I present several hypotheses to explain how the Mg sulfate detected in these nodules and pore-filling cements may have precipitated at depth, to complement current evaporite-driven models. These hypotheses could be tested in the coming years of Mars exploration by the rover, and will provide insights into the longevity of a groundwater system after surface water ceased to flow on ancient Mars. Overall, this work explores the well-studied terrestrial processes of surface modification, degradation, and diagenesis under distinctly alien conditions throughout the Solar System.", "doi": "10.7907/4mfp-zx56", "publication_date": "2025", "thesis_type": "phd", "thesis_year": "2025" } ]