The surfaces of icy satellites preserve a complex record of the processes that shape their formation and evolution, reflecting a combination of endogenic emplacement, exogenic delivery, and irradiation-driven modification. In this thesis, I use multi-wavelength spectroscopy to disentangle these competing processes and assess the extent to which surface compositions can be used to infer interior chemistry and system history across the outer solar system, from Jupiter to Neptune.
\r\n\r\nChapters 2 and 3 focus on Europa, whose surface may provide a window to the subsurface ocean composition, but is strongly altered by irradiation. Using VLT/SINFONI observations, I show that a 2.07 \u03bcm absorption feature on Europa\u2019s trailing hemisphere correlates with irradiation patterns but not with large-scale geology, indicating a predominantly exogenic origin and challenging models that invoke radiolytically altered ocean-derived salts to explain this band. I then use Galileo/NIMS observations of the young Pwyll and Manann\u2019an craters to constrain the equilibrium timescale of Europa\u2019s radiolytic sulfur cycle, finding that hydrated sulfuric acid is depleted in these regions and that equilibrium is reached on timescales \u22732\u20134 Myr\u2014orders of magnitude longer than laboratory estimates.
\r\n\r\nIn Chapter 4, I present near-global ultraviolet\u2013visible spectral mapping of Callisto with HST/STIS. These data reveal absorptions plausibly associated with iron-bearing silicates, organics, and potentially NaCl, and show little evidence for SO2 or other sulfur-bearing species. The spatial distribution of these spectral features reveals significant compositional heterogeneity, including localized features associated with major impact basins, challenging suggestions that Callisto\u2019s dark material is dominated by a thick (\u223c100 m) blanket of irregular satellite dust. Instead, these observations indicate that the dark material reflects a complex mixture of endogenic material excavated by impacts, delivered exogenic material, and radiolytic alteration.
\r\n\r\nIn Chapter 5, I use JWST/NIRSpec observations to show that Neptune\u2019s ring-moons exhibit a 2.72 \u03bcm absorption diagnostic of Mg-phyllosilicates, indicating extensive aqueous alteration within larger precursor bodies. These results suggest that Neptune\u2019s present-day moons are reaccreted fragments of differentiated satellites disrupted during Triton\u2019s capture, providing direct access to deep interior material from icy outer solar system bodies.
", "doi": "10.7907/605z-1m76", "publication_date": "2026", "thesis_type": "phd", "thesis_year": "2026" }, { "id": "thesis:18581", "collection": "thesis", "collection_id": "18581", "cite_using_url": "https://resolver.caltech.edu/CaltechTHESIS:05152026-163508815", "primary_object_url": { "basename": "milby_zachariah_2026.pdf", "content": "final", "filesize": 22140762, "license": "other", "mime_type": "application/pdf", "url": "/18581/1/milby_zachariah_2026.pdf", "version": "v4.0.0" }, "type": "thesis", "title": "The Optical Aurora of Jupiter's Galilean Satellites; or, What They Do in the Shadows", "author": [ { "family_name": "Milby", "given_name": "Zachariah", "orcid": "0000-0001-5683-0095", "clpid": "Milby-Zachariah" } ], "thesis_advisor": [ { "family_name": "de Kleer", "given_name": "Katherine R.", "orcid": "0000-0002-9068-3428", "clpid": "de-Kleer-K-R" } ], "thesis_committee": [ { "family_name": "Knutson", "given_name": "Heather A.", "orcid": "0000-0002-5375-4725", "clpid": "Knutson-H-A" }, { "family_name": "Brown", "given_name": "Michael E.", "orcid": "0000-0002-8255-0545", "clpid": "Brown-M-E" }, { "family_name": "de Kleer", "given_name": "Katherine R.", "orcid": "0000-0002-9068-3428", "clpid": "de-Kleer-K-R" }, { "family_name": "Blake", "given_name": "Geoffrey A.", "orcid": "0000-0003-0787-1610", "clpid": "Blake-G-A" } ], "local_group": [ { "literal": "div_gps" } ], "abstract": "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:18617", "collection": "thesis", "collection_id": "18617", "cite_using_url": "https://resolver.caltech.edu/CaltechTHESIS:05252026-061937325", "type": "thesis", "title": "Remote Sensing Aqueous Surface Processes on Planets Red and Blue: From Chemical Weathering Mineralogy on Mars to Water Temperature in Alaska's River Corridors", "author": [ { "family_name": "Baker", "given_name": "Samantha Rose", "orcid": "0009-0001-7705-3261", "clpid": "Baker-Samantha-Rose" } ], "thesis_advisor": [ { "family_name": "Ehlmann", "given_name": "Bethany L.", "orcid": "0000-0002-2745-3240", "clpid": "Ehlmann-B-L" }, { "family_name": "Lamb", "given_name": "Michael P.", "orcid": "0000-0002-5701-0504", "clpid": "Lamb-M-P" } ], "thesis_committee": [ { "family_name": "de Kleer", "given_name": "Katherine R.", "orcid": "0000-0002-9068-3428", "clpid": "de-Kleer-K-R" }, { "family_name": "Frankenberg", "given_name": "Christian", "orcid": "0000-0002-0546-5857", "clpid": "Frankenberg-C" }, { "family_name": "Fischer", "given_name": "Woodward W.", "orcid": "0000-0002-8836-3054", "clpid": "Fischer-W-W" }, { "family_name": "Ehlmann", "given_name": "Bethany L.", "orcid": "0000-0002-2745-3240", "clpid": "Ehlmann-B-L" }, { "family_name": "Lamb", "given_name": "Michael P.", "orcid": "0000-0002-5701-0504", "clpid": "Lamb-M-P" } ], "local_group": [ { "literal": "div_gps" } ], "abstract": "Ever since satellites were first launched into space in the 1950s, they have been utilized for critical science on Earth and eventually, around other planets as well. In Chapters 2 and 3, we use hyperspectral imagery and other remotely sensed data to examine the formation of aluminum phyllosilicate on Mars. Al-phyllosilicates across Mars have been proposed to have formed from extensive water leaching of basalt, though regional-specific studies suggest diversity between Al-phyllosilicate-bearing regions. Considering this complexity, in Chapter 2, we reexamine Al-phyllosilicate and its associated minerals in the Nili Fossae region. Using hyperspectral imagery from the CRISM instrument, we used Gaussian fitting to automatically identify and map Al-phyllosilicate and its associated minerals. We find that in contrast to prior interpretations, the Al-phyllosilicate in Nili Fossae most likely formed from weathering of an aluminous volcanic ash rather than from extensive leaching of basalt. In Chapter 3, we expand this analysis to a global survey of 6 Al-phyllosilicate-bearing regions on Mars. We examine the composition and stratigraphy of each region to determine their weathering histories, and we compare the regions to place them in a global context. We find that most of the Al-phyllosilicate on Mars is more consistent with volcanic ash alteration rather than extensive basalt leaching, though some regions are more consistent with basalt leaching. We conclude that the Al-phyllosilicate on Mars did not all form from one process, and our results demonstrate the potential for less extensive alteration and more widespread ash-producing volcanism in Mars\u2019 history than previously thought. In addition to composition, remotely sensed data can also be used to measure temperature. In Chapter 4, we use surface temperature derived from Landsat satellite data to examine the spatial and seasonal patterns of river temperature in Arctic rivers and their surrounding floodplains. Water temperature in the Arctic is interwoven with numerous environmental processes and is responding rapidly to the changing climate, but it is poorly monitored. We demonstrate that the Landsat temperature dataset is accurate for application to Arctic river environments, and we generate and examine water temperature maps of the Yukon River watershed. With these, we show that satellite-based temperature measurements can and should be used to address critical environmental questions in this region.", "doi": "10.7907/vz5n-qn22", "publication_date": "2026", "thesis_type": "phd", "thesis_year": "2026" }, { "id": "thesis:17717", "collection": "thesis", "collection_id": "17717", "cite_using_url": "https://resolver.caltech.edu/CaltechTHESIS:10082025-035835147", "primary_object_url": { "basename": "Kanine_Thesis_revision_with_supplement_reduced.pdf", "content": "final", "filesize": 79234832, "license": "other", "mime_type": "application/pdf", "url": "/17717/3/Kanine_Thesis_revision_with_supplement_reduced.pdf", "version": "v8.0.0" }, "type": "thesis", "title": "A Tale of Two Craters: Reconstructing Mars Paleoenvironment Using Orbital and Rover Data at Endeavour and Jezero", "author": [ { "family_name": "Kanine", "given_name": "Oak Arden", "orcid": "0000-0002-9204-6107", "clpid": "Kanine-Oak-Arden" } ], "thesis_advisor": [ { "family_name": "Ehlmann", "given_name": "Bethany L.", "orcid": "0000-0002-2745-3240", "clpid": "Ehlmann-B-L" }, { "family_name": "Lamb", "given_name": "Michael P.", "orcid": "0000-0002-5701-0504", "clpid": "Lamb-M-P" }, { "family_name": "Grotzinger", "given_name": "John P.", "orcid": "0000-0001-9324-1257", "clpid": "Grotzinger-J-P" } ], "thesis_committee": [ { "family_name": "Asimow", "given_name": "Paul David", "orcid": "0000-0001-6025-8925", "clpid": "Asimow-P-D" }, { "family_name": "Ehlmann", "given_name": "Bethany L.", "orcid": "0000-0002-2745-3240", "clpid": "Ehlmann-B-L" }, { "family_name": "Lamb", "given_name": "Michael P.", "orcid": "0000-0002-5701-0504", "clpid": "Lamb-M-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": "Understanding the evolution of planetary climates and corresponding changes to planetary surfaces involves the study of terrains of various ages and morphologies. Here, I study Endeavour and Jezero, two craters within the martian Noachian Highlands. I use multiple approaches \u2014 including sedimentology, stratigraphy, geomorphology, numerical modeling, and the quantitative study of geologic structures in 3D \u2014 and a combination of orbital images from satellites and on-the-ground or in-situ images from rover-based instruments to conduct detailed and ground-truthed studies at both craters. At each site, I characterize the role of water in shaping the surface and thus constrain the climate of ancient Mars. Also presented in this work is a methodological approach to optimize the usage of rover and orbital images for three-dimensional orientation of geologic structures.
\r\n\r\nIn Chapter 2, I find that rockfall is a potential source of erosion on Endeavour\u2019s crater rim. Repeated events of large clasts bouncing downslope and eroding bedrock could have generated incised landforms in the absence of flowing liquid water. Boulder deposits atop other crater-infilling units indicate the process of rockfall continued during or after the Noachian-Hesperian boundary (~2.5-2.7 Ga), potentially continuing to shape the crater rim morphology after regional surface desiccation. In contrast, the Jezero crater strata at Kodiak butte discussed in Chapter 3 are interpreted as either preserved fluvial bars in a braided river or mouth bars in a shallow lake, indicating water was abundant in the crater. Gravel- sized grains were transported by energetic flows, with minimal quiescent settling of fine-grained sediment. Deposits of such settings may have a lower biosignature preservation potential than those of Gilbert deltas, the previous depositional hypothesis. In Chapter 4, I performed traces of beds of the Jezero delta-fan structure in HiRISE and then used apparent dips of a given feature seen from multiple perspectives in rover images to estimate true surface orientations. I confirmed that the orientations of scarps, beds, or other quasi-planar surfaces measured from the highest-available resolution orbital datasets are accurate. However, it is challenging to distinguish hierarchical stratigraphic elements from each other and from erosional surfaces using orbital data, and therefore rover data adds key context for depositional interpretations that inform paleoenviroment. In particular, bedform internal structures could only be determined from rover datasets. In all, through a synthesis of methodological approaches and datasets, I find that the two studied craters potentially record disparate predominant morphology-shaping forces \u2014 dry rockfall in Endeavour, and fluvio-lacustrine activity at Jezero. These sites provide spatial and temporal snapshots of past surface conditions on Mars that can be used to build a more complete narrative of the planet\u2019s history.
", "doi": "10.7907/dry1-3w54", "publication_date": "2026", "thesis_type": "phd", "thesis_year": "2026" }, { "id": "thesis:18660", "collection": "thesis", "collection_id": "18660", "cite_using_url": "https://resolver.caltech.edu/CaltechTHESIS:05282026-024019971", "primary_object_url": { "basename": "thesis_abigail_keebler_2026.pdf", "content": "final", "filesize": 9559989, "license": "other", "mime_type": "application/pdf", "url": "/18660/1/thesis_abigail_keebler_2026.pdf", "version": "v4.0.0" }, "type": "thesis", "title": "From Spectra to Mineralogy: a Remote Sensing Approach to Earth's Arid Dust Source Regions", "author": [ { "family_name": "Keebler", "given_name": "Abigail May", "orcid": "0000-0001-6358-9834", "clpid": "Keebler-Abigail-May" } ], "thesis_advisor": [ { "family_name": "Ehlmann", "given_name": "Bethany L.", "orcid": "0000-0002-2745-3240", "clpid": "Ehlmann-B-L" } ], "thesis_committee": [ { "family_name": "de Kleer", "given_name": "Katherine R.", "orcid": "0000-0002-9068-3428", "clpid": "de-Kleer-K-R" }, { "family_name": "Ehlmann", "given_name": "Bethany L.", "orcid": "0000-0002-2745-3240", "clpid": "Ehlmann-B-L" }, { "family_name": "Frankenberg", "given_name": "Christian", "orcid": "0000-0002-0546-5857", "clpid": "Frankenberg-Christian" }, { "family_name": "Thompson", "given_name": "David R.", "orcid": "0000-0003-1100-7550", "clpid": "Thompson-David-R" } ], "local_group": [ { "literal": "div_gps" } ], "abstract": "Mineral dust plays a critical role in Earth\u2019s climate and biogeochemical systems, influencing radiative forcing, cloud microphysics, and nutrient fertilization of terrestrial and marine ecosystems. These impacts are strongly dependent on dust mineralogy, particularly the abundance and speciation of iron-bearing phases, clays, and carbonates, which control both the optical properties and chemical reactivity of dust aerosols. Despite this importance, mineralogical properties of dust source regions remain poorly constrained at regional to global scales, limiting the representation of dust processes in Earth system models. Hyperspectral visible to shortwave infrared (VSWIR) remote sensing offers a promising pathway for addressing this gap by enabling spectrally resolved characterization of surface mineralogy globally. However, translating reflectance spectra into quantitative mineral abundances remains challenging due to the nonlinear nature of VSWIR spectra of intimate mineral mixtures. \r\nThis dissertation combines hyperspectral remote sensing, field spectroscopy, and laboratory analyses to improve quantitative interpretation of mineral dust source regions and to evaluate the extent to which mineralogical properties can be retrieved from VSWIR observations. The work is structured around three complementary studies that collectively link global satellite observations, field measurements, and empirical modeling approaches.\r\nFirst, a global analysis of arid dust source regions is conducted using hyperspectral observations from the Earth Surface Mineral Dust Source Investigation (EMIT) mission. A systematic sampling and filtering framework is developed to extract a representative dataset of bare soil reflectance spectra from more than one billion observations. The resulting dataset characterizes global variability in surface albedo and mineralogical absorption features across major dust source regions, revealing distinct regional spectral endmembers associated with differences in mineralogy, including bright iron oxide- and kaolinite-rich Saharan surfaces and darker clay- and carbonate-dominated Asian surfaces. These results demonstrate substantial compositional diversity in dust source regions and provide constraints on surface radiative properties relevant to Earth system modeling.\r\nSecond, we investigate the physical and compositional controls on spectral variability using a novel dataset of co-located in situ VNIR reflectance spectra and laboratory measurements of mineralogy, grain size, and iron speciation. We compare the spectral variability captured in this dataset to that of the EMIT global dust source dataset to evaluate the range of global variability represented. We assess compositional controls on spectral variation, including absorption feature presence, position, and strength, as well as overall reflectance and continuum shape. Clay and carbonate absorption features show systematic but non-linear relationships with mineral presence and abundance, reflecting overlapping absorptions and mixed-phase effects. In contrast, iron oxide abundance exhibits strong, approximately linear relationships with diagnostic absorption features in fine-grained clay-rich sediments. We also identify a distinct population of hematite-bearing sands that display strong absorption features despite low hematite abundance. Radiative transfer modeling shows that grain size and bright mineral matrices significantly modulate iron oxide spectral expression. Overall, these results highlight that composition and sediment physical context in control spectral variability.\r\nThird, the dissertation evaluates empirical approaches for predicting quantitative mineral abundances from VSWIR spectra. Using the coupled spectral-mineralogical dataset, a partial least squares regression model is trained to estimate mineral and iron species abundances. Results show that mineral components which control continuum shape and albedo over the full spectral range, including quartz, feldspar, and iron oxides, can be predicted accurately, while phases that express diagnostic feature in a narrow spectral range, such as clays and carbonates, are less-well predicted. Application of the calibrated models to EMIT reflectance data demonstrates that PLSR models successfully identify the major minerals present in the ground-truth data. Compared with EMIT mineralogy products, the empirical approach provides several advantages. In particular, the models improve quantitative retrievals of carbonate abundance, and more frequently resolve complex multi-mineral assemblages containing combinations of clays, carbonates, and evaporite minerals within individual spectra. The models produce reasonable values when applied to spectra from sediment types outside the training dataset, suggesting promising transferability across heterogeneous dust source environments. Together, these findings demonstrate both the potential and current limitations of empirical inversion approaches for hyperspectral mineral retrieval and highlight their utility as a complement to existing feature-based remote sensing frameworks.\r\nTogether, these results provide a framework for improving quantitative interpretation of hyperspectral observations of Earth\u2019s bare sediment surface. By linking global-scale satellite data with field-based measurements and empirical modeling, this work advances the ability to retrieve physically meaningful mineralogical information from VSWIR remote sensing. These improvements are essential for better constraining the radiative and biogeochemical impacts of mineral dust in Earth system models and for extending hyperspectral approaches to future Earth and planetary remote sensing missions.", "doi": "10.7907/ce3x-6f96", "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: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" }, { "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:16271", "collection": "thesis", "collection_id": "16271", "cite_using_url": "https://resolver.caltech.edu/CaltechTHESIS:01012024-070706394", "primary_object_url": { "basename": "Zhang_Qicheng_2024.pdf", "content": "final", "filesize": 18942341, "license": "cc_by_sa", "mime_type": "application/pdf", "url": "/16271/1/Zhang_Qicheng_2024.pdf", "version": "v4.0.0" }, "type": "thesis", "title": "Sampling the Evolution of Solar System Cometoids", "author": [ { "family_name": "Zhang", "given_name": "Qicheng", "orcid": "0000-0002-6702-191X", "clpid": "Zhang-Qicheng" } ], "thesis_advisor": [ { "family_name": "Hallinan", "given_name": "Gregg W.", "orcid": "0000-0002-7083-4049", "clpid": "Hallinan-G-W" } ], "thesis_committee": [ { "family_name": "Brown", "given_name": "Michael E.", "orcid": "0000-0002-8255-0545", "clpid": "Brown-M-E" }, { "family_name": "de Kleer", "given_name": "Katherine R.", "orcid": "0000-0002-9068-3428", "clpid": "de-Kleer-K-R" }, { "family_name": "Knutson", "given_name": "Heather A.", "orcid": "0000-0002-5375-4725", "clpid": "Knutson-H-A" }, { "family_name": "Hallinan", "given_name": "Gregg W.", "orcid": "0000-0002-7083-4049", "clpid": "Hallinan-G-W" } ], "local_group": [ { "literal": "div_gps" } ], "abstract": "Comets are commonly defined to be planetesimals visibly losing mass through volatile sublimation. In the solar system, such behavior characterizes but a brief stage in the overall evolution of these objects, as limited by their supply of accessible volatile materials and the often short dynamical lifetimes of orbits sufficiently near the Sun for said volatiles to actually volatilize. In this thesis, I explore the characteristics of several different types of ``cometoids''---planetesimals visibly exhibiting comet-like mass loss sometime in their recent past, present, or near future---in both the outer and inner solar system at different stages in their physical and dynamical evolution. I first use stellar occultations---or rather, the lack thereof detected---to constrain their abundance of kilometer-scale objects in the Kuiper Belt, from which many comets are sourced. I then evaluate how the optical brightness, color, and polarization of dust ejected by a classical, currently active comet changes when exposed to the space environment in order to probe the material properties of its nucleus. Finally, I investigate an otherwise ordinary but active asteroid to explore how intense solar heating as it passes very near the Sun can volatilize its rocky surface to produce bright sodium emission explaining its comet-like behavior.", "doi": "10.7907/mjje-pc39", "publication_date": "2024", "thesis_type": "phd", "thesis_year": "2024" }, { "id": "thesis:14605", "collection": "thesis", "collection_id": "14605", "cite_using_url": "https://resolver.caltech.edu/CaltechTHESIS:05252022-004039193", "primary_object_url": { "basename": "JiazhengLi Thesis_20220524.pdf", "content": "final", "filesize": 3336484, "license": "other", "mime_type": "application/pdf", "url": "/14605/2/JiazhengLi Thesis_20220524.pdf", "version": "v5.0.0" }, "type": "thesis", "title": "The Chemistry of Europa and Venus, and Characterization of Earth-Like Exoplanets", "author": [ { "family_name": "Li", "given_name": "Jiazheng", "orcid": "0000-0002-2563-6289", "clpid": "Li-Jiazheng" } ], "thesis_advisor": [ { "family_name": "Yung", "given_name": "Yuk L.", "orcid": "0000-0002-4263-2562", "clpid": "Yung-Y-L" } ], "thesis_committee": [ { "family_name": "de Kleer", "given_name": "Katherine R.", "orcid": "0000-0002-9068-3428", "clpid": "de-Kleer-K-R" }, { "family_name": "Ingersoll", "given_name": "Andrew P.", "orcid": "0000-0002-2035-9198", "clpid": "Ingersoll-A-P" }, { "family_name": "Brown", "given_name": "Michael E.", "orcid": "0000-0002-8255-0545", "clpid": "Brown-M-E" }, { "family_name": "Yung", "given_name": "Yuk L.", "orcid": "0000-0002-4263-2562", "clpid": "Yung-Y-L" } ], "local_group": [ { "literal": "div_gps" } ], "abstract": "This thesis contains three parts of work, including oxidant sources on Europa, sulfur chemistry on Venus, and the characterization of Earth-like exoplanets. In the first part, we build two chemical-transport models to study the various oxidant-generation processes that occur in both Europa\u2019s atmosphere and surface ice. The atmospheric model focuses on the role that water plumes play in the formation of Europa\u2019s ionosphere. The simulation results, which show that the ionization reactions are initiated by electron-impact ionization and photoionization of water and continued by charge transfer between water and oxygen molecules, have successfully reproduced the observations. This model has also been used to study the dissociation processes of water molecules from the plumes, which can be regarded as an alternative source for the oxygen in the atmosphere. The chemical-transport model on Europa\u2019s surface ice is built to simulate chemical processes occurring in the ice during irradiation by electrons and to describe how the chemical species of interest (oxidants) are formed, transported, and distributed in the ice. This model also has implications for the chemical composition of Europa\u2019s subsurface ocean. Since the availability of oxidants could be the limiting factor for biologically useful chemical energy on Europa, the proposed research may give us insight into Europa\u2019s habitability.
\r\n\r\nThe second part of this thesis focuses on the unknown ultraviolet (UV) absorber(s) in the atmosphere of Venus. Ever since the detection of the enigmatic ultraviolet absorption in the upper atmosphere of Venus, questions have been raised about the identity of the unknown UV-visible absorber(s) and how it is formed on Venus. Our recent photochemical modelling study suggests that SO dimers may not be the major UV absorber(s) in Venus\u2019 upper atmosphere. However, SO dimers are important intermediaries in the formation of more complex S species (e.g., Sn (n = 1 to 7)). Polysulfur aerosol, which is formed from the nucleation process of Sn (n = 1 to 7), is a possible candidate for the unknown UV absorber(s). In this work, we compute that the mixing ratio of polysulfur aerosol is ~1.76\u00d710-14 in the upper atmosphere. By putting the polysulfur aerosol into the Spectral Mapping Atmospheric Radiative Transfer model (SMART), we find that the simulated spectrum of Venus agrees well with the observations. This result provides useful constraints for unraveling the identity(ies) of the unknown UV-visible absorber(s) on Venus.
\r\n\r\nThe third part of this thesis is devoted to the characterization of Earth-like exoplanets. In this part, we study the glints, a possible phenomenon on Earth-like exoplanets and the rotation period detection for Earth-like exoplanets. Small flashes of reflected light\u2014called glints\u2014are found in images taken by spacecraft observing the Earth and occur due to specularly reflected solar radiation. These glints have been found over both ocean and land. Using Deep Space Climate Observatory observations, we show that glints over land are due to specular reflection off horizontally oriented ice platelets floating in the air, while glints over ocean have contributions from reflection off either platelets floating above the ocean or a relatively smooth ocean surface. We use a radiative transfer model to simulate different kinds of glints and to explore their properties. This technique of comparing observations of terrestrial glints with model simulations may provide new information relevant to atmospheric dynamics and the search for habitable exoplanets. A terrestrial planet\u2019s rotation period is one of the key parameters that determines its climate and habitability. Here we demonstrate that, under certain conditions, the rotation period of an Earth-like exoplanet will be detectable using direct-imaging techniques. We use a global climate model that includes clouds to simulate reflected starlight from an Earth-like exoplanet and show that the rotation period of an Earth-like exoplanet is detectable using visible-wavelength channels with time-series monitoring at a signal-to-noise ratio (S/N) >20 with \u223c5\u201315 rotation periods of data, while the rotation period of a planet with full ocean coverage is unlikely to be detectable. To better detect the rotation period, one needs to plan the observation so that each individual integration would yield a S/N >10, while keeping the integration time shorter than 1/6 to 1/4 of the rotation period of the planet. Our results provide important guidance for rotation period detection of Earth-like exoplanets in reflected light using future space telescopes.
", "doi": "10.7907/1q9j-y297", "publication_date": "2022", "thesis_type": "phd", "thesis_year": "2022" }, { "id": "thesis:14462", "collection": "thesis", "collection_id": "14462", "cite_using_url": "https://resolver.caltech.edu/CaltechTHESIS:01032022-154242981", "type": "thesis", "title": "High-Resolution Imaging of Chemistry in Extreme Interstellar Environments", "author": [ { "family_name": "Wilkins", "given_name": "Olivia Harper", "orcid": "0000-0001-7794-7639", "clpid": "Wilkins-Olivia-Harper" } ], "thesis_advisor": [ { "family_name": "Blake", "given_name": "Geoffrey A.", "orcid": "0000-0003-0787-1610", "clpid": "Blake-G-A" } ], "thesis_committee": [ { "family_name": "Okumura", "given_name": "Mitchio", "orcid": "0000-0001-6874-1137", "clpid": "Okumura-M" }, { "family_name": "Cushing", "given_name": "Scott K.", "orcid": "0000-0003-3538-2259", "clpid": "Cushing-Scott-K" }, { "family_name": "de Kleer", "given_name": "Katherine R.", "orcid": "0000-0002-9068-3428", "clpid": "de-Kleer-K-R" }, { "family_name": "Blake", "given_name": "Geoffrey A.", "orcid": "0000-0003-0787-1610", "clpid": "Blake-G-A" } ], "local_group": [ { "literal": "div_chem" } ], "abstract": "Advancements in imaging technologies have changed the ways in which we see and understand our chemical universe. Given the extreme distances between Earth, from which we observe the wider chemical universe, and the molecular clouds where stars are born, we cannot resolve the chemical structure of these interstellar laboratories on the same scales as we can with samples on Earth. Nevertheless, with the advent of larger and more sophisticated telescopes, such as the Atacama Large Millimeter/submillimeter Array (ALMA) in Chile, we can now look at interstellar chemistry on scales much smaller than the solar system. The research presented here uses ALMA to investigate the chemistry in (massive) star-forming regions in different parts of the Milky Way with high spatial resolution.
\r\n\r\nThis thesis first focuses on the nearby Orion Kleinmann-Low nebula (Orion KL), the closest (at about 400 parsecs away) massive star-forming region to us, at spatial scales smaller than those of typical planetary systems. Using methanol and methyl cyanide as molecular probes, we provide new insight into the physical structure\u2014especially the thermal structure\u2014of the nebula by mapping physical parameters derived from observed spectra. We also use different isotopologues of methanol to understand its chemistry, specifically after it forms on the surfaces of icy dust grains. This work provides a new view of Orion KL by providing high-angular-resolution maps of parameters such as abundance and temperature, whereas much of the existing literature provides a single set of parameters for a region.
\r\n\r\nHowever, Orion KL is only one interstellar laboratory, and it is part of a cohort of star-forming regions that are the targets of repeated astrochemical observations. The second part of this thesis ventures into the so-called molecular ring, a region 4-8 kiloparsecs from the galactic center that has remained relatively unexplored in the context of astrochemistry. Using the Atacama Compact Array (ACA) component of ALMA, we observed a slew of millimeter-emitting objects across 11 giant molecular clouds at higher angular resolutions than most previous observations of our sample, and we characterize their chemistry for the first time. This pilot survey establishes a catalogue of interstellar laboratories for future higher-angular-resolution observations. Over time, this catalogue will drive a better understanding of the chemistry in molecular-ring young stellar objects, allowing us to see whether (and if so, how) chemistry varies across distance from the galactic center and significantly increasing the number of targets for astrochemical observations.
", "doi": "10.7907/rwj8-8683", "publication_date": "2022", "thesis_type": "phd", "thesis_year": "2022" } ]