de Kleer Research Group | Planetary Astronomy

Division of Geological and Planetary Sciences | Caltech

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Thermal Mapping of the Icy Galilean Satellites

The surface compositions of the solar system's icy moons arise from a combination of intrinsic properties (formation conditions, and endogenic processes such as past volcanism), and exogenic processing by their environments. The thermal properties of surface materials both provide compositional constraints, and determine the response of the materials to solar radiation. Millimeter wavelengths probe the upper tens of cm in icy surfaces, and are sensitive to the thermal properties of the upper subsurface beneath the most processed exterior. The Atacama Large Millimeter Array (ALMA) in Chile is an interferometric millimeter-wavelength observatory that is capable of spatially resolving solar system satellites, and hence providing maps of the thermal material properties of their surfaces. We have developed a thermal and radiative-transfer model for icy satellite surfaces that self-consistently links the dielectric and thermal surface properties and allows for a characterization of how properties such as temperature and ice porosity vary with depth in the subsurface. We apply this model to spatially-resolved ALMA amd the VLA observations to derive surface properties and to generate thermal maps of the Galilean satellites (Fig: residual brightness temperature map of Ganymede at a wavelength of 1.3 mm; de Kleer et al. 2021), in order to understand their subsurface composition and temperature profiles, and investigate the relative roles of exogenic vs. endogenic surface processing.

AAS Nova feature: "Peering at the surface of a nearby moon."

The Heating and Differentiation Histories of Planetesimals

Remnant fragments of the planetesimals remain today in the form of asteroids, preserving information from the early period of planet formation and revealing the evolution and processing of these bodies over the age of the Solar System. Remnants of large planetesimals that differentiated before undergoing catastrophic collisions should be present in the asteroid belt as heterogeneous fragments containing both metallic and silicate materials. We are looking for direct evidence of these processes through a characterization of the surface heterogeneity of large asteroids, particularly in the M ("metallic") and S ("stony") asteroid classes. The investigation uses high-resolution observations of asteroid thermal emission and its polarization from ALMA and the VLA. The degree and polarization of the thermal emission is heavily influenced by the dielectric properties of the surface, which are uniquely sensitive to the metal content. The high spatial resolution of ALMA (~30 km in the main asteroid belt) allows for mapping of thermal properties across the surface of these ~200 km objects. The figure at the left shows thermal emission images of the asteroid (16) Psyche at two points in its rotation; the circle at the lower left represents the size of a resolution element (see de Kleer et al. 2021 and Cambioni et al. 2022). Over 2022-2023 we will be conducting a survey of 12 additional asteroids across compositional classes using these techniques. In addition, upcoming data from our JWST Cycle 1 program will provide detailed compositional information on a class of asteroids that has been used as evidence for incomplete mixing of 26Al in the protoplanetary disk.

Press Releases (2021): Caltech
AAS Nova feature: "Surveying a Metal-Rich Asteroid."

The Volcanism and Tidal Heating of Io

Tidal heating plays a central role in geological activity on bodies all across the Solar System. On Io, tidal heat manifests in a clearly detectable way as large-scale volcanism, presenting a truly unique opportunity to connect interior processes to their surface expressions and unravel the mechanisms of tidal heating. The statistical properties of Io's eruptions discriminate between models of tidal heating and reveal the depth of tidal heat deposition in Io's interior. Since 2013, we have been characterizing these properties through a high-cadence adaptive optics imaging campaign at Keck and Gemini N. The dataset reveals the time-evolution of ~75 individual volcanoes over nearly 300 nights since the beginning of 2013 (de Kleer et al. 2014;, de Kleer & de Pater 2016a) and constitutes the highest-cadence database of high-resolution observations of Io's thermal emission to date. The observed spatial pattern of thermal activity is distinct and asymmetric (Fig; de Kleer & de Pater. 2016b; de Kleer et al. 2019), implying strong local geological controls on the locations of Io's eruptions, while the latitudinal distribution of high-power events is suggestive of a deep-mantle magma source. In addition, the high cadence dataset has provided suggestive evidence that the lava lake Loki Patera brightens periodically in sync with changes to Io’s orbital eccentricity induced by interactions with its neighbors Europa and Ganymede (de Kleer et al. 2019).

Press Releases (2016): UC Berkeley | Gemini N
NY Times: "This world is a simmer hellscape. They've been watching its explosions."

Galilean Satellite Atmospheres

The galilean satellites exhibit a range of atmospheric compositions, thermochemical histories, and magnetic field properties. These objects therefore present a laboratory for studying universal planetary processes including the creation and loss of tenuous atmospheres and the impact of geological activity and plasma environment on surfaces and atmospheres of objects with and without magnetic fields. The atmospheres of Europa and Ganymede are sourced from sputtering of their icy surfaces, while Io's sulfur-based atmosphere is sourced by direct volcanic outgassing and sublimation of surface frosts. The tenuous atmospheres of these worlds produce subtle signatures; we study these signatures at optical and near-infrared wavelengths by observing these satellites when they are in Jupiter's shadow and the sunlight normally reflected off of their bright surfaces is absent. Using this observing technique, we have used emission from hot volcanic SO gas on Io observed at high spectral resolution to infer gas reservoirs at two temperatures (de Kleer et al. 2019; de Pater et al. 2020). We are also using optical-wavelength auroral emissions from Io, Europa, Ganymede, and Callisto to constrain the atmospheric composition and column density, and to investigate the interactions of these species with the jovian magnetosphere and particle environment (Fig; de Kleer and Brown 2018; de Kleer and Brown 2019).

High-Resolution Mapping of Io's Volcanoes

The thermal output of Io's volcanoes reveals the properties of the eruption and magma, giving us a window into Io's interior and near-surface magma reservoirs. When individual volcanic features can be spatially resolved, we may distinguish multiple locations where magma is reaching the surface within a single volcanic complex and/or look for variations in properties across such a feature. Using this information, we can start to piece together characteristics of the magma plumbing system. In collaboration with a team at the Large Binocular Telescope, we have pushed the limits of current technology in order to achieve extreme spatial resolution on Io with the AO-assisted Large Binocular Telescope Interferometer (LBTI). In Conrad et al. (2015), we used LBTI to directly resolve a volcanic feature on Io for the first time from the Earth, revealing multiple distinct emission components within the lava lake Loki Patera. More recently, we used an occultation of Io by Europa to reconstruct the temperature distribution within Loki Patera at a spatial resolution of 10 km (Fig; de Kleer et al. 2017). The temperature map reveals evidence for two resurfacing waves traveling in opposite directions around the patera at different rates, indicating compositional differences across this 200-km feature. The same technique applied to other volcanoes on Io has resolved these volcanoes into multiple distinct emitting areas (Fig; de Kleer et al. 2021)

Press Releases (2017): UC Berkeley | LBTO
AAS Nova feature (2021): "Resolving Io's volcanoes through occultation localization."

Powerful Volcanic Eruptions on Io

The mechanisms that drive volcanism on Io are very different from on Earth, giving us the opportunity to investigate how the heat source and thermal history of a body impact its geophysical processes. Io's most powerful and puzzling volcanoes can serve as laboratories for probing the composition of Io's interior and the architecture of its sub-surface magma systems. Through time-domain coverage of Io's eruptions and simultaneous multi-telescope datasets, we have characterized some of Io's most energetic eruptions (Fig; de Kleer et al. 2014; de Kleer & de Pater 2017) and studied a unique class of eruptions on Io that are intense but very short-lived (de Kleer & de Pater 2016b).

Press Releases (2014): UC Berkeley | Gemini N | Keck

Ice Giant Atmospheres

The composition, circulation, and aerosol structure of planetary atmospheres inform planet formation and evolution models. I study these properties in the atmospheres of Uranus and Neptune using near-infrared adaptive optics observations from Lick Observatory and the Keck telescope. I have collaborated on the development of radiative transfer code to generate synthetic near-infrared spectra of planetary atmospheres. The synthetic models are fit to data using a Markov chain Monte Carlo (MCMC) sampler to determine the properties and distribution of atmospheric aerosols and molecular absorbers, which inform our understanding of global circulation patterns. By applying this code to observations of Uranus and Neptune with the OSIRIS integral-field spectrometer on the Keck telescope, we reconstruct 3D compositional maps of molecules and aerosols in the atmospheres of these planets. Despite major differences in heat flow and cloud activity between the two ice giants, our analysis indicates broad-scale similarities, including an extended haze layer above a compact condensation cloud (de Kleer et al. 2015; Luszcz-Cook et al. 2016). However, while high-latitude methane depletion on Uranus is broadly consistent with hemisphere-scale circulation cells, a lack of analogous trends on Neptune points to a multi-cell circulation pattern.

The Rings of Uranus

Ring systems are an ubiquitous feature of outer Solar System planets; the wide variety in their structure and particle properties indicates diverse formation scenarios, ages, and evolutionary histories. In de Kleer et al. (2013), I isolated the reflectance spectra of individual ring groupings within the Uranian ring system using the OSIRIS integral-field spectrometer on the Keck telescope and produced the first spectrally-resolved observations of the system, which confirm that Uranus' ring particles are neither dusty like Jupiter's nor pure water ice like Saturn's.