arXiv subjects

Kevin P. Hand

Publications and source records attributed to Kevin P. Hand.

17 recordsLinked to original sources

arXiv2026

Callisto from JWST: CO2-rich terrain on the leading hemisphere and global patterns of H2O ice

We present maps of H2O, CO2, and a 4.57 um spectral feature across Callisto's surface observed using the James Webb Space Telescope (JWST). H2O ice was mapped by measuring band parameters of the 3.1 um Fresnel peak across the leading and trailing hemispheres under a simplified assumption of crystalline ice. We update the CO2 solid-phase, CO2 gas, and 4.57 um feature band depth maps originally presented in Cartwright et al. (2024) with a new JWST observation of Callisto centered on Valhalla, the largest multi-ring impact basin in the solar system. Our H2O ice map shows that the Fresnel peak on the trailing hemisphere exhibits a bullseye pattern that is weaker at low latitudes, and on the leading hemisphere its strength is associated with impacts. This dichotomy is possibly related to the Jovian magnetospheric plasma impinging on the trailing hemisphere. Our solid-phase CO2 map reveals an enhancement in the vicinity of the Lofn/Heimdall impact craters, a region that may be the largest reservoir of non-radiolytic CO2 on Callisto's surface. The gas-phase CO2 exhibits a patchy spatial distribution and does not clearly correlate with solid CO2.

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arXiv2025

An Assessment of Organics Detection and Characterization on the Surface of Europa with Infrared Spectroscopy

Organics, if they do exist on Europa, may only be present in trace amounts on the surface. NASA's upcoming mission Europa Clipper is going to provide global, high quality data of the surface of Europa in the near-infrared (NIR), specifically the 3-5~$\mu$m region, where organics are rich in spectroscopic features. In this work we investigate Europa Clipper's ability to constrain the abundance of selected trace species of interest that span different chemical bonds found in organics, such as C-H, C=C, C$\equiv$C, C=O and C$\equiv$N, via NIR spectroscopy in the 3-5~$\mu$m wavelength region. We simulate reflectance spectra of these trace species mixed with water ice, at varying SNR and abundance fractions. The evidence for the trace species in a mixture is evaluated using two approaches: 1) calculating average strength of absorption feature(s), and 2) Bayesian model comparison (BMC) analysis. Our simulations show that sharp and strong spectroscopic features of trace ($\sim 5\%$ abundance by number) organic species should be detectable at $> 3\sigma$ significance in Europa Clipper quality data. A BMC analysis pushes the $3\sigma$ detection threshold of trace species even lower to $<1 \%$ abundance. We also consider an example with all trace species mixed together, with overlapping features, and BMC is able to retrieve strong evidence for all of them and also provide constraints on their abundance. These results are promising for Europa Clipper's capability to detect trace organic species, which would allow correlations to be drawn between the composition and geological regions with possibly endogenic material.

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arXiv2025

Experimental demonstration of electric power generation from Earth's rotation through its own magnetic field

Earth rotates through the axisymmetric part of its own magnetic field, but a simple proof shows that it is impossible to use this to generate electricity in a conductor rotating with Earth.However, we previously identified implicit assumptions underlying this proof and showed theoretically that these could be violated and the proof circumvented. This requires using a soft magnetic material with a topology satisfying a particular mathematical condition and a composition and scale favoring magnetic diffusion, i.e. having a low magnetic Reynolds number Rm (C.F. Chyba, K.P. Hand, Electric power generation from Earth's rotation through its own magnetic field. Phys. Rev. Applied 6, 014017-1-18 (2016)). Here we realize these requirements with a cylindrical shell of manganese-zinc ferrite. Controlling for thermoelectric and other potentially confounding effects (including 60 Hz and RF background), we show that this small demonstration system generates a continuous DC voltage and current of the (low) predicted magnitude. We test and verify other predictions of the theory: voltage and current peak when the cylindrical shell's long axis is orthogonal to both Earth's rotational velocity v and magnetic field; voltage and current go to zero when the entire apparatus (cylindrical shell together with current leads and multimeters) is rotated 90 degrees to orient the shell parallel to v; voltage and current again reach a maximum but of opposite sign when the apparatus is rotated a further 90 degrees; an otherwise-identical solid MnZn cylinder generates zero voltage at all orientations; and a highRm cylindrical shell produces zero voltage. We also reproduce the effect at a second experimental location. The purpose of these experiments was to test the existence of the predicted effect. Ways in which this effect might be scaled to generate higher voltage and current may now be investigated.

physics.app-ph↗

arXiv2024

Revealing Callisto's carbon-rich surface and CO2 atmosphere with JWST

We analyzed spectral cubes of Callisto's leading and trailing hemispheres, collected with the NIRSpec Integrated Field Unit (G395H) on the James Webb Space Telescope. These spatially resolved data show strong 4.25-micron absorption bands resulting from solid-state 12CO2, with the strongest spectral features at low latitudes near the center of its trailing hemisphere, consistent with radiolytic production spurred by magnetospheric plasma interacting with native H2O mixed with carbonaceous compounds. We detected CO2 rovibrational emission lines between 4.2 and 4.3 microns over both hemispheres, confirming the global presence of CO2 gas in Callisto's tenuous atmosphere. These results represent the first detection of CO2 gas over Callisto's trailing side. The distribution of CO2 gas is offset from the subsolar region on either hemisphere, suggesting that sputtering, radiolysis, and geologic processes help sustain Callisto's atmosphere. We detected a 4.38-micron absorption band that likely results from solid-state 13CO2. A prominent 4.57-micron absorption band that might result from CN-bearing organics is present and significantly stronger on Callisto's leading hemisphere, unlike 12CO2, suggesting these two spectral features are spatially anti-associated. The distribution of the 4.57-micron band is more consistent with a native origin and/or accumulation of dust from Jupiter's irregular satellites. Other, more subtle absorption features could result from CH-bearing organics, CO, carbonyl sulfide (OCS), and Na-bearing minerals. These results highlight the need for preparatory laboratory work and improved surface-atmosphere interaction models to better understand carbon chemistry on the icy Galilean moons before the arrival of NASA's Europa Clipper and ESA's JUICE spacecraft.

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arXiv2023

Detection of HCN and diverse redox chemistry in the plume of Enceladus

The Cassini spacecraft observed that Saturn's moon Enceladus possesses a series of jets erupting from its South Polar Terrain. Previous studies of in situ data collected by Cassini's Ion and Neutral Mass Spectrometer (INMS) have identified H$_2$O, CO$_2$, CH$_4$, NH$_3$, and H$_2$ within the plume of ejected material. Identification of minor species in the plume remains an ongoing challenge, owing to the large number of possible combinations that can be used to fit the INMS data. Here, we present the detection of several new compounds of strong importance to the habitability of Enceladus, including HCN, C$_2$H$_2$, C$_3$H$_6$, and C$_2$H$_6$. Our analyses of the low velocity INMS data, coupled with our detailed statistical framework, enable discrimination between previously ambiguous species in the plume by alleviating the effects of high dimensional model fitting. Together with plausible mineralogical catalysts and redox gradients derived from surface radiolysis, these compounds could potentially support extant microbial communities or drive complex organic synthesis leading to the origin of life.

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arXiv2022

Properties of the Nili Fossae Olivine-clay-carbonate lithology: orbital and in situ at S\'e\'itah

We examine the observed properties of the Nili Fossae olivine-clay-carbonate lithology from orbital data and in situ by the Mars 2020 rover at the S\'e\'itah unit in Jezero crater, including: 1) composition (Liu, 2022) 2) grain size (Tice, 2022) 3) inferred viscosity (calculated based on geochemistry collected by SuperCam (Wiens, 2022)). Based on the low viscosity and distribution of the unit we postulate a flood lava origin for the olivine-clay-carbonate at S\'e\'itah. We include a new CRISM map of the clay 2.38 {\mu}m band and use in situ data to show that the clay in the olivine cumulate in the S\'e\'itah formation is consistent with talc or serpentine from Mars 2020 SuperCam LIBS and VISIR and MastCam-Z observations. We discuss two intertwining aspects of the history of the lithology: 1) the emplacement and properties of the cumulate layer within a lava lake, based on terrestrial analogs in the Pilbara, Western Australia, and using previously published models of flood lavas and lava lakes, and 2) the limited extent of post emplacement alteration, including clay and carbonate alteration (Clave, 2022; Mandon, 2022).

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arXiv2021

Internal-Current Lorentz-Force Heating of Astrophysical Objects

Two forms of ohmic heating of astrophysical secondaries have received particular attention: unipolar-generator heating with currents running between the primary and secondary; and magnetic induction heating due to the primary's time-varying field. Neither appears to cause significant dissipation in the contemporary solar system. But these discussions have overlooked heating derived from the spatial variation of the primary's field across the interior of the secondary. This leads to Lorentz force-driven currents around paths entirely internal to the secondary, with resulting ohmic heating. We examine three ways to drive such currents, by the cross product of: (1) the secondary's azimuthal orbital velocity with the non-axially symmetric field of the primary; (2) the radial velocity (due to non-zero eccentricity) of the secondary with the primary's field; or (3) the out-of-plane velocity (due to non-zero inclination) with the primary's field. The first of these operates even for a spin-locked secondary whose orbit has zero eccentricity, in strong contrast to tidal dissipation. We show that Jupiter's moon Io today could dissipate about 600 GW (more than likely current radiogenic heating) in the outer hundred meters of its metallic core by this mechanism. Had Io ever been at 3 jovian radii instead of its current 5.9, it could have been dissipating 15,000 GW. Ohmic dissipation provides a mechanism that could operate in any solar system to drive inward migration of secondaries that then necessarily comes to a halt upon reaching a sufficiently close distance to the primary.

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arXiv2021

A comprehensive revisit of select Galileo/NIMS observations of Europa

The Galileo Near Infrared Mapping Spectrometer (NIMS) collected spectra of Europa in the 0.7-5.2 $μ$m wavelength region, which have been critical to improving our understanding of the surface composition of this moon. However, most of the work done to get constraints on abundances of species like water ice, hydrated sulfuric acid, hydrated salts and oxides have used proxy methods, such as absorption strength of spectral features or fitting a linear mixture of laboratory generated spectra. Such techniques neglect the effect of parameters degenerate with the abundances, such as the average grain-size of particles, or the porosity of the regolith. In this work we revisit three Galileo NIMS spectra, collected from observations of the trailing hemisphere of Europa, and use a Bayesian inference framework, with the Hapke reflectance model, to reassess Europa's surface composition. Our framework has several quantitative improvements relative to prior analyses: (1) simultaneous inclusion of amorphous and crystalline water ice, sulfuric-acid-octahydrate (SAO), CO$_2$, and SO$_2$; (2) physical parameters like regolith porosity and radiation-induced band-center shift; and (3) tools to quantify confidence in the presence of each species included in the model, constrain their parameters, and explore solution degeneracies. We find that SAO strongly dominates the composition in the spectra considered in this study, while both forms of water ice are detected at varying confidence levels. We find no evidence of either CO$_2$ or SO$_2$ in any of the spectra; we further show through a theoretical analysis that it is highly unlikely that these species are detectable in any 1-2.5 $μ$m Galileo NIMS data.

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arXiv2020

Endogenic and Exogenic Contributions to Visible-wavelength Spectra of Europa's Trailing Hemisphere

The composition of Europa's trailing hemisphere reflects the combined influences of endogenous geologic resurfacing and exogenous sulfur radiolysis. Using spatially resolved visible-wavelength spectra of Europa obtained with the Hubble Space Telescope, we map multiple spectral features across the trailing hemisphere and compare their geographies with the distributions of large-scale geology, magnetospheric bombardment, and surface color. Based on such comparisons, we interpret some aspects of our spectra as indicative of purely exogenous sulfur radiolysis products and other aspects as indicative of radiolysis products formed from a mixture of endogenous material and magnetospheric sulfur. The spatial distributions of two of the absorptions seen in our spectra -- a widespread downturn toward the near-UV and a distinct feature at 530 nm -- appear consistent with sulfur allotropes previously suggested from ground-based spectrophotometry. However, the geographies of two additional features -- an absorption feature at 360 nm and the spectral slope at red wavelengths -- are more consistent with endogenous material that has been altered by sulfur radiolysis. We suggest irradiated sulfate salts as potential candidates for this material, but we are unable to identify particular species with the available data.

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arXiv2020

Evidence for sulfur-bearing species on Callisto's leading hemisphere: Sourced from Jupiter's irregular satellites or Io?

We investigated whether sulfur-bearing species are present on the icy Galilean moon Callisto by analyzing eight near-infrared reflectance spectra collected over a wide range of sub-observer longitudes. We measured the band areas and depths of a 4-micron feature in these spectra, which has been attributed to sulfur dioxide (SO2), as well as carbonates, in previously collected datasets of this moon. All eight spectra we collected display the 4-micron band. The four spectra collected over Callisto's leading hemisphere display significantly stronger 4-micron bands compared to the four trailing hemisphere spectra (> 3-sigma difference). We compared the central wavelength position and shape of Callisto's 4-micron band to laboratory spectra of various sulfur-bearing species and carbonates. Our comparison demonstrates that Callisto's 4-micron band has a spectral signature similar to thermally-altered sulfur, as well as a 4.025 micron feature attributed to disulfanide (HS2). Our analysis therefore supports the presence of S-bearing species on Callisto but is not consistent with the presence of SO2. The significantly stronger 4-micron band detected on Callisto's leading hemisphere could result from collisions with H2S-rich dust grains that originate on Jupiter's retrograde irregular satellites or implantation of magnetospheric S ions that originate from volcanic activity on Io. Alternatively, S-bearing species could be native to Callisto and are exposed by dust collisions and larger impacts that drive regolith overturn, primarily on its leading side.

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arXiv2020

Evidence for ammonia-bearing species on the Uranian satellite Ariel supports recent geologic activity

We investigated whether ammonia-rich constituents are present on the surface of the Uranian moon Ariel by analyzing 32 near-infrared reflectance spectra collected over a wide range of sub-observer longitudes and latitudes. We measured the band areas and depths of a 2.2-{\micron} feature in these spectra, which has been attributed to ammonia-bearing species on other icy bodies. Ten spectra display prominent 2.2-{\micron} features with band areas and depths > 2σ. We determined the longitudinal distribution of the 2.2-{\micron} band, finding no statistically meaningful differences between Ariel's leading and trailing hemispheres, indicating that this band is distributed across Ariel's surface. We compared the band centers and shapes of the five Ariel spectra displaying the strongest 2.2-{\micron} bands to laboratory spectra of various ammonia-bearing and ammonium-bearing species, finding that the spectral signatures of the Ariel spectra are best matched by ammonia-hydrates and flash frozen ammonia-water solutions. Our analysis also revealed that four Ariel spectra display 2.24-{\micron} bands (> 2σ band areas and depths), with band centers and shapes that are best matched by ammonia ice. Because ammonia should be efficiently removed over short timescales by ultraviolet photons, cosmic rays, and charged particles trapped in Uranus' magnetosphere, the possible presence of this constituent supports geologic activity in the recent past, such as emplacement of ammonia-rich cryolavas and exposure of ammonia-rich deposits by tectonism, impact events, and mass wasting.

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arXiv2019

H$_2$O$_2$ within chaos terrain on Europa's leading hemisphere

Hydrogen peroxide is part of an important radiolytic cycle on Europa and may be a critical source of oxidants to the putative subsurface ocean. The surface geographic distribution of hydrogen peroxide may constrain the processes governing its abundance as well as its potential relevance to the subsurface chemistry. However, maps of Europa's hydrogen peroxide beyond hemispherical averages have never been published. Here, we present spatially resolved L-band (3.16 - 4 $μ$m) observations of Europa's 3.5 $μ$m hydrogen peroxide absorption, which we obtained using the near-infrared spectrometer NIRSPEC and the adaptive optics system on the Keck II telescope. Using these data, we map the strength of the 3.5 $μ$m absorption across the surface at a nominal spatial resolution of $\sim$300 km. Though previous disk-integrated data seemed consistent with the laboratory expectation that Europa's hydrogen peroxide exists primarily in its coldest and iciest regions, we find nearly the exact opposite at this finer spatial scale. Instead, we observe the largest hydrogen peroxide absorptions at low latitudes on the leading and anti-Jovian hemispheres, correlated with chaos terrain, and relative depletions toward the cold, icy high latitudes. This distribution may reflect the effects of decreased hydrogen peroxide destruction due to efficient electron scavenging by CO$_2$ within chaos terrain.

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arXiv2017

A New Spectral Feature on the Trailing Hemisphere of Europa at 3.78 microns

We present hemispherically resolved spectra of the surface of Europa from ~3.1--4.13 microns, which we obtained using the near infrared spectrometer NIRSPEC on the Keck II telescope. These include the first high-quality L-band spectra of the surface to extend beyond 4 microns. In our data we identify a previously unseen spectral feature at 3.78 microns on the trailing hemisphere. The longitudinal distribution of the feature is consistent with that of a radiolytic product created by electron or Iogenic ion bombardment. This feature is coincident with an absorption feature of SO2 frost seen in both laboratory spectra and spectra of Io. However, the corresponding, typically stronger 4.07 micron feature of SO2 frost is absent from our data. This result is contrary to the suggested detection of SO2 at 4.05 microns in Galileo NIMS data of the trailing hemisphere, which was severely affected by radiation noise. We use simple spectral modeling to argue that the 3.78 micron feature is not easily explained by the presence of SO2 frost on the surface. We explore alternative explanations and discuss other potential candidate species.

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arXiv2017

Active Cryovolcanism on Europa?

Evidence for plumes of water on Europa has previously been found using the Hubble Space Telescope (HST) using two different observing techniques. Roth et al. (2014) found line emission from the dissociation products of water. Sparks et al. (2016) found evidence for off-limb continuum absorption as Europa transited Jupiter. Here, we present a new transit observation of Europa that shows a second event at the same location as a previous plume candidate from Sparks et al. (2016), raising the possibility of a consistently active source of erupting material on Europa. This conclusion is bolstered by comparison with a nighttime thermal image from the Galileo Photopolarimeter-Radiometer (PPR) which shows a thermal anomaly at the same location, within the uncertainties (Spencer et al. 1999). The anomaly has the highest observed brightness temperature on the Europa nightside. If heat flow from a subsurface liquid water reservoir causes the thermal anomaly, its depth is ~1.8-2 km, under simple modeling assumptions, consistent with scenarios in which a liquid water reservoir has formed within a thick ice shell. Models that favor thin regions within the ice shell that connect directly to the ocean, however, cannot be excluded, nor modifications to surface thermal inertia by subsurface activity. Alternatively, vapor deposition surrounding an active vent could increase the thermal inertia of the surface and cause the thermal anomaly. This candidate plume region may offer a promising location for an initial characterization of Europa's internal water and ice and for seeking evidence of Europa's habitability. ~

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arXiv2015

Spatially Resolved Spectroscopy of Europa: The Distinct Spectrum of Large-scale Chaos

We present a comprehensive analysis of spatially resolved moderate spectral resolution near infrared spectra obtained with the adaptive optics system at the Keck Observatory. We identify three compositionally distinct end member regions: the trailing hemisphere bullseye, the leading hemisphere upper latitudes, and a third component associated with leading hemisphere chaos units. We interpret the composition of the three end member regions to be dominated by irradiation products, water ice, and evaporite deposits or salt brines, respectively. The third component is associated with geological features and distinct from the geography of irradiation, suggesting an endogenous identity. Identifying the endogenous composition is of particular interest for revealing the subsurface composition. However, its spectrum is not consistent with linear mixtures of the salt minerals previously considered relevant to Europa. The spectrum of this component is distinguished by distorted hydration features rather than distinct spectral features, indicating hydrated minerals but making unique identification difficult. In particular, it lacks features common to hydrated sulfate minerals, challenging the traditional view of an endogenous salty component dominated by Mg-sulfates. Chloride evaporite deposits are one possible alternative.

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arXiv2015

Silicates on Iapetus from Cassini's Composite Infrared Spectrometer

We present the first spectral features obtained from Cassini's Composite Infrared Spectrometer (CIRS) for any icy moon. The spectral region covered by CIRS focal planes (FP) 3 and 4 is rich in emissivity features, but previous studies at these wavelengths have been limited by low signal to noise ratios (S/Rs) for individual spectra. Our approach is to average CIRS FP3 spectra to increase the S/R and use emissivity spectra to constrain the composition of the dark material on Iapetus. We find an emissivity feature at ~855 cm-1 and a possible doublet at 660 and 690 cm-1 that do not correspond to any known instrument artifacts. We attribute the 855 cm-1 feature to fine-grained silicates, similar to those found in dust on Mars and in meteorites, which are nearly featureless at shorter wavelengths. Silicates on the dark terrains of Saturn's icy moons have been suspected for decades, but there have been no definitive detections until now. Serpentines reported in the literature at ambient temperature and pressure have features near 855 and 660 cm-1. However, peaks can shift depending on temperature and pressure, so measurements at Iapetus-like conditions are necessary for more positive feature identifications. As a first investigation, we measured muscovite at 125K in a vacuum and found that this spectrum does match the emissivity feature near 855 cm-1 and the location of the doublet. Further measurements are needed to robustly identify a specific silicate, which would provide clues regarding the origin and implications of the dark material.

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arXiv2013

Keck II Observations of Hemispherical Differences in H2O2 on Europa

We present results from Keck II observations of Europa over four consecutive nights using the near-infrared spectrograph (NIRSPEC). Spectra were collected in the 3.14--4.0 micron range, allowing detection and monitoring of the 3.5 micron feature due to hydrogen peroxide. Galileo Near-Infrared Spectrometer (NIMS) results first revealed hydrogen peroxide on Europa in the anti-jovian region of the leading hemisphere at an abundance of 0.13+/-0.07% by number abundance relative to water. We find comparable results for the two nights over which we observed the leading hemisphere. Significantly, we observed a small amount of hydrogen peroxide (~0.04%) during observations of Europa's anti- and sub-Jovian hemispheres. Almost no hydrogen peroxide was detected during observations of just the trailing hemisphere. We conclude that the Galileo observations likely represent the maximum hydrogen peroxide concentration, the exception potentially being the cold water ice regions of the poles, which are not readily observable from the ground. Our mapping of the peroxide abundance across Europa requires revisions to previous estimates for Europa's global surface abundance of oxidants and leads to a reduction in the total oxidant delivery expected for the sub-surface ocean, if exchange of surface material with the ocean occurs.

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