Murthy S. Gudipati

We have conducted a comprehensive study on trapping and sublimation of H2 in H2O ice. The results demonstrate that codeposited H2:H2O ices (1:5) at 7 K release H2 in two temperature regions during temperature-programmed desorption (TPD), at ∼25 K and ∼80 K, whereas H2 produced in H2O ices via electron irradiation is released in three regions, at ∼25 K, ∼80 K, and ∼110 K. The higher the electron energy, the greater the amount of H2 trapped in and released from H2O ice at ∼110 K. Following irradiation, the ∼80 K release of H2 is observed in amorphous but not crystalline ice. To better understand the trapping and release behavior of H2 in H2O ice, we carried out systematic laboratory studies under different astrophysical and planetary conditions, as presented here, and compared them with previous studies. The implications of our work will range from outgassing from cometary nuclei, protoplanetary disk ice chemistry, nongravitational acceleration of interstellar objects such as 1I/′Oumuamua, and outgassing of H2 from irradiated icy bodies such as Europa and Enceladus in the solar system. Our work also infers radiolytic production and storage of H2 in the lunar permanently shadowed regions.

The abundance of icy materials on or near the surface of planetary bodies in the outer solar system dictates the need for lab measurements of ice properties at relevant environmental conditions – to unravel their age, evolution, and surface physics/chemistry. Deposition of ice on the surface could occur due to a variety of surface-subsurface exchange processes, and once emplaced, it can undergo phase transitions, changes in grain size and other physical properties (e.g., 1-3). While water ice spectral properties have been extensively studied in the past (4), and absorption coefficients of relatively warm ice have been published, we need insights (at lab scale) into formation/transport mechanisms of surface icy materials, and evolution with time via active surface processes.NIR Spectroscopy: New infrastructure was custom-built (Fig. 1) to produce “thick” (~2 mm) pristine water ice samples and collect reflectance spectra in the near infrared (NIR) wavelength range, with grain-size ranging from 25-212 μm (under ultrahigh vacuum) at temperatures of 10-170 K (with annealing) for direct comparison to spectra of Solar System objects. Samples display spectral signatures of water ice, both crystalline and amorphous based on temperature and formation protocols and demonstrate the theorized evolution of absorption band features due to variations in sample thickness, grain size, and thermal cycling (Fig. 2). To demonstrate the efficacy of this new dataset for surface characterization, we determine bulk crystallinity of Europa’s leading hemisphere, and the environmental conditions required to meet current age estimates.Calculation of 1.65/1.5 μm Integrated Band Area Ratios (B): 𝐵 was calculated for our ice samples as the ratio of integrated area of the 1.65 μm and 1.5 μm region. New laboratory end members for pure amorphous (Bamorph = 0.0166) and crystalline ice (Bcryst = 0.0529) were calculated using transmission and reflection spectroscopy.Crystallinity: The amount of crystalline water ice compared to total abundance of both crystalline and amorphous ice phases, is referred to here as ‘‘crystallinity’’ (C) of a surface, defined by equation:C = (𝐵𝐺𝐵𝑂 - 𝐵𝑎𝑚𝑜𝑟𝑝ℎ) / (𝐵𝑐𝑟𝑦𝑠𝑡 - 𝐵𝑎𝑚𝑜𝑟𝑝ℎ)where BGBO is calculated for ground-based observations (GBO). Crystallinity captures the balance between crystallization and amorphization due to several processes operating on timescales of a few hours to millions of years [5]. Thus, our understanding of the environmental history of icy bodies (with limited observations) can be enhanced by constraining this crystallinity. Current crystallinity estimates of Europa’s leading hemisphere range from ~27 to 95% [6]. Using our new end members with 3 full-disk, spectroscopic GBO’s of Europa’s leading hemisphere, allows us to re-interpret its crystallinity, increasing from previously estimated 27-36% (using transmission spectra) to equal proportions as a full disk average of 46-52% (reflection spectra) - similar to [7] and global estimates of Ganymede. The discrepancies in measured crystallinities can be assigned to differences in methods of acquiring reflectance spectra from a real planetary surface and previous lab transmission spectra from thin films of water ice, grain sizes and thermal history contributing B values. The reflection spectroscopy technique with “thick” ice samples employed in this work should offer a more accurate comparison to planetary surface observations, with thicknesses far beyond 50 μm, and consistency with spectral mixing results increases our confidence. Spatial variations in crystallinity are expected due to changes in particle flux and thermal history with location, and due to other temporal resurfacing processes.Age Ruler: Over the past three decades, there have been several estimates of Europa’s surface age, with [8] estimating a best-fit average age of ~30-70 Ma and [9] estimating ~40-90 Ma. We can deploy an alternate approach here with the calculated bulk crystallinity, using an age equation established by [10], based on measured rate of ice phase transformation in environmental conditions:1-C = Φ Amax (1- exp [-kFt / N])Any determination of age requires tight constraints on surface conditions, which are captured in equation inputs, derived either from the methodology of [6], or from recent literature. [11] used this equation to determine the approximate ages of 2 craters on Saturn's moon Rhea. We have similarly computed an age ruler for Europa's leading hemisphere. The age range of ~ 40 Ma to 90 Ma can be obtained for our range of surface crystallinity values under a surface temperature of 110 K, and ~low particle flux (7.92 x 106 cm-2 s-1).Ongoing Work on Jovian System: Bulk crystallinity and age estimate techniques presented here provide first order approximations and constraints on surface conditions and evolution of ice. These techniques are immediately applicable to other icy moons in the Jovian system and will enable new discoveries with increased science return of past missions and support ongoing missions to Jovian moons such as Europa. Building on this work, we are performing calculations of spatial variation in the rate of water ice amorphization and thermal annealing in Europa’s radiation environment. We compare the rates of these two competing processes and identify a steady state condition for (location dependent) crystallinity on Europa. Overall, a broad global pattern of amorphous versus crystalline ice on both hemispheres will be established for Europa, and reported on here.Acknowledgments: A part of the work presented here was carried out at the Jet Propulsion Laboratory, California Institute of Technology under a...

Amorphous ice is understood to be the predominant phase of water in cometary nuclei. A significant number of other volatiles can be trapped in amorphous H₂O ice and released during the amorphous-to-crystalline phase transition (ACPT). This phase transition is an exothermic and is considered a potential cause of cometary outbursts, such as those observed in Comet 1P/Halley and Comet Hale–Bopp. However, only a single experimental study reported in the literature suggests that the presence of impurities (>2%) in amorphous H₂O ice suppresses the exothermic effect and results in an endothermic phase transition, which contradicts the hypothesis of exothermic-phase-transition-driven comet outbursts. To further explore this phenomenon, we conducted experiments on pure H₂O, CO:H₂O, and CO₂:H₂O ice mixtures with varying fractions (11%, 50%, and 80% CO, and 11%, 25%, and 50% CO₂, both relative to H₂O (100%)). Our experimental setup is a highly sensitive cryogenic differential scanning calorimeter with a μK noise floor and fifth-decimal resolution in temperature ratio of reference and ice. Our calorimetric data have been internally calibrated to ice sublimation endotherm to derive quantitative calorimetric data. We find that ACPT is exothermic in all CO:H₂O mixtures and CO₂:H₂O mixtures with lower CO₂ fractions. In mixtures with the highest CO₂ content (50% relative to H₂O (100%)) examined, the ACPT exotherm is weakened. Our results demonstrate that ACPT exothermicity persists throughout the CO and CO₂ mixing ratios observed in a majority of comets, and it should play an important role in comet outbursts when CO and CO₂ are the major volatiles trapped in amorphous H₂O ice.

Molecular size plays a crucial role in determining the physical, chemical, and biological properties of substances. However, traditional fluorescence polarization anisotropy methods struggle to capture fast transient processes or provide plane-specific details. To overcome these limitations, we introduce Compressed Ultrafast Planar Polarization Anisotropy Imaging (CUP2AI). This approach combines femtosecond laser-sheet illumination, molecular rotational diffusivity, and compressed sensing to enable real-time, non-invasive, wide-field anisotropy measurements in both liquid and gas phases. CUP2AI captures 2D molecular size mapping in a single acquisition, granting unprecedented insights into dynamic events across various excitation modes (i.e. both one- and two-photon) and environmental conditions. It enables mapping of molecular volume (500 ų–80,000 ų) and hydrodynamic diameter (10 Å–50 Å) based on anisotropy lifetimes. We imaged fluorescein-conjugated dextran in water and polycyclic aromatic hydrocarbons in flames. CUP2AI holds transformative potential for applications ranging from molecular biology and drug design to nanoparticle formation.

Electron irradiation of water-rich ices plays a significant role in initiating the chemical and physical processes on the surface of airless icy bodies in radiation environments such as Europa and Enceladus, as well as on the Moon, comets, and asteroids interacting with the solar wind. The sputtering process by electrons and ions leads to chemical modification and outgassing of their icy surfaces and the subsequent formation of a tenuous atmosphere. Though electron-sputtering yields are known to be lower compared to ion-sputtering yields, one needs to also account for their differential fluxes. In our experiments, the electron-induced sputtering yields of all the gaseous species H2, O, OH, H2O, and O2 are investigated for electron energies lower than 2 keV in terms of partial pressure vs. time of irradiation. The effective averaged change in partial pressure of the desorbed species is converted to the number of sputtered atoms or molecules per second per cm2 from the ice, and then to the sputtering yields (number of species sputtered per electron). Our data agrees well with the previously reported data for the sputtering of O2 and H2O yield for the amorphous ice. We also find that crystalline ice shows significantly lower sputtering yields when compared to amorphous ice, in agreement with the observation of similar trends in the literature. Our work indicates that sputtering yields per keV of O, OH, O2, and H2O drop with increasing electron energy from 0.5 keV to 2 keV.

The creation of carbonaceous nanoparticles and their dynamics in hydrocarbon flames are still debated in environmental, combustion, and material sciences. In this study, we introduce single-pulse femtosecond laser sheet-compressed ultrafast photography (fsLS-CUP), an ultrafast imaging technique specifically designed to shed light on and capture ultrafast dynamics stemming from interactions between femtosecond lasers and nanoparticles in flames in a single-shot. fsLS-CUP enables the first-time real-time billion frames-per-second (Gfps) simultaneous two-dimensional (2D) imaging of laser-induced fluorescence (LIF) and laser-induced heating (LIH) that are originated from polycyclic aromatic hydrocarbons (PAHs) and soot particles, respectively. Furthermore, fsLS-CUP provides the real-time spatiotemporal map of femtosecond laser-soot interaction as elastic light scattering (ELS) at an astonishing 250 Gfps. In contrast to existing single-shot ultrafast imaging approaches, which are limited to millions of frames per second only and require multiple laser pulses, our method employs only a single pulse and captures the entire dynamics of laser-induced signals at hundreds of Gfps. Using a single pulse does not change the optical properties of nanoparticles for a following pulse, thus allowing reliable spatiotemporal mapping. Moreover, we found that particle inception and growth are derived from precursors. In essence, as an imaging modality, fsLS-CUP offers ultrafast 2D diagnostics, contributing to the fundamental understanding of nanoparticle’s inception and broader applications across different fields, such as material science and biomedical engineering.

ESA’s Rosetta mission accompanied comet 67P/Churyumov-Gerasimenko (67P) for over two years between 2014 – 2016. On board was ROSINA, the Rosetta Orbiter Spectrometer for Ion and Neutral Analysis (Balsiger et al. 2007), which measured, in situ, the composition of the gaseous coma.The set of molecules monitored included highly volatile species, such as carbon monoxide and methane, together with the main coma gases, water and carbon dioxide. In our recent work (Rubin et al., 2023), we have shown that the local coma abundances of highly volatile species can be reproduced by a linear combination of H2O and CO2, indicating that they are also associated in the ices of the comet’s nucleus.In this presentation, we will report on these findings and further investigate correlations and variations between different volatile species. ReferencesBalsiger et al., ROSINA – Rosetta Orbiter Spectrometer for Ion and Neutral Analysis, SSR, 128(1), 745–801, 2007.Rubin et al., Volatiles in the H2O and CO2 ices of comet 67P/Churyumov-Gerasimenko, MNRAS, 526, 3, 4209–4233, 2023.

Jupiter’s icy moon, Europa, harbors a subsurface liquid water ocean; the prospect of this ocean being habitable motivates further exploration of the moon with the upcoming NASA Europa Clipper mission. Key among the mission goals is a comprehensive assessment of the moon’s composition, which is essential for assessing Europa’s habitability. Through powerful remote sensing andin situinvestigations, the Europa Clipper mission will explore the composition of Europa’s surface and subsurface, its tenuous atmosphere, and the local space environment surrounding the moon. Clues on the interior composition of Europa will be gathered through these assessments, especially in regions that may expose subsurface materials, including compelling geologic landforms or locations indicative of recent or current activity such as potential plumes. The planned reconnaissance of the icy world will constrain models that simulate the ongoing external and internal processes that act to alter its composition. This paper presents the composition-themed goals for the Europa Clipper mission, the synergistic, composition-focused investigations that will be conducted, and how the anticipated scientific return will advance our understanding of the origin, evolution, and current state of Europa.

Grains of ice are formed spontaneously when water vapor is injected into a weakly ionized laboratory plasma in which the background gas has been cooled to cryogenic temperatures comparable to those of deep space. These ice grains are levitated indefinitely within the plasma so that their time evolution can be observed under free-floating conditions. Using microscope imaging, ice grains are shown to have a spindle-like fractal structure and grow over time. Both crystalline and amorphous phases of ice are observed using Fourier transform infrared spectroscopy. A mix of crystalline and amorphous grains coexists under certain thermal conditions, and a linear mixing model is used on the ice absorption band surrounding 3.2 μm to examine the ice phase composition and its temporal stability. The extinction spectrum is also affected by inelastic scattering as grains grow, and characteristic grain radii are obtained from Mie scattering theory and compared to size measurements from direct imaging. Observations are used to compare possible ice nucleation mechanisms, and it is concluded that nucleation is likely catalyzed by ions, as ice does not nucleate in the absence of plasma and impurities are not detected. Ice grain properties and infrared extinction spectra show similarity to observations of some astrophysical ices observed in protoplanetary disks, implying that the fractal morphology of the ice and observed processes of homogeneous ice nucleation could occur as well in such astrophysical environments with weakly ionized conditions.

Context. A large fraction of the organic species produced photochemically in the atmosphere of Titan can condense to form ice particles in the stratosphere and in the troposphere. According to various studies, diacetylene (C₄H₂) condenses below 100 km where it can be exposed to ultraviolet radiation. Aims. We studied experimentally the photochemistry of diacetylene ice (C₄H₂) to evaluate its potential role in the lower altitude photochemistry of Titan’s atmospheric ices. Methods. C₄H₂ ice films were irradiated with near-ultraviolet (near-UV) photons (λ > 300 nm) with different UV sources to assess the impact of the wavelengths of photons on the photochemistry of C₄H₂. The evolution of the ice’s composition was monitored using spectroscopic techniques. Results. Our results reveal that diacetylene ice is reactive through singlet-triplet absorption, similar to the photochemistry of other organic ices of Titan (such as dicyanoacetylene C4N2 ice) that we investigated previously. Several chemical processes occurred during the photolysis: the hydrogenation of C₄H₂ to form other C₄ hydrocarbons (vinylacetylene C₄H₄ to butane C₄H₁₀); the formation of larger and highly polymerizable hydrocarbons, such as triacetylene (C₆H₂); and the formation of an organic polymer that is stable at room temperature. Conclusions. The nondetection of diacetylene ice in Titan’s atmosphere or surface could be rationalized based on our experimental results that C₄H₂ is photochemically highly reactive in the solid phase when exposed to near-UV radiation that reaches Titan’s lower altitudes and surface. C₄H₂ may be one of the key molecules promoting the chemistry in the ices and aerosols of Titan’s haze layers, especially in the case of co-condensation with other organic volatiles, with which it could initiate more complex solid-phase chemistry.