Rock, Dust and Ice: Interpreting planetary data
Abstracts for Pre-Recorded Talks
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Remote Boulder Counting and Thermal IR Temperature Curves Constrain Strength, Microporosity, Thermal Conductivity and Grain Dens
Jens Biele (DLR)
Abstract: In this paper, we summarize the findings and deductions for small asteroid Ryugu from Hayabusa2 remote sensing as well as from MASCOT radiometer (MARA) data. Observations cover the VIS (broadband) and MIR (broadband) wavelength ranges. For a typical rock on Ryugu’s surface, we find a thermal inertia of 295±18 Jm-2K-1s-1/2 (Hamm+ 2020), a microporosity of 50 ± 3% (Grott+ 2020, Hamm+, 2020), and assuming a CM composition and thus an inferred specific heat capacity of cp = 890 Jkg-1K-1 (± 10%, at an average temperature of 277 K), we estimate a thermal conductivity of 0.069 ± 0.012 Jm-1K-1 at ~277 K. These estimates are based on MARA surface brightness temperature measurements of an arguably (Biele+ 2019) dust-free boulder at MASCOT’s landing site obtained over a full diurnal cycle. Those values are consistent with the TIR instrument’s global findings (Okada+ 2020). The main source of uncertainty in the thermal inertia estimate is due to the uncertain surface orientation of the boulder top that determines the insolation power. Including a Digital Terrain Model (DTM) of the observed boulder, embedded in the MASCOT landing site (Scholten+, 2019), into the thermal model could reduce this uncertainty significantly. The very high deduced microporosity lets us reasonably estimate the tensile strength of those abundant “cauliflower rocks” (Jaumann+, 2019), ~200-280 kPa (Grott+, 2019).
Furthermore, also from orbital data (ONC imaging and counting, plus radiometric data for GM), we have estimated the macroporosity of Ryugu, assumed to be a homogeneous rubble pile, based on granular mixing theory and the size-frequency distribution of boulders ranging from ~0.1 m to ~100 m diameter. We find that the macroporosity of Ryugu is very low, 16±3% and that if the underlying homogeneity assumption is true, taken together with Ryugu’s bulk density and the average microporosity of its boulders, the average grain density can be estimated as 2.85±0.15 g/cm³, consistent with the mineralogy of CM meteorites or the ungrouped carbonaceous chondrite Tagish Lake. It will be exciting to compare these values to actual laboratory measurements of the returned samples (later in 2021). For example, if our values for Ryugu’s macroporosity and rock microporosity (and/or grain density) do not agree with what is found from the samples, the assumption of homogeneity might be wrong. This would mean that Ryugu’s surface has a significantly different boulder SFD than its interior implying regolith size sorting processes which may result in bulk density variations. Or, simpler, the assumed relationship between rock porosity and thermal conductivity is incorrect. As for the strength of rock pieces, besides possible size, i.e., scale dependencies and sampling bias (weak pieces tend not to survive the sampling process intact), a higher strength than predicted here would have to be reconciled with the very low thermal conductivity of Ryugu’s blocks, which dictates rather small grain-grain neck diameters, that are either sintered, volatile condensates, or salts. More laboratory data (and theory/simulations) on the thermal conductivity and strength of very porous rocks are urgently needed. To this end, we are currently studying UTPS, a cold pressed Phobos (Asteroid type) simulant (Miyamoto+ 2018) that can be produced with porosities of ~30 to ~50% and is competent, yet weak.
Values of Internal Scattering Coefficient Obtained from Reflectance Spectra of Meteorites and Minerals
Jorge Alejandro Gonzales Davalos (Grupo de Investigación en Astronomía, Facultad de Ciencias Físicas, Universidad Nacional Mayor de San Marcos)
Abstract: In this work was obtained values of the internal scattering coefficient s as a function of the wavelength from measurements of reflectance of a suite of eight particulate samples in the VIS/NIR from RELAB database. First of all, was found the imaginary part of the index of refraction inverting their reflectance spectra using the Hapke’s model considering the correction by porosity. For the inverse process was not assuming any a-priori value. The average value obtained for the internal scattering coefficient from the spectra inversion at the center of the first absoption band was 0.0692µm-1, which is in the same order of the value found in the literature for a synthetic sample of silicate glass. Also in this work, values of s were obtained for each particle size range fitting each spectra using the average of the imaginary index of refraction. Better values of s were found in the range 0-0.0963µm-1, correspondent at the range in particle diameter from 37.50 to 310.12µm. For this analysis, it is expected to find a positive correlation between s and D, but for this methodology the majority of the samples have negative or weak correlation, only one sample has a positive correlation. Therefore, we could not state whether there is some correlation between s and D, and more analysis is required.
Thermal Infrared Observations of the Moon from Diviner and L-CIRiS
Paul Hayne (University of Colorado Boulder)
Abstract: Earth’s Moon was among the first solar system objects to be observed systematically in the thermal IR (Pettit and Nicholson, 1931), and for decades was the only one from which regolith samples had been collected. Both lunar thermophysical and infrared spectral properties have been studied extensively through remote sensing, in-situ measurements, laboratory measurements, and modeling. At thermal IR wavelengths (~7 - 400 µm) the Diviner radiometer onboard NASA’s Lunar Reconnaissance Orbiter spacecraft has systematically mapped lunar surface temperatures (Williams et al., 2017) and derived rock abundance and regolith thermal inertia from multi-wavelength measurements with spatial resolution ~250 m (Bandfield et al., 2011; Hayne et al., 2017). Therefore, the Moon serves as a “ground truth” calibration point for models and observations of airless bodies more broadly. Yet, many of the characteristics of the Moon’s thermal IR emission are attributed to small-scale phenomena inferred but not yet directly observed. In late 2022, NASA will deliver the first thermal imager operating on the Moon’s surface, the Lunar Compact Infrared Imaging System (L-CIRiS). Here, we summarize the state-of-the-art thermophysical models for the Moon based on Diviner data, and describe how upcoming L-CIRiS data resolving features < 1 cm will push models toward higher spatial resolution and greater physical realism. We also suggest several modeling benchmarks based on: 1) existing constraints from Diviner data, and 2) testable hypotheses for the L-CIRiS investigation. Studies of the Moon and other airless bodies such as asteroids may benefit from both Diviner and L-CIRiS data. Furthermore, these datasets could be highly complementary to SOFIA observations of the Moon at similar wavelengths.
Revisiting (3200) Phaethon’s Thermal Inertia
Eric MacLennan (University of Helsinki)
Abstract: With a perihelion distance of q=0.14 au, the near-Earth asteroid (3200) Phaethon is the largest asteroid with q < 0.3 au. It’s repeatedly-observed dust tail at perihelion and orbital similarity to the Gemini meteor stream strongly suggest that it is the primary contributor of meteoroid material to the stream. Phaethon’s thermal inertia was previously estimated to be 600 +/- 200 (SI units) by Hanus et al. (2018) using a convex shape model and infrared data collected over three epochs at heliocentric distances ranging from 1.03 to 1.13 au. Using a radar-derived shape model of Phaethon and a larger thermal dataset (including NEOWISE data), we present new estimates of Phaethon’s thermal inertia at eight separate epochs at heliocentric distances, ranging from 1.03 to 2.32 au. We demonstrate that Phaethon’s thermal inertia is inversely-correlated with heliocentric distance and is lower than the Hanus et al. (2018) estimates at similar heliocentric distances. A function fit to these data show that the thermal inertia dependency on heliocentric distance is stronger than expected when accounting for the temperature-dependence relation of radiative thermal conductivity (e.g., Rozitis et al., 2018). As suggested by Rozitis et al. (2018), we explore the possibility that the observed thermal inertia variation is in part due to depth-dependent regolith properties as a result of changes in the thermal skin depth.
Interpretation of Short-Wavelength Thermal Observations: The Case of Ryugu
Thomas Müller (Max-Planck-Institut für extraterrestrische Physik)
Abstract: Thermalphysical models are widely used to interpret thermal measurements of near-Earth and main-belt asteroids (e.g., Delbo et al. 2015), and in some cases also for icy trans-Neptunian objects (e.g., Müller et al. 2020). The derived radiometric sizes, albedos and thermal properties are reliable when good-quality spin-shape solutions are available and when multi-wavelength thermal measurements are available for different phase angles. However, these techniques are not well tested for cases where the majority of the thermal measurements were taken in the short-wavelength regime below the thermal emission peak, e.g., from warm Spitzer-IRAC, WISE-W1/W2 bands, or groundbased observations up to the M-band around 5 μm.
Müller et al. (2017) analysed a collection of pre-mission thermal measurements of 162173 Ryugu, the Hayabusa-2 target asteroid. The data set was dominated by short-wavelength Spitzer-IRAC data at 3.55 and 4.49 μm, complemented by a Spitzer-IRS spectrum and a few individual data points at longer wavelengths. The best-fit radiometric size (850-880 m), albedo (0.044-0.050) and thermal inertia (150-300 Jm-2s-0.5K-1) agree very well with in-situ properties (896 m, 0.045, 300+/-100 Jm-2s-0.5K-1; Watanabe et al. 2019; Okada et al. 2020). However, the radiometric solution from 2017 pointed towards a smooth surface. A very low roughness in the TPM setup was required to explain the short- and long-wavelength data simultaneously. This is in strong contrast to the results derived from the Hayabusa-2! Close-proximity measurements of Ryugu obtained by the visual and infrared instruments of Hayabusa-2 revealed a surface roughness rms of 47° ± 5° (Shimaki et al. 2020), a high value when compared to lunar surface with a rms of surface slopes of 32° (Rozitis et al. 2011; Bandfield et al. 2015). So, what went wrong in the 2017 pre-mission TPM study? We repeated the analysis of the pre-mission IR measurements of Ryugu but now using the in-situ size, shape, spin (Watanabe et al. 2019) and surface roughness properties (47° ± 5°; Shimaki et al. (2020)). In a first step, we used a standard constant spectral emissivity of 0.9. This default e=0.90 assumption leads to a best-fit thermal inertia above 1000 Jm-2s-0.5K-1 in the radiometric minimalisation technique (e.g., Alí-Lagoa et al. 2020). In a second approach, we applied a spectral emissivity derived from high-accuracy disk-integrated measurements of the Moon (Müller et al. 2021). The lunar emissivity curve has a minimum value of about 0.7 around 4.5 μm and increasing to values close to 1.0 beyond 13 μm. As a result, we found a very consistent TPM solution for Ryugu for thermal inertias between 150 and 400 Jm-2s-0.5K-1, very close to the published values of about 300 Jm−2s−0.5K−1 (Okada et al. 2020) and 225 ± 45 Jm−2s−0.5K−1 (Shimaki et al. 2020). This example illustrates the importance of realistic spectral emissivity in the context of radiometric studies for short-wavelengh (<10 μm) thermal observations.
Near-Earth Object and Meteorite Physical Properties Online Database
Daniel Ostrowski (Bay Area Environmental Research Institute, NASA Ames Research Center)
Abstract: Information about the physical characteristics of near-Earth objects (NEOs) is needed to model behavior during atmospheric entry, to assess the risk of an impact, and to model possible mitigation techniques. NASA’s Asteroid Threat Assessment Project at Ames Research Center has built the Near-Earth Object Properties online database (neoproperties.arc.nasa.gov) that provides an easily accessible repository of aggregated data about the physical properties of near-earth objects and meteorites. The focus of the recorded data is on properties relevant to planetary defense, but can be useful for other planetary science studies. The meteorite data set contains one or more measurements of over 900 meteorites and more than 2300 unique samples. The tabulated physical properties include densities, porosity, acoustic velocity, elastic mechanical properties, electrical resistivity, magnetic susceptibility, and thermal properties. The database includes taxonomies for ~690 NEOs and thermally determined diameters for ~ 2300 NEOs. A literature-based mapping between asteroid taxonomic classes and related meteorite classes (and vice versa) is provided. This consolidated database can be used to support investigations into the properties of single asteroids and their likely meteorite analogs as well as facilitating analyses of the properties of the overall near-Earth asteroid population.
SBNAF Database for Thermal Infrared Observations of Small Bodies in the Solar System
Róbert Szakáts (Konkoly Observatory, Research Centre for Astronomy and Earth Sciences, ELKH)
Abstract: One of the goals of the Small Bodies: Near and Far (SBNAF) project was to create a database for thermal infrared observations of small bodies. We collected published thermal IR measurements for our selected samples of Solar System targets including data from large missions (e.g. catalogues based on Akari, IRAS and WISE observations) and also data from smaller scale and individual reductions (e.g. the Herschel Space Observatory measurements of near-Earth and main belt asteroids). A primary goal of this database is to help scientists working in the field of modeling the thermal emission of small bodies. However, the database has the option to include more data of Solar System small bodies which have been observed at thermal IR wavelengths from space or with ground-based instruments. Researchers who have infrared measurements can submit their published data to us and we can make it available in our database via our webpage, or VO tools.
Colour and Multi-Angular Observations of Martian Slope Streaks
Adomas Valantinas (Physikalisches Institut, Universität Bern)
Abstract: Colour and multi-angular observations can be used to determine properties such as roughness, grain size and relative composition of planetary regoliths. The Colour and Stereo Surface Imaging System (CaSSIS) on board ESA’s ExoMars Trace Gas Orbiter (TGO) can observe the Martian surface under various illumination geometries due to the non-Sun-synchronous orbit of TGO. Here, we use these capabilities to observe albedo variations of active mass wasting features on equatorial slopes, known as slope streaks. We detect a sharp increase of brightness relative to surrounding Martian dust in the BLU band (497 nm) under low phase angle observations. Slope streaks may be composed of Martian soil with unique properties that could be affected by opposition surge or coherent backscattering effects. To further investigate these phenomena, we contrast our observations with a Hapke-theory-based model of the spectral reflectance of Martian dust of different grain sizes, and with spectrometry and photo-goniometry measurements of different grain size distributions of a high-fidelity Martian soil analog.
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Simulating Icy Regolith for Exploration and Resource Development Efforts
Vincent Roux (Off Planet Research)
Abstract: Creating icy regolith simulants using cryogenic vapor deposition involves overcoming several challenges while providing several advantages. The ice formed within and upon regolith by this process has a different structure than other formation methods and can produce icy regolith simulants that more closely resemble intended mission destinations. A brief overview of the use of this type of simulant in 2019 to test the capabilities of an engineering version of the Near Infrared Volatile Spectrometer Subsystem (NIRVSS). The ability to produce simulated icy regolith for other worlds will be outlined. An NSF Phase 1 grant is to improve and expand production to provide these test materials is under way, and input from the community is requested to ensure that the process best meets researcher needs.
Dynamics of Water Adsorption on the Moon and Ceres: Theoretical Predictions
Norbert Schorghofer (Planetary Science Institute)
Abstract: On the airless bodies of the inner solar system, volatile water molecules can be adsorbed on silicate grains at sub-monolayer coverage. They may be found at higher concentration at the winter polar region of Ceres, where an optically thin seasonal water cap has been predicted. The concentration of adsorbed water molecules is also expected to increase with depth, due to a process known as “adsorbate pump”: The decrease of the diurnal temperature amplitude with depth leads to a decrease in the (time-averaged) vapor pressure, which in the long-term is compensated by an increase in adsorbate density. This process can even lead to the sequestration of ice, specifically in sunlit areas close to but outside of lunar cold traps, where the diurnal variation in the surface concentration of H2O water molecules is predicted to be largest.
What We Can Say about Ice of the Saturn’s Rings Particles
Vladimir V. Tchernyi (Modern Science Institute, SAIBR)
Abstract: The Cassini found particles of Saturn’s rings contain 93% ice. Ice in the rings has existed for billions of years. It is hardly possible to create such ice in a laboratory on Earth to simulate its properties. There are two possibilities. We can try to find some types of ice on Earth that can match the environmental parameters in Saturn’s rings. You can also try to understand what properties of ice particles can have from the theory of the origin of Saturn’s rings, if it is consistent with the measurements of the Cassini probe. We know 17 types of ice on Earth. As it turned out, type XI ice has stable parameters at the temperature of the rings. Such ice was discovered in Antarctica, its age estimated as 100 years. It may originate of ordinary ice below -32,8°F. On the other hand, we found that it is possible to construct a theory of the origin of Saturn’s rings if we assume that the ice in the particles of the protoplanetary cloud has diamagnetism. It also turned out that ice XI is diamagnetic. After emerging of Saturn’s magnetic field, all chaotic orbits of the ice particles due to the force of diamagnetic expulsion begin to shift to the magnetic equator plane, where the minimum magnetic energy of the particles is observed. Finally all particles are trapped in a three-dimensional magnetic well.
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Researching the Planetary Environment with an Interstellar Probe
Caitlin Ahrens (NASA Goddard Space Flight Center)
Abstract: In 2018, a study originated with the idea of a mission that would be feasible to launch in the 2030s, targeting 1000 AU within 50 years using current technology. While the primary objective of such an Interstellar Probe would be to understand the heliosphere and interstellar medium, this probe offers an excellent opportunity for rock, dust, and ice sciences. In the initial stages of its journey through the solar system, this Interstellar Probe would carry out a wide range of potential observations to study the planetary environment, particularly focusing on dust/ice analysis and planetary science through fly-bys of critical science targets, especially in the trans-Neptunian region. A flyby of a trans-Neptunian dwarf planet, such as Quaoar, would provide further geological, compositional, and geophysical context for Earth-based observations. Aside from in-situ remote sensing techniques, VISIR and dust analyses would also benefit in this region of the solar system to determine (i) dominant ice and dust compositions and potential variations depending on heliocentric distance (e.g., chemical or irradiated products); (ii) collections and identification of PAH-type components; and (iii) solar nebula chemical and mechanical processing, such as collisions. The purpose of our poster is to provide a background of the Interstellar Probe’s objectives and possible instrumentation to offer insight on current questions about the planetary environment.
Using ROCKE-3D General Circulation Model to Reveal Titan’s Atmospheric Dynamics
Maxwell Collins (University of Hong Kong)
Abstract: The Resolving Orbital and Climate Keys of Earth and Extraterrestrial Environments with Dynamics (ROCKE-3D) is a General Circulation Model adapted from the Goddard Institute for Space Studies ModelE2, which simulates modern and paleo-Earth climate. ROCKE-3D expands upon the base model to include the possibility of modeling extraterrestrial bodies such as Saturn’s moon Titan. Previous models of Titan have largely been focused on 1D or 2D radiative-convection models and have neglected large-scale spatial distributions. This model includes spectral input files within Titan’s range of received radiation and can be further improved upon, including updated topographical maps and orbital parameters adapted to synchronously rotating bodies (Way et al 2017). Titan’s nitrogen-rich atmosphere contains complex organic chemistry key to understanding the origins of life. This, combined with stable surface liquid methane/ethane bodies, creates an environment conducive to crucial prebiotic chemistry. ROCKE-3D contains extensive coupled atmospheric-surface interactions which provide insight into regions of habitability; atmospheric dynamics over long timescales in conjunction with updated topographical input describe essential features in surface deposition of heavy organic material, i.e., polycyclic aromatic hydrocarbons and tholins, given their production region and lifetime. The ROCKE-3D Titan GCM may be used to advise future missions in locations of interest and as an analogue to early Earth and origins of life studies as is identified by similar chemical constituents such as hydrogen cyanide (HCN).
Constraining Asteroid Regolith Grain Size with Hapke Radiative Transfer Modeling
Annika Gustafsson (Northern Arizona University)
Abstract: Radiative transfer models are some of the most widely used tools for compositional analyses of planetary bodies. For the application of silicate rich asteroids, radiative transfer models have been almost exclusively used to derive olivine to pyroxene abundance ratios. However, formulas for deriving mineralogies from visible and near-infrared spectra of asteroids with prominent olivine and pyroxene (1 and 2 micron) absorption bands have been developed (e.g Burbine et al. 2007; Reddy et al. 2011), and the effects of non-compositional parameters (temperature, phase angle, grain size) have been well characterized, allowing for more accurate interpretations of asteroid surface properties (Burbine et al. 2009; Sanchez et al. 2012; Reddy et al. 2012). The implementation of this technique into the small body field, where it has not yet been fully adopted, will vastly improve our understanding of surface properties in the population of S/Q type asteroids. A benefit of this technique is that it requires far fewer observations than thermal modeling and allows for the observation of much smaller targets using remote-sensing.
We have implemented radiative transfer modeling on visible and near-infrared spectra of unresolved silicate-rich asteroids using Hapke modeling to constrain surface grain sizes. We will discuss our assessment of the limitations of this technique utilizing visible and near-infrared spectra of ordinary chondrite meteorites from the RELAB database.
Multi-Wavelength, Spatial Resolved Modelling of Debris Discs
Shane Hengst (University of Southern Queensland)
Abstract: Debris discs are the dusty aftermath of planet formation processes around main-sequence stars. Debris discs are analogous to the Solar System’s debris belts (i.e., the asteroid and Kuiper belts) and studying these objects are one of the few ways to gain insight into the composition of other planetary systems. Recent observations have suggested that dusty debris discs also have ice and gas components; comparing them with the Solar system can point towards common formation mechanisms for the planetesimals which produce the detectable debris. Modelling of the disc structure and dust grain properties for those discs is often hindered by the absence of any meaningful constraint on the location and spatial extent of the disc. Multi-wavelength, spatially resolved imaging, and mid-infrared spectroscopy, are complementary to the continuum emission to refine the interpretation of these systems. In this talk I will present the results of studies of two debris disc systems resolved by Herschel, summarising the advances in our understanding of their architectures and properties obtained by this analysis. I will highlight ongoing and future work that will leverage the rich data sets available for some systems to provide points of comparison for planet formation processes in the Solar system and around other stars.
Radiative Transfer of Polarized Radiation for Investigating (Exo)planetary Atmospheres
Moritz Lietzow (Institute of Theoretical Physics and Astrophysics, Kiel University)
Abstract: Polarimetry is a powerful tool for determining the properties of planetary atmospheres, surface properties and the planetary environment. To provide the basis for preparatory studies and the interpretation of dedicated polarization measurements, sophisticated simulation and data analysis tools are required. For this purpose, we developed a radiative transfer simulation software that contains all relevant continuum polarization mechanisms for the comprehensive analysis of the polarized flux resulting from the scattering in the atmosphere, on the surface, and in the local planetary environment. Rayleigh scattering by small particles (e.g., gas molecules), Mie scattering by larger particles (e.g., clouds or dust), as well as surface reflections (e.g., by oceans or landmasses) are considered. The 3D Monte Carlo radiative transfer code POLARIS (Reissl et al. 2016) provides the platform for our simulation software. In a first case study, we investigate the impact of a circumplanetary ring consisting of micrometer-sized water-ice particles on the net polarization signal (Lietzow et al. 2021). While the focus is on the characterization of exoplanetary systems with polarization in the optical / near-infrared wavelength region, this tool can also be applied in the context of polarization studies of solar system objects.
(Sub-)Millimeter Dust Opacities from Laboratory Measurements
Harald Mutschke (Astrophysical Institute and University Observatory, Friedrich Schiller University)
Abstract: In this poster, we present measured low-temperature mass absorption coefficients of glassy and crystalline silicates, water ice, and carbonaceous materials in the far-infrared up to millimeter wavelengths. Some of these data have been transformed into complex refractive indices that can be directly used in the modeling of emissivities for thermal radiation of solid matter at low temperatures.
Understanding Cloud Formation on Titan
Xinting Yu (University of California Santa Cruz)
Abstract: Titan’s N2-CH4 atmosphere has enabled rich photochemistry to occur in its upper atmosphere. The photochemistry can create simple hydrocarbons and nitriles such as ethane, acetylene, benzene, and hydrogen cyanide. These simple organics are further processed to form complex organic haze particles. Many of the photochemically produced simple organics are condensable in certain altitudes of Titan’s atmosphere to form liquid or ice clouds. In order to form sufficient observable clouds, heterogeneous nucleation is needed for efficient cloud growth. On Titan, the haze particles are proposed to be the main heterogeneous cloud condensation nuclei (CCN) for the various cloud species. Previous efforts on studying Titan cloud formation focus on directly measuring methane/ethane adsorption on laboratory-produced Titan haze analogs, “tholin”, under cryogenic conditions. However, the viability to form other kinds of clouds remains unknown. We approach this question differently by first measuring the surface energy of tholin through the contact angle method, which then enables us to approach this question theoretically via the wetting theory. By using the measured surface energy of tholin and surface tensions of various organic species of interest, we can calculate contact angles formed between tholin and possible condensates. We find that Titan haze particles are likely good CCN for various cloud condensates on Titan, which suggests that we are expected to see more types of clouds on Titan.