ABSTRACTS of THERMOPSII presentations
1 - How to determine asteroid albedos: the role of Polarimetry
A. Cellino
The determination of asteroid albedos is an important task, because this parameter is related to surface composition and is very important for taxonomic classification purposes. The albedo values derived for most individual objects by means of the thermal radiometry technique are affected by high uncertainties due to a number of reasons. It seems that Polarimetry can be a better tool to derive more accurate estimates of the albedo, and also some other properties of the surface regolith. A better synergy between different techniques, including Polarimetry, Photometry, Spectroscopy and Thermal Radiometry is the key to strongly improve our capability to obtain accurate characterizations of the physical properties of the asteroids.
2- A New Analysis of Activity Drivers in the Enigmatic Comet 29P/Schwassmann-Wachmann 1
C. A. Schambeau
Introduction: 29P/Schwassmann-Wachmann 1 (SW1) is a unique comet (and Centaur) in an almost circular orbit just outside the orbit of Jupiter. This orbit results in SW1 receiving a nearly constant insolation, thus giving a simpler environment in which to study thermal properties and behaviors of this comet's nucleus. Such knowledge is crucial for improving our understanding of distant cometary activity drivers, drivers of cometary outbursts, nuclear thermal evolution, and nuclear structure. To this end, our overarching goal is to implement a thermophysical model of SW1's nucleus that makes use of realistic physical and structural properties as inputs. This model will help to explain the highly variable gas- and dust-production rates of this comet; SW1 is well known for it frequent but stochastic outbursts of mass loss [1,2,3]. Results: We present results from our ongoing effort to model the thermal evolution of cometary nuclei. In particular we are focusing on the enigmatic comet 29P/Schwassmann-Wachmann 1 (SW1) and the implementation of a thermophysical model of the comet's nucleus [4,5]. What makes our model different (compared with standard asteroid thermal models) is its incorporation of mass flux from the surface, its ability to handle compositional layering of the interior, its inclusion of porosity of the layers, its ability to handle compositional material phase changes such as crystallization of amorphous H2O ices, and sublimation of supervolatile species (e.g. CO, CO2, HCN, ...). The model also takes advantage of our knowledge of physical and dynamical properties of SW1's nucleus; in earlier work we have been able to put constraints on SW1's size, global-average beaming parameter, spin period, and spin axis direction [6,7]. ``Goodness of fit" of a model is tested by comparing model mass fluxes to dust mass loss rates measured from photometry; we have SW1 archival observations covering different positions all around its orbit from which to draw on. By comparing the observed and modeled dust production rates for matching trends, our overarching science goal is to gain a better understanding of what drives SW1's (and possibly all distant comet) activity. Of importance is our model's ability to possibly differentiate between the drivers of SW1's constant background coma (possibly sublimation of supervolatile species) and random outbursts (possibly thermal waves produced by crystallization fronts in amorphous water ice). We will present preliminary results from our initial model runs using various initial conditions and spin pole orientations. Acknowledgements: We acknowledge funding support from NASA's Outer Planets Research program (grant NNX12AK50G) and the Center for Lunar and Asteroid Surface Science (CLASS). References: [1] Kossacki, K. J. and Szutowicz, S.: 2013, Icarus, 225, 111-121. [2] Trigo-Rodriguez, J. M., et al.: 2008, A&A, 485, 599-606. [3] Trigo-Rodriguez, J. M., et al.: 2010, MNRAS, 409, 1682-1690. [4] Sarid, G., et al.: 2005, The Publications of the Astronomical Society of the Pacific, 117, issue 834. [5] Sarid, G., 2009, PhD thesis, Tel Aviv Univ. [6] Schambeau, C. A., et al.: 2014, Asteroids, Comets, Meteors Book of Abstracts, 46, 103.08. [7] Schambeau, C. A., et al.: 2015, Icarus, submitted (in review).
3 - Title : Surface thermal properties of 67/P inferred by VIRTIS/Rosetta
C. Leyrat
Since August 2014, the VIRTIS imaging spectrometer onboard the Rosetta ESA spacecraft has intensively observed both the nucleus and the coma environment of the comet 67P/Churyumov-Gerasimenko, in the 0.25-5 microns wavelength range. Nucleus observations are performed with both channels: VIRTIS-M for spectral mapping and VIRTIS-H for high spectral resolution . Each spectrum contained both reflected solar light and thermal emission of the surface itself. From the long wavelength range (typically 4-5 microns), we can derive the effective surface temperatures on the dayside at a variety of locations (latitude, longitude) on the surface under diverse conditions of illumination, local times and emission angles . This allows us to infer local thermal properties of the first millimeters of the regolith. Interestingly, the very irregular shape of 67P results in unusual patterns in the heating / cooling regime of the object (e.g. sudden transitions from day to night) which can be used to better understand sublimation processes when ice is present. CG-67/P is on the the small bodies, along with VESTA and CERES, that is observed continuously at high spatial resolution. In addition, the very elliptical orbit of the comet associated to its tilted spin axis implies strong seasonal effects that can be observed in thermal infrared. We will present thermal analyses of observations performed since mid-2014, with a focus on thermo-physical modeling of comet 67P on both regional and local scales. In particular, we will (i) model the diurnal curves in terms of global thermal properties (e.g. diurnal thermal inertia as a function of latitude and temperatures) and we will (ii) investigate possible changes of these properties with heliocentric distance.
4 - SHERMAN: A Shape-based Thermophysical Model for Near-Earth Asteroids
Ellen S. Howell
We have developed a thermophysical model, SHERMAN, which can produce temperature distributions and thermal fluxes for airless bodies. Although intended for near-Earth asteroids (NEAs), it has applications to the Moon and may be adapted for other situations. SHERMAN is based on the asteroid shape modeling code developed by Hudson (1993) and enhanced by Magri et al., (2007, 2011). The input model asteroid can have an arbitrary shape described as a polyhedral solid with triangular facets. The user specifies a spin state, which can include non-principal axis rotation. The asteroid orbit and positions of the Sun and Earth are input so that the proper heating and viewing geometry of the object are used. The user specifies the optical scattering law, IR emissivity, thermal inertia, specific heat, density and conductivity, all of which are permitted to vary across the surface if desired. We have applied this model to relative reflectance observations of NEAs with a wide range of compositions, sizes and shapes, in order to determine how much these properties can affect the thermal emission. We have selected objects that are also observed with radar, so that the actual shape and spin state is known, at least to some extent. We find that irregular shape can significantly affect the surface temperature distribution. Observations at several different viewing geometries can rarely be fit by a single set of thermal parameters, unless the detailed shape, spin and surface properties are included. Applications to thermal observations of NEAs (8567) 1996 HW1, 4769 Castalia and (285263) 1998 QE2 illustrate some of the situations this model allows us to explore. We present model results and discuss future applications to better understand the nature of asteroid surfaces.
5 - Constraining Thermal Inertia using a Spherical TPM
Eric M. MacLennan
When constraining thermal inertia from infrared flux measurements, spherical shapes are not often assumed. Techniques for incorporating modeled (non-spherical) shapes into the inversion process have been established [e.g., 1,2], yet shape models are not generally available for all (or most) objects that are observed in the thermal-IR. This is particularly true as the pace of infrared observations has recently dramatically increased, notably due to the WISE mission [3], while the time to acquire sufficient light curves for accurate shape inversion (e.g. DAMIT [4]) remains relatively long. Here, we investigate the accuracy of using a spherical TPM, combined with infrared observations obtained at pre- and post-opposition (hereafter multi-epoch) geometries to constrain the thermal inertias of a large number of asteroids. We test whether using this unique dataset of multi-epoch observations and stepping through a grid of spin vectors, the spherical TPM can reasonably constrain the thermal inertia of an object without a priori knowledge of its shape or spin state. The effectiveness of this technique is tested for 16 objects with shape models and WISE multi-epoch observations. For each object, a non-spherical TPM is used to generate synthetic fluxes for different values of thermal inertia. A spherical TPM stepped through different spin vectors is then fit to these synthetic flux “observations”, allowing for a direct assessment of its effectiveness for different thermal inertias and object shapes. We will discuss whether the precision of the thermal inertia constraints from the spherical TPM analysis of multi-epoch observations is comparable to works that use non-spherical shapes. The findings presented at the conference will be discussed in relation to works that also use different shape models (i.e. sphere, lightcurve-derived and radar-derived) to perform TPM analyses on asteroid thermal data (e.g. (101955) Bennu [5], (25143) Itokawa [6], and (1620) Geographos [7]).
References [1] Lagerros, J. S. V. (1996) A&A 310, 1011. [2] Mueller, M. (2007) Ph.D. Dissertation. [3] Mainzer, A., et al. (2011) ApJ, 731, 53. [4] Durech, J., et al. (2010) A&A, 513, A46. [5] Emery, J. P., et al. (2014) Icarus, 234, 17. [6] Muller, T. G., et al. (2014) PASJ, 66, 52. [7] Rozitis, B. and Green, S. F. (2014) A&A, 568, A43.
6 - Rosetta/MIRO Observations of Subsurface Temperatures of the Nucleus of 67P/Churyumov-Gerasimenko
F. P. Schloerb
Observations of the nucleus of 67P/Churyumov-Gerasimenko in the millimeter-wave continuum have been obtained by the Microwave Instrument for the Rosetta Orbiter (MIRO). We present data obtained at wavelengths of 0.5~mm and 1.6~mm during September 2014 when the nucleus was at heliocentric distances between 3.45 and 3.27~AU. The data are fit to simple models of the nucleus thermal emission in order to characterize the observed behavior and make quantitative estimates of important physical parameters. MIRO brightness temperatures on the irregular surface of 67P/C-G~are strongly affected by the local solar illumination conditions, and there is a strong latitudinal dependence of the mean brightness temperature as a result of the seasonal orientation of the comet's rotation axis with respect to the Sun. The MIRO emission exhibits strong diurnal variations, which indicate that it arises from within the thermally varying layer in the upper centimeters of the surface. The data are quantitatively consistent with very low thermal inertia values, between 10-30 $\rm{J} \, \rm{K}^{-1} \, \rm{m}^{-2} \, \rm{s}^{-\frac{1}{2}}$. Although the data are generally consistent with simple, homogeneous models, it is difficult to match all features of the data, suggesting that there may be some vertical structure within the upper few centimeters of the surface. The MIRO brightness temperatures at high Northern latitudes are consistent with sublimation of ice playing an important role in setting the temperatures of these regions where, in fact, ice is known to be sublimating from observations of gas and dust production.
7 - Infrared Asteroid Survey with AKARI
Fumihiko Usui
The physical properties of asteroids are fundamental to understanding the formation process of our solar system. Size and albedo are the basic physical properties of asteroids. The most effective method for measuring size and albedo is to find from radiometry. Using radiometric measurements, a large number of objects can be observed in a short period of time, thus providing uniform data for large populations of asteroids. Infrared measurements using space-borne telescopes are suitable for this method. The Japanese infrared astronomical satellite AKARI, launched on February 21, 2006 carried out the second generation infrared all-sky survey after IRAS. AKARI's liquid helium supply lasted until August 26, 2007 and enabled 550 days of fully cryogenic operations. During this cold mission phase, it surveyed more than 96% of the sky at 6 bands in mid- to far-infrared wavelengths. The mid-infrared part of the all-sky survey was conducted at 9 and 18 micron bands, using the on board InfraRed Camera (IRC). An asteroid survey was conducted from the mid-infrared survey data. The 16-month continuous cold mission allowed the inner edge of the main belt to be observed at least once. For each identified asteroid, we calculated the size and albedo using the Standard Thermal Model of asteroids. Then, we obtained an unbiased, homogeneous asteroid catalog named the Asteroid Catalog Using AKARI (AcuA). The catalog is publicly available at http://darts.jaxa.jp/ir/akari/catalogue/AcuA.html. AKARI had also the capability of performing deep imaging and/or spectroscopy in targeted observation mode. After the exhaustion of cryogen, i.e., in the AKARI's warm mission phase, the IRC carried out more than 12,000 targeted observations for a wide variety of astronomical targets ranging from solar system objects to galaxies at cosmological distances. Low resolution spectroscopic observations were performed using the near-infrared channel of the IRC from 2-5 micron, which provide valuable data in the next few decades because of its high sensitivity and unique wavelength coverage. As a part of the targeted spectroscopic observations, about 70 asteroids were observed. In this talk, we present the detail of the AKARI asteroid survey, and also show the results of spectroscopic observations of the selected asteroids.
8 - Origin of Sample-Rerurn Target Asteroids
Humberto Campins
Near-Earth asteroids (NEAs) 101955 Bennu and (162173) 1999 JU3 are both potentially hazardous to Earth and the targets of NASA’s OSIRIS-REx and JAXA’s Hayabusa-2 sample-return missions, respectively. These two objects are believed to originate in the inner asteroid belt and we use them as case studies for estimating the origin of NEAs using dynamical, spectral and thermal constraints.
9 - The Oxford Thermal Emissivity for Regolith Model: Comparison with laboratory measurements
J.A. Arnold
Mid-infrared (mid-IR), 2.5-100 μm or 100-4000 cm^{-1}, spectroscopy is a valuable tool for understanding regolith properties. However, mid-IR emissivity is controlled by many factors such as: composition; grain size distribution, shape, and packing [Hunt and Logan, 1972; Salisbury and Wald, 1992]; as well as atmospheric pressure and the thermal environment [Logan and Hunt, 1970; Henderson and Jakosky, 1997]. At present, models are not fully able to reproduce these dependencies, especially for very fine particulate powders (d~<60μm) typical of regolith of airless solar system bodies [Moersch and Christensen, 1995; Mustard and Hays, 1997]. Our group is developing and testing a model, the Oxford Thermal Emission for Regolith Model (O-ThERM), which calculates near-surface thermal gradients for a layered regolith as a function of various environmental conditions [see Lindsay et al., this meeting]. This is a continuation of the work presented in Millán et al. [2011] and Millán et al. [in preparation]. Here, we focus on the parameters required to model emissivity spectra of well-characterized mineral samples. We have tested several variables related to sample preparation and the laboratory environment including: lamp temperature, packing density, mineral composition and grain size (in both mono-disperse and log-normal distributions). Additionally, we have done a limited number of tests where packing fraction as a function of different log-normal size distributions is changed according to curves derived from experiments on glass beads [c.f. Wakeman et al., 1975]. O-ThERM results suggest that the responses of the reststrahlen bands (RB), Christiansen feature, and shorter wavelengths to grain size under cold, airless conditions differ substantially from those observed in an ambient or Earth-like environment. In ambient conditions, as particle size decreases, so does the spectral contrast of the RB [Salisbury and Wald, 1992], and our models are able to replicate this effect. However, our models of airless conditions do not follow a simple trend. We plan to test these results against spectra collected in the Simulated Lunar Environment Chamber in the Planetary Spectroscopy Facility at the University of Oxford, an emission chamber that simulates the pressure and temperature conditions of airless bodies (described in Thomas et al. [2012] and Donaldson Hanna et al. [2012]). For these measurements, we have prepared samples of olivine from San Carlos, AZ, a readily available, compositionally uniform type locality, for which optical constants [Jager et al., 1998] at the relevant wavelengths have been measured. Gem-quality millimeter-sized grains were crushed and dry sieved to obtain five different size separates (<45 μm, 45-75 μm, 75-125 μm, 125-250 μm). We plan to use an aerosizer to obtain the size distribution for each sample. We will also acquire SEM images of the grains to inform an investigation of grain shape. Preliminary results of these experiments compared to O-ThERM model results are presented.
Acknowledgements: The authors would like to thank Luis Millán for his permission to use, edit, and further his original code as described in Millán et al. [2011; in prep.]. The authors would also like to thank the Science and Technology Facilities Council (STFC) and the Leverhulme Trust for providing the funding for this work.
10 - Towards a “complete” regolith model
Jürgen Blum
Our access to the surfaces of small planetary bodies is usually by astronomical observations in the visible and infrared spectral range. To gain maximum information about the physical properties of the planetaryx regolith, like, e.g., grain size and packing density, we have to interprete the observations with the help of thermophysical models. We will present such a model, which treats the energy transport through the regolith by heat conduction and radiation. The model has been calibrated in laboratory experiments and will be presented with some exemplary cases. We will also reassess the role of the thermal inertia, which is often used in describing the thermal properties of the regolith.
11 - When comets sleep: size, albedo and beaming parameter distribution of Asteroids in Cometary Orbits
Javier Licandro
The classification criterion between asteroids and comets has evolved in recent decades, but the main phenomenological distinction remains unchanged: comets are active objects as they present gas and dust ejection from the surface at some point of their orbits, while asteroids are inert objects as they do not show any kind of large scale gas and dust ejection. Anyhow, there are some border cases between the asteroid and comet populations usually called comet-asteroid transition objects that includes: (1) objects in typical cometary orbits that never showed any kind of activity, the Asteroids in Cometary Orbits (ACOs); (2) objects in typical asteroidal orbits that presented some evidence of dust ejection, the it Active Asteroids (AAs) that include the so called Main Belt Comets (MBCs). To identify the transitional objects several classification schemes based on the orbital elements have been used. They are usually based on the Tisserand’s parameter (T_J), a constant of motion in the restricted three-body problem. Most comets have orbits with T_J < 3, while the great majority of asteroids have orbits with T_J>3. But more strict criteria is needed to decide which inactive asteroids are in orbits with an unlikely cometary origin. These objects are likely dormant comets, comets that developed an insulating dust mantle that avoid a significant ice sublimation. Tancredi (2013) presents a much more restrictive criteria to identify ACOs and a list of 316 of such objects. In this talk I present the surface physical properties of the ACOs in short and long period comet-like orbits base on multi-wavelenght (from visible to mid-infrared) spectroscopy and we will discuss the cumulative size distribution, albedo and beaming parameter of ACOs derived using WISE thermal data and a NEATM model. We will compare the observed properties to that of comet nuclei. We will show that this objects have very similar surface properties than comets, which strongly support their cometary nature.
12 - Thermophysical Model of 1627 Ivar
Jenna Crowell
1627 Ivar is an Amor class near Earth asteroid with a taxonomic type of Sqw [1] and a rotation period of 4.795162 ± 5.4 hours [2]. Ivar appears to be elongated with estimated dimensions of 11.32 x 5.51 km [2]. Its large size and close approach to Earth in 2013 (minimum distance 0.32 AU) provided an opportunity to observe the asteroid over many different viewing angles for an extended period of time. We have used lightcurves and the delay-Doppler images to generate an improved shape model of Ivar [2][3]. Here we present a thermophysical model of Ivar using this shape model and discuss the implications that these results have on the properties of the regolith of this asteroid. During Ivar’s apparition, we were able to obtain CCD lightcurves, radar data, and near-IR spectra. The radar data consists of Doppler spectra and delay-Doppler images with 300 m resolution obtained using the Arecibo Observatory’s 2380 MHz radar. Lightcurve data were gathered using the 0.35m telescope at the Palmer Divide Station. The NIR spectra encompass reflected and thermal wavelengths (0.8 – 4.1 µm) and were acquired using the SpeX instrument at the NASA IRTF [4]. We have used the software SHAPE [5] to incorporate these recent radar and lightcurve datasets in order to determine the best shape model for Ivar that updates the results presented by Kaasalainen et al. [6], which were based solely on lightcurves. Our approach was similar to that of Magri et al. [7] for 1580 Betulia. We will present our progress of using the shape model of Ivar with our thermophysical modeling code SHERMAN [3,8,9]. Input parameters for SHERMAN include the asteroid’s IR emissivity, optical scattering law and thermal inertia in order to complete thermal computations based on the shape model. We then create synthetic near-IR spectra that can be compared to our observed spectra, which cover a wide range of Ivar’s rotational longitudes and viewing geometries. SHERMAN lets us determine which reflective, thermal, and surface properties for Ivar best reproduce our spectra. From our derived best-fit thermal parameters, we will learn more about the detailed regolith and surface properties of Ivar and how those properties compare to those of other S-complex asteroids.
References: DeMeo et al. 2009, Icarus 202, 160-180. [2] Crowell, J. et al. 2015, LPSC 46. [3] Crowell, J. et al. 2014, AAS/DPS 46. [4] Rayner, J. et al. 2003, PASP 115, 362. [5] Magri, C. et al. 2011, Icarus 214, 210-227. [6] Kaasalainen, M. et al. 2004, Icarus 167, 178-196. [7] Magri C. et al. 2007, Icarus 186, 152-177. [8] Howell, E. et al. 2012, AAS/DPS 44. [9] Marshall, S. et al. 2013, AAS/DPS 45. We thank NSF (AST-1109855) and the CLASS SSERVI for their support of this work.
13 - Operational Comet surface thermal models: lessons learnt from Rosetta/Philae
Jens Biele
For the landing site selection and operations of Philae, operational CSTMs (comet surface thermal models) had to be calculated in a frenzy: Between August 2014 (orbit insertion) to September (final selection). Input relied on two Rosetta Orbiter instruments, VIRTIS (IR) and MIRO (microwave, subsurface) to estimate thermal inertia. Comet 67P/Churyumov-Gerasimenko was then, r> 3AU, rather inactive; so sublimation was not considered for the predictive models. We explain the model parameters, give some examples and report on the numerous unexpected difficulities to get a CSTM right on a small body with a complex shape.
14 - Effects of shape uncertainties in thermal modeling and revised WISE uncertainties
Josef Hanus
In the analysis of thermal infrared data of asteroids by means of thermophysical models (TPMs) it is a common practice to neglect the uncertainty of the shape model and the rotational state, which are taken as an input for the models. We will present a novel method of investigating the importance of the shape model and the pole orientation uncertainties in the thermophysical modeling - the varied shape TPM (VS-TPM). Our method uses optical photometric data to generate various shape models that map the uncertainty in the shape and the rotational state. The TPM procedure to the thermal infrared data acquired by the NASA's Wide-field Infrared Survey Explorer (WISE) is then run for all these shape models. We will also independently estimate the accuracy of the WISE thermal infrared data based on the examination of what we call double detections of asteroids - due to an overlap of about 10% between the areas of the field of view of two subsequently observed frames by WISE, we can sometimes find two flux measurements of an asteroid separated by ∼11 seconds, which is the cadence between the WISE frames. Moreover, we will discuss the possible implications of the Jarrett et al. (2011, Astrophysical Journal, 735, 112) paper where the authors study the accuracy of the absolute calibration of WISE data by performing an analysis of WISE-Spitzer flux cross-calibration of a number of calibration stars and one galaxy situated near the poles of the ecliptic.
15 - Spectroscopy in the Mid-IR of comets, asteroids and other airless bodies
Joshua Emery
Mid-infrared (~5 to 50 micron) spectroscopy is a proven tool for analysis of the silicate mineralogy of planetary bodies. Thermal emission spectra at these wavelengths of comet comae have long been used to decipher grain size distributions, crystalline fraction, and mineral phases of the dust enshrouding the nuclei. Analyses are facilitated by the optically thin nature of most comae, which allows components to be added linearly in spectral models. Observations of bedrock outcrops on various surfaces have been similarly fruitful, enabling remarkably precise constraints on modal mineralogy of these outcrops. These analyses are facilitated by the extreme optical density of the outcrops in the mid-IR; the resulting surface scattering again ensures that spectral components add linearly. Despite the successes on other bodies, mid-IR spectroscopy has yet to be widely applied to asteroid surfaces. Part of the reason has been technical – high thermal backgrounds along with atmospheric opacity and variability make it very difficult to obtain spectra of sufficiently high S/N and reliability to detect features at the one-to-few-percent level necessary for good analyses. Recently, Spitzer, during its cryogenic phase, provided a great platform for mid-IR spectral observations of asteroids, and good ground-based facilities and instruments (e.g., CanariCam on the GTC) are also providing reliable spectra. In a few years, JWST should be able to add significantly to this growing mid-IR database. Even with good mid-IR spectra of asteroids, however, the interpretation is not generally straightforward. For dusty surfaces, such as regolith-covered asteroids, the scattering regime is generally more complicated than either of the two endmembers mentioned above. In this presentation, I’ll review some issues related to scattering in regoliths, present some examples of mid-infrared spectra of asteroids (mostly from Spitzer), and discuss some laboratory work that has been published recently attempting to better understand the interpretation of these spectra.
16 - The effect of slope distribution on surface temperature on airless planetary bodies
Lior Rubanenko
The surface temperature of an airless planetary body such as the Moon depends on the amount of insolation it receives from the sun and the emitted and reflected radiation from nearby slopes. Motivated by the close relationship between temperature and volatile stability, we seek to consider the effect of the slope distribution at various scales on the temperature distribution. We numerically calculate the surface temperature distribution accounting for four effects: insolation reaching the surface, reflected solar radiation from other facets, emitted heat flux from other areas in proximity to the facet in question and its subsequent reflections, and conduction of heat into the ground. We use a ray casting technique in order to track the rays incident on a surface and its subsequent reflections. We have implemented heat diffusion into the subsurface in 1D using a highly efficient algorithm. We have validated our model and found good agreement with measurements acquired by the LRO DIVINER and LROC instruments, and by comparing it to previous works in the field. The model enables investigation of the effect of slopes and surface roughness on the temperature distribution both at the measured scale and at smaller scales.
17 - Brief history of the development and future perspectives of thermal and thermophysical modelling
Marco Delbo
We will describe the early efforts of obtaining diameters, and albedos of asteroids from ground-based thermal infrared observations primarily at 10 um using simple thermal models such as the STM, FRM and the like, and the IRAS experience. Next, we will show how, with the advent of more powerful computers and the necessary observational data, more sophisticated models, the so-called thermophysical models, became feasible. Thermophysical modeling led to asteroid size estimates that were confirmed at the few-percent level by later spacecraft visits. We present major breakthroughs achieved in studies of the thermal inertia, a sensitive indicator for the nature of asteroids soils, allowing us, for instance, to determine the grain size of asteroidal regoliths. Thermal inertia allow us, in a certain sense, to study the geology of asteroids from remote sensing data. Thermal inertia also governs non-gravitational effects on asteroid orbits, such as the Yarkovsky effect. Thermophysical modeling is thus required for precise asteroid dynamical studies. The Sun’s radiative heating of asteroids, meteoroids, and comets also controls the thermal stress in surface material, causing thermal cracking: only recently has it been recognized as a significant weathering process. Asteroid space missions with thermal infrared instruments are flying and will be launched in the next years. This will require a high level of sophistication of thermophysical models in order to analyze high-quality spacecraft data. We thank the contribution of M. Mueller, J. Emery, B. Rozitis, M.T. Capria, J. Hanus, V. Ali-Lagoa and A.W. Harris
18 - Asteroid shape modelling with ADAM: reconstructing Juno from ALMA observations
Matti Viikinkoski
ADAM, an acronym for all-data asteroid modelling, is a general procedure for combining disk-resolved observational data into a non-convex 3-D shape model. ADAM handles all disk-resolved data in a uniform manner via 2-D Fourier Transform. Almost all data sources are supported: lightcurves, adaptive optics and other images, range-Doppler radar data, and thermal infrared interferometry. To explore the viability of shape reconstruction of asteroids from interferometric data, we consider the large main-belt asteroid (3) Juno. Recent observations with the Atacama Large Millimeter Array, using baselines up to 10 km, provided disk-resolved thermal infrared data with a resolution sufficient for shape modelling. With an aid of a simple FFT-based thermal model, we combine the ALMA visibility data and disk-integrated photometry into a coherent 3-D shape solution. Furthermore, we show that the reconstructed model is consistent with the adaptive optics images.
19 - Thermal properties and models of comet dust
Michael S. P. Kelley
Solar system formation is an engine that can simultaneously preserve and process interstellar medium (ISM) dust: the ISM provides the raw material, and high-temperature nebular and disk processes (grain sublimation, condensation, and annealing) subsequently transformed the dust, and mixed it throughout the disk. Comets, being formed from outer-disk material, are potentially comprised of both primitive and evolved solar nebula dust grains. Thus, measuring the composition of comet dust today provides us an important connection to Solar System formation. Fortunately, comet comae are convenient laboratories for studying dust: they are optically thin, and occasionally bright enough for ground-based mid-infrared spectroscopy. I review observations and thermal models of comet dust, especially as they pertain to understanding the origins and evolution of comets.
20 - Binary TPM + ExploreNEOs results
Migo Müeller
Binary TPM: Observing the thermal response to eclipse events in a binary asteroid system gives a uniquely direct handle on thermal inertia, unobscured by roughness effects. Due to the timescales involved, eclipses probe thermal inertia to a shallower depth than the "traditional" method based on the diurnal heat wave. Planning and analyzing such observations is challenging, chiefly because it requires the mutual orbit and, to a lesser extent, the components' shape to be known to high fidelity. The first thermal observations of an eclipsing binary asteroid and their TPM analysis were reported by Mueller et al. (2010). Their target, (617) Patroclus, is a doubly tidally locked system, aiding the thermal modeling. Based on occultation observations, Buie et al. (2015) confirm the 2010 size estimate but report a component shape significantly more irregular than previously assumed. We report a re-analysis of the data using that updated shape model. Also, Spitzer-IRAC observations of eclipses in the binary NEA 1996 FG3 were taken. The signature of eclipses was clearly detected. Modeling is hampered by the rotational lightcurve of the primary component, which is not tidally locked. Different modeling strategies are discussed. ExploreNEOs and NEOsurvey: I briefly report the main results of our two large-scale NEO surveys using Warm Spitzer, ExploreNEOs (Trilling et al., 2010) and NEOsurvey (Trilling et al., submitted) sampling the thermal emission of more than 1,000 NEOs between them. The thermal-modeling pipeline employed in ExploreNEOs is described by Mueller et al. (2011), that for NEOsurvey builds upon it. In a nutshell, we use NEATM with an assumed eta value, linearly increasing with solar phase angle.
21 - Laboratory spectroscopy measurements to support thermal infrared observations of airless bodies in the solar system.
N. E. Bowles
To interpret observations of airless bodies in the thermal infrared, good supporting laboratory data are essential. Typical laboratory measurements include thermal infrared reflectance and emission spectroscopy of individual minerals, analogue materials and mixtures as well as meteorite samples. This presentation will review the techniques used to acquire thermal infrared spectra in the laboratory and discuss some of the complications that can arise when comparing these measurements with observations of airless bodies. The presentation will also describe some common sources of thermal-IR spectroscopic data for analysis of airless body data and detail current attempts to model the radiative transfer in the regolith of an airless body.
22 - Modelling Tangential YORP
Oleksiy Golubov
Rotation of small asteroids has long been recognized to be governed by YORP effect, a radiation torque, which appears due to asymmetry of the asteroid's shape. But recently it was found that even a symmetric asteroid can experience a thermal drag, originating from uneven heat radiation by eastern and western sides of stones on the asteroid's surface. This torque called tangential YORP, although produced by small decemetre-sized stones, appears to be comparable to the normal YORP. In our talk we shall cover numerical simulations of the tangential YORP for different geometries of stones, as well as analytical estimates for tangential YORP derived from simplified heat conductivity models. In the end we shall discuss implications of the tangential YORP for evolution of asteroids.
23 - Thermal light curves of trans-Neptunian objects with Herschel-PACS
P. Santos-Sanz
Trans-Neptunian objects (TNOs) are leftovers from the origin of the Solar System and the least physically-chemically evolved bodies we can access in our planetary system. Centaurs are related to TNOs and we believe that they link the latter with the Jupiter Family Comets. Thermal fluxes of TNOs (T $\sim$20-50 K) have their maxima in the Herschel Space Observatory-PACS wavelengths centred at 70, 100 and 160 microns. Herschel-PACS observations (either at 70/160 or 100/160 microns) covering the whole --or more than the whole-- rotational period have been obtained for few TNOs/Centaurs. Thermal light curves of four TNOs have been observed within the ``TNOs are Cool" Herschel key project [A],[B]: 136108 Haumea, 20000 Varuna, 2003 AZ84 and 2003 VS2. Other three thermal light curves have been observed out of this program: 136199 Eris and 50000 Quaoar --OT1--, and 2060 Chiron --must do observations--. Thermal light curves are clearly detected for Haumea and Varuna and only marginally for 2003 VS2, Quaoar and Chiron. An incomplete Eris light curve implying rotational variability is obtained. No thermal variation is detected for 2003 AZ84. Besides of the estimation of diameters and albedos, we can constrain the thermal inertias for these objects and derive some surface properties by means of thermal and/or thermo-physical models. Varuna and Haumea thermal light curves are correlated with the optical light curves (i.e. the variability is mainly shape-driven). 2003 VS2 and Quaoar's thermal light curves are also correlated with the optical ones. The effect of the dark spot on Haumea’s surface[C] is tentatively detected in the object thermal light curve. [A] Müller, T. G., Lellouch, E., Böhnhardt, H., et al. 2009, EM\&P, 105, 209 [B] Lellouch, E., Santos-Sanz, P., Lacerda, P., et al. 2013, A\&A, 557, A60 [C] Lacerda, P., Jewitt, D., & Peixinho, N. 2008, AJ, 135, 1749 (1) Instituto de Astrof\'{\i}sica de Andaluc\'{\i}a (CSIC), Glorieta de la Astronom\'{\i}a s/n, 18008-Granada, Spain (2) LESIA, Observatoire de Paris, CNRS, Meudon, France (3) Konkoly Observatory, Budapest, Hungary (4) Max Planck Institute for extraterrestrial Physics, Garching, Germany (5) Max Planck Institute for Solar System Research, Gottingen, Germany (6) Laboratoire d’Astrophysique de Marseille, CNRS, Marseille, France (7) Space Telescope Science Institute, Baltimore, USA (8) 40 members of 19 Institutions in 9 Countries.
24 - L-type: rare remains of a first asteroid generation?
P. Tanga
Some L-type asteroids (collectively called "Barbarians") are known to exhibit an anomalous polarimetric behavior, whose origin - still to be elucidated - can be related to compositional and/or scattering effects. The fact that these asteroids belong to the same taxonomic class (following the De Meo 2009 classification, including NIR) implies that composition must play a role. Sunshine et al. 2008 showed that these asteroids contain high amounts of CAIs, possibly hinting to a formation in an early proto-planetary environment, very rich in refractory material. On the base of this evidence, we started an observational campaign to increase the data coverage of these objects, by obtaining new NIR spectra, photometric and polarimetric measurements. Our first results show that the peculiar features are not restricted to polarimetry. In particular we show the existence of an anomalous distribution of the rotation periods, and a possible relation between CAI abundance and albedos determined by WISE. We tentatively discuss a possible scenario justifying the different observed features. Also, we stress the interest of obtaining the thermal inertia of such objects, a very difficult task given their relatively long rotation periods.
25 - Three-dimensional heat diffusion in boulders and the influence on the YORP effect
Pavel Sevecek
The influence of small topographic features on the YORP effect seems to be of utmost importance. It is already known that a lateral heat diffusion in boulders of suitable sizes leads to an emergence of a local YORP effect, causing an angular acceleration of the asteroid. Our aim is to solve the three-dimensional heat diffusion equation in a realistic boulder (and its surroundings) by the finite element method, using the FreeFem++ code. The contribution to the total torque is then inferred from the computed temperature distribution. Our general approach allows us to compute the torque induced by an arbitrarily irregular boulder. We also assess the influence of shadowing, self-heating and scattering. Finally, extrapolating the observed size-frequency distribution of boulders, we estimate the total torque boulders can exert on asteroid (25143) Itokawa.
26 - The Oxford Thermal Emissivity for Regolith Model (O-Therm): Model tests and improvements
Sean S. Lindsay
The determination of compositional and regolith structure information from mid-infrared (MIR; 2.5 – 100 µm or 100 – 4000 cm-1) remote sensing data of airless bodies is complicated by steep thermal gradients in the upper few millimeters of regolith, especially if the grains are < 100 µm. MIR emissivity of surfaces is further complicated by factors such as particle size, particle shape, and porosity of the regolith surface (packing fraction). In order to accurately retrieve compositional and regolith properties from MIR emissivity remote sensing data sets, a model that accounts for these variables is required. Here we present improvements to a multi-layer thermal radiative transfer model (Millán et al., 2011; forthcoming), renamed the Oxford Thermal Emissivity for Regolith Model (O-ThERM). O-ThERM calculates the near-surface thermal gradient for a regolith surface as a function of environmental conditions (pressure, temperature, and illumination), and it accounts for particle scattering, particle size and shape, regolith porosity, and composition. The resulting temperature gradient is used to calculate the emissivity following Henderson and Jakosky (1994, 1997). In this work, we test the computational capabilities of O-ThERM. To this end, we investigate: 1) time-scale feasibility of achieving thermal equilibrium; 2) the effect of subsurface temperature over a range of temperatures relevant to Solar System bodies; 3) the effect of variable regolith packing fraction; 4) the stability and computation time optimization with respect to the integration time step; and 5) the computational stability as a function of particle size to layer thickness ratio. These tests are performed using optical constants for quartz (Spitzer and Kleinman, 1961) in airless body conditions where the simulated regolith is heated from below, and they include cases with and without simulated illumination from the top. The thermal equilibrium tests establish the number of integration time steps needed to achieve an “effective” equilibrium, defined as the point where continued integration toward equilibrium no longer significantly alters the emissivity. Interestingly, this ‘effective’ equilibrium is achieved in very few time steps when simulated solar illumination is included. This indicates that the main factor affecting the emissivity is the uppermost (~1-2 mm depth) portion of the regolith. Many additions have also been made to the Millán et al. (2011; forthcoming) version of the model have been made including an option to run O-ThERM in a batch mode and a module to compute the optical properties of the grains using a Continuous Distribution of Ellipsoids (CDE; Bohren and Huffman, 1983) methodology to simulate particle shapes. The computational efficiency increases and testing of model dependent parameters presented here are exploited by Arnold et al. (this meeting), where O-ThERM is employed to test laboratory sample parameters (grain size distribution, packing density, and mineral composition) and used to evaluate the emissivity of laboratory samples measured in a simulated airless body environmental chamber (described in Thomas et al., 2012). Acknowledgements: The authors would like to thank Luis Millán for his permission to use, edit, and further his original code as described in Millán et al (2011; forthcoming). The authors would also like to thank the Science and Technology Facilities Council (STFC) and the Leverhulme Trust for providing the funding for this work.
27 - The Search for Observational Detections of the YORP Effect
Stephen Lowry
Thermal radiative torques can modify the rotation rates and spin-axis orientations of small asteroidal bodies, and the effect can explain many observed phenomena in asteroidal science. Despite the importance of YORP, there existed only indirect evidence for its presence on solar system objects until relatively recently (Lowry et al. 2007, Science 316; Taylor et al. 2007, Science 316; Kaasalainen et al. 2007, Nature 446). Theoretical studies are progressing well but it is crucial that these studies have observational calibrations of the strength of the effect. We are conducting a large observational programme to acquire YORP detections on a wide selection of near-Earth asteroids. From 2010-2014 our programme was awarded Large Programme (LP) status at the European Southern Observatory’s NTT facility, to acquire high quality photometric lightcurve data on NEAs, with the goal of detecting spin-rate changes due to YORP. Supporting observations are also being acquired at a range a telescope facilities in Europe and the US, including the Palomar 5m Hale Telescope (California), the 2.5m INT (La Palma), and the Liverpool Telescope (La Palma), among many others. The optical component is supported by a programme of thermal-IR observations, conducted primarily at the ESO VLT facility using the VISIR thermal imager. Derivation of thermal parameters is crucial for determining more accurate theoretical YORP strengths for our targets, for comparison with that observed. For thermal modelling we use the Advanced Thermophysical Model (ATPM) (Rozitis and Green, 2011, MNRAS 415). Observations and data analysis for the programme is on-going. We will discuss the latest status of the programme and our first results. This includes our recent detection of YORP spin-up on NEA Itokawa. This NEA was visited by the Hayabusa spacecraft in 2005, resulting in a highly detailed surface shape and topography model. This model has led to several predictions for the expected radiative torques on this asteroid, suggesting that its spin rate should be decelerating. Through an observational survey spanning 2001 to 2013 we have successfully measured an acceleration in its spin rate, equivalent to a decrease of its rotation period of ~45 ms/year. Using the shape model determined from the Hayabusa spacecraft, we applied a detailed thermophysical analysis, to reconcile the predicted YORP strength with that observed. We find that the center-of-mass for Itokawa must be shifted by ~20 m along the long-axis of the asteroid to reconcile observations with theory. This can be explained if Itokawa is composed of two separate bodies with very different bulk densities of 1740 ± 110 kg/m3 and 2730 ± 440 kg/m3, and was formed from the merger of two separate bodies, consistent with the collapse of a binary system or the re-accumulation of material from a catastrophic collisional disruption. We demonstrate that an observational measurement of radiative torques, when combined with a detailed shape model, can provide insight into the interior structure of an asteroid. We will present more details of this study and others at the meeting, along with a discussion of the future outlook for this area.
28 - OXFORD SPACE ENVIRONMENT GONIOMETER: Surface Roughness effects on the Directional Emissivity
T. J. Warren
Measurements of the light scattering properties of the regolith of airless bodies in the Solar system, across wavelengths from the visible to the far infrared are essential to understanding their surface properties. This presentation will describe a new experimental setup, the Oxford Space Environment Goniometer (OSEG). The OSEG allows phase function measurements of regolith simulant samples to be made under vacuum (<10-6 mbar) whilst enclosed by a cooled (<150 K) radiation shield. The cooled radiation shield reduces the thermal background allowing phase measurements from the visible to the thermal infrared (TIR) to be made. Although significant progress has been made in determining the scattering properties of the lunar soil in the visible and near infrared [e.g. 2,3], there is still limited or no data available on the scattering properties in the TIR. We have made some of the first phase function measurements of black painted targets and the first TIR phase function measurements of powdered samples (JSC1 and albite). The TIR Phase functions measurements of black painted surfaces cannot be fitted with Fresnel’s law. Measured from the normal of the surface at large emission angles the emissivity of the surface is considerably higher than that predicted by Fresnel law. We believe the deviation from Fresnel law is due to the small scale surface roughness (1e-6m length scale) of the paint and have focused on investigating how the surface roughness of a sample affects its emission phase function. Using a Monte Carlo ray tracing model outlined in [4] we have made accurate models of TIR emission phase functions measurements from rough surfaces that agree within experimental error with the measurements made by the OSEG. References [1] Paige, D. A. et al., Space Sci. Rev.150, 125-160, 2009. [2] Foote, E. J., LPSC XXXXIII abstract #2357, 2012. [3] Pommerol, A. et al., Planetary and Space Science., 59, 1601-1612, 2011. [4] Tang, K.; Dimenna, R.; Buckius, R., International Journal of Heat and Mass Transfer, Volume 40, Issue 1, pp. 49-59 (1997).
29 - Thermal Infrared Experiments in Hayabusa2
T. Okada
TIR is a thermal infrared imager on Hayabsua2, the 2nd Japanese sample return mission to C-class, Near-Earth asteroid (162173) 1999JU3. It is to image thermal emission off the surface of the asteroid every 5 minute during a 7.63 hours asteroid rotation period. Temporal profile of thermal emission intensity at each geological site is traced, and the thermal inertia is informed from the peak temperature and its delayed time from the local noon. A thermal inertia map will be consequently derived from the TIR data, which strongly contributes to scientific and mission objectives. Thermo-physical properties are among the most essential information for understanding the nature of C-class asteroid and for selecting the sampling sites scientifically. In addition, it enables to predict thermal environments at the time of touchdown operation and to estimate the surface condition, which is crucial for safe operation of spacecraft. In Hayabusa2, three small landing robot rovers will measure the surface temperature, and an European small lander MASCOT will observe the thermal radiation with 6 band radiometer MARA. TIR data is compared with such local site temperature measurements. In this talk, science objectives and the observation plan will be reviewed, as well as the outline and the current status of Hayabsua2 will be briefly presented.
30 - Thermal effects in comet nuclei. The effects of thermal cracking
V. Ali-Lagoa
We use a thermophysical model with global-shape shadowing and self-heating to study the surface temperature distribution of the target body of the European Space Agency's Rosetta mission, comet 67P/Churyumov-Gerasimenko, at different parts of its orbit with the objective of shedding light on the characteristics of its so-called early activity (Gulkis et al. 2015, Sierks et al. 2015). Studies of resolved data have revealed that the activity originates mostly from the neck region, called the Hapi region (Thomas et al. 2015). This is a puzzling result since the Hapi region received the least amount of heat per rotational period at the onset of activity and therefore constituted the coolest parts of the comet's surface at those epochs (Sierks et al. 2015). While the energy balance relevant to the sublimation of volatiles is mostly dependent on the surface's absolute temperature (see, e.g., Skorov et al. 2011 and references therein), we investigate whether other effects such as temperature variation throughout the day, topography, rotation rate, and spin state might be playing relevant roles when the bodies are far way from the Sun.
31 - Review of Observed Thermal Properties of Active Cometary Nuclei
Y. R. Fernandez
With the advent of several spacecraft visitations to comets and of powerful infrared observational facilities such as the Spitzer Space Telescope, we are seeing a surge in the number of cometary nuclei whose thermal emissions are being measured. This work has the potential to provide insight into the ensemble structural and thermophysical properties of comets and, ultimately, the circumstances of their formation and evolution. As is often the case, there is a trade-off between observing a few comets frequently and in-depth to understand the detailed behavior of individuals and studying a larger fraction of the known population more superficially but with an eye toward learning overall ``average" properties. In this review I will summarize recent observational work and some of the key interpretations. There are several lines of evidence that suggest that nuclei have low thermal inertia, although there are further experiments that could be done to test this idea more thoroughly across the comet population as a whole. Furthermore, observational limitations can make it difficult to understand the thermal properties of the cometary nuclei population -- or, even worse, make it easy to fool us into unknowingly misinterpreting data. I will discuss some of these issues, as well as present some possible futures in observational studies of thermal emission from nuclei.
32 - M-type Asteroids in the Mid-Infrared
Zoe Landsman
Studies across several wavelength ranges have shown that many compositions are represented among the M-type asteroids, and only some M-types are likely iron core candidates. Analysis of spectral bands in the visible and near-infrared, interpretation of radar reflectivity, and comparisons with meteorite spectra have provided some constraints on the mineralogy these asteroids. Still, unambiguous compositional interpretation of most remains hindered by seemingly contradictory results. For example, spectra have shown the coexistence of hydrated phases and high-temperature silicates on M-types, including some with metal-like radar albedos. We plan to better constrain the compositions of M-type asteroids using mid-infrared (5 – 40 $\mu$m) spectroscopy and thermophysical modeling (TPM). Physical properties revealed through TPM, such as thermal inertia and surface roughness, can help clarify the composition of M-type asteroids. Additionally, mid-infrared emissivity spectra may provide additional mineralogical insight.
33 - Strategies and pitfalls of interpreting radiometric IR measurements
E. Kührt
Infrared radiometer and spectrometer onboard of spacecrafts measure spatially resolved fluxes from the near to the far IR wavelengths. Scientific goals of such experiments are the characterization of the composition and the derivation of the temperatures and of thermo-physical properties of the ground. IR instruments on many space missions provided valuable scientific data.
However, the interpretation of the measured fluxes from real targets is tricky, needs considerable efforts in data reduction and modeling and has its limitations. Rough and inhomogeneous planetary surfaces are basically neither black nor Lambertian emitters. This is particularly the case for atmosphereless bodies like asteroids and comets.
The talk gives an overview over methods for analyzing remote sensing thermal measurements and explains some pitfalls of their interpretation.
35 - Strategies and pitfalls of interpreting radiometric IR measurements
P. Hayne
The Earth's Moon is the nearest and best-studied airless body in the Solar System. We have mapped thermal emission over the full diurnal and seasonal cycle using the Diviner multi-spectral radiometer, on board the Lunar Reconnaissance Orbiter (LRO). Using this extensive dataset, we have characterized and quantified: 1) rock abundance, 2) regolith structure and thermal inertia, 3) surface roughness, 4) reflectance and emission phase functions, and 5) the extent and temperatures of polar cold traps.
In addition to the standard Diviner dataset, we have also acquired multi-spectral IR observations during lunar eclipses using Diviner and the ground-based AEOS telescope. AEOS is 3.7-m telescope operated by the US Air Force on Maui, Hawaii, and utilizes a multi-band thermal imager with excellent angular resolution and wide field of view. These eclipse observations are critical for constraining the thermophysical properties of the upper ~1 cm of regolith. Further, the lunar eclipse observations are most comparable to thermal emission observations of asteroids, whose rotation periods are typically of order hours.