IAU XXIX General Assembly

Body Solar system bodies are observed at many scattering angles. The reflection and polarization change withphase angle of light scattered from particulates has been studied for a century in the lab in efforts tounderstand clouds, aerosols, planetary ring systems and planetary regoliths. These effects must beunderstood in order to infer surface properties from astronomical data. The increase in reflectance withdecreasing phase angle, the ‘Opposition Effect’ (OE), has been well documented in astronomicalobservations and laboratory studies. Variations in linear polarization with phase angle have also beenwell studied. Nevertheless, there is no generally accepted physical explanation. Our lab studies showthat the OE in particulate materials is due to two processes, Shadow Hiding (SHOE) and CoherentBackscattering (CBOE). SHOE arises because, as phase angle nears zero, shadows cast by regolith grainsupon one another become less visible. CBOE results from constructive interference between raystraveling the same path but in opposite directions. The CBOE process assumes the returned radiation is multiply scattered. We have deconstructed the scattering process using a goniometric photopolarimeter(GPP). This permits us to present samples with light that is linearly polarized in, and perpendicular to,the scattering plane. We make angular scattering measurements of the light scattered from a simulatedplanetary surface. The GPP also illuminates samples with both right handed and left handed circularlypolarized light. This permits us to measure the phase curve, the linear and circular polarization ratiosand the linear polarization as a function of phase angle. These GPP measurements permit us to quantifythe amount of multiple scattering in a particulate medium in the laboratory. At smaller phase angles inhighly reflective material such as Al2O3, multiple scattering increases. This is a consequence of coherentbackscattering of photons that are multiply scattered in the medium. These photometric properties aredependent on the size and the albedo of the regolith particles.Author(s): Robert M. Nelson, Bruce W. Hapke, Mark D. Boryta, Ken S. Manatt, William D. Smythe Institution(s): 1. Jet Propulsion Lab, 2. Jet Propulsion Laboratory, 3. Mt. San Antonio College, 4. Planetary Science Institute, 5. University of Pittsburgh FM12.5 – FM 12: Planetary II and Plasma I FM12.5.01 – Meteorites Meteorites have long been known to offer a unique window into planetary formation processes at thetime of solar system formation and into the materials that rained down on Earth at the time of theorigin of life. Their material properties determine the impact hazard of Near Earth Asteroids. Someinsight into how future laboratory studies of meteorites and laboratory astrophysics simulations ofrelevant physical processes can help address open questions in these areas and generate newastronomical observations, comes from what was learned from the recent laboratory studies of freshlyfallen meteorites. The rapid recovery of Almahata Sitta (a polymict Ureilite), Sutter's Mill (a CMchondrite regolith breccia), Novato (an L6 chondrite), and Chelyabinsk (an LL5 chondrite) each werefollowed by the creation of a meteorite consortium, which grew to over 50 researchers in the case ofChelyabinsk. New technologies were used to probe the organic content of the meteorites as well astheir magnetic signatures, isotopic abundances, trapped noble gasses, and cosmogenic radio nucleides,amongst others. This has resulted in fascinating insight into the nature of the Ureilite parent body, thelikely source region of the CM chondrites in the main asteroid belt, and the collisional environment ofthe CM parent body. This work has encouraged follow-up in the hope of catching more uniquematerials. Rapid response efforts are being developed that aim to recover meteorites as pristinely aspossible from falls for which the approach orbit was measured. A significant increase in the number ofknown approach orbits for different meteorite types will help tie meteorite types to their asteroid familysource regions. Work so far suggests that future laboratory studies may recognize multiple sourceregions for iron-rich ordinary chondrites, for example. Hope is that these source regions will give insightinto the material properties of impacting asteroids. At least some future laboratory astrophysicsexperiments are expected to focus on clarifying the physical conditions during small asteroid impactssuch as the one responsible for the Chelyabinsk airburst and the over 1200 injured who needed medicalattention.Author(s): Peter Jenniskens Institution(s): 1. SETI Institute FM12.5.02 – The THS: Simulating Titan’s atmospheric chemistry at low temperature In Titan’s atmosphere, composed mainly of N2 (95-98%) and CH4 (2-5%), a complex chemistry occurs atlow temperature, and leads to the production of heavy organic molecules and subsequently solidaerosols. Here, we used the Titan Haze Simulation (THS) experiment, an experimental setup developedat the NASA Ames COSmIC simulation facility to study Titan’s atmospheric chemistry at lowtemperature. In the THS, the chemistry is simulated by plasma in the stream of a supersonic expansion.With this unique design, the gas is cooled to Titan-like temperature (~150K) before inducing thechemistry by plasma, and remains at low temperature in the plasma discharge (~200K). Different N2-CH4-based gas mixtures can be injected in the plasma, with or without the addition of heavier precursorspresent as trace elements on Titan, in order to monitor the evolution of the chemical growth. Both thegasand solid phase products resulting from the plasma-induced chemistry can be monitored andanalyzed using a combination of complementary in situ and ex situ diagnostics.A recent mass spectrometry study of the gas phase has demonstrated that the THS is a unique tool toprobe the first and intermediate steps of Titan’s atmospheric chemistry at Titan-like temperature. Inparticular, the mass spectra obtained in aN2-CH4-C2H2-C6H6 mixture are relevant for comparison toCassini’s CAPS-IBS instrument. The results of a complementary study of the solid phase are consistentwith the chemical growth evolution observed in the gas phase. Grains and aggregates form in the gasphase and can be jet deposited on various substrates for ex situ analysis. Scanning Electron Microscopyimages show that more complex mixtures produce larger aggregates. A DART mass spectrometryanalysis of the solid phase has detected the presence of aminoacetonitrile, a precursor of glycine, in theTHS aerosols. X-ray Absorption Near Edge Structure (XANES) measurements also show the presence ofimine and nitrile functional groups, showing evidence of nitrogen chemistry. These complementarystudies show the high potential of the THS to better understand Titan’s chemistry and the origin ofaerosol formation.Author(s): Ella Sciamma-O'Brien, Kathleen T. Upton, Jack L. Beauchamp, Farid Salama Institution(s): 1. Caltech, 2. NASA Ames Research Center FM12.5.03 – Magnetic field generation, Weibel-mediated collisionless shocks, and magnetic reconnection in colliding laser-produced plasmas Colliding plasmas are ubiquitous in astrophysical environments and allow conversion of kinetic energyinto heat and, most importantly, the acceleration of particles to extremely high energies to form thecosmic ray spectrum. In collisionless astrophysical plasmas, kinetic plasma processes govern theinteraction and particle acceleration processes, including shock formation, self-generation of magneticfields by kinetic plasma instabilities, and magnetic field compression and reconnection. How each ofthese contribute to the observed spectra of cosmic rays is not fully understood, in particular both shockacceleration processes and magnetic reconnection have been proposed. We will review recent results oflaboratory astrophysics experiments conducted at high-power, inertial-fusion-class laser facilities, whichhave uncovered significant results relevant to these processes. Recent experiments have now observedthe long-sought Weibel instability between two interpenetrating high temperature plasma plumes,which has been proposed to generate the magnetic field necessary for shock formation in unmagnetizedregimes. A second set of experiments has demonstrated magnetized shock formation in pre-magnetizedplasmas. Finally, magnetic reconnection has been studied in systems of colliding plasmas using eitherself-generated magnetic fields or externally applied magnetic fields, and show extremely fastreconnection rates, indicating fast destruction of magnetic energy and further possibilities to accelerateparticles. Finally, we highlight kinetic plasma simulations, which have proven to be essential tools in thedesign and interpretation of these experiments.Author(s): William Fox Institution(s): 1. Princeton Plasma Physics Laboratory FM12.5.04 – Generation of collisionless shock in laser-produced plasmas Collisionless shocks are ubiquitous in astrophysical environments and are tightly connected withmagnetic-field amplification and particle acceleration. The fast progress in high-power laser technologyis bringing the study of high Mach number shocks into the realm of laboratory plasmas, where in situmeasurements can be made helping us understand the fundamental kinetic processes behind shocks. Iwill discuss the recent progress in laser-driven shock experiments at state-of-the-art facilities like NIFand Omega and how these results, together with ab initio massively parallel simulations, can impact ourunderstanding of magnetic field amplification and particle acceleration in astrophysical plasmas.Author(s): Frederico Fiuza Institution(s): 1. SLAC National Accelerator Laboratory FM12.5.05 – The emerging understanding of magnetic reconnection through laboratory experiments, theory and modeling and in situ satellite measurements Magnetic reconnection is the driver of explosive energy release in laboratory, space and astrophysicalplasma systems. It plays a centralrole in such diverse phenomena as solar and stellar flares, flares in pulsar nebulae, gamma ray burstsand possibly even in the productionof energetic particles in supernova shocks. The close interaction of scientists doing laboratoryexperiments, in situ satellite measurements and theory and modeling has led to remarkable progress onkey issues such as the mechanisms for fast energy release and heating and particle acceleration. Thereare, however, many open issues. The talk will address the emerging understanding of reconnection aswell as areas where significant uncertainty remains. The role of new laboratory experiments such asFLARE at PPPL and the recently launched four-spacecraft MMS mission in resolving open issues will bediscussed.Author(s): James F. Drake Institution(s): 1. University of Maryland FM12.5.06 – Influence of Multiple Ionization on Charge State Distributions The spectrum emitted by a plasma depends on the charge state distribution (CSD) of the gas. Forcollisionally ionized plasmas, the CSD is is determined by the corresponding rates for electron-impactionization and recombination. In astrophysics, such plasmas are formed in stars, supernova remnants,galaxies, and galaxy clusters. Current CSD calculations generally do not account for electron-impactmultiple ionization (EIMI), a process in which multiple electrons are ejected by a single electron-ioncollision. We have estimated the EIMI cross sections for all charge states of iron using a combination ofthe available experimental data and semi-empirical formulae. We then modeled the CSD and observedthe influence of EIMI compared to only including single ionization. One case of interest for astrophysicsis nanoflare heating, which is a leading theory to explain the heating of the solar corona. In order todetermine whether this theory can indeed explain coronal heating, spectroscopic measurements arebeing compared to model nanoflare spectra. Such models have attempted to predict the spectra ofimpulsively heated plasmas in which the CSD is time dependent. These nonequilbirium ionizationcalculations have so far ignored EIMI, but our findings suggest that EIMI can have a significant effect onthe CSD of a nanoflare-heated plasma, changing the ion abundances by up to about 50%.Author(s): Michael Hahn, Daniel Wolf Savin Institution(s): 1. Columbia University FM12.6 – FM 12: Plasma II and Nuclei and Particles I FM12.6.01 – The Wisconsin Plasma Astrophysics Laboratory (WiPAL): A New Experimental User