Description: Publication date: 1 September 2018 Source: Icarus, Volume 311 Author(s): Eri Tatsumi, Deborah Domingue, Naru Hirata, Kohei Kitazato, Faith Vilas, Susan Lederer, Paul R. Weissman, Stephen C. Lowry, Seiji Sugita We present photometry of the S-type near-Earth asteroid 25143 Itokawa based on both ground-based observations in the UBVRI bands and measurements from the AMICA/Hayabusa spacecraft observations with ul-, b-, v-, w-, x-, and p-filters. Hayabusa observed Itokawa around opposition during the rendezvous, thus providing a unique set of observations of this asteroid. We fit the phase curve measurements with both the Classic Hapke Model (Hapke, 1981, 1984, 1986) and Modern Hapke Model (Hapke, 2002, 2008, 2012a) and thereby extract the physical properties of Itokawa's surface regolith. The single-scattering albedo (0.57 ± 0.05) is larger than that derived for Eros (0.43 ± 0.02), another S-type near-Earth asteroid visited by a spacecraft. Both models indicate a regolith that is forward-scattering in nature. From the hockey stick relationship derived for the single-particle phase function (Hapke, 2012b), both modeling results suggest a regolith comprised of rough surfaced particles with a low density of internal scatterers. Application of the Modern Hapke model derives porosity parameter values from 1 to 1.1, for BVR bands, which corresponds to porosity values between 77–79%. This suggests the surface of Itokawa is very fluffy and the large boulders may be bonded with smaller size particles, typical of the particle sizes observed in Muses Sea. Both models also provide similar geometric albedo values (0.27 ± 0.02) at the V-band wavelength, which are equivalent to Eros’ geometric albedo.

Description: Publication date: 15 July 2018 Source: Icarus, Volume 309 Author(s): M.D. Paton, A.-M. Harri, H. Savijärvi Martian boundary layer wind speed and direction measurements, from a variety of locations, seasons and times, are provided. For each lander sent to Mars over the last four decades a unique record of the winds blowing during their descent is preserved at each landing site. By comparing images acquired from orbiting spacecraft of the impact points of jettisoned hardware, such as heat shields and parachutes, to a trajectory model the winds can be measured. We start our investigations with the Viking lander 1 mission and end with Schiaparelli. In-between we extract wind measurements based on observations of the Beagle 2, Spirit, Opportunity, Phoenix and Curiosity landing sites. With one exception the wind at each site during the lander’s descent were found to be  〈 8 m s − 1 . High speed winds were required to explain the displacement of jettisoned hardware at the Phoenix landing site. We found a tail wind ( > 20 m s − 1 ), blowing from the north-west was required at a high altitude ( > 2 km) together with a gust close to the surface ( 〈 500 m altitude) originating from the north. All in all our investigations yielded a total of ten unique wind measurements in the PBL. One each from the Viking landers and one each from Beagle 2, Spirit, Opportunity and Schiaparelli. Two wind measurements, one above about 1 km altitude and one below, were possible from observations of the Curiosity and Phoenix landing site. Our findings are consistent with a turbulent PBL in the afternoon and calm PBL in the morning. When comparing our results to a GCM we found a good match in wind direction but not for wind speed. The information provided here makes available wind measurements previously unavailable to Mars atmosphere modellers and investigators.

Description: Publication date: 15 July 2018 Source: Icarus, Volume 309 Author(s): Joana R.C. Voigt, Christopher W. Hamilton The Elysium Volcanic Province consists of numerous overlapping flow units and may include the youngest lava flows on Mars. However, it is possible that these volcanic units have been modified or overprinted by aqueous processes. Understanding the timing of the igneous and aqueous events in this region is therefore essential for constraining the geological and environmental history of Mars during the Amazonian Period. We investigate the geologic evolution of Eastern Elysium Planitia to determine the relationship between major units, with the support of a geological map and chronological constraints from crater size–frequency distributions. We also evaluate the hypothesized origin of these units via volcanic, fluvial, and/or fluvioglacial processes using a detailed facies-mapping approach. The study area includes the Eastern Cerberus Fossae, Rahway Valles, and Marte Vallis. The surficial deposits in Rahway Valles were formerly interpreted to be modified by fluvial and fluvioglacial processes. However, our facies map reveals that the surface of Eastern Elysium Planitia includes nineteen morphologically distinct regions (i.e., facies), which are interpreted to be the products of flood lava volcanism, including: ʻaʻā, pāhoehoe, and transitional lava flow types. In contrast to previous studies, which determined that Rahway Valles and Marte Vallis consist of two distinct geologic units with Middle to Late Amazonian ages, the results of this work show that the region was resurfaced by at least two volcanic flows with much younger ages of 20.0 Ma and 8.8 Ma. Furthermore, by coupling results of our geologic and facies mapping with chronological constraints as well as subsurface information provided by Shallow Radar reflectors, we show that there is an erosional unconformity located between the two youngest lava flow units in Marte Vallis. We interpret that this unconformity was generated by a catastrophic aqueous flooding event that occurred only 8.8 − 20.0 Ma ago. This implies alternating episodes of volcanism and aqueous flooding that have continued into the geologically recent past on Mars, and may again occur within Elysium Planitia.

Description: Publication date: June 2018 Source: Icarus, Volume 307 Author(s): D.P. Hinson, I.R. Linscott, D.F. Strobel, G.L. Tyler, M.K. Bird, M. Pätzold, M.E. Summers, S.A. Stern, K. Ennico, G.R. Gladstone, C.B. Olkin, H.A. Weaver, W.W. Woods, L.A. Young On 14 July 2015 New Horizons performed a radio occultation (RO) that sounded Pluto’s neutral atmosphere and ionosphere. The solar zenith angle was 90.2° (sunset) at entry and 89.8° (sunrise) at exit. We examined the data for evidence of an ionosphere, using the same method of analysis as in a previous investigation of the neutral atmosphere (Hinson et al., 2017). No ionosphere was detected. The measurements are more accurate at occultation exit, where the 1-sigma sensitivity in integrated electron content (IEC) is 2.3 × 10 11 cm − 2 . The corresponding upper bound on the peak electron density at the terminator is about 1000 cm − 3 . We constructed a model for the ionosphere and used it to guide the analysis and interpretation of the RO data. Owing to the large abundance of CH 4 at ionospheric heights, the dominant ions are molecular and the electron densities are relatively small. The model predicts a peak IEC of 1.8 × 10 11 cm − 2 for an occultation at the terminator, slightly smaller than the threshold of detection by New Horizons.

Description: Publication date: June 2018 Source: Icarus, Volume 307 Author(s): Peng Hong, Yasuhito Sekine, Tsutoni Sasamori, Seiji Sugita Formation of organic aerosols driven by photochemical reactions has been observed and suggested in CH 4 -containing atmospheres, including Titan and early Earth. However, the detailed production and growth mechanisms of organic aerosols driven by solar far ultraviolet (FUV) light remain poorly constrained. We conducted laboratory experiments simulating photochemical reactions in a CH 4 CO 2 atmosphere driven by the FUV radiations dominated by the Lyman-α line. In the experiments, we analyzed time variations in thickness and infrared spectra of solid organic film formed on an optical window in a reaction cell. Gas species formed by FUV irradiation were also analyzed and compared with photochemical model calculations. Our experimental results show that the growth rate of the organic film decreases as the CH 4 /CO 2 ratio of reactant gas mixture decreases, and that the decrease becomes very steep for CH 4 /CO 2  〈 1. Comparison with photochemical model calculations suggests that polymerizations of gas-phase hydrocarbons, such as polyynes and aromatics, cannot account for the growth rate of the organic film but that the addition reaction of CH 3 radicals onto the organic film with the reaction probability around 10 −2 can explain the growth rate. At CH 4 /CO 2  〈 1, etching by O atom formed by CO 2 photolysis would reduce or inhibit the growth of the organic film. Our results suggest that organic aerosols would grow through CH 3 addition onto the surface during the precipitation of aerosol particles in the middle atmosphere of Titan and early Earth. On Titan, effective CH 3 addition would reduce C 2 H 6 production in the atmosphere. On early Earth, growth of aerosol particles would be less efficient than those on Titan, possibly resulting in small-sized monomers and influencing UV shielding.

Description: Publication date: June 2018 Source: Icarus, Volume 307 Author(s): Alexis Coyette, Rose-Marie Baland, Tim Van Hoolst Observation of the rotation of synchronously rotating satellites can help to probe their interior. Previous studies mostly assume that these large icy satellites are in hydrostatic equilibrium, although several measurements indicate that they deviate from such a state. Here we investigate the effect of non-hydrostatic equilibrium and of flow in the subsurface ocean on the rotation of Titan. We consider the variations in rotation rate and the polar motion due to (1) the gravitational force exerted by Saturn at orbital period and (2) exchanges of angular momentum between the seasonally varying atmosphere and the solid surface. The deviation of the mass distribution from hydrostaticity can significantly increase the diurnal libration and decrease the amplitude of the seasonal libration. The effect of the non-hydrostatic mass distribution is less important for polar motion, which is more sensitive to flow in the subsurface ocean. By including a large spectrum of atmospheric perturbations, the smaller than synchronous rotation rate measured by Cassini in the 2004–2009 period (Meriggiola et al., 2016) could be explained by the atmospheric forcing. If our interpretation is correct, we predict a larger than synchronous rotation rate in the 2009–2014 period.

Description: Publication date: June 2018 Source: Icarus, Volume 307 Author(s): Daniel M. Applin, Matthew R.M. Izawa, Edward A. Cloutis, Jeffrey J. Gillis-Davis, Karly M. Pitman, Ted L. Roush, Amanda R. Hendrix, Paul G. Lucey A number of planetary spacecraft missions have carried instruments with sensors covering the ultraviolet (UV) wavelength range. However, there exists a general lack of relevant UV reflectance laboratory data to compare against these planetary surface remote sensing observations in order to make confident material identifications. To address this need, we have systematically analyzed reflectance spectra of carbonaceous materials in the 200–500 nm spectral range, and found spectral-compositional-structural relationships that suggest this wavelength region could distinguish between otherwise difficult-to-identify carbon phases. In particular (and by analogy with the infrared spectral region), large changes over short wavelength intervals in the refractive indices associated with the trigonal sp 2 π – π * transition of carbon can lead to Fresnel peaks and Christiansen-like features in reflectance. Previous studies extending to shorter wavelengths also show that anomalous dispersion caused by the σ–σ* transition associated with both the trigonal sp 2 and tetrahedral sp 3 sites causes these features below λ  = 200 nm. The peak wavelength positions and shapes of π – π * and σ–σ* features contain information on sp 3 /sp 2 , structure, crystallinity, and powder grain size. A brief comparison with existing observational data indicates that the carbon fraction of the surface of Mercury is likely amorphous and submicroscopic, as is that on the surface of the martian satellites Phobos and Deimos, and possibly comet 67P/Churyumov–Gerasimenko, while further coordinated observations and laboratory experiments should refine these feature assignments and compositional hypotheses. The new laboratory diffuse reflectance data reported here provide an important new resource for interpreting UV reflectance measurements from planetary surfaces throughout the solar system, and confirm that the UV can be rich in important spectral information.

Description: Publication date: 1 September 2018 Source: Icarus, Volume 311 Author(s): Christopher Lee, Mark I. Richardson, Claire E. Newman, Michael A. Mischna Mars exhibits less atmospheric variability at the solstices than it does during periods nearer the equinoxes. Much of this variability in air temperature and dust activity is attributable to a significant decrease in eastward traveling transient wave amplitudes in the lower atmosphere near the solstice. Previous versions of the Mars Weather Research and Forecasting (MarsWRF) model using only dust radiative forcing have reproduced the nature but not the magnitude of this ‘solsticial pause’ in atmospheric variability. In this paper, we use a version of MarsWRF that includes a fully-interactive dust and water cycle to simulate winter solsticial pauses under a range of dust and water ice conditions. The upgraded model specifically includes a new hybrid binned/two-moment microphysics model that simulates dust, water ice, and cloud condensation nuclei. The scheme tracks mass and number density for the three particle types throughout the atmosphere and allows advection by resolved winds, mixing by unresolved processes, and sedimentation that depends on particle size and density. Ice and dust particles interact with radiation in the atmosphere using a Mie scattering parameterization that allows for variable particle size and composition. Heterogeneous nucleation and condensation use an adaptive bin size scheme to accurately track the particle size during condensation and sublimation processes. All microphysical processes in the model are calculated within the dynamical timesteps using stability-guaranteed implicit calculations with no sub-timestepping. The impact of the addition of water processes to the model was assessed by comparing simulations with only interactive dust (dry simulations) and ones with a fully-interactive dust and water cycle (wet simulations). In dry simulations with dust storms a solsticial pause occurs in the northern winter with a magnitude (or ‘depth’) that depends on the opacity of the southern summer dust storms. In wet simulations that include water ice and dust particles, deep solsticial pauses are found in both winter hemispheres. In all simulations that reproduce the solsticial pause, energy and instability analysis suggest that a decrease in baroclinic instability and increase in barotropic energy conversion occurs during the solsticial pause. In dry simulations the decrease in baroclinic instability is caused by increased dust opacity leading to increased thermal static stability. In wet simulations, additional opacity from local cap-edge ice clouds reduces the near surface wind shear and further inhibits baroclinic eddy growth. The wet simulations are in better agreement with observations and tend to support results from other models that include ice cloud radiative effects.

Description: Publication date: 1 September 2018 Source: Icarus, Volume 311 Author(s): J.E. Wickham-Eade, M.J. Burchell, M.C. Price, K.H. Harriss Results are presented for the fragmentation of projectiles in laboratory experiments. 1.5 mm cubes and spheres of basalt and shale were impacted onto water at normal incidence and speeds from 0.39 to 6.13 km s −1 ; corresponding to peak shock pressures 0.7–32 GPa. Projectile fragments were collected and measured (over 100,000 fragments in some impacts, at sizes down to 10 µm). Power laws were fitted to the cumulative fragment size distributions and the evolution of the exponent vs. impact speed and peak shock pressure found. The gradient of each of these power laws increased with increasing impact speed/peak shock pressure. The percentage of the projectiles recovered in the impacts was found and used to estimate projectile remnant survival in different solar system impact scenarios at the mean impact speed appropriate to that scenario. For Pluto, the Moon and in the asteroid belt approximately 55%, 40% and 15%, respectively, of an impactor could survive and be recovered at an impact site. Finally, the catastrophic disruption energy densities of basalt and shale were measured and found to be 24 × 10 4  J kg −1 and 9 × 10 4  J kg −1 , respectively, a factor of ∼2.5 difference. These corresponded to peak shock pressures of 1 to 1.5 GPa (basalt), and 0.8 GPa (shale). This is for near normal-incidence impacts where tensile strength is dominant. For shallow angle impacts we suggest shear effects dominate, resulting in lower critical energy densities and peak shock pressures. We also determine a method to ascertain information about fragment sizes in solar system impact events using a known size of impactor. The results are used to predict projectile fragments sizes for the Veneneia and Rheasilvia crater forming impacts on Vesta, and similar impacts on Ceres.

Description: Publication date: 1 September 2018 Source: Icarus, Volume 311 Author(s): S.-B. Gerig, R. Marschall, N. Thomas, I. Bertini, D. Bodewits, B. Davidsson, M. Fulle, W.-H. Ip, H.U. Keller, M. Küppers, F. Preusker, F. Scholten, C.C. Su, I. Toth, C. Tubiana, J.-S. Wu, H. Sierks, C. Barbieri, P.L. Lamy, R. Rodrigo, D. Koschny, H. Rickman, J. Agarwal, M.A. Barucci, J.-L. Bertaux, G. Cremonese, V. Da Deppo, S. Debei, M. De Cecco, J. Deller, S. Fornasier, O. Groussin, P.J. Gutierrez, C. Güttler, S.F. Hviid, L. Jorda, J. Knollenberg, J.-R. Kramm, E. Kührt, L.M. Lara, M. Lazzarin, J.J. Lopez Moreno, F. Marzari, S. Mottola, G. Naletto, N. Oklay, J.-B. Vincent The Optical, Spectroscopic, and Infrared Remote Imaging System (OSIRIS) onboard the European Space Agency's Rosetta spacecraft acquired images of comet 67P/Churyumov–Gerasimenko (67P) and its surrounding dust coma starting from May 2014 until September 2016. In this paper we present methods and results from analysis of OSIRIS images regarding the dust outflow in the innermost coma of 67P. The aim is to determine the global dust outflow behaviour and place constraints on physical processes affecting particles in the inner coma. We study the coma region right above the nucleus surface, spanning from the nucleus centre out to a distance of about 50 km comet centric distance (approximately 25 average comet radii). We primarily adopt an approach used by Thomas and Keller (1990) to study the dust outflow. We present the effects on azimuthally-averaged values of the dust reflectance of non-radial flow and non-point-source geometry, acceleration of dust particles, sublimation of icy dust particles after ejection from the surface, dust particle fragmentation, optical depth effects and the influence of gravitationally bound particles. All of these physical processes could modify the observed distribution of light scattered by the dust coma. In the image analysis, profiles of azimuthally averaged dust brightness as a function of impact parameter b (azimuthal average, “Ā-curve”) were fitted with a simple function that best fits the shape of our profile curves ( f ( b ; u , v , w , z ) = u / b v + w b + z ). The analytical fit parameters ( u, v, w, z ), which hold the key information about the dust outflow behaviour, were saved in a comprehensive database. Through statistical analysis of these information, we show that the spatial distribution of dust follows free-radial outflow behaviour (i.e. force-free radial outflow with constant velocity) beyond distances larger than ∼11.9 km from the comet centre, which corresponds to a relative distance of about 6 average comet radii from the comet centre. Hence, we conclude that beyond this distance, and on average, fragmentation and gravitationally bound particles are negligible processes in determining the optically scattered light distribution in the innermost coma. Closer to the nucleus we observe dust outflow behaviour that deviates from free-radial outflow. A comparison of our result profiles with numerical models using a Direct Simulation Monte Carlo (DSMC) approach with dust particle distributions calculated using a test particle approach has been used to demonstrate the influence of a complex shape and particle acceleration on the azimuthal average profiles. We demonstrate that, while other effects such as fragmentation or sublimation of dust particles cannot be ruled out, acceleration of the dust particles and effects arising from the shape of the irregular nucleus (non-point source geometry) are sufficient to explain the observed dust outflow behaviour from image data analysis. As a by-product of this work, we have calculated “Afρ” values for the 1/r regime. We found a peak in the coma activity in terms of Afρ (normalised to a phase angle of 90°) of ∼210 cm 20 days after perihelion. Furthermore, based on simplified models of particle motion within bound orbits, it is shown that limits on the total cross-sectional area of bound particles might be derived through further analysis. An example is given.