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): M. Voelker, E. Hauber, K. Stephan, R. Jaumann Hellas Planitia is one of the major topographic sinks on Mars for the deposition of any kind of sediments. We report on our observations of sheet deposits in the eastern part of the basin that are apparently related to the Dao Vallis outflow channel. The deposits have lobate flow fronts and a thickness of a few decameters. Despite their generally smooth surface, some distinctive textures and patterns can be identified, such as longitudinal lineations, distributive channels, and polygons. We compared these deposits to other sheet deposits on Mars and tested three hypotheses of their origin: volcanic flows as well as water- and ice-related mass wastings. Despite some similarities to volcanic sheet flows on Mars, we found several morphological characteristics that are not known for sheet lava flows; for example conically arranged lineations and channel systems very similar to fluvial incisions. We also reject an ice-related formation similar to terrestrial rock-ice avalanches, as there is no sufficient relief energy to explain their extent and location. A water-related origin appears most consistent with our observations, and we favor an emplacement by fluvially-driven mass wasting processes, e.g., debris flows. Assuming a water-related origin, we calculated the amount of water that would be required to deposit such large sedimentary bodies for different flow types. Our calculations show a large range of possible water volumes, from 64 to 2,042 km³, depending on the specific flow mechanism. The close link to Dao Vallis makes these deposits a unique place to study the deposition of outflow channel sediments, as the deposits of other outflow channels on Mars, such as those around Chryse Planitia, are mostly buried by younger sediments and volcanic flows.

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: June 2018 Source: Icarus, Volume 307 Author(s): V. Hue, F. Hersant, T. Cavalié, M. Dobrijevic, J.A. Sinclair In this work, we aim at constraining the diffusive and advective transport processes in Jupiter’s stratosphere, using Cassini/CIRS observations published by Nixon et al. (2007,2010). The Cassini–Huygens flyby of Jupiter on December 2000 provided the highest spatially resolved IR observations of Jupiter so far, with the CIRS instrument. The IR spectrum contains the fingerprints of several atmospheric constituents and allows probing the tropospheric and stratospheric composition. In particular, the abundances of C 2 H 2 and C 2 H 6 , the main compounds produced by methane photochemistry, can be retrieved as a function of latitude in the pressure range at which CIRS is sensitive to. CIRS observations suggest a very different meridional distribution for these two species. This is difficult to reconcile with their photochemical histories, which are thought to be tightly coupled to the methane photolysis. While the overall abundance of C 2 H 2 decreases with latitude, C 2 H 6 becomes more abundant at high latitudes. In this work, a new 2D (latitude-altitude) seasonal photochemical model of Jupiter is developed. The model is used to investigate whether the addition of stratospheric transport processes, such as meridional diffusion and advection, are able to explain the latitudinal behavior of C 2 H 2 and C 2 H 6 . We find that the C 2 H 2 observations are fairly well reproduced without meridional diffusion. Adding meridional diffusion to the model provides an improved agreement with the C 2 H 6 observations by flattening its meridional distribution, at the cost of a degradation of the fit to the C 2 H 2 distribution. However, meridional diffusion alone cannot produce the observed increase with latitude of the C 2 H 6 abundance. When adding 2D advective transport between roughly 30 mbar and 0.01 mbar, with upwelling winds at the equator and downwelling winds at high latitudes, we can, for the first time, reproduce the C 2 H 6 abundance increase with latitude. In parallel, the fit to the C 2 H 2 distribution is degraded. The strength of the advective winds needed to reproduce the C 2 H 6 abundances is particularly sensitive to the value of the meridional eddy diffusion coefficient. The coupled fate of these methane photolysis by-products suggests that an additional process is missing in the model. Ion-neutral chemistry was not accounted for in this work and might be a good candidate to solve this issue.

Description: Publication date: June 2018 Source: Icarus, Volume 307 Author(s): Ralph D. Lorenz Measurements of discharge currents on the Venera 13 and 14 landers during their descent in the lowest 35 km of the Venus atmosphere are interpreted as driven either by an ambient electric field, or by deposition of charge from aerosols. The latter hypothesis is favored (`triboelectric charging' in aeronautical parlance), and would entail an aerosol opacity and charge density somewhat higher than that observed in Saharan dust transported over long distances on Earth.