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: 1 September 2018 Source: Icarus, Volume 311 Author(s): M. Luginin, A. Fedorova, D. Belyaev, F. Montmessin, O. Korablev, J.-L. Bertaux SPICAV IR, one channel of SPICAV/SOIR instrument suite onboard Venus Express, performed solar occultation measurements of the atmosphere at terminators in 0.65–1.7 µm spectral range. We analyze the properties of the upper part of the Venus aerosol layer (upper haze, 70 − 95 km altitude) from 798 observations performed from May 2006 through November 2014. Vertical profiles of slant optical depth, extinction coefficient, effective radius, and number density of haze particles from 222 orbits were analyzed in a previous publication (Luginin et al., 2016); their diurnal, latitudinal, and interannual variabilities were investigated. The present paper is devoted to analysis of scale heights and properties of detached haze layers from 147 orbits at mid-to-high northern latitudes, where the best spatial resolution was obtained. Scale heights retrieved from 43 orbits were equal to 4 − 5.5 km at the North Pole (82°N-90°N) decreasing to 2 − 4 km at 60°N − 80°N latitudes. As an explanation of such latitudinal variations, we propose a mechanism based on vertical transport driven by winds that are directed upward at the North Pole and downward at 60°N − 80°N latitudes. Detached layers were detected in 93 occultations at 58°N − 90°N. The detached layers are presumably formed through condensation of water vapor on droplets of sulfuric acid water solution; they were mostly seen at 80 − 88 km at the morning terminator, and at 84 − 90 km at the evening one. This difference in altitude of the detached layers can be explained by diurnal variations in thermal structure of Venusian mesosphere. The vertical optical depth of detached layers varies broadly around the mean τ DL ∼ 0.8 − 3·10 −3 ; no difference between the morning and the evening terminators was observed. The effective radius and number density of aerosol particles in the detached layers group around a very wide maximum at the morning terminator (0.65 ± 0.25 µm and 0.6 ± 0.4 cm −3 ) and two maxima at the evening terminator (0.4 ± 0.1 µm and 0.85 ± 0.15 µm; 0.3 ± 0.2 cm −3 and 4.5 ± 2.5 cm −3 ). This could be explained by differences in initial altitudes at which condensation of particles occurs.

Description: Publication date: 1 September 2018 Source: Icarus, Volume 311 Author(s): Wladimir Neumann, Stephan Henke, Doris Breuer, Hans-Peter Gail, Winfried H. Schwarz, Mario Trieloff, Jens Hopp, Tilman Spohn The acapulcoites and lodranites are rare groups of primitive achondrites that originate from a common parent body and are of particular interest since they experienced only partial melting. We calculated thermal evolution and differentiation models of the parent body of the Acapulco-Lodran meteorite clan. The models were compared to the maximum metamorphic temperatures, differentiation degree, and thermo-chronological data available. An optimized set of parameters which fits to the data was determined: A radius of ≈ 260 km, a formation time of ≈ 1.7 Ma after CAIs and an initial temperature of ≈ 250 K. The burial depths derived are 7–13 km. The respective layers experienced minor melting and small-scale melt migration, matching the differentiation degree of the meteorites. The resulting structure has an iron core, a silicate mantle, a partially differentiated layer, and an undifferentiated outer shell. Our results indicate a larger size, an earlier formation time, and a formation closer to the sun of the parent body of acapulcoites and lodranites than typical estimates for ordinary chondritic parent bodies, consistent with a stronger thermal metamorphism. The burial depths support excavation by a single impact. The presence of core and mantle indicates that these meteorites could share a common parent body with differentiated stony and iron meteorites.

Description: Publication date: 15 July 2018 Source: Icarus, Volume 309 Author(s): Adam J. McKay, Anita L. Cochran, Michael A. DiSanti, Neil Dello Russo, Harold Weaver, Ronald J. Vervack, Walter M. Harris, Hideyo Kawakita We present H 2 O production rates for comet C/2012 S1 (ISON) derived from observations of [O I] and OH emission during its inbound leg, covering a heliocentric distance range of 1.8–0.44 AU. Our production rates are in agreement with previous measurements using a variety of instruments and techniques and with data from the various observatories greatly differing in their projected fields of view. The consistent results across all data suggest the absence of an extended source of H 2 O production, for example sublimation of icy grains in the coma, or a source with spatial extent confined to the dimensions of the smallest projected field of view (in this case  〈 1000 km). We find that ISON had an active area of around 10 km 2 for heliocentric distances R h  >  1.2 AU, which then decreased to about half this value from R h = 1.2–0.9 AU. This was followed by a rapid increase in active area at about R h = 0.6 AU, corresponding to the first of three major outbursts ISON experienced inside of 1 AU. The combination of a detected outburst in the light curve and rapid increase in active area likely indicates a major nucleus fragmentation event. The 5–10 km 2 active area observed outside of R h = 0.6 AU is consistent with a 50–100% active fraction for the nucleus, larger than typically observed for cometary nuclei. Although the absolute value of the active area is somewhat dependent on the thermal model employed, the changes in active area observed are consistent among models. The conclusion of a 50–100+% active fraction is robust for realistic thermal models of the nucleus. However the possibility of a contribution of a spatially unresolved distribution of icy grains cannot be discounted. As our [OI]-derived H 2 O production rates are consistent with values derived using other methods, we conclude that the contribution of O 2 photodissociation to the observed [O I] emission is at most 5–10% that of the contribution of H 2 O for ISON. This is consistent with the expected contribution of O 2 photodissociation if O 2 /H 2 O  ∼  4%, meaning [O I] emission can still be utilized as a reliable proxy for H 2 O production in comets as long as O 2 /H 2 O  ≲  4%, similar to the abundance measured by the ROSINA instrument on Rosetta at comet 67P/Churyumov–Gerasimenko.

Description: Publication date: 15 July 2018 Source: Icarus, Volume 309 Author(s): Naoya Sakatani, Kazunori Ogawa, Masahiko Arakawa, Satoshi Tanaka Many air-less planetary bodies, including the Moon, asteroids, and comets, are covered by regolith. The thermal conductivity of the regolith is an essential parameter controlling the surface temperature variation. A thermal conductivity model applicable to natural soils as well as planetary surface regolith is required to analyze infrared remote sensing data. In this study, we investigated the temperature and compressional stress dependence of the thermal conductivity of the lunar regolith simulant JSC-1A, and the temperature dependence of sieved JSC-1A samples under vacuum conditions. We confirmed that a series of the experimental data for JSC-1A are fitted well by our analytical model of the thermal conductivity (Sakatani et al., 2017). Comparison with the calibration data of the sieved samples with those for original JSC-1A indicates that the thermal conductivity of natural samples with a wide grain size distribution can be modeled as mono-sized grains with a volumetric median size. The calibrated model can be used to estimate the volumetric median grain size from infrared remote sensing data. Our experiments and the calibrated model indicates that uncompressed JSC-1A has similar thermal conductivity to lunar top-surface materials, but the lunar subsurface thermal conductivity cannot be explained only by the effects of the density and self-weighted compressional stress. We infer that the nature of the lunar subsurface regolith grains is much different from JSC-1A and lunar top-surface regolith, and/or the lunar subsurface regolith is over-consolidated and the compressional stress higher than the hydrostatic pressure is stored in the lunar regolith layer.

Description: Publication date: 15 July 2018 Source: Icarus, Volume 309 Author(s): Rebecca M.E. Williams, Michael C. Malin, Kathryn M. Stack, David M. Rubin The stratigraphic context of rock layers is a critical piece of information needed for accurate reconstruction of their geologic history. Although sedimentary rocks are widespread in Gale crater, efforts to deduce stratigraphic relationships of rocks were challenging early in the Mars Science Laboratory mission because vertical bedrock exposures were relatively rare along the first ∼3 km the rover traversed across Aeolis Palus. Potential insights into the three-dimensional configuration of rock layers were made once the rover passed Dingo Gap, especially in the informally-named Kylie and Kimberley regions. Here, the terrain exhibits low relief ( 〈 10 m) cliffs, some of which are continuous over lengths > 75 m. Curiosity Mastcam and Navcam images show that the cliffs are capped by resistant, bench-forming rock layers corresponding to two facies: a poorly sorted, weakly stratified pebble conglomerate, and a massive, dark-toned, vuggy sandstone. In places, the inclination of the topographic surface (northward ∼2° to 3°) is similar to the apparent dip of the underlying strata, suggesting the presence of dip slopes in an area inferred to be generally flat-lying, conformable rock units. Further, we assessed potential strata correlations via plane-fitting exercises and a regional comparison to other capping strata. We speculate that bench-forming strata in the study region could be part of a widespread package of draping strata (the Siccar Point group) that post-dates deposition and exhumation of the lower strata of Mount Sharp.

Description: Publication date: 15 July 2018 Source: Icarus, Volume 309 Author(s): Kateryna Frantseva, Michael Mueller, Inge Loes ten Kate, Floris F.S. van der Tak, Sarah Greenstreet Given rapid photodissociation and photodegradation, the recently discovered organics in the Martian subsurface and atmosphere were probably delivered in geologically recent times. Possible parent bodies are C-type asteroids, comets, and interplanetary dust particles (IDPs). The dust infall rate was estimated, using different methods, to be between 0.71 and 2.96 × 10 6 kg/yr (Nesvorny et al., 2011; Borin et al., 2017; Crismani et al., 2017); assuming a carbon content of 10% (Flynn, 1996), this implies an IDP carbon flux of 0.07 − 0.3 × 10 6 kg/yr. We calculate for the first time the carbon flux from impacts of asteroids and comets. To this end, we perform dynamical simulations of impact rates on Mars. We use the N-body integrator RMVS/Swifter to propagate the Sun and the eight planets from their current positions. We separately add comets and asteroids to the simulations as massless test particles, based on their current orbital elements, yielding Mars impact rates of 4.34 × 10 − 3 comets/Myr and 3.3 asteroids/Myr. We estimate the delivered amount of carbon using published carbon content values. In asteroids, only C types contain appreciable amounts of carbon. Given the absence of direct taxonomic information on the Mars impactors, we base ourselves on the measured distribution of taxonomic types in combination with dynamic models of the origin of Mars-crossing asteroids. We estimate the global carbon flux on Mars from cometary impacts to be  ∼ 0.013 × 10 6  kg/yr within an order of magnitude, while asteroids deliver  ∼ 0.05 × 10 6  kg/yr. These values correspond to ∼ 4 − 19 % and ∼ 17 − 71 % , respectively, of the IDP-borne carbon flux estimated by Nesvorny et al., Borin et al. and Crismani et al. Unlike the spatially homogeneous IDP infall, impact ejecta are distributed locally, concentrated around the impact site. We find organics from asteroids and comets to dominate over IDP-borne organics at distances up to 150 km from the crater center. Our results may be important for the interpretation of in situ detections of organics on Mars.

Description: Publication date: 15 July 2018 Source: Icarus, Volume 309 Author(s): C.J. Bierson, F. Nimmo, W.B. McKinnon Observations by the New Horizons spacecraft have determined that Pluto has a larger bulk density than Charon by 153 ± 44 kg m − 3 (2 σ uncertainty). We use a thermal model of Pluto and Charon to determine if this density contrast could be due to porosity variations alone, with Pluto and Charon having the same bulk composition. We find that Charon can preserve a larger porous ice layer than Pluto due to its lower gravity and lower heat flux but that the density contrast can only be explained if the initial ice porosity is  ≳ 30%, extends to ≳100 km depth and Pluto retains a subsurface ocean today. We also find that other processes such as a modern ocean on Pluto, self-compression, water-rock interactions, and volatile (e.g., CO) loss cannot, even in combination, explain this difference in density. Although an initially high porosity cannot be completely ruled out, we conclude that it is more probable that Pluto and Charon have different bulk compositions. This difference could arise either from forming Charon via a giant impact, or via preferential loss of H 2 O on Pluto due to heating during rapid accretion.

Description: Publication date: 15 July 2018 Source: Icarus, Volume 309 Author(s): Toshihiko Kadono, Takayuki Tanigawa, Kosuke Kurosawa, Takaya Okamoto, Takafumi Matsui, Hitoshi Mizutani We propose that the shape of impact fragments reflects their fragmentation mechanisms; the fragmentation process that generates smaller fragments (fractal crack bifurcation) produces the shapes frequently observed in the previous studies, and those that generate larger fragments (spallation, random tessellation, and geometrical effects) produce flatter fragments. Fragment shape analyses derived from hypervelocity impact experiments in a variety of mass distribution ranges qualitatively support this view.

Description: Publication date: 15 July 2018 Source: Icarus, Volume 309 Author(s): Uwe Fink, Lyn Doose A phase curve is derived for the dust coma of comet 67P/Churyumov-Gerasimenko (67P) from 1.2° to 74° using images from the OSIRIS camera system on board the Rosetta mission during the period 2014 July 25 to 2015 February 23 as the spacecraft approached the comet. We analyzed 123 images of the continuum filter at 612.6 nm and 60 images of the 375 nm UV continuum filter of the Wide Angle Camera. Our method of extracting a phase curve, close to the nucleus, taking into account illumination conditions, activity of the comet, strong radial radiance intensity decrease and varying phase angles across the image, is described in detail. Our derived backscattering phase curve is considerably steeper than earlier published data. The radiance of the scattering dust in the 612.6 nm filter increases by about a factor of 12 going from a phase angle of 75° to a phase angle of 2.0°. The phase curve for the 375 nm filter is similar but there is reasonable evidence that the I/F color ratio between the two filters changes from a roughly neutral color ratio of 1.2 to a more typical red color of ∼ 2.0 as the activity of the comet increases. No substantial change in the shape of the phase curve could be discerned between 2014 August and 2015 February 19–23 when the comet increased considerably in activity. The phase curve behavior on the illuminated side of the comet and the dark side is in general similar. A comparison of our phase curve with a recent phase curve for 67P by Bertini et al. for the phase angle range ∼15°–80°, where our two reductions overlap, shows good agreement (as does our color ratio between the 612.6 nm and the 375 nm filters) despite the fact that the two phase curve determinations observed the comet at different dust activity levels, at different distances from the nucleus and used completely different observing and data reduction methodologies. Trial scattering calculations demonstrate that the observed strong backscattering most likely arises from particles in the size range 1–20 µm. Our observed backscattering phase curve gives no constraints on the real index of refraction, the particle size distribution or the minimum and maximum particle size cut-offs. However, an upper limit to the imaginary index of refraction of ∼0.01 was required, making these particles quite transparent. Simple spherical scattering calculations including particle size distributions can fit the general characteristics of the phase curve but cannot produce a satisfactory detailed fit.