Description: Publication date: 1 September 2018 Source: Icarus, Volume 311 Author(s): James A. Kwiecinski, Andrew L. Krause, Robert A. Van Gorder As observations of ‘Oumuamua were collected well into the outbound component of its hyperbolic orbit, it is not obvious what effects Sol had on its rotational dynamics. Therefore, we simulate ‘Oumuamua as a prolate spheroid and triaxial ellipsoid of uniform mass density and show that the experimentally observed angular velocities remain largely unchanged during ‘Oumuamua’s flyby, supporting previous work suggesting that, in the absence of a collision during its interstellar journey, the asteroid was tumbling in the same manner as when it left its original solar system.

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): 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): Christopher O. Johnston, Eric C. Stern, Lorien F. Wheeler A high-fidelity approach for simulating the aerothermodynamic environments of meteor entries was developed, which allows the commonly assumed heat transfer coefficient of 0.1 to be assessed. This model uses chemically reacting computational fluid dynamics (CFD), coupled with radiation transport and surface ablation. Coupled radiation accounts for the impact of radiation on the flowfield energy equations, while coupled ablation explicitly models the injection of ablation products within the flowfield and radiation simulations. For a meteoroid with a velocity of 20 km/s, coupled radiation is shown to reduce the stagnation point radiative heating by over 60%. The impact of coupled ablation (with coupled radiation) is shown to provide at least a 70% reduction in the radiative heating relative to cases with only coupled radiation. This large reduction is partially the result of the low ionization energies of meteoric ablation products relative to air species. The low ionization energies of ablation products, such as Mg and Ca, provide strong photoionization and atomic line absorption in regions of the spectrum that air species do not. MgO and CaO are also shown to provide significant absorption. Turbulence is shown to impact the distribution of ablation products through the shock-layer, which results in up to a 100% increase in the radiative heating downstream of the stagnation point. To create a database of heat transfer coefficients, the developed model was applied to a range of cases. This database considered velocities ranging from 14 to 20 km/s, altitudes ranging from 20 to 50 km, and nose radii ranging from 1 to 100 m. The heat transfer coefficients from these simulations are below 0.045 for the range of cases, for both laminar and turbulent, which is significantly lower than the canonical value of 0.1. When the new heat transfer model is applied to a Tunguska-like 15 Mt entry, the effect of the new model is to lower the height of burst by up to 2 km, depending on assumed entry angle. This, in turn, results in a significantly larger ground damage footprint than when the canonical heating assumption is used.

Description: Publication date: 15 July 2018 Source: Icarus, Volume 309 Author(s): Shujuan Sun, Zongyu Yue, Kaichang Di The depth and diameter relationship is one of the most important characteristics of craters; however, previous studies have focused mostly on large-diameter craters because of the limitations of image resolution. Recently, very high resolution images have been obtained that make it possible to expand this field of study to craters with diameters of 〈 1 km. Using images with resolution of up to 0.5 m, acquired by the Lunar Reconnaissance Orbiter, we investigated the depth and diameter relationship of fresh craters with subkilometer diameters. We selected craters from lunar maria and highlands, and we made precise measurements of their diameters and depths. The results show that the d/D ratio of small craters in the lunar maria and highlands, which varies from ∼0.2 to ∼0.1, is generally shallower than that of larger craters. We propose that the reason for the difference is because of the low strength of the lunar surface material. The fitted power law parameters of lunar mare and highland craters were found to be different, and that might be explained by terrain-related differences.

Description: Publication date: 15 July 2018 Source: Icarus, Volume 309 Author(s): Colman Gallagher, Matt Balme, Richard Soare, Susan J. Conway Galaxias Chaos is a region of low plateaus separated by narrow fractures – a chaotic terrain. Galaxias Mensae and Galaxias Colles are characterised by mesa and knobby terrains of individual landforms, or small assemblages, separated by plains. Galaxias Chaos has been attributed to ground disturbance due to sublimation in shallow subsurface ice-rich deposits, Galaxias Mensae and Galaxias Colles to sublimation and degradation of icy surface materials, without production of chaotic terrain. Liquid water has not been regarded as a product of the degradation of these icy terrains. This paper asks two research questions: (1) what was the total extent of the different modes of landscape degradation, especially chaotic terrain, involved in producing the present landscapes of Galaxias Chaos and Galaxias Mensae–Colles; (2) can the generation of liquid water as a product of landscape degradation be ruled-out? Using a morphological-statistical approach, including power spectrum analysis of relief, our observations and analyses show that present mesa-knobby terrains of Galaxias Mensae–Colles evolved from a landscape that had the same directional pattern and relief as presently found in Galaxias Chaos. This terrain extended across ∼440,000 km 2 but ∼22,000 km 3 (average thickness, 77 m) have been lost across ∼285,000 km 2 . This represents a significant loss of ice-bearing deposits. Moreover, this surface degradation was spatially partitioned by landforms associated with elevated ground heating and the transmission of a fluid in the shallow subsurface towards a distal channel. In answer to research question 2, it cannot be determined definitively if the fluid involved was groundwater, generated by the thermal destabilisation of the icy deposits, or low viscosity lava. However, it is likely that the degradation of Galaxias Mensae–Colles was not a consequence of sublimation alone. These findings underscore the significance of cryo-volcanic interactions in the cycling of water between the Martian surface and the atmosphere.

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): 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): M. Pajuelo, B. Carry, F. Vachier, M. Marsset, J. Berthier, P. Descamps, W.J. Merline, P.M. Tamblyn, J. Grice, A. Conrad, A. Storrs, B. Timerson, D. Dunham, S. Preston, A. Vigan, B. Yang, P. Vernazza, S. Fauvaud, L. Bernasconi, D. Romeuf, R. Behrend, C. Dumas, J.D. Drummond, J.-L. Margot, P. Kervella, F. Marchis, J.H. Girard The population of large 100+ km asteroids is thought to be primordial. As such, they are the most direct witnesses of the early history of our Solar System available. Those among them with satellites allow study of the mass, and hence density and internal structure. We study here the dynamical, physical, and spectral properties of the triple asteroid (107) Camilla from lightcurves, stellar occultations, optical spectroscopy, and high-contrast and high-angular-resolution images and spectro-images. Using 80 positions measured over 15 years, we determine the orbit of its larger satellite, S/2001 (107) 1 , to be circular, equatorial, and prograde, with root-mean-square residuals of 7.8 mas, corresponding to a sub-pixel accuracy. From 11 positions spread over three epochs only, in 2015 and 2016, we determine a preliminary orbit for the second satellite S/2016 (107) 1 . We find the orbit to be somewhat eccentric and slightly inclined to the primary’s equatorial plane, reminiscent of the properties of inner satellites of other asteroid triple systems. Comparison of the near-infrared spectrum of the larger satellite reveals no significant difference with Camilla. Hence, both dynamical and surface properties argue for a formation of the satellites by excavation from impact and re-accumulation of ejecta in orbit. We determine the spin and 3-D shape of Camilla. The model fits well each data set: lightcurves, adaptive-optics images, and stellar occultations. We determine Camilla to be larger than reported from modeling of mid-infrared photometry, with a spherical-volume-equivalent diameter of 254 ± 36 km (3 σ uncertainty), in agreement with recent results from shape modeling (Hanus et al., 2017, A&A 601). Combining the mass of (1.12 ± 0.01) × 10 19  kg (3 σ uncertainty) determined from the dynamics of the satellites and the volume from the 3-D shape model, we determine a density of 1,280 ± 130 kg · m − 3 (3 σ uncertainty). From this density, and considering Camilla’s spectral similarities with (24) Themis and (65) Cybele (for which water ice coating on surface grains was reported), we infer a silicate-to-ice mass ratio of 1–6, with a 10–30% macroporosity.

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.