Description: Publication date: 1 January 2017 Source: Icarus, Volume 281 Author(s): Michael K. Shepard, James Richardson, Patrick A. Taylor, Linda A. Rodriguez-Ford, Al Conrad, Imke de Pater, Mate Adamkovics, Katherine de Kleer, Jared R. Males, Katie M. Morzinski, Laird M. Close, Mikko Kaasalainen, Matti Viikinkoski, Bradley Timerson, Vishnu Reddy, Christopher Magri, Michael C. Nolan, Ellen S. Howell, Lance A.M. Benner, Jon D. Giorgini, Brian D. Warner, Alan W. Harris Using the S-band radar at Arecibo Observatory, we observed 16 Psyche, the largest M-class asteroid in the main belt. We obtained 18 radar imaging and 6 continuous wave runs in November and December 2015, and combined these with 16 continuous wave runs from 2005 and 6 recent adaptive-optics (AO) images ( Drummond et al., 2016 ) to generate a three-dimensional shape model of Psyche. Our model is consistent with a previously published AO image (Hanus et al., 2013) and three multi-chord occultations. Our shape model has dimensions 279 × 232 × 189 km (± 10%), D eff = 226 ± 23 km, and is 6% larger than, but within the uncertainties of, the most recently published size and shape model generated from the inversion of lightcurves (Hanus et al., 2013). Psyche is roughly ellipsoidal but displays a mass-deficit over a region spanning 90° of longitude. There is also evidence for two ∼50–70 km wide depressions near its south pole. Our size and published masses lead to an overall bulk density estimate of 4500 ± 1400 kgm −3 . Psyche's mean radar albedo of 0.37 ± 0.09 is consistent with a near-surface regolith composed largely of iron-nickel and ∼40% porosity. Its radar reflectivity varies by a factor of 1.6 as the asteroid rotates, suggesting global variations in metal abundance or bulk density in the near surface. The variations in radar albedo appear to correlate with large and small-scale shape features. Our size and Psyche's published absolute magnitude lead to an optical albedo of p v = 0.15 ± 0.03, and there is evidence for albedo variegations that correlate with shape features.

Description: Publication date: 1 January 2017 Source: Icarus, Volume 281 Author(s): Ryuji Morishima We investigate planetary accretion that starts from equal-mass planetesimals using an analytic theory and numerical simulations. We particularly focus on how the planetary mass M oli at the onset of oligarchic growth depends on the initial mass m 0 of a planetesimal. Oligarchic growth commences when the velocity dispersion relative to the Hill velocity of the protoplanet takes its minimum. We find that if m 0 is small enough, this normalized velocity dispersion becomes as low as unity during the intermediate stage between the runaway and oligarchic growth stages. In this case, M oli is independent of m 0 . If m 0 is large, on the other hand, oligarchic growth commences directly after runaway growth, and M oli ∝ m 0 3 / 7 . The planetary mass M oli for the solid surface density of the Minimum Mass Solar Nebula is close to the masses of the dwarf planets in a reasonable range of m 0 . This indicates that they are likely to be the largest remnant planetesimals that failed to become planets. The power-law exponent q of the differential mass distribution of remnant planetesimals is typically − 2.0 and − 2.7 to − 2.5 for small and large m 0 . The slope, q ≃ − 2.7 , and the bump at 10 21 g (or 50 km in radius) for the mass distribution of hot Kuiper belt objects are reproduced if m 0 is the bump mass. On the other hand, small initial planetesimals with m 0 ∼ 10 13 g or less are favored to explain the slope of large asteroids, q ≃ − 2.0 , while the bump at 10 21 g can be reproduced by introducing a small number of asteroid seeds each with mass of 10 19 g.

Description: Publication date: December 2016 Source: Icarus, Volume 280 Author(s): J.A. Grant, T.J. Parker, L.S. Crumpler, S.A. Wilson, M.P. Golombek, D.W. Mittlefehldt Endeavour crater (2.28°S, 354.77°E) is a Noachian-aged 22 km-diameter impact structure of complex morphology in southern Meridiani Planum. The degradation state of the crater has been studied using orbital data from the Mars Reconnaissance Orbiter and in situ data from the Opportunity rover. Multiple exposed crater rim segments range in elevation from ∼10 m to over 100 m above the level of the embaying Burns Formation. The crater is 200–500 m deep and the interior wall exposes over ∼300 m of relief around the southern half of the crater. Slopes of 6–16% flank the exterior of the largest western rim segment. On the west side of the crater, both pre-impact rocks (Matijevic Formation) and Endeavour impact ejecta (Shoemaker Formation) are present at Cape York, but only the Shoemaker Formation (up to ∼140 m section) outcrops at Cape Tribulation. Study of similar sized pristine craters Bopolu and Tooting (with complex morphology) and use of metrics for describing the morphometry of martian craters suggest the original rim of Endeavour averaged 410 m in elevation, but relief varied about ±200 m around the circumference. A 250–275 m section of ejecta (±50–60 m) would have comprised a significant fraction of the rim height. The original crater was likely 1.5–2.2 km deep and may have had a central peak (no obvious evidence is present) between 200 and 500 m high. Comparison between the predicted original and current form of Endeavour suggests 100–200 m of rim degradation ranging from nearly complete ejecta removal in some locations to preservation of a thick ejecta section in others. Differences in rim relief are at least partially due to degradation and not just original rim relief and (or) due to offsets along rim faults. Most degradation occurred prior to deposition of the Burns Formation which is ∼200 m thick outside the crater, but likely thicker inside the crater. Aeolian stripping of the Burns Formation continues today via prevailing winds and lesser mass wasting is important on steeper walls. However, analogy with degraded Noachian craters south of Meridiani suggests fluvial processes were most important in early degradation and is consistent with the nearly complete removal of ejecta from some rim segments, gaps in the rim, formation of Marathon Valley, and interpretation of a pediment flanking the western rim. Slope processes likely accompanied incision that may have accounted for tens of metres rim lowering near Marathon Valley to more than 100 m at Cape York.

Description: Publication date: December 2016 Source: Icarus, Volume 280 Author(s): R.A. Yingst, K. Cropper, S. Gupta, L.C. Kah, R.M.E. Williams, J. Blank, F. Calef, V.E. Hamilton, K. Lewis, J. Shechet, M. McBride, N. Bridges, J. Martinez Frias, H. Newsom We combine the results of orbitally-derived morphologic and thermal inertia data with in situ observations of abundance, size, morphologic characteristics, and distribution of pebble- to cobble-sized clasts along the Curiosity rover traverse. Our goals are to characterize rock sources and transport history, and improve our ability to predict upcoming terrain. There are ten clast types, with nine types interpreted as sedimentary rocks. Only Type 3 clasts had morphologies indicative of significant wear through transport; thus, most clast types are indicative of nearby outcrops or prior presence of laterally extensive sedimentary rock layers, consistent with the erosional landscape. A minor component may reflect impact delivery of more distant material. Types 1 and 4 are heavily-cemented sandstones, likely associated with a “caprock” layer. Types 5 and 6 (and possibly 7) are pebble-rich sandstones, with varying amounts of cement leading to varying susceptibility to erosion/wear. Type 3 clasts are rounded pebbles likely transported and deposited alluvially, then worn out of pebbly sandstone/conglomerate. Types 9 and 10 are poorly-sorted sandstones, with Type 9 representing fragments of Square Top-type layers, and Type 10 deriving from basal or other Mt. Sharp layers. Types 2, 8 and 9 are considered exotics. There are few clear links between clast type and terrain surface roughness (particularly in identifying terrain that is challenging for the rover to navigate). Orbital data may provide a reasonable prediction of certain end-member terrains but the complex interplay between variables that contribute to surface characteristics makes discriminating between terrain types from orbital data problematic. Prediction would likely be improved through higher-resolution thermal inertia data.

Description: Publication date: December 2016 Source: Icarus, Volume 280 Author(s): G.M. Martínez, E. Fischer, N.O. Rennó, E. Sebastián, O. Kemppinen, N. Bridges, C.S. Borlina, P.-Y. Meslin, M. Genzer, A.-H. Harri, A. Vicente-Retortillo, M. Ramos,  M. de la Torre Juárez, F. Gómez, J. Gómez-Elvira We provide indirect evidence for the formation of frost at the surface of Gale crater by analyzing the highest confidence data from simultaneous measurements of relative humidity and ground temperature during the first 1000 sols of the Mars Science Laboratory (MSL) mission. We find that except for sol 44, frost events could have occurred only between sols 400 and 710, corresponding to the most humid and coldest time of the year (from early fall to late winter). In particular, measurements at Dingo Gap during sols 529–535, at an unnamed place during sols 554–560, at Kimberley during sols 609–617 and at an unnamed place during sols 673–676 showed the largest likelihood of the occurrence of frost events. At these four locations, the terrain is composed of fine-grained and loosely packed material with thermal inertia values of ∼200 SI units, much lower than the 365 ± 50 SI units value found at the landing ellipse. This is important because terrains with exceptionally low thermal inertia favor the formation of frost by lowering minimum daily ground temperatures. An order-of-magnitude calculation to determine the thickness of the frost layer at these four locations results in values of tenths of µm, while the precipitable water content is a few pr-µm. Therefore, surface frost events can have important implications for the local water cycle at Gale crater. In addition, frost is the most likely type of water that can be temporarily found in bulk amounts on the surface of Mars at low latitudes and therefore can cause weathering, influencing the geology of Gale crater.

Description: Publication date: 1 January 2017 Source: Icarus, Volume 281

Description: Publication date: 1 January 2017 Source: Icarus, Volume 281 Author(s): Jurriën Sebastiaan Knibbe, Wim van Westrenen We have simulated in-orbit variations of the impact flux and spatial distributions of >100 km diameter ( D ) crater production for Mercury in its current 3:2 and hypothetical 2:1 and 1:1 spin–orbit resonances. Results show that impact fluxes and D > 100 km cratering are non-uniform for these rotational states when Mercury's orbit is significantly eccentric. Variations in the impact flux and D > 100 km cratering depend on the orbital elements of Mercury and its impactors. The observed spatial distribution of large Mercurian craters is difficult to generate by cratering in Mercury's current 3:2 spin–orbit resonance, but can be produced by cratering in a former 1:1 (as previously proposed by Wieczorek et al., 2012 ) or 2:1 spin–orbit resonance. We have calculated capture probabilities at spin–orbit resonances for a rigid Mercury. If Mercury's initial rotation was prograde, we find that a higher order spin–orbit resonance is the most likely first capture for feasible (low) values of Mercury's past triaxiality. In light of Mercury's crater record, we examined the possibility that impacts have initiated transitions in past spin–orbit resonances. Although the number of craters whose generating impact would have destabilized a spin–orbit resonance is sensitive to the crater scaling procedure, any initial rotational state of Mercury has likely been destabilized by impacts. An initial and permanent 3:2 spin–orbit resonance capture seems untenable. Mercury's tidal torque decelerates Mercury's rotation for the most likely range of Mercury's orbital eccentricity. Only one or two craters are candidate relics of an impact-event that facilitates an instantaneous transition from a former synchronous rotation to the 3:2 spin–orbit resonance, and only for a small crater scaling factor. We propose a rotational evolution trajectory for Mercury with visits to spin–orbit resonances of decreasing order including a substantial period in the 2:1 spin–orbit resonance, which can account for the observed spatial distribution of large craters.

Description: Publication date: 1 January 2017 Source: Icarus, Volume 281 Author(s): Aimee W. Merkel, Timothy A. Cassidy, Ronald J. Vervack, William E. McClintock, Menelaos Sarantos, Matthew H. Burger, Rosemary M. Killen The Ultraviolet and Visible Spectrometer channel of the Mercury Atmospheric and Surface Composition Spectrometer instrument aboard the MErcury Surface, Space ENvironment, GEochemistry, and Ranging spacecraft made near-daily observations of solar-scattered resonant emission from magnesium in Mercury's exosphere during the mission's orbital phase (March 2011–April 2015, ∼17 Mercury years). In this paper, a subset of these data (March 2013–April 2015) is described and analyzed to illustrate Mg's spatial and temporal variations. Dayside altitude profiles of emission are used to make estimates of the Mg density and temperature. The main characteristics of the Mg exosphere are (a) a predominant enhancement of emission in the morning (6 am–10 am) near perihelion, (b) a bulk temperature of ∼6000 K, consistent with impact vaporization as the predominant ejection process, (c) a near-surface density that varies from 5 cm −3 to 50 cm −3 and (d) a production rate that is strongest in the morning on the inbound leg of Mercury's orbit with rates ranging from 1 × 10 5 cm −2 s −1 to 8 × 10 5 cm −2 s −1 .

Description: Publication date: 1 January 2017 Source: Icarus, Volume 281 Author(s): Yury Aglyamov, Dustin M. Schroeder, Steven D. Vance The surface of Europa has been hypothesized to include an ice regolith layer from hundreds of meters to kilometers in thickness. However, contrary to previous claims, it does not present a significant obstacle to searching for Europa’s ocean with radar sounding. This note corrects prior volume scattering loss analyses and expands them to includes observational and thermo-mechanical constraints on pore size and regolith depth. This provides a more physically realistic range of potential ice-regolith volume-scattering losses for radar sounding observations of Europa’s ice shell in the HF and VHF frequency bands. We conclude that, for the range of physical processes and material properties observed or hypothesized for Europa, volume scattering losses are not likely to pose a major obstacle to radar penetration.

Description: Publication date: 1 January 2017 Source: Icarus, Volume 281 Author(s): S. De Angelis, C. Carli, F. Tosi, P. Beck, B. Schmitt, G. Piccioni, M.C. De Sanctis, F. Capaccioni, T. Di Iorio, Sylvain Philippe We investigate two poly-hydrated magnesium sulfates, hexahydrite (MgSO 4 · 6H 2 O) and epsomite (MgSO 4 · 7H 2 O), in the visible and infrared (VNIR) spectral range 0.5/4.0 µm, as particulate for three different grain size ranges: 20–50 µm, 75–100 µm and 125–150 µm. All samples were measured in the 93–298 K temperature range. The spectra of these hydrated salts are characterized by strong OH absorption bands in the 1.0–1.5 µm region, and by H 2 O absorption bands near 2 and 3 µm. Other weak features show up at low temperatures near 1.75 µm (in both hexahydrite and epsomite) and 2.2 µm (only in hexahydrite). The spectral behavior of the absorption bands of these two minerals has been analyzed as a function of both grain size and temperature, deriving trends related to specific spectral parameters such as band center, band depth, band area, and band width. Hydrated minerals, in particular mono- and poly-hydrated sulfates, are present in planetary objects such as Mars and the icy Galilean satellites. Safe detection of these minerals shall rely on detailed laboratory investigation of these materials in different environmental conditions. Hence an accurate spectral analysis of such minerals as a function of temperature is key to better understand and constrain future observations.