ALBERT

Description: We analyze the shapes of twenty eight hypothesized scoria cones in three regions on Mars, i.e. Ulysses and Hydraotes Colles and Coprates Chasma. Using available HiRISE and CTX Digital Elevation Models, we determine the basic morphometric characteristics of the cones and estimate from ballistic modelling the physical parameters of volcanic eruptions that could have formed them. When compared to terrestrial scoria cones, most of the studied cones show larger volumes (up to 4.2 × 10 9  m 3 ), larger heights (up to 573 m) and smaller average slopes. The average slopes of the Ulysses, Hydraotes and Coprates cones range between 7° and 25°, and the maximum slopes only rarely exceed 30°, which suggests only a minor role of scoria redistribution by avalanching. Ballistic analysis indicates that all cones were formed in a similar way and their shapes are consistent with an ejection velocity about two times larger and a particle size about twenty times smaller than on Earth. Our results support the hypothesis that the investigated edifices were formed by low energy Strombolian volcanic eruptions and hence are equivalent to terrestrial scoria cones. The cones in Hydraotes Colles and Coprates Chasma are on average smaller and steeper than the cones in Ulysses Colles, which is likely due to the difference in topographic elevation and the associated difference in atmospheric pressure. This study provides the expected morphometric characteristics of Martian scoria cones, which can be used to identify landforms consistent with this type of activity elsewhere on Mars and distinguish them from other conical edifices.

Description: No abstract is available for this article.

Description: NASA's New Horizons spacecraft flies past Pluto on July 14, 2015, carrying two instruments that detect charged particles. Pluto has a tenuous, extended atmosphere that is escaping the planet's weak gravity. The interaction of the solar wind with Pluto's escaping atmosphere depends on solar wind conditions as well as the vertical structure of Pluto's atmosphere. We have analyzed Voyager 2 particles and fields measurements between 25 and 39 AU and present their statistical variations. We have adjusted these predictions to allow for the Sun's declining activity and solar wind output. We summarize the range of SW conditions that can be expected at 33 AU and survey the range of scales of interaction that New Horizons might experience. Model estimates for the solar wind stand-off distance vary from ~7 to ~1000 R P with our best estimate being around 40 R P (where we take Pluto's radius to be R P  = 1184 km).

Description: No abstract is available for this article.

Description: The Pb isotopic compositions of maskelynite and pyroxene grains were measured in ALH84001 and three enriched Shergottites (Zagami, RBT04262, and LAR12011) by Secondary Ion Mass Spectrometry (SIMS). A maskelynite-pyroxene isochron for ALH84001 defines a crystallization age of 4089±73 Ma (2σ). The initial Pb isotopic composition of each meteorite was measured in multiple maskelynite grains. ALH84001 has the least radiogenic initial Pb isotopic composition of any Martian meteorite measured to date (i.e., 206 Pb/ 204 Pb=10.07±0.17, 2σ). Assuming an age of reservoir formation for ALH84001 and the enriched Shergottites of 4513 Ma (Borg et al., 2003, Lapen et al., 2010), a two stage Pb isotopic model has been constructed. This model links ALH84001 and the enriched Shergottites by their similar μ-value ( 238 U/ 204 Pb) of 4.1-4.6 from 4.51 Ga to 4.1 Ga and 0.17 Ga, respectively. The model employed here is dependent on a chondritic μ-value (~1.2) from 4567–4513 Ma, which implies core segregation had little to no effect on the μ-value(s) of the Martian mantle. The proposed Pb isotopic model here can be used to calculate ages that are in agreement with Rb-Sr, Lu-Hf and Sm-Nd ages previously determined in the meteorites and confirm the young (~170 Ma) ages of the enriched Shergottites and ancient, 〉4 Ga, age of ALH84001.

Description: The physical processes active during the crystallization of a low-pressure, low-gravity planetesimal core are poorly understood but have implications for asteroidal magnetic fields and large-scale asteroidal structure. We consider a core with only a thin silicate shell, which could be analogous to some M-type asteroids including Psyche, and use a parameterized thermal model to predict a solidification timeline and the resulting chemical profile upon complete solidification. We then explore the potential strength and longevity of a dynamo in the planetesimal's early history. We find that cumulate inner core solidification would be capable of sustaining a dynamo during solidification, but less power would be available for a dynamo in an inward dendritic solidification scenario. We also model and suggest limits on crystal settling and compaction of a possible cumulate inner core.

Description: We report the global distribution of areas exhibiting no absorption features (featureless or FL) on the lunar surface, based on the reflectance spectral data set obtained by the Spectral Profiler onboard Kaguya/SELENE. We found that FL sites are located in impact basins and large impact craters in the Feldspathic Highlands Terrane (FHT), while there are no FL sites in the Procellarum regions nor the South Pole–Aitken basin. FL sites in each impact basin/crater are mainly found at the peak rings or rims, where the purest anorthosite (PAN) sites are also found. At the local scale, most of the FL and PAN points are associated with impact craters and peaks. Most of the FL spectra show a steeper (redder) continuum than the PAN spectra, suggesting the occurrence of space weathering effects. We propose that most of the material exhibiting a FL spectrum originate from space weathered PAN. Taking into account all the occurrence trends of FL sites on the Moon, we propose that both the FL and PAN materials were excavated from the primordial lunar crust during ancient basin formations below the megaregolith in the highlands. Since the FL and PAN sites are widely distributed over the lunar surface, our new data may support the existence of a massive PAN layer below the lunar surface.

Description: Measurements of the flow directions of 80 long lava flows in the flexural basin that surrounds Olympus Mons reveal that most are no longer aligned with the regional downslope direction, but diverge by an average of 15.4° ± 9.8° in the counterclockwise direction. This misalignment is consistent with topographic change in the basin after the flows were emplaced due to subsidence centered on nearby Olympus Mons. The observed tilting indicates subsidence of 0.62 to 2.13 km at the center of Olympus Mons induced by the addition of 1.33 x 10 5 to 1.35 x 10 6  km 3 of volcanic material, or about 1-10% of the total volume of the present-day edifice. New crater size-frequency measurements for selected areas within the flexural basin indicate the subsidence took place in the past 210 ± 40 Ma. These findings indicate that previously recognized Late Amazonian volcanic and tectonic events at Olympus Mons signify an important period of magmatic activity during which a substantial portion of the present-day edifice may have been added.

Description: We use a 3D general circulation model to compare the primitive Martian hydrological cycle in “warm and wet” and “cold and icy” scenarios. In the “warm and wet” scenario, an anomalously high solar flux or intense greenhouse warming artificially added to the climate model are required to maintain warm conditions and an ice-free northern ocean. Precipitation shows strong surface variations, with high rates around Hellas basin and west of Tharsis but low rates around Margaritifer Sinus(where the observed valley network drainage density is nonetheless high). In the “cold and icy” scenario, snow migration is a function of both obliquity and surface pressure, and limited episodic melting is possible through combinations of seasonal, volcanic and impact forcing. At surface pressures above those required to avoid atmospheric collapse (~0.5 bar) and moderate to high obliquity, snow is transported to the equatorial highland regions where the concentration of valley networks is highest. Snow accumulation in the Aeolis quadrangle is high, indicating an ice-free northern ocean is not required to supply water to Gale crater. At lower surface pressures and obliquities, both H2O and CO2 are trapped as ice at the poles and the equatorial regions become extremely dry. The valley network distribution is positively correlated with snow accumulation produced by the “cold and icy” simulation at 41.8° obliquity but uncorrelated with precipitation produced by the “warm and wet” simulation. Because our simulations make specific predictions for precipitation patterns under different climate scenarios, they motivate future targeted geological studies.

Description: Tidal deformation of icy satellites provides crucial information on their subsurface structures. In this study, we investigate the parameter dependence of the tidal displacement and potential Love numbers (i.e., h 2 and k 2 , respectively) of Ganymede. Our results indicate that Love numbers for Ganymede models without a subsurface ocean are not necessarily smaller than those with a subsurface ocean. The phase lag, however, depends primarily on the presence/absence of a subsurface ocean. Thus, the determination of the phase lag would be of importance to infer whether Ganymede possesses a subsurface ocean or not based only on geodetic measurements. Our results also indicate that the major control on Love numbers is the thickness of the ice shell if Ganymede possesses a subsurface ocean. This result, however, does not necessarily indicate that measurement of either of h 2 or k 2 alone is sufficient to estimate the shell thickness; while a thin shell leads to large h 2 and k 2 independent of parameters, a thick shell does not necessarily lead to small h 2 and k 2 . We found that, to reduce the uncertainty in the shell thickness, constraining k 2 in addition to h 2 is necessary, highlighting the importance of collaborative analyses of topography and gravity field data.