A theoretical model for the formation of Ring Moat Dome Structures:Products of second boiling in lunar basaltic lava flows

Newly documented Ring Moat Dome Structures (RMDSs), low mounds typically several hundred meters across with a median height of ~3.5 m and surrounded by moats, occur in the lunar maria. They appear to have formed synchronously with the surrounding mare basalt deposits. It has been hypothesized that they formed on the surfaces of lava flows by the extrusion of magmatic foams generated in the flow interiors as the last stage of the eruption and flow emplacement process. We develop a theoretical model for the emplacement and cooling of mare basalts in which the molten cores of cooling flows are inflated during the late stages of eruptions by injection of additional hot lava containing dissolved volatiles. Crystallization of this lava causes second boiling (an increase in vapor pressure to the point of supersaturation due to crystallization of the melt), generating copious quantities of vesicles (magmatic foam layers) at the top and bottom of the central core of the flow. Flow inflation of many meters is predicted to accompany the formation of the foam layers, flexing the cooled upper crustal layer, and forming fractures that permit extrusions of the magmatic foams onto the surface to form domes, with subsidence of the subjacent and surrounding surface forming the moats. By modeling the evolution of the internal flow structure we predict the properties of RMDSs and the conditions in which they are most likely to form. We outline several tests of this hypothesis

Duchenne Muscular Dystrophy and Brain Function

Muscular dystrophies have historically been characterised according to clinical criteria, however in the genomic age the muscular dystrophies are now subdivided into groups according to the primary gene defect. Currently identified are 29 different loci and encoded proteins, giving rise to 34 distinct forms of muscular dystrophy (Dalkilic & Kunkel 2003;Hsu 2004). The majority of these types of muscular dystrophy are caused by perturbations of different components of the dystrophin-glycoprotein complex (DGC) an integral component of the cellular cytoskeleton (see below). Dystrophin is the largest component of the DGC and is absent in Duchenne muscular dystrophy (DMD), and severely truncated with decreased levels in Becker muscular dystrophy (BMD) (Hoffman & Kunkel 1989). DMD and the allelic BMD are the most common forms of muscular dystrophy in humans and together they are termed dystrophinopathies (Kingston et al. 1984; Shaw & Dreifuss1969). DMD alone accounts for approximately 80% of all the myopathies in the muscular dystrophy group (Culligan et al. 1998).The dystrophin gene is the second largest described to date, totalling 1.5% of the X chromosome, 0.1% of the entire genome. The DMD gene is 99% introns, with a coding sequence of 86 exons (including the promoters) and remains the only known human metagene (Blake et al. 2002; Burmeister et al. 1988; Hamed & Hoffmann 2006; Kenwrick et al. 1987; Koenig et al. 1987; Kunkel et al. 1986; Muntoni et al. 2003; Roberts et al. 1993; Smith et al. 2006; Van Ommen et al. 1987; Wallis et al. 2004). Dystrophin wasdemonstrated to be localised at the sarcolemma in human skeletal muscle after its’ genetic characterisation (Arahata et al. 1988; Sugita et al. 1988; Zubrzycka-Gaarn et al. 1988). This discovery was followed by a report of dystrophin messenger RNA in brain, with the protein being specifically localised at postsynaptic densities (PSD) in the CNS, in particular in the hippocampus, cerebral cortex and in cerebellar Purkinje cells (PC) (Chamberlain et al. 1988; Chelly et al. 1988, 1989; Lidov et al. 1990, Nudel et al. 1988)

The Cauchy 5 Small, Low-Volume Lunar Shield Volcano:Evidence for Volatile Exsolution-Eruption Patterns and Type 1/Type 2 Hybrid Irregular Mare Patch Formation

The lunar shield volcano Cauchy 5, sitting at the low diameter‐height‐volume end of the population, is the only known example containing two different types of Irregular Mare Patches (IMPs) in very close association: (1) the pit crater interior Type 1 IMP composed of bleb‐like mounds surrounded by a hummocky and blocky floor unit and (2) Type 2 IMPs, small, often optically immature pits less than ~5 m deep, located on the generally block‐deficient shield flanks. A four‐phase lunar magma ascent/eruption model predicts that during a relatively brief eruption, low magma rise rates maximize volatile exsolution in lava filling the pit crater. Bubble‐rich magmas overtop the pit crater and form extremely vesicular flows on the shield flanks. Exposure of the flanking flows to vacuum produces a fragmental layer of exploded glassy bubble walls. Subsequent second boiling upon cooling of the flanking flow interiors releases additional volatiles which migrate and collect, forming magmatic foams and gas pockets. As magma rise rates slow, trapped gas and magmatic foam build up below the cooling pit crater floor. Magmatic foams are extruded to form Type 1 IMP deposits. Type 2 IMPs on the flanks are interpreted to be due primarily to subsequent impacts causing collapse of the flow surface layer into the extremely vesicle‐ and void‐rich flow interior. Anomalously young pit crater floor/shield flank crater retention ages compared with surrounding maria ages may be due to effects of Cauchy 5 substrate characteristics (extreme micro‐ and macroporosity, foamy nature, and glassy auto‐regolith) on superposed crater formation and retention

Rethinking Lunar Mare Basalt Regolith Formation:New Concepts of Lava Flow Protolith and Evolution of Regolith Thickness and Internal Structure

Lunar mare regolith is traditionally thought to have formed by impact bombardment of newly emplaced coherent solidified basaltic lava. We use new models for initial emplacement of basalt magma to predict and map out thicknesses, surface topographies and internal structures of the fresh lava flows, and pyroclastic deposits that form the lunar mare regolith parent rock, or protolith. The range of basaltic eruption types produce widely varying initial conditions for regolith protolith, including (1) autoregolith, a fragmental meter-thick surface deposit that forms upon eruption and mimics impact-generated regolith in physical properties, (2) lava flows with significant near-surface vesicularity and macroporosity, (3) magmatic foams, and (4) dense, vesicle-poor flows. Each protolith has important implications for the subsequent growth, maturation, and regional variability of regolith deposits, suggesting wide spatial variations in the properties and thickness of regolith of similar age. Regolith may thus provide key insights into mare basalt protolith and its mode of emplacement

Rainfall on Noachian Mars:Nature, timing, and influence on geologic processes and climate history

The formation of martian geologic features, including degraded impact craters, valley networks, and lakes, has been interpreted to require a continuously “warm and wet” Noachian climate, with above-freezing surface temperatures and rainfall. More specifically, it has been argued that a change in the nature of rainfall in the Noachian, from a diffusive rain splash-dominated erosional regime to an advective runoff-dominated erosional regime, is the best explanation for the observed temporal differences of erosion style: the degradation of craters has been interpreted to be due to rain splash throughout the Noachian, while the formation of valley networks and lakes has been interpreted to be due to more erosive activity and more abundant fluvial activity at the Noachian/Hesperian transition. However, the presence of a long-lived “warm and wet” climate with rainfall is difficult to reconcile with climate models which instead suggest that the long-lived climate may have been “cold and icy”, with surface temperatures far below freezing, precipitation limited to snowfall, and most water trapped as ice in the highlands. In such a “cold and icy” climate scenario, fluvial and lacustrine activity would only be possible during transient warm periods, which could produce “warm and wet” conditions for relatively short periods of time. In this work, we (1) review the geomorphic evidence for Noachian rainfall and the various rainfall-related erosional regimes, (2) explore climate model predictions for a “cold and icy” climate and the potential for short-lived “warm and wet” excursions, and (3) attempt to characterize the transition from diffusive to advective erosional rainfall regimes through analysis of atmospheric pressure and rainfall dynamics with the goal of providing insight into the nature of the Noachian hydrological cycle and thus, the Noachian climate. We conclude that (1) if rainfall occurred on early Mars, raindrops would have been capable of transferring sufficient energy to initiate sediment transport regardless of atmospheric pressure, implying that rain splash would have been possible throughout the Noachian, and (2) in contrast to previous findings, maximum possible raindrop size does not depend on atmospheric pressure and, as a result, simple parameterized relationships suggest that rainfall intensity (rainfall rate) does not depend on atmospheric pressure. Therefore, our results, based on the implementation of a simple parameterized relationship for rainfall intensity, predict that there would not have been a transition from rain splash-dominated erosion to runoff-dominated erosion related solely to decreasing atmospheric pressure in the Noachian. This finding is not consistent with the hypothesis of Craddock and Lorenz (2017) that the long-lived Noachian climate was “warm and wet” with rainfall throughout the Noachian and that rainfall intensity changed as a function of atmospheric pressure declining through time; our findings do not preclude the possibility that early Mars was predominantly “cold and icy”. Remaining unknown is the mechanism(s) for the observed geomorphic transition in erosion style, and whether melting of surface snow/ice and runoff during a punctuated heating episode in an otherwise “cold and icy” climate could explain the formation of the valley networks and lakes in the absence of rainfall. We conclude by outlining future work that introduces more advanced methodology to further explore a possible relationship between rainfall intensity and atmospheric pressure

Rheological Chaos in a Scalar Shear-Thickening Model

We study a simple scalar constitutive equation for a shear-thickening material at zero Reynolds number, in which the shear stress \sigma is driven at a constant shear rate \dot\gamma and relaxes by two parallel decay processes: a nonlinear decay at a nonmonotonic rate R(\sigma_1) and a linear decay at rate \lambda\sigma_2. Here \sigma_{1,2}(t) = \tau_{1,2}^{-1}\int_0^t\sigma(t')\exp[-(t-t')/\tau_{1,2}] {\rm d}t' are two retarded stresses. For suitable parameters, the steady state flow curve is monotonic but unstable; this arises when \tau_2>\tau_1 and 0>R'(\sigma)>-\lambda so that monotonicity is restored only through the strongly retarded term (which might model a slow evolution of material structure under stress). Within the unstable region we find a period-doubling sequence leading to chaos. Instability, but not chaos, persists even for the case \tau_1\to 0. A similar generic mechanism might also arise in shear thinning systems and in some banded flows.Comment: Reference added; typos corrected. To appear in PRE Rap. Com

Lunar Irregular Mare Patches:Classification, Characteristics, Geologic Settings, Updated Catalog, Origin, and Outstanding Questions

One of the most mysterious lunar features discovered during the Apollo era was Ina, a ~2 × 3-km depression composed of bleb-like mounds surrounded by hummocky and blocky terrains. Subsequent studies identified dozens of similar features in lunar maria, describing them as Irregular Mare Patches (IMPs). Due to the unusual and complex characteristics of IMPs, their specific formation mechanism is debated. To improve our understanding of the nature and origin of IMPs, we undertook an updated search and geological characterization of all IMPs and established a classification approach encompassing the full spectrum of IMPs. We present an updated catalog of 91 IMPs and survey the detailed characteristics of each IMP. We find that the majority of IMPs occur in maria emplaced over three billion years ago, contemporaneous with the peak period of global lunar volcanism. We utilized geologic context information and characteristics to establish two classification schemes for lunar IMPs: (1) geologic context: IMPs are categorized into (a) small shield volcano summit pit floor and flank, (b) linear/sinuous rille interior and adjacent exterior, and (c) typical maria; (2) characteristics: IMPs are classified into (a) “mound + floor” and (b) “pit only” types. We showed the range of characteristics of lunar IMPs was consistent with the waning-stage magmatic foam formation and extrusion scenario in different environments. Our updated catalog and classification raise several outstanding questions concerning the nature and origin of lunar IMPs. Assessing these questions will improve our knowledge of lunar thermal and geologic evolution. ©2020. American Geophysical Union. All Rights Reserved

Global modelling of the early Martian climate under a denser CO2 atmosphere: Water cycle and ice evolution

We discuss 3D global simulations of the early Martian climate that we have performed assuming a faint young Sun and denser CO2 atmosphere. We include a self-consistent representation of the water cycle, with atmosphere-surface interactions, atmospheric transport, and the radiative effects of CO2 and H2O gas and clouds taken into account. We find that for atmospheric pressures greater than a fraction of a bar, the adiabatic cooling effect causes temperatures in the southern highland valley network regions to fall significantly below the global average. Long-term climate evolution simulations indicate that in these circumstances, water ice is transported to the highlands from low-lying regions for a wide range of orbital obliquities, regardless of the extent of the Tharsis bulge. In addition, an extended water ice cap forms on the southern pole, approximately corresponding to the location of the Noachian/Hesperian era Dorsa Argentea Formation. Even for a multiple-bar CO2 atmosphere, conditions are too cold to allow long-term surface liquid water. Limited melting occurs on warm summer days in some locations, but only for surface albedo and thermal inertia conditions that may be unrealistic for water ice. Nonetheless, meteorite impacts and volcanism could potentially cause intense episodic melting under such conditions. Because ice migration to higher altitudes is a robust mechanism for recharging highland water sources after such events, we suggest that this globally sub-zero, `icy highlands' scenario for the late Noachian climate may be sufficient to explain most of the fluvial geology without the need to invoke additional long-term warming mechanisms or an early warm, wet Mars.Comment: Minor revisions to text, one new table, figs. 1,3 11 and 18 redon

Mare Domes in Mare Tranquillitatis:Identification, Characterization, and Implications for Their Origin

Mare domes, small shield volcanoes typically <∼30 km diameter, are part of the spectrum of lunar volcanic features that characterize extrusive basalt deposits. We used new spacecraft data to document these in Mare Tranquillitatis, among the oldest maria and the site commonly interpreted as an ancient degraded non-mascon impact basin. We found 283 known and suspected mare domes, with the majority (n = 229) concentrated on a broad, ∼450 km circular topographic rise in eastern Mare Tranquillitatis. The domes (median diameter 5.6 km, height 68 m, volume 0.7 km3) contain summit pits (74%; median diameter 0.8 km), and exhibit minor compositional variability between domes and surrounding flows, suggesting that domes both supply and are embayed by these flows. Based on their characteristics and associations, we interpret the small shield volcanoes to have been built from individual low-volume (<∼10–100 km3), low volatile content, short duration, cooling-limited eruptions. The ∼450 km broad volcanic rise is ∼920 m high (volume ∼1.6 × 105 km3) and is interpreted to be built from multiple occurrences of small shield eruptions, a shield plains volcanism style. This implies a shallow mantle source region capable of supplying distributed dike-emplacement and eruption events over an area of 1.75 × 105 km2 early in mare volcanism history (∼3.7 Ga). The difference between Mare Tranquillitatis and younger mare-filled mascon basins is attributed to the more ancient thermal state and crustal structure of the viscously relaxed Tranquillitatis basin, and a shallower broad magma source region present in earlier lunar thermal history

Volcanically Induced Transient Atmospheres on the Moon:Assessment of Duration, Significance, and Contributions to Polar Volatile Traps

A transient lunar atmosphere formed during a peak period of volcanic outgassing and lasting up to about ~70 Ma was recently proposed. We utilize forward-modeling of individual lunar basaltic eruptions and the observed geologic record to predict eruption frequency, magma volumes, and rates of volcanic volatile release. Typical lunar mare basalt eruptions have volumes of ~102–103 km3, last less than a year, and have a rapidly decreasing volatile release rate. The total volume of lunar mare basalts erupted is small, and the repose period between individual eruptions is predicted to range from 20,000 to 60,000 years. Only under very exceptional circumstances could sufficient volatiles be released in a single eruption to create a transient atmosphere with a pressure as large as ~0.5 Pa. The frequency of eruptions was likely too low to sustain any such atmosphere for more than a few thousand years. Transient, volcanically induced atmospheres were probably inefficient sources for volatile delivery to permanently shadowed lunar polar regions. ©2020. American Geophysical Union. All Rights Reserved