Enceladus's degree 2 gravity, determined by Cassini, is nominally nonhydrostatic to 3σ (J2/C22 = 3.38–3.63, as opposed to 10/3). Iess et al. (2014) interpret this in terms of a hydrostatic interior ...(core) and isostatic (not hydrostatic) floating ice shell. Enceladus's rapid (1.37 d) synchronous spin and tide distorts its shape substantially, though, enough that the predicted hydrostatic J2/C22 is not 10/3 but closer to 3.25. This leads to the following revision to the internal picture of Enceladus, compared with Iess et al.: (1) the satellite's core is somewhat smaller and slightly denser (190 km radius and 2450 kg/m3); (2) the compensation depth (shell thickness) of the global (degree 2) ice shell is ≈ 50 km, rather close to the base of the modeled ice + water layer; and (3) the compensation depth (shell thickness) beneath the South Polar Terrain (from J3) remains shallower (thinner) at ≈ 30 km, independent of but influenced by the degree 2 solution.
Key Points
Enceladus' rapid spin requires higher‐order theory for degree 2 gravity and shape
Cassini gravity and shape imply a global ocean nearly but not completely frozen
Ice shell much thinner over the South Polar Terrain facilitating plume venting
The Pluto System After New Horizons Stern, S. Alan; Grundy, William M; McKinnon, William B ...
Annual review of astronomy and astrophysics,
09/2018, Letnik:
56, Številka:
1
Journal Article
Recenzirano
Odprti dostop
The
New Horizons
(NH) flyby of the Pluto-Charon binary planet and its system of four small surrounding satellites in mid-2015 revolutionized our knowledge of this distant planet and its moons. Beyond ...providing rich geo-logical, compositional, and atmospheric data sets, NH demonstrated that Pluto has been surprisingly geologically and climatologically active throughout the past 4+ Gyr and that the planet exhibits a remarkably complex range of atmospheric phenomenology and geological expressions that rival Mars in their richness. In contrast, Pluto's large, planet-sized satellite Charon, though also geologically complex, has no detected active surface volatiles, has no detectable atmosphere, has much more muted colors, has lower albedo, and exhibits only ancient terrains. Pluto's system of four small satellites orbiting outside of Charon is itself dynamically complex and geologically interesting. Here, we review both what was known about the Pluto system before NH and what it has taught us about the Pluto system specifically and, by inference, other small planets in the Kuiper Belt. We go on to examine the natural next steps in Kuiper Belt exploration.
Highlights • Neuromuscular adaptations with age result in reduced muscle contraction velocity. • This leads to an accelerated loss of muscle power compared to isometric strength. • The failure to ...produce force rapidly puts older adults at increased risk for falls. • Muscle power represents a functionally relevant outcome for rehabilitation programs. • Power training may have advantages over strength training for improving mobility.
Near-threshold
J
/ψ photoproduction is a key aspect of the physics program at the Thomas Jefferson National Accelerator Facility (JLab) 12 GeV upgrade due to the wealth of information it has to ...offer.
J
/ψ photoproduction proceeds through the exchange of gluons in the
t
-channel and is expected to provide unique insight about the nucleon gravitational form factors and the nucleon mass radius. The JLab based CLAS Collaboration, which uses the CEBAF Large Acceptance Spectrometer (CLAS12), aims to measure the
J
/ψ near threshold photoproduction cross section using both a proton and a deuteron target, from threshold up to 10.6 GeV. The deuteron target further offers the possibility of comparing the proton and neutron gluonic form factors and mass radii in a first measurement of the cross sections off a proton or neutron within the deuteron target. The analysis towards these measurements is ongoing and well advanced, with machine learning based techniques for particle identification already designed and validated with CLAS12 data.
Enceladus' cratered terrains contain large numbers of unusually shallow craters consistent with deformation by viscous relaxation of water ice under conditions of elevated heat flow. Here we use ...high‐resolution topography to measure the relaxation fraction of craters on Enceladus far from the active South Pole. We find that many craters are shallower than expected, with craters as small as 2 km in diameter having relaxation fractions in excess of 90%. These measurements are compared with numerical simulations of crater relaxation to constrain the minimum heat flux required to reproduce these observations. We find that Enceladus' nominal cold surface temperature (70 K) and low surface gravity strongly inhibit viscous relaxation. Under such conditions less than 3% relaxation occurs over 2 Ga even for relatively large craters (diameter 24 km) and high, constant heat fluxes (150 mW m−2). Greater viscous relaxation occurs if the effective temperature at the top of the lithosphere is greater than the surface temperature due to insulating regolith and/or plume material. Even for an effective temperature of 120 K, however, heat fluxes in excess of 150 mW m−2are required to produce the degree of relaxation observed. Simulations of viscous relaxation of Enceladus' largest craters suggest that relaxation is best explained by a relatively short‐lived period of intense heating that decayed quickly. We show that infilling of craters by plume material cannot explain the extremely shallow craters at equatorial and higher northern latitudes. Thus, like Enceladus' tectonic terrains, the cratered regions of Enceladus have experienced periods of extreme heat flux.
Key Points
Many craters (some ~2‐km in diameter) on Enceladus are highly viscously relaxed
The degree of relaxation observed requires extremely high heat fluxes
Infilling by plume material cannot account for Enceladus' shallow craters
Highlights • Motor unit potential (MUP) shape variability was quantified across consecutive motor unit (MU) discharges in healthy older men compared to young control subjects. • Near fiber (NF) ...jiggle was significantly higher in the older age group, and was significantly correlated with multiple MUP parameters indicative of MU loss. • NF jiggle may be a valuable quantitative measure used in conjunction with other MUP parameters indicative of MU remodeling and the stability of neuromuscular transmission.
•Ceres is 2.5 km more oblate than its rotational geoid.•Faster paleospin can reconcile Ceres’ present-day (i.e., fossil) shape and J2 gravity.•Ceres could have been modestly despun by impacts or by ...satellite tidal evolution and loss.•Deep-seated global mass anomalies can also explain Ceres’ nonhydrostatic degree-2 shape and gravity.•Ceres’ mean moment-of-inertia lies between 0.345 and 0.375 depending on paleospin.
We show that Ceres’ measured degree-2 zonal gravity, J2, is smaller by about 10% than that derived assuming Ceres’ rotational flattening, as measured by Dawn, is hydrostatic. Irrespective of Ceres’ radial density variation, as long as its internal structure is hydrostatic the J2 predicted from the shape model is consistently larger than measured. As an explanation, we suggest that Ceres’ current shape may be a fossil remnant of faster rotation in the geologic past. We propose that up to ∼7% of Ceres’ previous spin angular momentum has been removed by dynamic perturbations such as a random walk due to impacts or a loss of satellite that slowed Ceres spin as it tidally evolved outward. As an alternative, we also consider a formal degree-2 admittance solution, from which we infer a range of possible non-hydrostatic contributions to J2 from uncompensated, deep-seated density anomalies. We show that such density anomalies could be due to low order convection or upwelling. The normalized moments-of-inertia derived for the two explanations – faster paleospin and deep-seated density anomalies – range between 0.353 ± 0.009 and 0.375 ± 0.001 for a spherically equivalent Ceres, which can be used as constraints on more complex Ceres interior models.
Comprehensive pyrolysis models that are integral to computational fire codes have improved significantly over the past decade as the demand for improved predictive capabilities has increased. High ...fidelity pyrolysis models may improve the design of engineered materials for better fire response, the design of the built environment, and may be used in forensic investigations of fire events. A major limitation to widespread use of comprehensive pyrolysis models is the large number of parameters required to fully define a material and the lack of effective methodologies for measurement of these parameters, especially for complex materials. The work presented here details a methodology used to characterize the pyrolysis of a low-pile carpet tile, an engineered composite material that is common in commercial and institutional occupancies. The studied material includes three distinct layers of varying composition and physical structure. The methodology utilized a comprehensive pyrolysis model (ThermaKin) to conduct inverse analyses on data collected through several experimental techniques. Each layer of the composite was individually parameterized to identify its contribution to the overall response of the composite. The set of properties measured to define the carpet composite were validated against mass loss rate curves collected at conditions outside the range of calibration conditions to demonstrate the predictive capabilities of the model. The mean error between the predicted curve and the mean experimental mass loss rate curve was calculated as approximately 20% on average for heat fluxes ranging from 30 to 70 kW·m
, which is within the mean experimental uncertainty.
We report the detection of ammonia (NH
) on Pluto's surface in spectral images obtained with the New Horizons spacecraft that show absorption bands at 1.65 and 2.2 μm. The ammonia signature is ...spatially coincident with a region of past extensional tectonic activity (Virgil Fossae) where the presence of H
O ice is prominent. Ammonia in liquid water profoundly depresses the freezing point of the mixture. Ammoniated ices are believed to be geologically short lived when irradiated with ultraviolet photons or charged particles. Thus, the presence of NH
on a planetary surface is indicative of a relatively recent deposition or possibly through exposure by some geological process. In the present case, the areal distribution is more suggestive of cryovolcanic emplacement, however, adding to the evidence for ongoing geological activity on Pluto and the possible presence of liquid water at depth today.
PDF
