Variations in Earth's 1D viscosity structure in different tectonic regimes

Earth’s long-wavelength geoid provides insights into the thermal, structural, and compositional evolution of the mantle. Historically, most estimates of mantle viscosity using the long-wavelength geoid have considered radial variations with depth in a symmetric Earth. Global estimates of this kind suggest an increase in viscosity from the upper mantle to lower mantle of roughly 2 -- 3 orders of magnitude. Using a spatio-spectral localization technique with the geoid, here we estimate a series of locally constrained viscosity-depth profiles covering two unique regions, the Pacific and Atlantic hemispheres, which show distinct rheological properties. The Pacific region exhibits the conventional Earth's 1D rheology with a factor of roughly 80-100 increase in viscosity occurring at transition zone depths (400 - 800 km). The Atlantic region in contrast does not show significant viscosity jumps with depth, and instead has a near uniform viscosity in the top 1000~km. The inferred viscosity variations between our two regions could be due to the prevalence of present-day subduction in the Pacific and the infrequence of slabs in the Atlantic, combined with a possible hydrated transition zone and mid-mantle of the Atlantic region by ancient subduction during recent supercontinent cycles. Rigid slab material within the top 800 km, with about 90\% Majoritic garnet in the form of subducted oceanic crust, coupled with unique regional mantle structures, may be generating a strong transition zone viscosity interface for the Pacific region. These effective lateral variations in mantle viscosity could play a role in the observed deformation differences between the Pacific and Atlantic hemispheres.

The Abundances of Zinc and Copper in the Atmospheres of 78 Vir (A2p SrCrEu) and θ Leo (A2 IV)

Zinc and Copper abundances are rarely available for normal and Chemically Peculiar A stars because the strongest transitions of Zn ii and Cu ii fall in the mid-UV. Estimates of the abundances of zinc and copper are derived for 78 Vir (A2p SrCrEu) and θ Leo (A2 IV) using mean mid-ultraviolet spectra constructed by coadding individual spectra collected with the Long Wave Prime and Long wavelength Redundant cameras over the 18 yr of the IUE mission. The strong transition of Cu ii at 2135.98 Å is present in 78 Vir and θ Leo but definitely stronger in 78 Vir, whereas all Zn ii lines are blended. Spectral synthesis of the least blended lines yields estimates of the abundances of zinc and copper of about 4.92 and 4.95 respectively in θ Leo and 5.82 and 5.19 in 78 Vir (on a scale where log(H) = 12). There is no convincing evidence that these lines varied in the spectra analyzed for both stars.

Making An Observation Without Making An Observation: “Seeing” Knowledge Embedded In The Physical World With One’s Own Eyes

In electron shelving, one kind of quantum jump in which an electron in an atom moves in a discontinuous fashion to another energy level, a person can "see" knowledge that a weak transition (which is very slow) has occurred since it is proven that a strong transition (which is very fast) has not occurred in the time in which the strong transition can occur. This is a null measurement. There is no physical detection of the photon in the weak transition. There is only the logical deduction that the weak transition occurred since one cannot detect the photon in the strong transition, the only other possible transition. Looking at the interruption in the fluorescence that occurs in the strong transition, a “dark” interval, is "seeing" the knowledge resulting from the logical deduction. The significance of a null measurement in leading to the conclusion that one can "see" the knowledge resulting from a logical deduction is discussed. The null measurement in the weak transition is one form of Einstein’s spooky action where something not physical occurring in the possible strong transition leads to the occurrence of the weak transition. (2021 April Meeting of the American Physical Society, https://meetings.aps.org/Meeting/APR21/Session/KP01.48)

Towards Automatic Recognition of Wakes Generated by Dark Vessels in Sentinel-1 Images

The recognition of wakes generated by dark vessels is a tremendous and interesting challenge in the field of maritime surveillance by Synthetic Aperture Radar (SAR) images. The paper aims at assessing the detection performance in different scenarios by processing Sentinel-1 SAR images along with ground truth data. Results confirm that the Radon-based approach is an effective technique for wake-based detection of dark vessels, and they lead to a deeper understanding of the effects of different sea and wind conditions. In general, the best applicative scenario is a marine image characterized by homogeneous sea clutter; the presence of natural surface film or strong transition from low wind speed areas to more windy zones worsen the detection performance. Nonetheless, the proposed approach features dark vessel detection capabilities by identifying their wakes, without any a priori knowledge of their positions.

Model Description of the Unusual Temperature Dependence of the Viscosity of Metallic Glass-Forming Liquids

The temperature dependence of the viscosity of some metallic glass forming liquids (MGFLs) exhibits an unusual behavior. At high temperature, the temperature dependence is quite weak, whereas at low temperature, the viscosity varies exponentially. Recently, this type of behavior are attracting much attention, because it can be considered as a manifestation of the fragile-to-strong transition. Well known classic viscosity models do not describe such kind of behavior over a wide temperature range. In the present report, it is shown that a modified version of the Bond Strength-Coordination Number Fluctuation (BSCNF) model describes the behavior observed in MGFLs. For the convenience of the readers, a brief review of the BSCNF model is also given.

Unraveling a Force-Generating Allosteric Pathway of Actomyosin Communication Associated with ADP and Pi Release

The actomyosin system generates mechanical work with the execution of the power stroke, an ATP-driven, two-step rotational swing of the myosin-neck that occurs post ATP hydrolysis during the transition from weakly to strongly actin-bound myosin states concomitant with Pi release and prior to ADP dissociation. The activating role of actin on product release and force generation is well documented; however, the communication paths associated with weak-to-strong transitions are poorly characterized. With the aid of mutant analyses based on kinetic investigations and simulations, we identified the W-helix as an important hub coupling the structural changes of switch elements during ATP hydrolysis to temporally controlled interactions with actin that are passed to the central transducer and converter. Disturbing the W-helix/transducer pathway increased actin-activated ATP turnover and reduced motor performance as a consequence of prolonged duration of the strongly actin-attached states. Actin-triggered Pi release was accelerated, while ADP release considerably decelerated, both limiting maximum ATPase, thus transforming myosin-2 into a high-duty-ratio motor. This kinetic signature of the mutant allowed us to define the fractional occupancies of intermediate states during the ATPase cycle providing evidence that myosin populates a cleft-closure state of strong actin interaction during the weak-to-strong transition with bound hydrolysis products before accomplishing the power stroke.