Non-monotonicity of the frictional bimaterial effect

Sliding along frictional interfaces separating dissimilar elastic materials is qualitatively different from sliding along interfaces separating identical materials due to the existence of an elastodynamic coupling between interfacial slip and normal stress perturbations in the former case. This bimaterial coupling has important implications for the dynamics of frictional interfaces, including their stability and rupture propagation along them. We show that while this bimaterial coupling is a monotonically increasing function of the bimaterial contrast, when it is coupled to interfacial shear stress perturbations through a friction law, various physical quantities exhibit a non-monotonic dependence on the bimaterial contrast. In particular, we show that for a regularized Coulomb friction, the maximal growth rate of unstable interfacial perturbations of homogeneous sliding is a non-monotonic function of the bimaterial contrast, and provide analytic insight into the origin of this non-monotonicity. We further show that for velocity-strengthening rate-and-state friction, the maximal growth rate of unstable interfacial perturbations of homogeneous sliding is also a non-monotonic function of the bimaterial contrast. Results from simulations of dynamic rupture along a bimaterial interface with slip-weakening friction provide evidence that the theoretically predicted non-monotonicity persists in non-steady, transient frictional dynamics.Comment: 14 pages, 5 figure

Olivine friction at the base of oceanic seismogenic zones

Author Posting. © American Geophysical Union, 2007. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Journal of Geophysical Research 112 (2007): B01205, doi:10.1029/2006JB004301.We investigate the strength and frictional behavior of olivine aggregates at temperatures and effective confining pressures similar to those at the base of the seismogenic zone on a typical ridge transform fault. Triaxial compression tests were conducted on dry olivine powder (grain size ≤ 60 μm) at effective confining pressures between 50 and 300 MPa (using Argon as a pore fluid), temperatures between 600°C and 1000°C, and axial displacement rates from 0.06 to 60 μm/s (axial strain rates from 3 × 10−6 to 3 × 10−3 s−1). Yielding shows a negative pressure dependence, consistent with predictions for shear enhanced compaction and with the observation that samples exhibit compaction during the initial stages of the experiments. A combination of mechanical data and microstructural observations demonstrate that deformation was accommodated by frictional processes. Sample strengths were pressure-dependent and nearly independent of temperature. Localized shear zones formed in initially homogeneous aggregates early in the experiments. The frictional response to changes in loading rate is well described by rate and state constitutive laws, with a transition from velocity-weakening to velocitystrengthening at 1000°C. Microstructural observations and physical models indicate that plastic yielding of asperities at high temperatures and low axial strain rates stabilizes frictional sliding. Extrapolation of our experimental data to geologic strain rates indicates that a transition from velocity weakening to velocity strengthening occurs at approximately 600°C, consistent with the focal depths of earthquakes in the oceanic lithosphere.This research was supported by the Deep Ocean Exploration Institute at WHOI and NSF grants to Greg Hirth and Brian Evans

The Lunar Polar Hydrogen Mapper (LunaH-Map) Mission

The Lunar Polar Hydrogen Mapper (LunaH-Map) mission will map hydrogen enrichments within permanently shadowed regions at the lunar south pole using a miniature neutron spectrometer. While hydrogen enrichments have been identified regionally from previous orbital missions, the spatial extent of these regions are often below the resolution of the neutron instruments that have flown on lunar missions. LunaH-Map will enter into an elliptical, low altitude perseline orbit which will enable the mission to spatially isolate and constrain the hydrogen enrichments within permanently shadowed regions. LunaH-Map will use a solid iodine ion propulsion system, X-Band radio communications through the NASA Deep Space Network, star tracker, C&DH and EPS systems from Blue Canyon Technologies, solar arrays from MMA Designs, LLC, mission design and navigation by KinetX. Spacecraft systems design, integration, qualification, test and mission operations are performed by Arizona State University

LunaH-Map: Revealing Lunar Water with a New Radiation Sensor Array

A new type of neutron and gamma-ray spectrometer called the Miniature Neutron Spectrometer (Mini-NS) has been developed, assembled, qualified and delivered as part of the Lunar Polar Hydrogen Mapper (LunaH-Map) cubesat mission. The LunaH-Map spacecraft is currently manifested as a secondary payload on the Space Launch System (SLS) Artemis-1 rocket. LunaH-Map will deploy from Artemis-1 and enter a low altitude perilune elliptical orbit around the Moon. The Mini-NS will measure the lunar epithermal neutron albedo, and measurements around perilune will be used to produce maps of hydrogen enrichments and depletions across the lunar South Pole region including both within and outside of permanently shadowed regions (PSRs). The Min-NS was designed to achieve twice the epithermal neutron count rate of the Lunar Prospector Neutron Spectrometer (LP-NS). The instrument response was characterized through the collection of pre-flight neutron counting data with a Cf-252 neutron source at Arizona State University across hundreds of power cycles, as well as across the expected temperature range. The instrument spatial response was characterized at the Los Alamos National Laboratories (LANL) Neutron Free In-Air Facility. The LunaH-Map Mini-NS was designed to fit within the cubesat form-factor and uses two detectors with eight sensor heads that can be operated independently. For future missions with different science goals that can be achieved with epithermal neutron detection, the number of Mini-NS sensor heads can easily be modified without requiring a complete re-design and re-qualification

Targeted Amplicon Sequencing (TAS): A Scalable Next-Gen Approach to Multilocus, Multitaxa Phylogenetics

Next-gen sequencing technologies have revolutionized data collection in genetic studies and advanced genome biology to novel frontiers. However, to date, next-gen technologies have been used principally for whole genome sequencing and transcriptome sequencing. Yet many questions in population genetics and systematics rely on sequencing specific genes of known function or diversity levels. Here, we describe a targeted amplicon sequencing (TAS) approach capitalizing on next-gen capacity to sequence large numbers of targeted gene regions from a large number of samples. Our TAS approach is easily scalable, simple in execution, neither time-nor labor-intensive, relatively inexpensive, and can be applied to a broad diversity of organisms and/or genes. Our TAS approach includes a bioinformatic application, BarcodeCrucher, to take raw next-gen sequence reads and perform quality control checks and convert the data into FASTA format organized by gene and sample, ready for phylogenetic analyses. We demonstrate our approach by sequencing targeted genes of known phylogenetic utility to estimate a phylogeny for the Pancrustacea. We generated data from 44 taxa using 68 different 10-bp multiplexing identifiers. The overall quality of data produced was robust and was informative for phylogeny estimation. The potential for this method to produce copious amounts of data from a single 454 plate (e.g., 325 taxa for 24 loci) significantly reduces sequencing expenses incurred from traditional Sanger sequencing. We further discuss the advantages and disadvantages of this method, while offering suggestions to enhance the approach

Broadly sampled multigene analyses yield a well-resolved eukaryotic tree of life

Author Posting. © The Authors, 2010. This is the author's version of the work. It is posted here by permission of Oxford University Press for personal use, not for redistribution. The definitive version was published in Systematic Biology 59 (2010): 518-533, doi:10.1093/sysbio/syq037.An accurate reconstruction of the eukaryotic tree of life is essential to identify the innovations underlying the diversity of microbial and macroscopic (e.g. plants and animals) eukaryotes. Previous work has divided eukaryotic diversity into a small number of high-level ‘supergroups’, many of which receive strong support in phylogenomic analyses. However, the abundance of data in phylogenomic analyses can lead to highly supported but incorrect relationships due to systematic phylogenetic error. Further, the paucity of major eukaryotic lineages (19 or fewer) included in these genomic studies may exaggerate systematic error and reduces power to evaluate hypotheses. Here, we use a taxon-rich strategy to assess eukaryotic relationships. We show that analyses emphasizing broad taxonomic sampling (up to 451 taxa representing 72 major lineages) combined with a moderate number of genes yield a well-resolved eukaryotic tree of life. The consistency across analyses with varying numbers of taxa (88-451) and levels of missing data (17-69%) supports the accuracy of the resulting topologies. The resulting stable topology emerges without the removal of rapidly evolving genes or taxa, a practice common to phylogenomic analyses. Several major groups are stable and strongly supported in these analyses (e.g. SAR, Rhizaria, Excavata), while the proposed supergroup ‘Chromalveolata’ is rejected. Further, extensive instability among photosynthetic lineages suggests the presence of systematic biases including endosymbiotic gene transfer from symbiont (nucleus or plastid) to host. Our analyses demonstrate that stable topologies of ancient evolutionary relationships can be achieved with broad taxonomic sampling and a moderate number of genes. Finally, taxonrich analyses such as presented here provide a method for testing the accuracy of relationships that receive high bootstrap support in phylogenomic analyses and enable placement of the multitude of lineages that lack genome scale data

Limitations and pitfalls of using family letters to communicate genetic risk: a qualitative study with patients and healthcare professionals

European genetic testing guidelines recommend that healthcare professionals (HCPs) discuss the familial implications of any test with a patient and offer written material to help them share the information with family members. Giving patients these “family letters” to alert any relatives of their risk has become part of standard practice and has gone relatively unquestioned over the years. Communication with at-risk relatives will become an increasingly pressing issue as mainstream and routine practice incorporates broad genome tests and as the number of findings potentially relevant to relatives increases. This study therefore explores problems around the use of family letters to communicate about genetic risk. We conducted 16 focus groups with 80 HCPs, and 35 interviews with patients, recruited from across the UK. Data were analyzed thematically and we constructed four themes: 1) HCPs writing family letters: how to write them and why?, 2) Patients’ issues with handing out family letters, 3) Dissemination becomes an uncontrolled form of communication, and 4) When the relative has the letter, is the patient’s and HCP’s duty discharged? We conclude by suggesting alternative and supplementary methods of communication, for example through digital tools, and propose that in comparison to communication by family letter, direct contact by HCPs might be a more appropriate and successful option

Reasonable expectations of privacy in non-disclosure of familial genetic risk: What is it reasonable to expect?

Where there is conflict between a patient's interests in non-disclosure of their genetic information to relatives and the relative's interest in knowing the information because it indicates their genetic risk, clinicians have customarily been able to protect themselves against legal action by maintaining confidence even if, professionally, they did not consider this to be the right thing to do. In ABC v St Georges Healthcare NHS Trust ([2017] EWCA Civ 336) the healthcare team recorded their concern about the wisdom of the patient's decision to withhold genetic risk information from his relative, but chose to respect what they considered to be an unwise choice. Even though professional guidance considers that clinicians have the discretion to breach confidence where they believe this to be justified, (Royal College of Physicians, Royal College of Pathologists and the British Society of Human Genetics, 2006; GMC, 2017) clinicians find it difficult to exercise this discretion in line with their convictions against the backdrop of the legal prioritisation of the duty to maintain confidence. Thus, the professional discretion is not being freely exercised because of doubts about the legal protection available in the event of disclosure. The reliance on consent as the legal basis for setting aside the duty of confidence often vetoes sharing information with relatives. This paper argues that an objective approach based on privacy, rather than a subjective consent-based approach, would give greater freedom to clinicians to exercise the discretion which their professional guidance affords