Catastrophic Disruption of Hollow Ice Spheres

Catastrophic disruption is a possible outcome of high-speed collisions in the solar system. The critical energy density Q* (impact energy/mass of the target), which is taken to mark the onset of catastrophic disruption, occurs when the largest intact fragment post-impact is 50% of the original target mass. Studies of Q* usually suppose the target body is a solid, rigid object. However, what if the body has a rigid shell and a hollow interior? Here, hollow ice spheres (a diameter of 19–20 cm with an ice thickness of 2.5–3.6 cm) were impacted at speeds up to ∼5 km/s. Catastrophic disruption occurred at Q* ∼ 25.5 ± 0.5 J kg−1, greater than that for similar size solid, or water-filled ice spheres (16–18 J kg−1). However, while the Q* value has increased, the actual impact energy associated with the new value of Q* has not, and the change in Q* arises due to the lower mass of the hollow target bodie

Hypervelocity impacts into ice?topped layered targets: Investigating the effects of ice crust thickness and subsurface density on crater morphology

Many bodies in the outer solar system are theorized to have an ice shell with a different subsurface material below, be it chondritic, regolith, or a subsurface ocean. This layering can have a significant influence on the morphology of impact craters. Accordingly, we have undertaken laboratory hypervelocity impact experiments on a range of multilayered targets, with interiors of water, sand, and basalt. Impact experiments were undertaken using impact speeds in the range of 0.8–5.3 km s?1, a 1.5 mm Al ball bearing projectile, and an impact incidence of 45°. The surface ice crust had a thickness between 5 and 50 mm, i.e., some 3–30 times the projectile diameter. The thickness of the ice crust as well as the nature of the subsurface layer (liquid, well consolidated, etc.) have a marked effect on the morphology of the resulting impact crater, with thicker ice producing a larger crater diameter (at a given impact velocity), and the crater diameter scaling with impact speed to the power 0.72 for semi-infinite ice, but with 0.37 for thin ice. The density of the subsurface material changes the structure of the crater, with flat crater floors if there is a dense, well-consolidated subsurface layer (basalt) or steep, narrow craters if there is a less cohesive subsurface (sand). The associated faulting in the ice surface is also dependent on ice thickness and the substrate material. We find that the ice layer (in impacts at 5 km s?1) is effectively semi-infinite if its thickness is more than 15.5 times the projectile diameter. Below this, the crater diameter is reduced by 4% for each reduction in ice layer thickness equal to the impactor diameter. Crater depth is also affected. In the ice thickness region, 7–15.5 times the projectile diameter, the crater shape in the ice is modified even when the subsurface layer is not penetrated. For ice thicknesses, <7 times the projectile diameter, the ice layer is breached, but the nature of the resulting crater depends heavily on the subsurface material. If the subsurface is noncohesive (loose) material, a crater forms in it. If it is dense, well-consolidated basalt, no crater forms in the exposed subsurface layer

Survival of Fossilised Diatoms and Forams in Hypervelocity Impacts with Peak Shock Pressures in the 1 – 19 GPa Range

Previously it has been shown that diatom fossils embedded in ice could survive impacts at speeds of up to 5 km s-1 and peak shock pressures up to 12 GPa. Here we confirm these results using a different technique, with diatoms carried in liquid water suspensions at impact speeds of 2 to 6 km s-1. These correspond to peak shock pressures of 3.8 to 19.8 GPa. We also report on the results of similar experiments using forams, at impact speeds of 4.67 km s-1 (when carried in water) and 4.73 km s-1 (when carried in ice), corresponding to peak shock pressures of 11.6 and 13.1 GPa respectively. In all cases we again find survival of recognisable fragments, with mean fragment size of order 20 – 25 µm. We compare our results to the peak shock pressures that ejecta from giant impacts on the Earth would experience if it subsequently impacted the Moon. We find that 98% of impacts of terrestrial ejecta on the Moon would have experienced peak pressures less than 20 GPa if the ejecta were a soft rock (sandstone). This falls to 82% of meteorites if the ejecta were a hard rock (granite). This assumes impacts on a solid lunar surface. If we approximate the surface as a loose regolith, over 99% of the impacts involve peak shock pressures below 20 GPa. Either way, the results show that a significant fraction of terrestrial meteorites impacting the Moon will do so with peak shock pressures which in our experiments permit the survival of recognisable fossil fragments

Analysis of Impact Craters Using Optical Coherence Tomography

We present our work on the use of OCT to analyse impact samples

Hypervelocity impact fragmentation of basalt and shale projectiles

Results are presented for the fragmentation of projectiles in laboratory experiments. 1.5 mm cubes and spheres of basalt and shale were impacted onto water at normal incidence and speeds from 0.39 to 6.13 km s−1; corresponding to peak shock pressures 0.7–32 GPa. Projectile fragments were collected and measured (over 100,000 fragments in some impacts, at sizes down to 10 µm). Power laws were fitted to the cumulative fragment size distributions and the evolution of the exponent vs. impact speed and peak shock pressure found. The gradient of each of these power laws increased with increasing impact speed/peak shock pressure. The percentage of the projectiles recovered in the impacts was found and used to estimate projectile remnant survival in different solar system impact scenarios at the mean impact speed appropriate to that scenario. For Pluto, the Moon and in the asteroid belt approximately 55%, 40% and 15%, respectively, of an impactor could survive and be recovered at an impact site. Finally, the catastrophic disruption energy densities of basalt and shale were measured and found to be 24 × 104 J kg−1 and 9 × 104 J kg−1, respectively, a factor of ∼2.5 difference. These corresponded to peak shock pressures of 1 to 1.5 GPa (basalt), and 0.8 GPa (shale). This is for near normal-incidence impacts where tensile strength is dominant. For shallow angle impacts we suggest shear effects dominate, resulting in lower critical energy densities and peak shock pressures. We also determine a method to ascertain information about fragment sizes in solar system impact events using a known size of impactor. The results are used to predict projectile fragments sizes for the Veneneia and Rheasilvia crater forming impacts on Vesta, and similar impacts on Ceres

Suicide and attempted suicide in bipolar disorder: a systematic review of risk factors

Objective: To determine the main risk factors for suicide and nonfatal suicidal behavior in patients with bipolar disorder through a systematic review of the international literature. Data Sources: Studies were identified through electronic searches of MEDLINE (1966-December 2003), EMBASE (1980-December 2003), PsycINFO (1872-November 2003), and Biological Abstracts (1985-December 2003) using index and free-text search terms for bipolar disorder, bipolar depression, manic depression, mania, and affective disorders; combined with terms for self-harm, self-injury, suicide, attempted suicide, automutilation, self-mutilation, self-poisoning, and self-cutting; and combined with terms for risk, case control, cohort, comparative, longitudinal, and follow-up studies. No language restrictions were applied to the search. Study Selection: Included studies were cohort, case-control, and cross-sectional investigations of patients with bipolar disorder in which suicide (13 studies) or attempted suicide (23 studies) was reported as an outcome. The selected studies also used diagnostic tools including the DSM, International Classification of Diseases, and Research Diagnostic Criteria. Data Synthesis: Meta-analysis of factors reported in more than 1 study identified the main risk factors for suicide as a previous suicide attempt and hopelessness. The main risk factors for nonfatal suicidal behavior included family history of suicide, early onset of bipolar disorder, extent of depressive symptoms, increasing severity of affective episodes, the presence of mixed affective states, rapid cycling, comorbid Axis I disorders, and abuse of alcohol or drugs. Conclusions: Prevention of suicidal behavior in patients with bipolar disorder should include attention to these risk factors in assessment and treatment, including when deciding whether to initiate treatment aimed specifically at reducing suicide risk

A new compact, self-compressing, vertical one and two-stage gas gun at the University of Kent

A new gas gun configuration has been developed at the Centre for Astrophysics and Planetary Science, University of Kent, to produce vertical impacts at up to 2 km s−1. The vertical arrangement allows impact into non-cohesive target materials such as sand (representing loose regolith on the surface of solar system bodies) and water. Three key constraints had to be met: (1) the vertical gun had to fit in a laboratory with a ceiling of 3.5 m height, (2) the gun had to operate without a chemical propellant (i.e. gunpowder) and (3) it had to fire into the same target chamber as the existing horizontal Kent gun. In addition, the use of standard pipeline elements was adopted. The result was a self-compressing right angle shaped gun design, with single and two-stage variants for low and high speeds respectively. The gun assembly and its operation are described, together with ancillary components. The self-compressing first stage of the gun (using pressure generated by boiling liquid nitrogen) is horizontal leading into a vertical second stage. The gun has performed as expected, with shots in the speed range 0.3 to 2 km s−1. Results from the first study using the gun are presented for impacts of 1 mm diameter stainless steel spheres into sand over the full speed range, and are found to be compatible with previous work at low and high speed