Meteoroid Impacts as a Source of Bennu's Particle Ejection Events

Asteroid (101955) Bennu, a near-Earth object with a primitive carbonaceous chondrite-like composition, was observed by the Origins, Spectral Interpretation, Resource Identification, and Security-Regolith Explorer (OSIRIS-REx) spacecraft to undergo multiple particle ejection events near perihelion between December 2018 and February 2019. The three largest events observed during this period, which all occurred 3.5 to 6 hr after local noon, placed numerous particles <10 cm on temporary orbits around Bennu. Here we examine whether these events could have been produced by sporadic meteoroid impacts using the National Aeronautics and Space Administration's (NASA) Meteoroid Engineering Model 3.0. Most projectiles that impact Bennu come from nearly isotropic or Jupiter-family comets and have evolved toward the Sun by Poynting-Robertson drag. We find that 7,000-J impacts on Bennu occur with a biweekly cadence near perihelion, with a preference to strike in the late afternoon (~6 pm local time). This timing matches observations. Crater scaling laws also indicate that these impact energies can reproduce the sizes and masses of the largest observed particles, provided the surface has the cohesive properties of weak, porous materials. Bennu's ejection events could be caused by the same kinds of meteoroid impacts that created the Moon's asymmetric debris cloud observed by the Lunar Atmosphere and Dust Environment Explorer (LADEE). Our findings also suggest that fewer ejection events should take place as Bennu moves further away from the Sun, a result that can be tested with future observations.

Thermal Fatigue as a Driving Mechanism for Activity on Asteroid Bennu

Many boulders on (101955) Bennu, a near-Earth rubble pile asteroid, show signs of in situ disaggregation and exfoliation, indicating that thermal fatigue plays an important role in its landscape evolution. Observations of particle ejections from its surface also show it to be an active asteroid, though the driving mechanism of these events is yet to be determined. Exfoliation has been shown to mobilize disaggregated particles in terrestrial environments, suggesting that it may be capable of ejecting material from Bennu's surface. We investigate the nature of thermal fatigue on the asteroid, and the efficacy of fatigue-driven exfoliation as a mechanism for generating asteroid activity, by performing finite element modeling of stress fields induced in boulders from diurnal cycling. We develop a model to predict the spacing of exfoliation fractures and the number and speed of particles that may be ejected during exfoliation events. We find that crack spacing ranges from ~1 mm to 10 cm and disaggregated particles have ejection speeds up to ~2 m/s. Exfoliation events are most likely to occur in the late afternoon. These predictions are consistent with observed ejection events at Bennu and indicate that thermal fatigue is a viable mechanism for driving asteroid activity. Crack propagation rates and ejection speeds are greatest at perihelion when the diurnal temperature variation is largest, suggesting that events should be more energetic and more frequent when closer to the Sun. Annual thermal stresses that arise in large boulders may influence the spacing of exfoliation cracks or frequency of ejection events.

In situ evidence of thermally induced rock breakdown widespread on Bennu's surface

Rock breakdown due to diurnal thermal cycling has been hypothesized to drive boulder degradation and regolith production on airless bodies. Numerous studies have invoked its importance in driving landscape evolution, yet morphological features produced by thermal fracture processes have never been definitively observed on an airless body, or any surface where other weathering mechanisms may be ruled out. The Origins, Spectral Interpretation, Resource Identification, and Security–Regolith Explorer (OSIRIS-REx) mission provides an opportunity to search for evidence of thermal breakdown and assess its significance on asteroid surfaces. Here we show boulder morphologies observed on Bennu that are consistent with terrestrial observations and models of fatigue-driven exfoliation and demonstrate how crack propagation via thermal stress can lead to their development. The rate and expression of this process will vary with asteroid composition and location, influencing how different bodies evolve and their apparent relative surface ages from space weathering and cratering records.

In their study, the authors discuss the potential of thermal weathering on airless bodies. As a case study, they use boulder and fracture morphologies on asteroid Bennu.

Collisional formation of top-shaped asteroids and implications for the origins of Ryugu and Bennu

Asteroid shapes and hydration levels can serve as tracers of their history and origin. For instance, the asteroids (162173) Ryugu and (101955) Bennu have an oblate spheroidal shape with a pronounced equator, but contain different surface hydration levels. Here we show, through numerical simulations of large asteroid disruptions, that oblate spheroids, some of which have a pronounced equator defining a spinning top shape, can form directly through gravitational reaccumulation. We further show that rubble piles formed in a single disruption can have similar porosities but variable degrees of hydration. The direct formation of top shapes from single disruption alone can explain the relatively old crater-retention ages of the equatorial features of Ryugu and Bennu. Two separate parent-body disruptions are not necessarily required to explain their different hydration levels.

Correction to: Leveraging Geospatial Approaches to Characterize the HIV Prevention and Treatment Needs of Out-of-School Adolescent Girls and Young Women in Ethiopia

The article "Leveraging Geospatial Approaches to Characterize the HIV Prevention and Treatment Needs of Out-of-School Adolescent Girls and Young Women in Ethiopia" written by Y. Wang, C. A. Comins, A. Mulu, S. A. Abebe, K. Belete, T. T. Balcha, S. Baral, S. R. Schwartz, was originally published electronically on the publisher's internet portal (currently SpringerLink) on 27th May 2019 without open access. With the author(s)' decision to opt for Open Choice the copyright of the article changed on 19th September 2019 to © The Author(s) 2019 and the article is forthwith distributed under the terms of the Creative Commons Attribution 4.0 International License (http://creativecommons.org/licenses/by/4.0/), which permits use, duplication, adaptation, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license and indicate if changes were made.

Shape of (101955) Bennu indicative of a rubble pile with internal stiffness

The shapes of asteroids reflect interplay between their interior properties and the processes responsible for their formation and evolution as they journey through the Solar System. Prior to the OSIRIS-REx (Origins, Spectral Interpretation, Resource Identification, and Security-Regolith Explorer) mission, Earth-based radar imaging gave an overview of (101955) Bennu's shape. Here, we construct a high-resolution shape model from OSIRIS-REx images. We find that Bennu's top-like shape, considerable macroporosity, and prominent surface boulders suggest that it is a rubble pile. High-standing, north-south ridges that extend from pole to pole, many long grooves, and surface mass wasting indicate some low levels of internal friction and/or cohesion. Our shape model indicates that, similar to other top-shaped asteroids, Bennu formed by reaccumulation and underwent past periods of fast spin leading to its current shape. Today, Bennu might follow a different evolutionary pathway, with interior stiffness permitting surface cracking and mass wasting.

Human papillomavirus infection and survival in oral squamous cell cancer: a population-based study

OBJECTIVE

To determine whether human papillomavirus (HPV) type 16 affects survival in oral squamous cell carcinoma.

STUDY DESIGN

Two hundred fifty-four patients diagnosed with primary oral cancer were studied for survival in relation to tumor HPV type 16 status. Kaplan-Meier analysis and Cox proportional hazard models were used to assess survival and estimate hazard ratios adjusted for potential confounders.

RESULTS

HPV type 16 DNA was detected in 15.1% of tumors. HPV 16 positive patients had significantly reduced all-cause mortality (hazard ratio [HR] estimates = 0.34, 95% CI = 0.14, 0.83) and disease-specific mortality (HR = 0.17, 95% CI = 0.04, 0.76) compared with HPV 16 negative patients after adjustment for age, stage, treatment, smoking, alcohol, education, and comorbid disease.

CONCLUSIONS

The presence of HPV type 16 DNA is independently associated with a favorable prognosis in patients with oral squamous cell carcinoma.

CLINICAL SIGNIFICANCE

Although HPV genotyping is currently not widely available, it may provide important prognostic information.

Choroidal nonperfusion in giant cell arteritis

A 68-year-old man had visual loss secondary to isolated choroidal nonperfusion as a clinical manifestation of giant cell arteritis. Ophthalmoscopy disclosed scattered yellow-white lesions at the level of the retinal pigment epithelium in the posterior pole of the right eye. Intravenous fluorescein angiography demonstrated marked delay in choroidal filling of the macula in the right eye. There was no ophthalmoscopic or angiographic evidence of anterior ischemic optic neuropathy or central retinal artery occlusion. After approximately 72 hours of intravenous corticosteroid therapy, the patient's visual acuity improved and repeat intravenous fluorescein angiography showed normal choroidal circulation. Isolated choroidal ischemia is a potential cause of reversible visual loss in patients with giant cell arteritis.

Assessment of epithelial thickness by ultrasonic imaging

Moisturizing creams and lotions are an important aspect of preventing pressure ulcers. This study reports the increased thickening of the epithelium on 11 subjects who used a moisturizing cream (treatment) for 20 days. The thickening of the epithelium was measured before use of the moisturizing cream, on day 1, day 8, and day 20 of the treatment, and 12 days after the treatment. The ultrasound measurements were similar before the treatment and 12 days post treatment; the measurements increased for the days of treatment. Current research would lead us to believe that short-term increase in epithelial thickness as produced by a moisturizer is a result of increased water content. Ultrasound imaging may be the new standard for comparing the effects of moisturing products.

A basic residency curriculum concerning the chronically mentally ill

In this paper a group of knowledgeable individuals with expertise in psychiatric education present their recommendations for a basic psychiatric residency curriculum concerning the chronically mentally ill. The proposed curriculum consists of knowledge, skill, and attitude educational objectives, as well as clinical experiences, faculty supervision, didactics and seminars, and evaluation mechanisms. Recommendations are also made concerning changes in the Accreditation Council for Graduate Medical Education's Special Requirements for Residency Training in Psychiatry, which would require residency programs to place more emphasis on training to meet the needs of the chronically mentally ill. Obstacles to the implementation of the proposed recommendations are presented and possible solutions are discussed.

Human safety study of body lotion containing Kathon CG

The safety of Kathon CG biocide as a preservative in leave-on body lotions was assessed by 2 double-blind studies, using similar protocols. A total of 209 healthy male and female subjects aged 18 to 65 years, 100 in California (72 test subjects, 28 controls) and 109 in Florida (88 test subjects, 21 controls) completed the studies which included pre- and post-use phase diagnostic patch testing with Kathon CG 100 ppm active ingredient, and 13 weeks daily applications of either a test lotion containing Kathon CG 15 ppm active ingredient or a control lotion without Kathon CG. No evidence of irritation or sensitization attributable to use of the biocide was found during regular dermatological examinations during the use phase. Post-use phase patch testing produced negative results in all subjects with the exception of 1 control subject in Florida who had positive readings at the 2- and 4-week post-use phase patch testing. Overall, these studies show there is minimal, if any, risk of adverse effects associated with the use of Kathon CG 15 ppm active ingredient in a leave-on application.

Case management for the chronic mentally ill: models and dimensions

Community-based care of the chronic mentally ill requires the integrative efforts that generally fall under the rubric of case management. The authors describe models of case management according to three dimensions: the manager's degree of involvement in direct service, the type of caseload, and the source and extent of the manager's control over services and resources. The last dimension can be affected by such factors as contracts with private service providers and the case manager's rapport with clinical service providers. Various systemic problems, such as insufficient funds and duplication of services, may impede case management. Effective case management depends on adequate resources and clear communication among the components of the system.

The new chronic patient: clinical characteristics of an emerging subgroup

A subgroup of chronic mentally ill persons who have had little or no state hospitalization and who are difficult to engage in existing systems of community care is emerging in major urban areas. Observations made at a large municipal general hospital indicate the patients are typically young, more likely to be male, and highly transient. They have frequent interactions with emergency psychiatric and crisis units, coupled with intermittent involuntary short-term stays in local inpatient units. They have few skills and virtually no natural support systems. Under stress they show disorders in reality testing, exhibit anger and depression, and are prone to impulsive aggressive and self-destructive behaviors. They are typically unwilling to voluntarily accept continuing care. There is a lack of fit between this group's characteristic style of interaction and existing community-based programs. The costs of this lack of fit, both in staff morale and inappropriate use of expensive services, make the exploration of approaches to these "new chronic patients" a matter of immediate importance.