ABSTRACT
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.
ABSTRACT
We explored the illumination exposure of middle-aged adults to determine normal values and to explore several correlates of daily light exposures. Subjects aged 40-64 years in San Diego, CA were recruited by random telephone dialing. Subjects completed a demographic interview and the Center for Epidemiologic Studies Depression self-rating scale (the CES-D) supplemented with eight questions related to seasonal affective disorders (SAD). Data were analyzed for 106 volunteers who wore a device that monitors illumination exposures and activity. The median subject was exposed to illumination > or = 1000 lux for only 4% of the time observed, that is, only about 58 min per day were spent in daylight. Subjects scoring higher on the atypical SAD mood symptoms spent less time in bright illumination rs = -0.266, p = 0.003. The CES-D depression score was similarly correlated with illumination but of borderline significance (rs = -0.150, p = 0.063). These results suggest the hypothesis that many Americans may be receiving insufficient light exposure to maintain optimal mood.
