Michael K. Barker

Bio: Michael K. Barker is an academic researcher from Goddard Space Flight Center. The author has contributed to research in topics: Orbiter & Altimeter. The author has an hindex of 11, co-authored 30 publications receiving 660 citations.

A new lunar digital elevation model from the Lunar Orbiter Laser Altimeter and SELENE Terrain Camera

Abstract: United States. National Aeronautics and Space Administration. Lunar Reconnaissance Orbiter Project and Planetary Geology and Geophysics Program

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

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, which led to its current shape. Today, Bennu might follow a different evolutionary pathway, with an interior stiffness that permits surface cracking and mass wasting.Near-Earth asteroid Bennu has a top-like shape with longitudinal ridges, macroporosity, prominent boulders and surface mass wasting, suggesting that it is a stiff rubble pile, according to early observations by the OSIRIS-REx mission.

A new lunar digital elevation model from the Lunar Orbiter Laser Altimeter and SELENE Terrain Camera

Abstract: United States. National Aeronautics and Space Administration. Lunar Reconnaissance Orbiter Project and Planetary Geology and Geophysics Program

Crater degradation on the lunar maria: Topographic diffusion and the rate of erosion on the Moon

Abstract: Landscape evolution on the Moon is dominated by impact cratering in the post-maria period. In this study, we mapped 800 m to 5 km diameter craters on >30% of the lunar maria and extracted their topographic profiles from digital terrain models created using the Kaguya Terrain Camera. We then characterized the degradation of these craters using a topographic diffusion model. Because craters have a well-understood initial morphometry, these data provide insight into erosion on the Moon and the topographic diffusivity of the lunar surface as a function of time. The average diffusivity we calculate over the past 3 Ga is ~5.5 m2/Myr. With this diffusivity, after 3 Ga, a 1 km diameter crater is reduced to approximately ~52% of its initial depth and a 300 m diameter crater is reduced to only ~7% of its initial depth, and craters smaller than ~200–300 m are degraded beyond recognition. Our results also allow estimation of the age of individual craters on the basis of their degradation state, provide a constraint on the age of mare units, and enable modeling of how lunar terrain evolves as a function of its topography.

Episodes of particle ejection from the surface of the active asteroid (101955) Bennu

Abstract: INTRODUCTION Active asteroids are small bodies in the Solar System that show ongoing mass loss, such as the ejection of dust, which may be caused by large impacts, volatile release, or rotational acceleration. Studying them informs our understanding of the evolution and destruction of asteroids and the origin of volatile materials such as water on Earth. The OSIRIS-REx (Origins, Spectral Interpretation, Resource Identification, and Security–Regolith Explorer) spacecraft has rendezvoused with the near-Earth asteroid (101955) Bennu. The selection of Bennu as the OSIRIS-REx target was partially based on its spectral similarity to some active asteroids. Observations designed to detect mass loss at Bennu were conducted from Earth and during the spacecraft’s approach, but no signs of asteroid activity were found. However, when the spacecraft entered orbit in January 2019, we serendipitously observed particles in the vicinity of Bennu that had apparently been ejected from its surface. RATIONALE We analyzed the properties and behavior of particles ejected from Bennu to determine the possible mechanisms of ejection and provide understanding of the broader population of active asteroids. Images obtained by the spacecraft indicate multiple discrete ejection events with a range of energies and resultant particle trajectories. We characterized three large ejection events that respectively occurred on 6 January, 19 January, and 11 February 2019. Tracking of individual particles across multiple images by means of optical navigation techniques provided the initial conditions for orbit determination modeling. By combining these approaches, we estimated the locations and times of ejection events and determined initial velocity vectors of particles. We estimated the particle sizes and the minimum energies of the ejection events using a particle albedo and density consistent with observations of Bennu. RESULTS Particles with diameters from 3 m s–1. Estimated energies ranged from 270 mJ for the 6 January event to 8 mJ for the 11 February event. The three events arose from widely separated sites, which do not show any obvious geological distinction from the rest of Bennu’s surface. However, these events all occurred in the late afternoon, between about 15:00 and 18:00 local solar time. In addition to discrete ejection events, we detected a persistent background of particles in the Bennu environment. Some of these background particles have been observed to persist on temporary orbits that last several days—in one case, with a semimajor axis >1 km. The orbital characteristics of these gravitationally bound objects make it possible to determine the ratio of their cross-sectional area to their mass. Combined with their photometric phase functions, this information constrains the parameter space of the particles’ diameters, densities, and albedos. CONCLUSION Plausible mechanisms for the large ejection events include thermal fracturing, volatile release through dehydration of phyllosilicates, and meteoroid impacts. The late-afternoon timing of the events is consistent with any of these mechanisms. Bennu’s boulder geology indicates that thermal fracturing, perhaps enhanced by volatile release, could occur on the asteroid surface. Smaller events, especially those that occur on the night side of Bennu, could be attributable to reimpacting particles. Our observations classify Bennu as an active asteroid. Active asteroids are commonly identified by major mass loss events observable with telescopes, on scales much greater than we observed at Bennu. Our findings indicate that there is a continuum of mass loss event magnitudes among active asteroids.