Takahiro Hiroi

Chelyabinsk airburst, damage assessment, meteorite recovery, and characterization

Deep Impact? On 15 February 2013, the Russian district of Chelyabinsk, with a population of more than 1 million, suffered the impact and atmospheric explosion of a 20-meter-wide asteroid—the largest impact on Earth by an asteroid since 1908. Popova et al. (p. 1069, published online 7 November; see the Perspective by Chapman) provide a comprehensive description of this event and of the body that caused it, including detailed information on the asteroid orbit and atmospheric trajectory, damage assessment, and meteorite recovery and characterization. A detailed study of a recent asteroid impact provides an opportunity to calibrate the damage caused by these rare events. [Also see Perspective by Chapman] The asteroid impact near the Russian city of Chelyabinsk on 15 February 2013 was the largest airburst on Earth since the 1908 Tunguska event, causing a natural disaster in an area with a population exceeding one million. Because it occurred in an era with modern consumer electronics, field sensors, and laboratory techniques, unprecedented measurements were made of the impact event and the meteoroid that caused it. Here, we document the account of what happened, as understood now, using comprehensive data obtained from astronomy, planetary science, geophysics, meteorology, meteoritics, and cosmochemistry and from social science surveys. A good understanding of the Chelyabinsk incident provides an opportunity to calibrate the event, with implications for the study of near-Earth objects and developing hazard mitigation strategies for planetary protection.

The global distribution of pure anorthosite on the Moon

It has been thought that the lunar highland crust was formed by the crystallization and floatation of plagioclase from a global magma ocean, although the actual generation mechanisms are still debated. The composition of the lunar highland crust is therefore important for understanding the formation of such a magma ocean and the subsequent evolution of the Moon. The Multiband Imager on the Selenological and Engineering Explorer (SELENE) has a high spatial resolution of optimized spectral coverage, which should allow a clear view of the composition of the lunar crust. Here we report the global distribution of rocks of high plagioclase abundance (approaching 100 vol.%), using an unambiguous plagioclase absorption band recorded by the SELENE Multiband Imager. If the upper crust indeed consists of nearly 100 vol.% plagioclase, this is significantly higher than previous estimates of 82–92 vol.% (refs 2, 6, 7), providing a valuable constraint on models of lunar magma ocean evolution.

Simulation of space weathering of planet-forming materials: Nanosecond pulse laser irradiation and proton implantation on olivine and pyroxene samples

For the purpose of simulating the surface alteration process called “space weathering”, experiments of pulse laser irradiation, proton implantation, and laser irradiation to proton implanted samples were performed and reflectance spectra of altered materials were measured. To simulate the impact heating by micrometeorite bombardments, we made a new apparatus using a pulse laser whose pulse duration is 6–8 nanoseconds, comparable with a timescale of micrometeorite impacts. We find that the degree of space weathering, i.e., change of reflectance spectrum should depend on mineral composition. Laser irradiation onto olivine produces the largest reduction of albedo and the highest reddening of reflectance spectrum. In general, variation of olivine spectra is much larger than that of pyroxenes. Depths of absorption bands do not change in the scaled spectra. The olivine spectrum after the laser irradiation can match spectra of some olivine asteroids within a subtype of S-type asteroids. Comparison of Vesta spectrum with altered pyroxene spectra suggests that Vesta surface would be relatively older than olivine asteroids. We also investigate the influence of solar wind proton and pyroxene FeO content. The proton implantation causes small changes in olivine and enstatite spectra. Implanted protons do not influence spectral change by the laser irradiation: the laser irradiation and the proton implantation do not produce multiplicative but additive changes on the reflectance spectra. FeO content of pyroxenes does not relate to the degree of reflectance change.

Near-Infrared Spectral Results of Asteroid Itokawa from the Hayabusa Spacecraft

The near-infrared spectrometer on board the Japanese Hayabusa spacecraft found a variation of more than 10% in albedo and absorption band depth in the surface reflectance of asteroid 25143 Itokawa. Spectral shape over the 1-micrometer absorption band indicates that the surface of this body has an olivine-rich mineral assemblage potentially similar to that of LL5 or LL6 chondrites. Diversity in the physical condition of Itokawas surface appears to be larger than for other S-type asteroids previously explored by spacecraft, such as 433 Eros.

Radar-Enabled Recovery of the Sutter’s Mill Meteorite, a Carbonaceous Chondrite Regolith Breccia

The Meteor That Fell to Earth In April 2012, a meteor was witnessed over the Sierra Nevada Mountains in California. Jenniskens et al. (p. 1583) used a combination of photographic and video images of the fireball coupled with Doppler weather radar images to facilitate the rapid recovery of meteorite fragments. A comprehensive analysis of some of these fragments shows that the Sutters Mill meteorite represents a new type of carbonaceous chondrite, a rare and primitive class of meteorites that contain clues to the origin and evolution of primitive materials in the solar system. The unexpected and complex nature of the fragments suggests that the surfaces of C-class asteroids, the presumed parent bodies of carbonaceous chondrites, are more complex than previously assumed. Analysis of this rare meteorite implies that the surfaces of C-class asteroids can be more complex than previously assumed. Doppler weather radar imaging enabled the rapid recovery of the Sutter’s Mill meteorite after a rare 4-kiloton of TNT–equivalent asteroid impact over the foothills of the Sierra Nevada in northern California. The recovered meteorites survived a record high-speed entry of 28.6 kilometers per second from an orbit close to that of Jupiter-family comets (Tisserand’s parameter = 2.8 ± 0.3). Sutter’s Mill is a regolith breccia composed of CM (Mighei)–type carbonaceous chondrite and highly reduced xenolithic materials. It exhibits considerable diversity of mineralogy, petrography, and isotope and organic chemistry, resulting from a complex formation history of the parent body surface. That diversity is quickly masked by alteration once in the terrestrial environment but will need to be considered when samples returned by missions to C-class asteroids are interpreted.

Evidence of thermal metamorphism on the C, g, B, and f asteroids.

Reflectance spectra (0.3 to 2.6 micrometers) of 14 C, G, B, and F asteroids and 21 carbonaceous chondrite powders are compared in detail. Only three thermally metamorphosed CM-Cl chondrites that have a weak ultraviolet absorption are shown to have close counterparts among those asteroids. Reflectance spectra of heated Murchison CM2 chondrite are compared with the average C and G type asteroid spectra. Murchison heated at 600� to 1000�C exhibits a similar weak ultraviolet absorption and provides the best analog for those spectra. Comparison of ultraviolet absorption strengths between 160 C, G, B, and F asteroids and carbonaceous chondrites suggests that surface minerals of most of those asteroids are thermally metamorphosed at temperatures around 600� to 1000�C.

Estimation of grain sizes and mixing ratios of fine powder mixtures of common geologic minerals

Two different approaches for modeling reflectance spectra of intimate mixtures, Hapkes model and the isograin model, are used to estimate grain sizes and mixing ratios of powder mixtures of three geologic minerals: olivine, orthopyroxene, and plagioclase. In Hapkes model, scattering and extinction efficiencies are mixed separately, and both models employ semiempirical refractive index spectra for component minerals. Mixing ratios of mixtures of grain size 45–75 μm are well estimated by both models assuming a common grain size of 60 μm and optimizing the constants for the single-particle scattering. For each model, effective grain size ratios for mineral constituents in mixtures of grain size <25 μm, are derived successfully that allows mineral abundances to be accurately predicted within ∼4 wt%. On the other hand, neither model can accurately predict mineral reflectance spectra for its smaller grain sizes (<25 and 25–45 μm) using an absorption coefficient spectrum derived from a larger grain size (45–75 μm). The errors in both models are significantly reduced if surface roughness effects of the smaller grain-size fractions are modeled.

Developing space weathering on the asteroid 25143 Itokawa

Puzzlingly, the parent bodies of ordinary chondrites (the most abundant type of meteorites) do not seem to be abundant among asteroids. One possible explanation is that surfaces of the parent bodies become optically altered, to become the S-type asteroids which are abundant in the main asteroid belt. The process is called ‘space weathering’—it makes the visible and near-infrared reflectance spectrum of a body darker and redder. A recent survey of small, near-Earth asteroids suggests that the surfaces of small S asteroids may have developing stages of space weathering. Here we report that a dark region on a small (550-metre) asteroid—25143 Itokawa—is significantly more space-weathered than a nearby bright region. Spectra of both regions are consistent with those of LL5-6 chondrites after continuum removal. A simple calculation suggests that the dark area has a shorter mean optical path length and about 0.04 per cent by volume more nanophase metallic iron particles than the bright area. This clearly shows that space-weathered materials accumulate on small asteroids, which are likely to be the parent bodies of LL chondrites. We conclude that, because LL meteorites are the least abundant of ordinary (H, L, and LL) chondrites, there must be many asteroids with ordinary-chondrite compositions in near-Earth orbits.

Spectroscopy of B-type asteroids: Subgroups and meteorite analogs

[1] B-type asteroids have a negative slope from -0.5 to ∼1.1 μm and beyond. What causes this? Visible to near-infrared reflectance spectra (0.4-2.5 μm) are assembled for 22 B-type asteroids. The spectra fall naturally into three groups: (1) those with negative (blue) spectral shapes like 2 Pallas (7 objects), (2) those with concave curve shapes like 24 Themis (11 objects), and (3) everything else (4 objects). The asteroid spectra are compared to mineral and meteorite spectra from the Reflectance Experiment Laboratory library of 15,000 samples, in a least squares search for particulate analogs, constrained by spectral brightness. The Pallas group objects show a trend of analogs from the CV, CO, and CK meteorite groups. Only three of the seven Pallas-like objects are determined to be dynamically related (2, 1508, and 6411). The Themis group objects show a trend of analogs from the CI, CM, CR, CI-Unusual, and CM-Unusual meteorites (as expected from the work of Hiroi et al. (1996)). Seven of the 11 Themis-like objects are dynamically related (24, 62, 222, 316, 379, 383, and 431). Allowing for reasonable uncertainties in the spectral matches, we find no need to invoke mineralogies that do not exist in the meteorite collection to explain B-type spectra or their negative slopes. Our Themis group results are as expected and are consistent with previous work, but our Pallas group results are new and, in some cases, in conflict with previous work.

Simulation of space weathering by nanosecond pulse laser heating: dependence on mineral composition, weathering trend of asteroids and discovery of nanophase iron particles

Abstract The spectral mismatch between asteroids and ordinary chondrites, is explained by a so-called “space weathering” process where impacts of interplanetary dust altered the optical properties of asteroid surfaces. To simulate the heating by micrometeorite impacts, pellet samples of olivine, pyroxene, and anorthite are irradiated by a pulse laser beam (1064nm) with a pulse duration of 6–8 nanoseconds, which is comparable with a micron-sized particle impact. After the laser irradiation, bidirectional reflectance spectra between 250 and 2600nm of samples are measured by step of 10nm. Laser-irradiated samples show significant reddening: the reduction of spectra is much larger in the visible region than in the near-infrared region. Changes of pyroxene spectra are much smaller than those of olivine. Some asteroid spectra such as 349 Dembowska and 446 Aeternitas can be reproduced by the mixing of spectra of irradiated samples. To clarify the microscopic process and cause of reflectance change, we observed the irradiated samples using transmission electron microscopy (TEM). In rim regions of irradiated olivine grains from pellet samples, nanophase iron particles (several to 30 nm in size) are widely spread. In contrast, no trace of structural change is found on irradiated olivine crystal samples. This suggests that the presence of regoligh-like surface would be essential for the effective space weathering on asteroids. Very small asteroids without regolith could be hardly weathered.