Most large (over a kilometre in diameter) near-Earth asteroids are now known, but recognition that airbursts (or fireballs resulting from nuclear-weapon-sized detonations of meteoroids in the ...atmosphere) have the potential to do greater damage than previously thought has shifted an increasing portion of the residual impact risk (the risk of impact from an unknown object) to smaller objects. Above the threshold size of impactor at which the atmosphere absorbs sufficient energy to prevent a ground impact, most of the damage is thought to be caused by the airburst shock wave, but owing to lack of observations this is uncertain. Here we report an analysis of the damage from the airburst of an asteroid about 19 metres (17 to 20 metres) in diameter southeast of Chelyabinsk, Russia, on 15 February 2013, estimated to have an energy equivalent of approximately 500 (±100) kilotons of trinitrotoluene (TNT, where 1 kiloton of TNT = 4.185×10(12) joules). We show that a widely referenced technique of estimating airburst damage does not reproduce the observations, and that the mathematical relations based on the effects of nuclear weapons--almost always used with this technique--overestimate blast damage. This suggests that earlier damage estimates near the threshold impactor size are too high. We performed a global survey of airbursts of a kiloton or more (including Chelyabinsk), and find that the number of impactors with diameters of tens of metres may be an order of magnitude higher than estimates based on other techniques. This suggests a non-equilibrium (if the population were in a long-term collisional steady state the size-frequency distribution would either follow a single power law or there must be a size-dependent bias in other surveys) in the near-Earth asteroid population for objects 10 to 50 metres in diameter, and shifts more of the residual impact risk to these sizes.
Resolving the timing of crustal processes and meteorite impact events is central to understanding the formation, evolution and habitability of planetary bodies. However, identifying multi-stage ...events from complex planetary materials is highly challenging at the length scales of current isotopic techniques. Here we show that accurate U-Pb isotopic analysis of nanoscale domains of baddeleyite can be achieved by atom probe tomography. Within individual crystals of highly shocked baddeleyite from the Sudbury impact structure, three discrete nanostructural domains have been isolated yielding average
Pb/
U ages of 2,436±94 Ma (protolith crystallization) from homogenous-Fe domains, 1,852±45 Ma (impact) from clustered-Fe domains and 1,412±56 Ma (tectonic metamorphism) from planar and subgrain boundary structures. Baddeleyite is a common phase in terrestrial, Martian, Lunar and asteroidal materials, meaning this atomic-scale approach holds great potential in establishing a more accurate chronology of the formation and evolution of planetary crusts.
•Lunar impact monitoring has been used to determine the flux of meteoroids in the 30g to a few kilograms size range.•A catalog of 126 photometrically calibrated impact flashes is presented.•A ...rigorous approach for photometric calibration of impact flash observations and calculation of flash energy is described.•Rates of observed flashes are correlated with meteor showers.•A large impact flash in March 2013 gave crater size estimates consistent with the crater measured by LRO.
The flashes from meteoroid impacts on the Moon are useful in determining the flux of impactors with masses as low as a few tens of grams. A routine monitoring program at NASA’s Marshall Space Flight Center has recorded over 300 impacts since 2006. A selection of 126 flashes recorded during periods of photometric skies was analyzed, creating the largest and most homogeneous dataset of lunar impact flashes to date. Standard CCD photometric techniques were applied to the video and the luminous energy, kinetic energy, and mass are estimated for each impactor. Shower associations were determined for most of the impactors and a range of luminous efficiencies was considered. The flux to a limiting energy of 2.5×10−6kT TNT or 1.05×107J is 1.03×10−7km−2h−1 and the flux to a limiting mass of 30g is 6.14×10−10m−2yr−1 at the Moon. Comparisons made with measurements and models of the meteoroid population indicate that the flux of objects in this size range is slightly lower (but within the error bars) than flux at this size from the power law distribution determined for the near Earth object and fireball population by Brown et al. (Brown, P.G., Spalding, R., ReVelle, D., Tagliaferri, E., Worden, S. 2002. Nature 420, 294–296). Size estimates for the crater detected by Lunar Reconnaissance Orbiter from a large impact observed on March 17, 2013 are also briefly discussed.
Since the advent of atomic force microscopy, mechanical resonators have been used to study a wide variety of phenomena, including the dynamics of individual electron spins, persistent currents in ...normal metal rings and the Casimir force. Key to these experiments is the ability to measure weak forces. Here, we report on force sensing experiments with a sensitivity of 12 zN Hz(-1/2) at a temperature of 1.2 K using a resonator made of a carbon nanotube. An ultrasensitive method based on cross-correlated electrical noise measurements, in combination with parametric downconversion, is used to detect the low-amplitude vibrations of the nanotube induced by weak forces. The force sensitivity is quantified by applying a known capacitive force. This detection method also allows us to measure the Brownian vibrations of the nanotube down to cryogenic temperatures. Force sensing with nanotube resonators offers new opportunities for detecting and manipulating individual nuclear spins as well as for magnetometry measurements.
Zircon microcrystals are found in many planetary crustal rocks and nanoscale research on grains from well-characterized impact environments on Earth provide a baseline for reconstructing extreme ...shock and thermal histories elsewhere. However, using zircon to date large impact events can be challenging given that shock-related isotopic re-setting of U–Pb ratios, when measured at micrometre scale, is often incomplete and difficult to interpret as the underlying Pb migration mechanisms are unclear. To better understand shocked zircon U–Pb systematics, we performed atom probe tomography and electron microscopy with the previous SIMS analyses of two shock metamorphosed Mesoarchean zircon grains from deep (≥ 15 km) beneath the centre of the 2.020 Ga giant Vredefort impact structure. We find evidence of two types of impact-related nanoscale Pb mobility. In one grain, clustering has produced ~ 10 nm diameter bodies of radiogenic Pb, unsupported by U and co-located with Al, with an average
207
Pb/
206
Pb ratio of ~ 0.50 (
n
= 4); the value extant in the grain at the time of impact. Conversely, nearby nanodomains exhibit randomly distributed radiogenic Pb, U and Al and yield
206
Pb/
238
U dates consistent with 100% loss of pre-impact radiogenic Pb atoms during shock metamorphic processes. Notably, domains with multiple Pb clusters occur within micrometres of domains that experienced 100% Pb loss, precluding a uniform radial pattern of thermally-driven Pb diffusion at the grain scale. These cases of broadly coeval clustering and outward Pb mobility during geologically instantaneous shock metamorphism point to unusually rapid, multi-path diffusion processes within sub-micrometre volumes which, when averaged, yield normally discordant U–Pb dates. The isolation of spatially variable styles of Pb retention and loss at nanoscale amidst classical grain-scale shock microstructures shows promise for recognizing and resolving bombardment histories in planetary crusts using zircon.
Identifying and dating large impact structures is challenging, as many of the traditional shock indicator phases can be modified by post-impact processes. Refractory accessory phases, such as zircon, ...while faithful recorders of shock wave passage, commonly respond with partial U–Pb age resetting during impact events. Titanite is an accessory phase with lower Pb closure temperature than many other robust chronometers, but its potential as indicator and chronometer of impact-related processes remains poorly constrained. In this study, we examined titanite grains from the Sudbury (Ontario, Canada) and Vredefort (South Africa) impact structures, combining quantitative microstructural and U–Pb dating techniques. Titanite grains from both craters host planar microstructures and microtwins that show a common twin–host disorientation relationship of 74° about . In the Vredefort impact structure, the microtwins deformed internally and developed high- and low-angle grain boundaries that resulted in the growth of neoblastic crystallites. U–Pb isotopic dating of magmatic titanite grains with deformation microtwins from the Sudbury impact structure yielded a
207
Pb/
206
Pb age of 1851 ± 12 Ma that records either the shock heating or the crater modification stage of the impact event. The titanite grains from the Vredefort impact structure yielded primarily pre-impact ages recording the cooling of the ultra-high-temperature Ventersdorp event, but domains with microtwins or planar microstructures show evidence of U–Pb isotopic disturbance. Despite that the identified microtwins are not diagnostic of shock-metamorphic processes, our contribution demonstrates that titanite has great potential to inform studies of the terrestrial impact crater record.
Invaluable records of planetary dynamics and evolution can be recovered from the geochemical systematics of single meteorites. However, the interpreted ages of the ejected igneous crust of Mars ...differ by up to four billion years, a conundrum due in part to the difficulty of using geochemistry alone to distinguish between the ages of formation and the ages of the impact events that launched debris towards Earth. Here we solve the conundrum by combining in situ electron-beam nanostructural analyses and U-Pb (uranium-lead) isotopic measurements of the resistant micromineral baddeleyite (ZrO2) and host igneous minerals in the highly shock-metamorphosed shergottite Northwest Africa 5298 (ref. 8), which is a basaltic Martian meteorite. We establish that the micro-baddeleyite grains pre-date the launch event because they are shocked, cogenetic with host igneous minerals, and preserve primary igneous growth zoning. The grains least affected by shock disturbance, and which are rich in radiogenic Pb, date the basalt crystallization near the Martian surface to 187 ± 33 million years before present. Primitive, non-radiogenic Pb isotope compositions of the host minerals, common to most shergottites, do not help us to date the meteorite, instead indicating a magma source region that was fractionated more than four billion years ago to form a persistent reservoir so far unique to Mars. Local impact melting during ejection from Mars less than 22 ± 2 million years ago caused the growth of unshocked, launch-generated zircon and the partial disturbance of baddeleyite dates. We can thus confirm the presence of ancient, non-convecting mantle beneath young volcanic Mars, place an upper bound on the interplanetary travel time of the ejected Martian crust, and validate a new approach to the geochronology of the inner Solar System.
Despite the high prevalence and potentially serious outcomes associated with concussion in athletes, there is little systematic research examining risk factors and short- and long-term outcomes.
To ...assess the relationship between concussion history and learning disability (LD) and the association of these variables with neuropsychological performance and to evaluate postconcussion recovery in a sample of college football players.
A total of 393 athletes from 4 university football programs across the United States received preseason baseline evaluations between May 1997 and February 1999. Subjects who had subsequent football-related acute concussions (n = 16) underwent neuropsychological comparison with matched control athletes from within the sample (n = 10).
Clinical interview, 8 neuropsychological measures, and concussion symptom scale ratings at baseline and after concussion.
Of the 393 players, 129 (34%) had experienced 1 previous concussion and 79 (20%) had experienced 2 or more concussions. Multivariate analysis of variance yielded significant main effects for both LD (P<.001) and concussion history (P=.009), resulting in lowered baseline neuropsychological performance. A significant interaction was found between LD and history of multiple concussions and LD on 2 neuropsychological measures (Trail-Making Test, Form B P=.007 and Symbol Digit Modalities Test P=.009), indicating poorer performance for the group with LD and multiple concussions compared with other groups. A discriminant function analysis using neuropsychological testing of athletes 24 hours after acute in-season concussion compared with controls resulted in an overall 89.5% correct classification rate.
Our study suggests that neuropsychological assessment is a useful indicator of cognitive functioning in athletes and that both history of multiple concussions and LD are associated with reduced cognitive performance. These variables may be detrimentally synergistic and should receive further study.
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