SciCombinator

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Concept: Meteoroid

220

Large meteors (or superbolides [Ceplecha Z, et al. (1999) Meteoroids 1998:37-54]), although rare in recorded history, give sobering testimony to civilization’s inherent vulnerability. A not-so-subtle reminder came on the morning of February 15, 2013, when a large meteoroid hurtled into the Earth’s atmosphere, forming a superbolide near the city of Chelyabinsnk, Russia, ∼1,500 km east of Moscow, Russia [Ivanova MA, et al. (2013) Abstracts of the 76th Annual Meeting of the Meteoritical Society, 5366]. The object exploded in the stratosphere, and the ensuing shock wave blasted the city of Chelyabinsk, damaging structures and injuring hundreds. Details of trajectory are important for determining its specific source, the likelihood of future events, and potential mitigation measures. Earth-viewing environmental satellites can assist in these assessments. Here we examine satellite observations of the Chelyabinsk superbolide debris trail, collected within minutes of its entry. Estimates of trajectory are derived from differential views of the significantly parallax-displaced [e.g., Hasler AF (1981) Bull Am Meteor Soc 52:194-212] debris trail. The 282.7 ± 2.3° azimuth of trajectory, 18.5 ± 3.8° slope to the horizontal, and 17.7 ± 0.5 km/s velocity derived from these satellites agree well with parameters inferred from the wealth of surface-based photographs and amateur videos. More importantly, the results demonstrate the general ability of Earth-viewing satellites to provide valuable insight on trajectory reconstruction in the more likely scenario of sparse or nonexistent surface observations.

Concepts: Near-Earth object, Atmosphere, Meteoroids, Russia, Comet, Meteoroid, Meteorite

54

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.

Concepts: Thermodynamics, Estimation, Shock wave, Impact event, Mars, Near-Earth object, Meteoroid, Asteroid

42

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 a unique opportunity to calibrate the event, with implications for the study of near-Earth objects and developing hazard mitigation strategies for planetary protection.

Concepts: Meteoroid, Planetary science, Tunguska event, Asteroid, Mars, Comet, Meteorite, Impact event

11

The Chelyabinsk asteroid impact is the second largest asteroid airburst in our recorded history. To prepare for a potential threat from asteroid impacts, it is important to understand the nature and formational history of Near-Earth Objects (NEOs) like Chelyabinsk asteroid. In orbital evolution of an asteroid, collision with other asteroids is a key process. Here, we show the existence of a high-pressure mineral jadeite in shock-melt veins of Chelyabinsk meteorite. Based on the mineral assemblage and calculated solidification time of the shock-melt veins, the equilibrium shock pressure and its duration were estimated to be at least 3-12 GPa and longer than 70 ms, respectively. This suggests that an impactor larger than 0.15-0.19 km in diameter collided with the Chelyabinsk parent body at a speed of at least 0.4-1.5 km/s. This impact might have separated the Chelyabinsk asteroid from its parent body and delivered it to the Earth.

Concepts: Spaceguard, Meteorite, Earth, Comet, Mars, Meteoroid, Asteroid, Impact event

10

The catastrophic disruption of the L chondrite parent body in the asteroid belt c. 470 Ma initiated a prolonged meteorite bombardment of Earth that started in the Ordovician and continues today. Abundant L chondrite meteorites in Middle Ordovician strata have been interpreted to be the consequence of the asteroid breakup event. Here we report a zircon U-Pb date of 467.50±0.28 Ma from a distinct bed within the meteorite-bearing interval of southern Sweden that, combined with published cosmic-ray exposure ages of co-occurring meteoritic material, provides a precise age for the L chondrite breakup at 468.0±0.3 Ma. The new zircon date requires significant revision of the Ordovician timescale that has implications for the understanding of the astrogeobiologic development during this period. It has been suggested that the Middle Ordovician meteorite bombardment played a crucial role in the Great Ordovician Biodiversification Event, but this study shows that the two phenomena were unrelated.

Concepts: Meteoroid, Mars, Asteroid belt, Comet, Asteroid, Middle Ages, Chondrite, Meteorite

10

Space missions and thermal infrared observations have shown that small asteroids (kilometre-sized or smaller) are covered by a layer of centimetre-sized or smaller particles, which constitute the regolith. Regolith generation has traditionally been attributed to the fall back of impact ejecta and by the break-up of boulders by micrometeoroid impact. Laboratory experiments and impact models, however, show that crater ejecta velocities are typically greater than several tens of centimetres per second, which corresponds to the gravitational escape velocity of kilometre-sized asteroids. Therefore, impact debris cannot be the main source of regolith on small asteroids. Here we report that thermal fatigue, a mechanism of rock weathering and fragmentation with no subsequent ejection, is the dominant process governing regolith generation on small asteroids. We find that thermal fragmentation induced by the diurnal temperature variations breaks up rocks larger than a few centimetres more quickly than do micrometeoroid impacts. Because thermal fragmentation is independent of asteroid size, this process can also contribute to regolith production on larger asteroids. Production of fresh regolith originating in thermal fatigue fragmentation may be an important process for the rejuvenation of the surfaces of near-Earth asteroids, and may explain the observed lack of low-perihelion, carbonaceous, near-Earth asteroids.

Concepts: Fracture mechanics, Speed, Space exploration, Mars, Impact crater, Meteoroid, Asteroid, Diurnal temperature variation

6

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.

Concepts: Radar, Meteoroid, Asteroid, Chondrite, Comet, Impact event, Mars, Meteorite

2

Meteorite impact craters are one of the most common geological features in the solar system. An impact event is a near-instantaneous process that releases a huge amount of energy over a very small region on a planetary surface. This results in characteristic changes in the target rocks, from vaporization and melting to solid-state effects, such as fracturing and shock metamorphism. Shatter cones are distinctive striated conical fractures that are considered unequivocal evidence of impact events. They are one of the most used and trusted shock-metamorphic effects for the recognition of meteorite impact structures. Despite this, there is still considerable debate regarding their formation. We show that shatter cones are present in several stratigraphic settings within and around impact structures. Together with the occurrence of complete and “double” cones, our observations are most consistent with shatter cone formation due to tensional stresses generated by scattering of the shock wave due to heterogeneities in the rock. On the basis of field mapping, we derive the relationship D sc = 0.4 D a, where D sc is the maximum spatial extent of in situ shatter cones, and D a is the apparent crater diameter. This provides an important, new, more accurate method to estimate the apparent diameter of eroded complex craters on Earth. We have reestimated the diameter of eight well-known impact craters as part of this study. Finally, we suggest that shatter cones may reduce the strength of the target, thus aiding crater collapse, and that their distribution in central uplifts also records the obliquity of impact.

Concepts: Planetary science, Meteoroid, Meteorite, Asteroid, Comet, Impact event, Mars, Impact crater

2

We report observations of dusty clouds in Saturn’s rings, which we interpret as resulting from impacts onto the rings that occurred between 1 and 50 hours before the clouds were observed. The largest of these clouds was observed twice; its brightness and cant angle evolved in a manner consistent with this hypothesis. Several arguments suggest that these clouds cannot be due to the primary impact of one solid meteoroid onto the rings, but rather are due to the impact of a compact stream of Saturn-orbiting material derived from previous breakup of a meteoroid. The responsible interplanetary meteoroids were initially between 1 centimeter and several meters in size, and their influx rate is consistent with the sparse prior knowledge of smaller meteoroids in the outer solar system.

Concepts: Asteroid belt, Asteroid, Orbital resonance, Meteoroid, Saturn, Comet, Scientific method, Solar System

1

Boulders are ubiquitously found on the surfaces of small rocky bodies in the inner solar system and their spatial and size distributions give insight into the geological evolution and collisional history of the parent bodies. Using images acquired by the Chang'e-2 spacecraft, more than 200 boulders have been identified over the imaged area of the near-Earth asteroid Toutatis. The cumulative boulder size frequency distribution (SFD) shows a steep slope of -4.4 ± 0.1, which is indicative of a high degree of fragmentation. Similar to Itokawa, Toutatis probably has a rubble-pile structure, as most boulders on its surface cannot solely be explained by impact cratering. The significantly steeper slope for Toutatis' boulder SFD compared to Itokawa may imply a different preservation state or diverse formation scenarios. In addition, the cumulative crater SFD has been used to estimate a surface crater retention age of approximately 1.6 ± 0.3 Gyr.

Concepts: Meteoroid, Mars, Differential calculus, Asteroid, Slope, Analytic geometry, Impact crater, Solar System