Meteoroid Fragmentation in the Martian Atmosphere and the Formation of Crater Clusters

The current rate of small impacts on Mars is informed by more than one thousand impact sites formed in the last 20 years, detected in images of the martian surface. More than half of these impacts produced a cluster of small craters formed by fragmentation of the meteoroid in the martian atmosphere. The spatial distributions, number and sizes of craters in these clusters provide valuable constraints on the properties of the impacting meteoroid population as well as the meteoroid fragmentation process. In this paper, we use a recently compiled database of crater cluster observations to calibrate a model of meteoroid fragmentation in Mars' atmosphere and constrain key model parameters, including the lift coefficient and fragment separation velocity, as well as meteoroid property distributions. The model distribution of dynamic meteoroid strength that produces the best match to observations has a minimum strength of 10-90 kPa, a maximum strength of 3-6 MPa and a median strength of 0.2-0.5 MPa. An important feature of the model is that individual fragmentation events are able to produce fragments with a wide range of dynamic strengths as much as 10 times stronger or weaker than the parent fragment. The calibrated model suggests that the rate of small impacts on Mars is 1.5-4 times higher than recent observation-based estimates. It also shows how impactor properties relevant to seismic wave generation, such as the total impact momentum, can be inferred from cluster characteristics.

A unique stone skipping-like trajectory of asteroid Aletai

Meteoroids/asteroids could deposit energy to Earth during their entries, which arouses great concerns. Strewn field, as a product of meteoroids/asteroids breakup, comprehensively reflects the trajectory, dynamics, and physical properties of meteoroids/asteroids. It typically has a length of several to a dozen kilometers. Nevertheless, the recently found massive Aletai irons in the northwest China comprise the longest known strewn field of ~430 kilometers. This implies that the dynamics of Aletai could be unique. Petrographic and trace elemental studies suggest that all the Aletai masses exhibit unique compositions (IIIE anomalous), indicating that they were from the same fall event. Numerical modeling suggests that the stone skipping-like trajectory associated with a shallow entry angle (e.g., ~6.5° to 7.3°) is responsible for Aletai's exceptionally long strewn field if a single-body entry scenario is considered. The stone skipping-like trajectory would not result in the deposition of large impact energy on the ground but may lead to the dissipation of energy during its extremely long-distance flight.

Evidence for a large Paleozoic Impact Crater Strewn Field in the Rocky Mountains

The Earth is constantly bombarded by meteoroids of various sizes. During hypervelocity collisions a large amount of energy is coupled to the Earth's atmosphere leading to disruption of decimeter to hundred meter-sized meteoroids. Smaller meteoroids may form meteorite strewn fields while larger initial bodies and high-strength iron meteoroids may form impact crater strewn fields. Impact crater strewn fields are ephemeral and none documented to date are older than about 63,500 years. Here we report on a newly discovered impact crater strewn field, about 280 Myr old, in tilted strata of the Rocky Mountains near Douglas, Wyoming. It is the oldest and among the largest of impact crater strewn fields discovered to date, extending for a minimum of 7.5 km along a SE-NW trajectory. The apparent width of the strewn field is 1.5 km, but the full extent of the crater strewn field is not yet constrained owing to restricted exposure. We probably see only a small section of the entire crater strewn field. The cascade of impacts occurred in an environment that preserved the craters from destruction. Shock lithification aided this process.

The trajectory, structure and origin of the Chelyabinsk asteroidal impactor

Earth is continuously colliding with fragments of asteroids and comets of various sizes. The largest encounter in historical times occurred over the Tunguska river in Siberia in 1908, producing an airburst of energy equivalent to 5-15 megatons of trinitrotoluene (1 kiloton of trinitrotoluene represents an energy of 4.185 × 10(12) joules). Until recently, the next most energetic airburst events occurred over Indonesia in 2009 and near the Marshall Islands in 1994, both with energies of several tens of kilotons. Here we report an analysis of selected video records of the Chelyabinsk superbolide of 15 February 2013, with energy equivalent to 500 kilotons of trinitrotoluene, and details of its atmospheric passage. We found that its orbit was similar to the orbit of the two-kilometre-diameter asteroid 86039 (1999 NC43), to a degree of statistical significance sufficient to suggest that the two were once part of the same object. The bulk strength--the ability to resist breakage--of the Chelyabinsk asteroid, of about one megapascal, was similar to that of smaller meteoroids and corresponds to a heavily fractured single stone. The asteroid broke into small pieces between the altitudes of 45 and 30 kilometres, preventing more-serious damage on the ground. The total mass of surviving fragments larger than 100 grams was lower than expected.

Hydrogen cyanide production due to mid-size impacts in a redox-neutral N2-rich atmosphere

Cyanide compounds are amongst the most important molecules of the origin of life. Here, we demonstrate the importance of mid-size (0.1-1 km in diameter) hence frequent meteoritic impacts to the cyanide inventory on the early Earth. Subsequent aerodynamic ablation and chemical reactions with the ambient atmosphere after oblique impacts were investigated by both impact and laser experiments. A polycarbonate projectile and graphite were used as laboratory analogs of meteoritic organic matter. Spectroscopic observations of impact-generated ablation vapors show that laser irradiation to graphite within an N2-rich gas can produce a thermodynamic environment similar to that produced by oblique impacts. Thus, laser ablation was used to investigate the final chemical products after this aerodynamic process. We found that a significant fraction (>0.1 mol%) of the vaporized carbon is converted to HCN and cyanide condensates, even when the ambient gas contains as much as a few hundred mbar of CO2. As such, the column density of cyanides after carbon-rich meteoritic impacts with diameters of 600 m would reach ~10 mol/m(2) over ~10(2) km(2) under early Earth conditions. Such a temporally and spatially concentrated supply of cyanides may have played an important role in the origin of life.

Meteoritic dust from the atmospheric disintegration of a large meteoroid

Much of the mass of most meteoroids entering the Earth's atmosphere is consumed in the process of ablation. Larger meteoroids (> 10 cm), which in some cases reach the ground as meteorites, typically have survival fractions near 1-25 per cent of their initial mass. The fate of the remaining ablated material is unclear, but theory suggests that much of it should recondense through coagulation as nanometre-sized particles. No direct measurements of such meteoric 'smoke' have hitherto been made. Here we report the disintegration of one of the largest meteoroids to have entered the Earth's atmosphere during the past decade, and show that the dominant contribution to the mass of the residual atmospheric aerosol was in the form of micrometre-sized particles. This result is contrary to the usual view that most of the material in large meteoroids is efficiently converted to particles of much smaller size through ablation. Assuming that our observations are of a typical event, we suggest that large meteoroids provide the dominant source of micrometre-sized meteoritic dust at the Earth's surface over long timescales.

Efficient disruption of small asteroids by Earth's atmosphere

Accurate modelling of the interaction between the atmosphere and an incoming bolide is a complex task, but crucial to determining the fraction of small asteroids that actually hit the Earth's surface. Most semi-analytical approaches have simplified the problem by considering the impactor as a strengthless liquid-like object ('pancake' models), but recently a more realistic model has been developed that calculates motion, aerodynamic loading and ablation for each separate particle or fragment in a disrupted impactor. Here we report the results of a large number of simulations in which we use both models to develop a statistical picture of atmosphere-bolide interaction for iron and stony objects with initial diameters up to approximately 1 km. We show that the separated-fragments model predicts the total atmospheric disruption of much larger stony bodies than previously thought. In addition, our data set of >1,000 simulated impacts, combined with the known pre-atmospheric flux of asteroids with diameters less than 1 km, elucidates the flux of small bolides at the Earth's surface. We estimate that bodies >220 m in diameter will impact every 170,000 years.