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Natasha Artemieva

I. Report on research
 
1.  Terrestrial impact ejecta layers and climate (CoI in PSI-ATM Exobiology project, PI in a new project submitted to PGG in 2013 – not funded). However, some results have been obtained without any funding.
Collaboration with Imperial College, London, GB; University of Edinburgh, GB; University of Muenster, Germany; Oberlin University; American Museum of Natural History, NYC); Museum fuer Naturkunde, Berlin. 
 
K-Pg layer (Chixculub impact) and global fires. 
The discovery of large amounts of soot in clays deposited at the Cretaceous-Paleogene (K-Pg) boundary and linked to the ~65Ma Chicxulub impact crater led to the hypothesis that major wildfires were a consequence of the asteroid impact. Subsequently, several lines of evidence, including the lack of charcoal in North American sites, were used to argue against global wildfires. Close to the impact site fires are likely to be directly ignited by the impact fireball, whereas globally they could be ignited by radiation from the reentry of hypervelocity ejecta. To-date, models of the latter have yet to take into account that ejection —and thus the emission of thermal radiation—is asymmetric and dependent on impact angle. We model: (1) the impact and ejection of material, (2) the ballistic continuation of ejecta around a spherical Earth, and (3) the thermal pulse delivered to the Earth’s surface when ejecta reenters the atmosphere. We find that thermal pulses in the downrange direction are sufficient to ignite flora several thousand kilometers from Chicxulub, whereas pulses at most sites in the uprange direction are too low to ignite even the most susceptible plant matter. Our analyses and models suggest some fires were ignited by the impact fireball and ejecta reentry, but that the nonuniform distribution of thermal radiation across the surface of the Earth is inconsistent with the ignition of fires globally as a direct and immediate result of the Chicxulub impact. Instead, we propose that the desiccation of flora by ejecta reentry, as well as the effects of postimpact global cooling/darkness, left much of the terrestrial flora prone to fires, and that the volume of soot in the global K-Pg layer is explained by a combination of syn- and postimpact wildfires. 
The paper (Morgan et al 2013) has been published in JGR. The MS, describing experiments, is currently in preparation.
 
Formation of accretionary lapilli after large impacts
Accretionary lapilli (AL) are 2-64 mm in diameter aggregates composed of agglomerated ash particles that occur in volcanic deposits, distal ejecta of the largest impact craters (Sudbury and Chicxulub), and within crater-filling deposits (Ries, Elgygytgyn). Intensive study of volcanic AL and laboratory experiments revealed crucial conditions allowing accretion of fine particles into aggregates: 1) collisions of particles within a high density dusty flow; 2) presence of a binding agent, usually liquid water, ice, or electrostatic forces. In impact cratering, three possible scenarios are usually considered: 1) formation within an ejecta plume rising above the growing crater; 2) accretion within a turbulent ejecta curtain; 3) formation in a density current associated with ejecta propagation. In our model we consider AL forming during re-entry of ejecta in the upper atmosphere.
During massive re-entry (the total mass of ejecta is comparable with the mass of gas along their trajectory) a complex flow arises in the atmosphere. The largest particles tend to penetrate to low altitudes, keeping their initial temperature, while smaller particles are decelerated at high altitudes, heat atmosphere to T> 600 K, and are at elevated temperatures themselves. As a result, these particles can release water if it was present in pores or if water was ejected and has been frozen in space (after the impact into a water-covered area. Large cold particles entering later may be coated by this water forming a liquid bridge to other particles. Substantial mixing of particles at temperatures and densities suitable for AL formation retains as long as ejecta are arriving at the top of the atmosphere, i.e., within a few minutes. When the flux ceases, the cloud becomes stratified with largest particles at its bottom and the smallest at the top. Shock waves reflected from the surface stir the cloud a few times, strongly affect the smallest particles, which may be thrown away from the site back to high altitudes. 
At distances between 400 – 1600 km from the crater (re-entry velocities of 2-4 km/s) conditions are suitable for AL formation. Closer to the crater, low temperatures and a deficiency of small particles prevent AL formation. At larger distances, the temperature is too high and the density of small particles is very low. The LPSC abstract was submitted in Jan. 2014.
 
Projectile material in Australasian tektites
The Australasian (AA) tektite strewn field covers more than 10% of the Earth’s surface. This areal distribution, together with the size of individual tektites (up to ~25 kg) and total recovered mass, indicates that the AA “strewn field–forming event” produced several orders of magnitude more tektites than other tektite-forming events (i.e., Bosumtwi, Ries, Chesapeake Bay). Despite indications for the location of the impact site near the Indochina peninsula, the young age of ~0.8 Ma, and an estimated crater size between 20 and 100 km Ø, no AA tektite related impact structure has been identified so far.
Recent geochemical analysis revealed PGE abundances in tektites allowing the identification of an extraterrestrial component. The same analysis showed an extremely thorough mixing of target/projectile materials. Numerical models confirmed the mixing process and the development of Kelvin-Helmholtz instabilities at the target/projectile boundary during the compression stage (see Figure). The LPSC abstract was submitted in Jan. 2014.
 
2. Impact ejecta on the Moon (PI of the LASER project, 3nd year, CoI of the LASER project, University of Texas, 4th year).
 
Ejection of lunar meteorites and lunar dust
The main goal of this study is to re-evaluate the amount of solid escape ejecta from the Moon (SEEM) taking into account the highly porous upper regolith layer. Presence of a porous regolith layer decreases the total mass of SEEM – from ~1 projectile mass after an impact into consolidated non-porous target to 0.06M after an impact into a pure regolith layer with a constant density of 1.6 g/cm3. This dramatic, almost twentyfold, decrease is mainly related to substantial decrease in the value of shock pressure causing rock melting– from 60 GPa in solid rocks to 15 GPa in 40% porous regolith. As a result, in nonporous rocks 55% of all escape ejecta are solid, while in porous rocks this parameter is below 10%. 
The excavation depth of SEEM does not exceed 10-20% of the projectile diameter, i.e., is substantially smaller than the total excavation depth (1/10 of crater diameter) as it was assumed earlier. For example, a 10-m projectile excavates SEEM from the depth of ~1-2 m, but makes a ~1-km-diameter crater with the total excavation depth of about 80 m.
The amount of ejecta enriched in 3He nearly coincides with the total amount of SEEM for small, <10 m in diameter projectiles, and is proportional to the projectile volume. For larger projectiles this volume increases as squared projectile diameter as only the upper few meters of the Moon are enriched in 3He. During the atmospheric entry large (>1 cm) particles lose 20 – 50% (entry velocities 11.4 – 18 km/s) of their initial mass due to ablation, but remain cold inside. Smaller particles (<1 mm) are isothermal and are heated to 1700, 980, and 500 K. (diameters 100, 10, and 1 µm, respectively). As 10% of lunar soil particles are smaller than 10 µm (they are also the richest in 3He), these particles can deliver 3He to the Earth without substantial loss.
The LPSC abstract was submitted in Jan. 2014.
 
3. The Chelyabinsk meteorite fall (no funding) 
On the early morning of February 15, 2013, thousands of people observed a bright flash in the sky over the city of Chelyabinsk. The flash was followed by a powerful sonic boom which destroyed windows across the area of ~ 5000 km2 injuring more than 1500 people. Numerous video recordings of the event have allowed to reconstruct the trajectory and fragmentation history. The size (15-20 m) and energy of meteorite (100-500 kt TNT) were estimated based on its infrasound signal, the energy of the brightest flash, and ground effects. Only a small fraction of the total mass was found on the surface near Chelyabinsk — mainly just tiny pieces with the most sizeable fragment weighting ~3 kg. The largest fragment (~600 kg) landed into the nearby lake was recovered by divers from a depth of ~18 m in October 2013. 
Simple entry models taking into account multiple fragmentation processes allowed to reproduce a light flash in the atmosphere; the model of a cylindrical explosion in the atmosphere produced damage on the surface comparable with observations and reasonable wake evolution. A deficiency of the total recovered mass (<0.01% of the pre-atmospheric mass) is still a problem for any available model.
One popular paper published in Meteorites, News & Views in Nature; two invited talks, one oral presentation at EPSC. 
 
II. Publications
 
Papers (5)
1. Artemieva N., Wünnemann K., Krien F., Reimold W.U., Stöffler D. (2013) Ries crater and suevite revisited—Observations and modeling Part II: Modeling. Meteoritics and Planetary Science 48, 590-627.
2. Stöffler D., Artemieva N., Wünnemann K., Reimold W.U., Jacob J., Hansen B.K., Summerson I.A. (2013) Ries crater and suevite revisited - Observations and modeling Part I: Observations. Meteoritics and Planetary Science 48, 515-589.
3. Morgan J., Artemieva N., Goldin T. (2013) Revisiting wildfires at the K-Pg boundary. JGR: Biogeosciences 118, doi:10.1002/2013JG002428
4. Artemieva N. (2013) Russian Skyfall. Nature 503, 202-203
5. Artemieva N., Shuvalov V. (2013) Let the sky fall. Meteorite 19, 12-16.
 
Conference abstracts (7)
1. Ivanova M. and 10 co-authors (2013) Fall, Searching and First Study of the Chelyabinsk Meteorite. 76th Annual Meeting of the Meteoritical Society, held July 29-August 7, 2013 in Edmonton, Canada. Published in Meteoritics and Planetary Science Supplement, id.5366 (invited 30 minutes talk, opening of the special session).
2. Artemieva N. and Shuvalov V (2013) Chelyabinsk meteorite entry model and damage on the surface. European Planetary Science Congress 2013, held 8-13 September in London, UK, id.EPSC2013-1039.
3. Morgan J., Artemieva N., Belcher C., Hadden R., Rein G., Goldin T. (2013) K-Pg Wildfires: modeling, experiments and observations. European Planetary Science Congress 2013, held 8-13 September in London, UK, id.EPSC2013-586
4. Bray V. J., Artemieva N. A., Neish C. D., McEwen A. S., McElwaine J. N.(2013) Impact Melt Entrained in the Ballistic Ejecta of Lunar Craters. LPSC-44, Abstract #2782.
5. Artemieva N. (2013) Tycho crater ejecta. LPSC-44, Abstract #1413. 
6. Artemieva N. (2013) Numerical Modeling of the Australasian Strewn Field. LPSC-44, Abstract #1410.
7. Artemieva N. (2013) Airbursts - from Tunguska to Chelyabinsk. XXV IUPAP Conference on Computational Physics August 20-24, 2013, Moscow (invited plenary talk, 40 min)
 
III. Service to the Science Community
 
1. Associate Editor, M&PS
2. Reviewer for Icarus, M&PS, EPSL, Geology, GSA Special Paper, etc.
3. Co-convener and co-chairperson of the impact session at EPSC-2013 (London).
3. Member of ISSI (International Space Science Institute in Bern, Switzerland) team “Updating The Lunar Chronology And Stratigraphy: New Laboratory And Remote Sensing Data”.
 
Research Year: 
2013
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fischer [at] psi.edu (A. Fischer)

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