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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 2012). 
Collaboration with Imperial College, London, GB; University of Edinburgh, GB; University of Muenster, Germany; Oberlin University; American Museum of Natural History, NYC). 
 
K-Pg layer (Chixculub impact) and global fires. The discovery of large amounts of soot in K-Pg boundary clays led to the hypothesis that major wildfires were a consequence of impact.  Subsequently, several lines of evidence, including the lack of charcoal in North American sites, were used to argue against global wildfires. Current widely used models fail to take into account that ejection is asymmetric and dependent on impact angle, and that ejecta travel significant distances laterally after its arrival at the top of the Earth's atmosphere. Importantly, radiation is delivered to the Earth's surface where the ejecta re-enters the atmosphere, not its final destination. 
For the first time in the K-Pg study, we have modeled the entire process: 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 re-enters the atmosphere. To model the latter process we substantially modified the SOVA code to include the radiation transfer in a gas-particle mixture. This modification allows to model a wide range of particle sizes with different optical properties. 
Our models predict thermal pulses of over 55 kW/m2 to the surface in the downrange direction up to a few thousand kilometers from Chicxulub, but radiation is less severe in other directions and at distal sites. Although thermal pulses may be extremely high, their duration at intermediate (4000-5000 km) and proximal sites (2000-2500 km) is quite short, only a few minutes, because of a so-called self-shielding (cooled particles absorb radiation emitted by hot particles) – see Figure.
 
Thermal flux to the surface for various distances from the Chicxulub crater (distal – 7000 km, intermediate – 4000 km, proximal – 2000 km) and various azimuths (numbers near the curves).
Experiments with similar thermal fluxes show that healthy flora are resistant to ignition, but do undergo substantial pyrolysis when subjected to higher thermal pulses, leading to the release of charred material and a high potential for tree mortality. Flora in distal and uprange sites are likely to be dried, but not charred or ignited. Our combined results suggest that some wildfires may have been ignited by the impact fireball and ejecta re-entry, but that wildfires were not started globally immediately after the Chicxulub impact. Given the volumes of soot in the global K-Pg layer, the dessication of flora by ejecta re-entry, as well as the effects of post-impact global cooling/darkness, probably left much of the terrestrial flora prone to post-impact fires. 
 
An influence of small impacts on climate. 
We have carried out a simulation with the SOVA model to describe the effects of a 1 km asteroid impact on land. The mass of ejecta reaching the upper atmosphere, above the tropopause, has been estimated. We are now working out the best way to map the results of the SOVA simulation into the size distributions that are needed to provide the initial condition for the CESM/CARMA run. There is considerable uncertainty in making this determination, but we will scale the SOVA results with estimates based upon on the estimates of historical impacts and also from measurements of surface nuclear weapons tests Sensitivity tests may be necessary to quantify the importance of this uncertainty.
Dust aerosols from a meteor impact are expected to cover a wide range of sizes and can be produced from several mechanisms including: pulverization, melting and vaporization of both the impactor and the surface. The smallest sub-micron sized particles are expected to provide the biggest long-term effect, because they sediment more slowly. These particles are similar in size and to those produced by ablated micrometeorites, and the production from ablation of melted debris and condensation of vaporized material is a similar process. Because we have verified that we can perform simulations of meteoric dust from ablated micrometeorites, we feel the model is ready to simulate the dust and soot produced by a land based meteor impact.
 
The biggest and the youngest tektite strewn field, Australasian], is the single strewn field on Earth for which the parent crater is still not identified, although many researches point to a relatively small area in Indochina (104-109°E, 10-17°N). An estimated total mass of Australasian tekites (AAT) ranges from 1011 kg up to 3.2·1013 kg; parent crater size varies from the first tens of km up to 300 km. 
Due to the fact that such a big parent crater remains unknown, it is reasonable to suggest that it was destroyed by water currents immediately after its formation on the oceanic shelf. On one hand, 10Be content in tektites and the results of numerical modeling demand a very shallow initial depth of tektite-forming sediments. On the other hand, the presence of a water layer (and/or water-saturated sediments) leads to a higher ejection velocities, and, finally, to a wider distribution of ejecta. In particular, the recent drilling project and numerical modeling of the Chesapeake Bay structure confirmed that North-American tektites have been ejected from a crater covered by water. The main goals of this study are: (1) to estimate the minimal crater size, allowing production of reasonable amount of AAT; (2) to discuss post-impact evolution of this crater and possible geophysical signatures which can help to identify its remnants.
Three impacts have been modeled: 300-m-diameter projectile impacting into 150-m-deep water (S), 1-km-diameter projectile and 300-m-deep water (M), 5-km-diameter projectile and 600-m-deep water (L). In all cases an impact velocity was equal to 18 km/s and an impact angle – to 45° or 30°. Tracer particles are used to follow trajectories of molten impact ejecta (shock compression > 40 GPa). The presence of a water layer leads to a substantial decrease in the amount of high-velocity (>5 km/s) ejecta, and to total absence of ejecta with velocities above 7 km/s. However, the total amount of ejecta with velocities > 1 km/s exceeds geological estimates of the AAT total mass (except of the smallest impact). The final distribution of tektites after a medium-size impact is shown in the Figure; this distribution correlates well with geological observations. The final crater diameter is of ~20 km. Its geophysical signatures may be similar to the Mjolnir marine impact crater. The results show a strong decrease of the initial (pre-impact) depth of tektite materials with increasing distance from the impact point. This result is in a good correlation with 10Be content in AAT (Ma et al. 2004). Average shock compression of melt increases with the increasing distance, i.e., average tektites’ size should decrease with no tektites (but microtektites) beyond 8,000 km.
The LPSC abstract has been submitted (Jan. 2013).
 
2.  Impact ejecta on the Moon (PI of the LASER project, 2nd/3rd year, CoI of the LASER project, University of Texas, 4th year).
 
Tycho crater antipodal ejecta
Tycho is an 86-km-diameter lunar impact crater located in the southern lunar highlands. The crater is surrounded by a distinctive ray system forming long spokes that reach as long as 1,500 kilometers. Lunar Reconnaissance Orbiter Camera has recently revealed a large region (>3000 km2, at 41°N, 167°E) containing hundreds of young (probably <100 Ma), discrete smooth deposits with total volume > 1km3 (Robinson et al. 2011). The images show that the viscid material was emplaced with velocities high enough to allow uphill movement of still molten material. The authors consider impact melt from Tycho as a possible source of these ponds, although with a certain share of skepticism.
The modeling includes two stages: 1) modeling of impact cratering; 2) ballistic continuation on a sphere for all ejected materials. A 1D heat transfer equation has been solved to estimate melt cooling and solidification in space. Tracer particles are used to quantify the amount of ejected material, its depth of origin, maximum shock compression, and its ejection velocity. We keep the impact velocity equal to 18 km/s; vary the projectile size (from 16 to 7 km) and the impact angle (from 15° to 90°) to keep the transient cavity size constant and equal to ~70 km.
The total volume of initially molten ejecta at the antipode increases quickly with the impact angle decrease (3.4, 5.4 and 10.2 km3 after a 45°, 30°, and 15° impact, respectively). An increase of volume of fast ejecta (arriving to the antipode within 5 hours) is even sharper: 0.12, 1.75, and 10.2 km3 for the respective impact angles. It is necessary to mention that projectile melt prevails at all impact angles except for 15°.
Approximately half of the ejecta is a melt-vapor mixture and has to be excluded from our model, as it expands non-ballistically, condenses, and cools quickly. We assume that the rest of impact melt is ejected in the form of spherical blobs. Using standard data for silicate rocks, total solidification of particles in space may be estimated as follows: 10-cm-diameter particles are solid after 10 minutes, 1-m-diameter – after 10 hours, 10-m-diameter fragments – after 100 hours. After 2 (5) hours in space, 31 (8) % of all meter-sized blobs remain solid; only a minor, <10%, fraction of larger, 10-m-blobs, is solidified creating an outer shell filled by the impact melt. Partially molten blobs arrive to the antipode with a velocity of ~ 2 km/s. Simple estimates show that this velocity is too low to cause additional melting. However, it may be high enough to partially disperse the blobs and to allow uphill movement of the melt. 
These estimates are in a quantitative agreement with observations: > 1 km3 of impact melt from Tycho re-impacts the antipode as partially (or totally) molten material. It seems interesting to check chemical composition of such deposits, for they may be rich in PGEs.
The LPSC abstract has been submitted.
 
Cometary impact on the Moon and water retention 
Several missions have yielded observations that could indicate the presence of water ice in lunar polar regions. Our work aims to investigate cometary impacts as a mechanism for the delivery of water to permanently shadowed craters (‘cold traps’) at the lunar poles. Of particular interest is the influence of parameters such as impact angle, velocity and location on the long-term retention of cometary water. Our 3D, unsteady simulations use the SOVA hydrocode to model the impact and vaporization of a cometary nucleus composed of pure water ice, 2km in diameter, impacting at 30 km/s. Subsequently, a Direct Simulation Monte Carlo code, designed to handle rarefied planetary flows, is used to simulate the transient water vapor atmosphere that develops. Molecules in this atmosphere collide and migrate across the lunar surface, driven by diurnal variations in surface temperature, and may land in permanently shadowed craters, cold enough to trap water over geological time scales. We consider the dynamic development of the transient atmosphere and compare initial deposition patterns as gravitationally bound water vapor begins to fall back to the lunar surface, for two different impact angles: 45° and 60° from the horizontal. A greater fraction of water remains gravitationally bound to the Moon in the 60° case, and a less pronounced downrange focusing of the vapor results in a more symmetric initial deposition pattern. On the cold night-side of the Moon, water simply sticks to the surface. However, on the warm day-side, where residence times are much shorter, we observe the development of a relatively dense, low-speed, surface-hugging flow. A particularly interesting depositional feature is the concentration of mass at a point almost antipodal to the point of impact, where a convergence of streamlines results in a shock that channels water to the surface.
 
II. Publications
 
Papers (3)
1. Pierazzo E. and Artemieva N. 2012. Local and Global Environmental Effects of Impacts on Earth. Elements, vol. 8, pp. 56-60.
2. Reimold W.U., Hansen, B. K.; Jacob, J.; Artemieva, N. A.; Wunnemann, K.; Meyer, C. 2012. Petrography of the impact breccias of the Enkingen (SUBO 18) drill core, southern Ries crater, Germany: New estimate of impact melt volume. Geological Society of America Bulletin, vol. 124, issue 1-2, pp. 104-132.
 
Conference abstracts (6)
1. Deutsch A., Artemieva N. 2012. Tracking down tracers of the Chicxulub projectile in K-Pg boundary deposits. LPSC-2012, Abstract 2087.
2. Artemieva N. and Simonson B. 2012. Elucidating the formation of Archean-Proterozoic boundary spherule layers. LPSC-2012, Abstract 1372.
3. Artemieva N., K.Wünnemann, D.Stöffler, and W.U. Reimold. 2012. Ries suevite – plume ejecta, melt flow or something else? LPSC-2012, Abstract 1364.
4. Fernandes V., Artemieva N. 2012. Impact Ejecta Temperature Profile on the Moon — What are the Effects on the Ar-Ar Dating Method? LPSC-2012, Abstract 1367.
5. Artemieva N. 2012. Tycho crater ejecta. EPSC meeting in Madrid. 
6. Prem, Parvathy; Artemieva, N. A.; Pierazzo, E.; Stewart, B. D.; Goldstein, D. B.; Varghese, P. L.; Trafton, L. M. 2012. Cometary Delivery of Lunar Water: Transient Atmosphere Dynamics and Deposition Patterns. American Astronomical Society, DPS meeting #44, #401.03.
 
III. Awards and Honors
 
2012 Berlin-Brandenburg Academy of Science Prize (Perigrinus Prize). I included it in my previous report (2011), but the prize was awarded in 2012 (November 30).
 
 
IV. 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-2012 (Madrid).
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: 
2012
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