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Natasha Artemieva

2009 Research Report

1. Impact plume and ejecta re-distribution.
Ejecta curtain, plume, and atmosphere are the three main factors defining impact ejecta deposition. Interaction of ejecta curtains with the plume depends on particles' size distribution (SFD) and the gas-solid ratio in the ejecta. Simplified models of the ejection process with various SFD and gas-solid ratios show that substantial redistribution of ejecta occurs if the gas content is higher than 1-10% and particles are smaller than 1 cm. A short animation illustrates ejecta separation into ballistic ejecta (1-m-diameter fragments, in magenta) and plume ejecta (1-mm-diameter particles, in yellow).

In real impact events high gas content and small fragments are typical for the early ejecta (e.g. tektites). On contrary, late ejecta contain very little gas-vapor, while the fragments are much larger, and are deposited ballistically. Interaction of ejecta curtains with the atmosphere becomes important if the mass of the ejected material is comparable with the mass of atmosphere through which it travels, i.e., usually occurs beyond continuous ejecta blankets. Within a few minutes after the impact, gravity causes the plume collapse. Although the plume is diluted and contains very little ejecta, it's heavy enough to cause strong oscillations and horizontal winds in the upper atmosphere that can last from a few hours to a few days. Consequently, all particles, suspended in the atmosphere long enough (mainly, because of their small size and low precipitation velocity) may be transported by these winds thousands of km away from their nominal destination.

Starting with the pioneering paper of Alvarez group, it was widely accepted that the K-Pg boundary is the ejecta from the K-Pg plume. However, our modeling have shown that at least shocked quartz grains in this layer could not belong to the Chicxulub impact plume; instead, they have been part of a dense and relatively low-velocity (< 3km/s) ejecta curtain. Nonetheless, shocked grains (and some spherules) were distributed world-wide because of the plume collapse.

2. Interaction of impact melt with water.
The early hypothesis for the Ries suevite origin is based on the separation of ejected material into ballistic ejecta (coarse, unshocked sediment), and plume-related ejecta (fine particles derived from the basement that are highly shocked or molten). The numerical modeling of impact cratering shows that 1) ejecta from all stratigraphic units with velocity < 1 km/s are deposited ballistically, without separation; 2) deposition of sediment prevails by an order of magnitude at any distance from the crater; 3) the impact plume above the crater consists mainly of a sediment-derived vapor/melt mixture, with the total thickness of plume deposits inside the crater < 2 m (much less outside); 4) the crater floor is covered by a 100-200 m thick layer of impact melt. Thus, the plume hypothesis consistent with tektite formation and deposition can not explain the proximal double-layered ejecta and prominence of suevite within the crater.

A sharp Bunte Breccia/suevite boundary means a substantial hiatus between ballistic ejecta emplacement and highly turbulent low-velocity suevitic flow. It seems that the single source of suevitic material is the Ries melt pool. Interaction of hot rocks with volatiles (similar to fuel-coolant interaction or phreato-magmatic explosion) may be a powerful driver of the flow. However, to initiate the flow we need at least 10-20 wt% of water, i.e. an order of magnitude more than the amount of water bound in the crystalline or metamorphic target rocks. Possible sources include: water condensing from the impact plume; water in sediments around the crater; rivers or lakes which feed into the Ries basin within a few thousand years after the impact.

3. Fate of organic molecules on Titan
Saturn's largest moon Titan should have a layer 200-500 meters thick of ethane averaged over the surface of the planet if methane photolysis has proceeded in a continuous fashion over geologic time. The Cassini-Huygens observations militate against the presence of exposures of liquid hydrocarbons extensive enough to constitute the expected reservoir. We explore the hypothesis that high velocity cometary impacts have removed the reservoir.

There are three ways to mix water with hydrocarbons to form clathrates: 1) ethane near an impact site (directly beneath the impact point) is pushed down and mixed with abundant mantle water; 2) ejected water and hydrocarbons are mixed in the plume and ejecta curtains; 3) deposited water (analogous to terrestrial ejecta blankets) interacts with local hydrocarbons. The last two scenarios result in the deposition of a layer of ethane-rich clathrate onto the surface. As the ethane-rich layer has a density equivalent to liquid water, much larger than the underlying ice/methane clathrate crust, a crustal overturn can occur. Impacts themselves by intensively fracturing the crust may favor the gravitational destabilization of the upper crust and the recycling of ethane-rich clathrate in the interior.

4. Tunguska enigma is finally solved
The Tunguska explosion happened in 1908 in Siberia and since then was a subject of numerous speculations. A hundred years later numerical models revealed that all observed Tunguska-related effects are typical for 50-100 m bodies entering the Earth's atmosphere at cosmic velocity. These effects are defined mainly by projectile size and the entry angle, not by its structure or composition. Recent speculations about the cometary nature of the Tunguska object are based solely on the presence of water (and NLC) in the mesosphere and may not be valid. Our modeling shows that this water was of tropospheric origin (it has been lifted by the rising plume) and is not the result of comet vaporization in atmosphere. The type of the Tunguska object can not be defined without geochemical analysis of its material, which has not been identified so far. The average concentration of this widely dispersed material (see Fig.1) is comparable with the annual flux of cosmic spherules onto the Earth. It may probably be detected in the regions with extremely accurate stratigraphic records (glaciers in Greenland could possibly be candidates). Nowadays Tunguska-sized objects represent the most hazardous cosmic bodies. The probability of such event is quite high, while the size is too small to predict the collision in advance. Damage on the surface may reach thousands of sq.km. Disturbances in ionosphere may destroy modern telecommunication systems and cause a world-wide chaos.

Artemieva Fig1

Fig. 1 The Tunguska plume evolution. The atmosphere is in blue color, the projectile material (10-100-mkm-diameter particles, resulting from the projectile ablation-condensation) are green and yellow. A) total deceleration of the projectile, the frame is 20x20 km; B and C) 1 and 3 min later, a small square in the bottom left corner corresponds to the A-frame, 400x400 km); D) 10 min later, the plume collapse, 1500km x 400 km.

5. Update of the numerical code
Probably, this is the most boring part of my job. However, it has to be done for scientific progress in the future. The 3D hydroocode SOVA is currently updated to include magneto-hydrodynamic effects (which are important for L. Hood project "Lunar crustal magnetization"). Also spherical geometry has been included to model world-wide distribution of impact ejecta.

Papers:

1. Artemieva N. and Pierazzo E. (2009) The Canyon Diablo impact event: 1. Projectile motion through atmosphere. M&PS 44, 25-42.

2. Artemieva N. and Morgan J. (2009) Modeling the formation of the K-Pg boundary layer. Icarus 201,768-780.

3. Hood L., Ciesla F., Artemieva N., Marzari F., and Weidenschillung (2009) Nebular shock waves generated by planetesimals passing through Jovian resonances: Possible sites for chondrule formation. M&PS 44, 327-342.

4. Kenkmann Th., Artemieva N. and 4 co-authors (2009) The Carancas meteorite impact crater, Peru: Geologic surveying and modeling of crater formation and atmospheric passage. M&PS 44, 985-1000.

Conference abstracts:

1. Artemieva, N. A.; Wünnemann, K.; Meyer, C.; Reimold, W. U.; Stöffler, D. (2009) Ries Crater and Suevite Revisited: Part II Modelling, LPSC-40, abstr. 1526 (oral)

2. Stöffler, D.; Meyer, C.; Reimold, W. U.; Artemieva, N. A.; Wünnemann, K. (2009) Ries Crater and Suevite Revisited: Part I Observations, LPSC-40, abstr. 1504 (oral)

3. Hood, L. L.; Ciesla, F. J.; Artemieva, N. A.; Marzari, F.; Weidenschilling, S. J.(2009) Chondrule Formation in Nebular Shock Waves Generated by Planetesimals Passing Through Jovian Resonances: Relative Importance of Bow Shocks and Impact Shocks, LPSC-40, abstr. 1775 (oral)

Research Year: 
2009
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