15-19 May 2017, Tokyo, Japan IAA-PDC THREE-DIMENSIONAL SIMULATIONS OF OBLIQUE ASTEROID IMPACTS INTO WATER

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1 5th IAA Planetary Defense Conference PDC May 2017, Tokyo, Japan IAA-PDC THREE-DIMENSIONAL SIMULATIONS OF OBLIQUE ASTEROID IMPACTS INTO WATER Galen R Gisler, Tamra Heberling, Catherine S Plesko, and Robert P Weaver Los Alamos National Laboratory, Los Alamos NM USA galengisler@mac.com, theberling@lanl.gov, plesko@lanl.gov, rpw@lanl.gov Keywords: Asteroid ocean impacts, hydrocode simulations, impact wave generation ABSTRACT Waves generated by impacts into oceans may represent the most significant danger from near-earth asteroids and comets. For impacts near populated shores, the crown splash and subsequent waves, accompanied by sediment lofting and high winds, are more damaging than storm surges from the strongest hurricanes. Asteroids less than 500m in diameter, impacting deep water far from shores, produce waves that may be detectable over large distances, but are probably not significantly dangerous. We present new three-dimensional simulations of oblique impacts into deep water, with trajectory angles ranging from 27 degrees to 60 degrees (where 90 degrees is vertical). These simulations are performed with the Los Alamos Rage hydrocode, and include atmospheric effects including ablation and airbursts. These oblique impact simulations are specifically performed in order to help determine whether there are additional dangers from the obliquity of impact not covered by previous two-dimensional studies. Water surface elevation profiles, surface pressures, and depth-averaged mass fluxes within the water are prepared for use in propagation studies. Introduction Tsunamis have been considered to be the most important hazard from asteroid impacts into the ocean. Yet, although oceans cover three-quarters of the Earth s surface, the geological evidence for tsunamis from impacts has been scarce. The suggestions previously presented have been hotly contested (Masse et al. 2006, Pinter and Ishman 2008). The shallow-water impacts at Mjølnir north of Norway (Tsikalas 2005) and at Montagnais off Nova Scotia (Dypvik and Jansa 2003) should have left tsunami deposits on nearby shores, but searches for such deposits have been inconclusive (Dypvik et al. 2006). Of course there are well-known tsunami deposits associated with the very large Chicxulub event at the end of the Cretaceous period, but these were likely caused by submarine slope failures subsequent to the massive impact on the continental shelf (Matsui et al. 2002). In the planetary defense community, far-field propagation of tsunamis from oceanic impacts had been thought to be the principle danger from the impact of small (<500 m diameter) asteroids (Ward and Asphaug 2000, Chesley and Ward 2006). Other 1

2 work (Strelitz 1979, Melosh 2003, Gusiakov 2007, Wünnemann et al. 2007, Gisler et al. 2003, Gisler 2007, 2008) has downplayed the risk of far-field tsunamis from small asteroids. A classical tsunami is a wave with amplitude small compared to the ocean depth and wavelength very much greater than the depth. Earthquake-generated tsunamis can have wavelengths greater than 100 km while having mid-ocean amplitudes of centimeters or less. In contrast, waves produced by impacts have wavelengths only on the order of the transient crater size (see, e.g. Strelitz 1979). It is the very long wavelengths that pose the greatest danger from tsunamis: the water just keeps on coming. Waves on water are dispersion-free only for wavelengths greater than about 20 times the water depth; in effect, this is the limit of validity for the shallow-water equations. Classical tsunamis propagate at the shallow-water wave speed, obtained from the eigenvalues of the shallow-water equations, ± (gd), where g is the acceleration due to gravity and D is the ocean depth. In shallower depths near shores, the waves slow and pile up (this is called shoaling ). Our Earlier Calculations In previous work (Gisler et al. 2010) we showed that the danger of tsunamis at a location far from the impact site of a small asteroid is minimal and that the true danger lies in the local and mainly atmospheric effects. Just as in impacts of small asteroids on land, it is the near-field effects that are most concerning. That work and the present work use the SAGE/RAGE hydrocode, a multi-material finite-volume Eulerian code with a high-resolution Godunov scheme originally developed by Michael Gittings (Gittings et al. 2008) and subsequently adapted at Los Alamos National Laboratory. The grid is continuously refined, cell-by-cell and cycle-by-cycle, throughout the problem run, focusing computational resources on the places in the grid where the action is most dynamic. The code makes use of tabular equations of state from the LANL Sesame library (Holian 1984, Lyon and Johnson 1992) and a high-quality equation of state for water provided by SAIC and derived from the NBS/ NRC Steam Tables (Haar et al. 1984). In Gisler et al we reported a series of axisymmetric calculations of asteroid impacts into 5-km deep water, with the asteroid diameter varying from 100 meters to 1000 meters. The assumption of axisymmetry restricted consideration to vertical impacts. The results from that study are displayed in Table 1. The inputs for those runs were the ocean depth (5 km of water, underlain by sediment), the asteroid speed (20 km/s), density (2.63 g/cm -3 ), and asteroid diameter, shown in the first row of Table 1. The second row is the calculated kinetic energy in GigaTons TNT equivalent, and subsequent rows are measured from the outputs of the calculations. In yellow are highlighted the cases in which the transient crater penetrates the layer of sediment at the bottom of the sea. Only in these cases could one hope to observe a sea-floor crater, but subsequent sediment remobilization would obscure it relatively quickly. The cells highlighted in magenta represent near-field dangers. Wave heights at 30 kilometers (row 5) are hazardous even for the smallest of these impactors; for the 2

3 largest, they could induce flooding several kilometers inland. These initial waves, being of relatively short wavelengths, are more like storm surges than classical tsunamis. Rows 6-8 illustrate dangerous atmospheric effects: hurricane-force winds produced by impacts can extend more than 30 kilometers from the site of an impact of 100 meters or greater; places within more than 30 kilometers from an impact of 200 meters or greater would experience direct fallout from the initial splash megatons of water excavated from the transient crater falling back to the surface at terminal velocity; and an impact of 600 meters or greater can raise air temperatures to greater than 100 C more than 30 kilometers away. Table 1. Results from vertical-impact calculations in Gisler et al asteroid diameter (m) kinetic energy (GT) transient crater diameter (km) transient crater depth (km) wave height at 30 km (m) extent of hurricane force winds (km) fallout zone radius (km) air temp > 100 C (km) wavelength (km) wave speed (m/s) Finally, the last two rows give characteristics of the resultant wave produced by an impact, as measured from tracer particles distributed initially along the unperturbed surface of the water at distances of tens of kilometers from the impact site. Wavelengths produced by these impacts are only on the order of twice the diameter of the transient crater (comparing rows 3 and 9) and are always less than 5 times the 5-km ocean depth for impacts of less than 1 kilometer in diameter. Only the largest impacts in Table 1 approach within 3/4 of the shallow-water wave speed, 221 km/s in a 5-km deep ocean. The cells highlighted in cyan are the cases in which one might be concerned about setting up a long wave. 3

4 A second set of calculations also presented in Gisler et al. 2010, found that the nearfield effects are even more dangerous in the case of impacts into shallower water (continental shelves, for example), being compounded by the excavation of sea-floor sediment that is subsequently hurled onto coastal structures with hurricane force. The New Calculations The calculations presented in the present study are performed for oblique impacts. We had argued previously (Gisler 2007, Gisler et al. 2010) that two-dimensional axisymmetric calculations were sufficient for studying wave development because the explosive vaporization (of water and asteroid) rapidly symmetrizes the transient crater produced by an oblique impact, resulting in a wave with cylindrical symmetry (Gisler et al. 2003). Focussing on atmospheric and near-field effects for smaller impacts, however, requires us to consider the complications of oblique impacts. An asteroid on an oblique trajectory passes through, and preheats, much more of the atmosphere, spreading effects over a wider area, while at the same time suffering greater ablation from the passage. Oblique impacts are also more likely to produce airbursts whose explosive effects can damage property, as occurred in the Chelyabinsk event of 15 February 2013 (Popova et al. 2013). Moreover, downrangeuprange asymmetries may have consequences for civil defense measures in coastal communities near the impact site. Relaxing the assumption of axisymmetry results in considerably more complex simulations, requiring much more computing power. For angles low to the horizon, the computational domain must be lengthened considerably to encompass the trajectory. Some of the new runs have exceeded 4 billion computational cells and have taken months of wall-clock time to conclude. Further, the computational data produced by fully three-dimensional runs is enormously greater than in twodimensional axisymmetric runs and is therefore much more difficult to store, curate, post-process, and analyze. More powerful and more efficient tools and methods for extracting knowledge from the data are being developed (Patchett et al. 2017; Samsel et al. 2017). Input variables for the new set of oblique-impact runs are presented in Table 2, with the airburst runs shaded in magenta. The nomenclature for these runs is as follows: the initial letter (y or x) refers to the initial asteroid speed (x at 16.7 km/s, y at 20 km/ s). The second letter (A,B,C, or D) allows for an artificial airburst produced by converting 10% of the asteroid kinetic energy into thermal energy as the asteroid passes through a specific altitude (D at 15 km, C at 10 km, B at 5 km, and A with no airburst). The number in third position indicates the asteroid diameter (5 is 500 meter, 3 is 250 meter, 1 is 100 meter), and the number in fourth position refers to the angle of the trajectory (2 at 60 degrees, 1 at 45 degrees, 0 at 27 degrees). The lowest obliquity was chosen to approximately represent the trajectory of the fictitious asteroid 2017PDC on an ocean impact east of Japan. The smaller of the two 27- degree trajectories was also set into a water depth of 11 km, as the deepest part of the Japan trench, while all other runs are done with a standard water depth of 5 km corresponding to the abyssal plain in the Pacific Ocean. 4

5 Table 2. Input parameters for the new oblique-impact calculations run name airburst altitude asteroid size (m) trajectory angle water depth (km) asteroid kinetic energy (MT) ya ya yd ya yb yc ya yb yc xa xa I briefly describe the phenomenology of these runs, broadly similar for all of them, and illustrated in Figures 1 and 2 in the case of ya31. The spherical, homogeneous, and strengthless asteroid is initialized with specified velocity at an altitude of 20 km in a static atmosphere in hydrostatic equilibrium. The atmospheric pressuretemperature-density profile is based on the US Standard Atmosphere (NASA 1976). The asteroid s passage through the atmosphere creates an evacuated channel in its wake. Some ablation of the asteroid s mass deposits a trail of asteroid material within the wake. In the event of an airburst, most of the asteroid s mass is scattered nearly isotropically. On impact with the water, the asteroid (or what remains of it) is very quickly vaporized. Much of it is expelled up the evacuated wake, the rest scattered fairly isotropically with respect to the asteroid s momentum. Because the energy per unit mass of the impacting body is much greater than the latent heat of vaporization of water, a very large quantity of water, significantly larger than the mass of the impactor, is instantly vaporized. The energy of this explosion creates a transient cavity in the water, whose depth and width depend on the characteristics of the impact. Much of the vaporized water is lofted up into the stratosphere. 5

6 a 1.25 sec Run ya b 1.60 sec e 95.0 sec 0 Log Density d c 43.0 sec sec 5 km Figure 1. Montage of snapshots from a slice along the asteroid trajectory for run ya31. All frames have the same spatial scale and log-density color palette. a) Atmospheric passage, showing evacuated wake. b) Contact with the water. c) Transient crater and splash curtain. d) Rebound jet, collapse of splash curtain, and lofting of vapor into the stratosphere. e) Breaking rim wave and second rebound sec Figure 2. Volume rendering of water volume fraction (top) and pressure (bottom) for run ya31 at 16.6 seconds. Water vapor is ejected along the evacuated channel left by the asteroid s trajectory and upward into the stratosphere. The pressure wave from the vaporization shock is weaker in the uprange direction, but the shock from the atmospheric passage adds to it. 6

7 Water that is not vaporized is expelled from the cavity by the force of the explosion, leading to the formation of a splash curtain around the transient crater. This curtain is asymmetric, avoiding the uprange side of the crater, and highest on the downrange side. The splash curtain extends several kilometers up into the atmosphere, but remains liquid. It subsequently falls to the surface at speeds near terminal velocity, causing secondary splashes. If near a populated coast, these would cause significant flooding and damage to human infrastructures. Table 3. Output measurements for the runs of Table 2 run name water KE air KE splash height (m) transient crater depth (m) central jet height (m) initial rim wave height (m) water in stratosphere (10 9 kg) ya % 2.2% ya % 7.0% yd % 20.5% ya % 9.9% yb % 18.8% yc % 18.0% ya % 19% yb % 19.7% yc % 20.9% xa % 12.9% xa % 14.3% The transient cavity, which in these small-asteroid runs never reaches the seafloor, is refilled by water rushing in from the sides. Meeting in the middle of the cavity, the water then jets upward a few kilometers. The collapse of this central jet leads to a highly turbulent and dissipative rim wave that propagates. Subsequent damped rebounds produce a relatively short train of waves, nearly circularly symmetric. For more oblique impacts, the downrange portion of the wave train has noticeably higher amplitude than the uprange portion, and typical surface elevation contours appear slightly elliptical. Most of the runs of Table 2 have progressed far enough that the initial impact splash, the transient crater, and the central jet have subsided and the subsequent rim wave 7

8 has begun to develop and propagate. The exceptions are the two low-angle runs, in which the central jet is still developing. A selection of output quantities from the runs of Table 2 is presented in Table 3. The numbers in columns 2 and 3 are expressed as percentages of the initial asteroid kinetic energy delivered to the water and the atmosphere respectively. The rest of the energy goes into heat. Essentially no transient water craters were produced by any of the 100-meter diameter asteroid runs (yd12, ya11, yb11, yc11) whether or not airbursts were included, because very little asteroid material survived the passage through the atmosphere, even at the 60-degree entry angle of yd12. Further, the 300-meter diameter runs with airbursts (yb31, yc31) were considerably less effective than the corresponding non-airburst run (ya31) at delivering kinetic energy to the water, excavating a crater, and generating a wave. Comparing nonairburst runs of the same diameter asteroid but different angles of entry (ya52 and xa50; ya32, ya31, and xa30) we see that the trajectories with steeper entry angles deliver less kinetic energy to the atmosphere, more kinetic energy to the water, and excavate deeper transient craters, as expected. Also not surprising is that the airburst runs deliver orders of magnitude more energy to the atmosphere than to the water. The Table lacks data for jet height and rim wave for the 27-degree angle runs xa30 and xa50 because at this writing they are still making progress. Figure 3. Slice plots in the trajectory plane illustrating pressure (bluegreen) and asteroid volume fraction (yellow-red) shortly after the time of impact for the three runs ya31, yb31, and yc31. Cratering is most efficient in the non-airburst run ya31, with a strong pressure pulse in the water heading toward the seafloor. Pressure pulses in the other two runs are more diffuse in both the atmosphere and the water. Asteroid material in all three cases is seen streaming up the evacuated channel of the wake. In the non-airburst case the remaining asteroid material seems to be nearly specularly reflected off the cavity, while in the airburst cases it splays out on the downrange side. 8

9 The remarks above regarding cratering efficiency and airburst are best illustrated by referring to Figure 3, a direct comparison of the three 45-degree 300-meter diameter runs. The initial rim wave heights given in Table 3 are much larger than the wave heights at 30 kilometers given in Table 1 for the axisymmetric runs. These are not directly comparable, however, due to dissipation that occurs during the initial propagation. Computational demands preclude the long-time calculations necessary for the waves to reach 30 kilometers in the 3-dimensional runs. However, very similar initial rim wave heights were seen in the axisymmetric runs at similar asteroid diameters Airburst kinetic energy (erg) Kinetic Energy kinetic energy (erg) Impact ya m asteroid no airburst 45 degree trajectory asteroid air water yb m asteroid 5 km airburst 45 degree trajectory asteroid air water time (sec) time (sec) change in internal energy (erg) ya m asteroid no airburst 45 degree trajectory Thermal Energy Gain asteroid air water change in internal energy (erg) yb m asteroid 5 km airburst 45 degree trajectory asteroid air water time (sec) time (sec) Figure 4. Tallies of kinetic energy and thermal energy gain versus time for two asteroid impact runs. Left panels are for the non-airburst run ya31, right for the corresponding yb31 run with an airburst at 5 km altitude. In each panel the top frame is kinetic energy versus time, the bottom is the gain in thermal energy versus time. Dashed lines showing the time of airburst and impact are shown for yb31; the impact time in ya31 is the time of the sudden rise of the water s kinetic energy. The kinetic energy deposition efficiencies in columns 2 and 3 of Table 3 are obtained by querying the hydrocode output at frequent intervals for mass and energy statistics for each material component of the problem. A graphical illustration of these tallies 9

10 for the two runs ya31 and yb31, including also the change in internal energy of each material component, is given in Figure 4. The deposition of water vapor into the stratosphere, recorded in the 8th column of Table 3, and illustrated in Figure 5 for ya31, was computed by recording the quantity of water vapor that ascended above the 7 km level as a function of time. (Missing values are still being computed at this writing.) In all cases, water vapor was continuing to ascend above this level as the calculations were terminated. We regard the reported amounts as lower limits. The stratosphere is normally very dry, so the sudden local injection of half a billion metric tons of water may have some significant regional effects, leading to cooling or warming depending on whether condensation into cirrus subsequently occurs. 20 km 7 km 1 km Figure 5. Volume rendering of water mass fraction 75 seconds after the impact of a 250-meter diameter asteroid into a 5-km depth ocean, from run ya31. This run results in a quarter of a billion metric tons of water vapor into the stratosphere, possibly producing regional climate effects. Near-field tropospheric effects from oceanic impacts can be more severe and exhibit interesting uprange/downrange differences. In run ya31, surface temperatures 5 km downrange from the impact point can briefly be above 1200 C as the initial shock passes. At 30 seconds after impact, the shock passes 20 km with a local downrange surface temperature of 80 C and an uprange surface temperature of 58 C. After a full minute, temperatures exceeding 55 C still extend 7 km downrange and 3 km uprange, accompanied by horizontal wind speeds of 30 m/s, hurricane force, inwards 10

11 toward the impact point, and even greater speeds upward. The distribution of total wind speeds at 1 km altitude in this run is illustrated in Figure 6. Local pressures both uprange and downrange at 5 km distance from the impact point briefly exceed 10 atmospheres as the shock from the explosive vaporization of water passes. By the time the shock has passed the 10 km distance, the uprange region sees additional pressure from the wake shock. The combination of the two shocks uprange give a total surface pressure of 4.4 atmospheres at 10 km, while the downrange shock at the same distance gives a slightly smaller surface pressure of 4.05 atmospheres. Figure 6. Illustration of wind speeds at approximately 1 km altitude in run ya31, 43 seconds after impact. The numbers 1 5 in the WindSpeed legend refer to the Saffir-Simpson scale of hurricane force, the lower boundaries of which are 33 m/s, 43 m/s, 49 m/s, 58 m/s, and 70 m/s. In magenta are wind speeds greater than 200 m/s. Near the impact point, the airflow is directed strongly upward, similar to the updraft associated with mushroom clouds. Downrange, where the most violent winds are, is to the right. In all the runs presented here, the asteroid is assumed strengthless. Competent rocks have cohesive strengths on the order of 100 MPa or less. The ram pressure experienced by an asteroid passing through the atmosphere at 20 km/s exceeds this strength already at 15 km altitude. Since most asteroids are thought not to be fully competent, the strengthless assumption may in general be roughly valid. Indeed airbursts are thought to occur for fairly competent asteroids when they encounter ram pressure in excess of their cohesive strength, and these airbursts are usually at much higher altitudes. Because the non-airbursting case is of interest, we use our simulations to study atmospheric ablation of a strengthless asteroid as it proceeds 11

12 through the atmosphere. This study is just beginning now. As an illustration, we specifically examine the low-angle and massive xa50 run of Table 1. Figure 7 is a volume rendering showing the asteroid a fraction of a second before impact. The initially spherical asteroid has pancaked and has shed ~15% of its initial kinetic energy and ~5% of its mass. 1 km Figure 7. Volume rendering of asteroid partial density shortly before impact in run xa50. Roughly 95% of the mass of the initially spherical and strengthless asteroid remains in the pancaked core, while the rest stretches out as a tail into the evacuated wake. The asteroid still retains some 85% of its initial kinetic energy. Conclusions An ocean impact within a few tens of kilometers of a populated coastline would be devastating. The crown splash and rebounding jet reach many kilometers into the air and will lead to severe flooding. High temperatures generated by the disintegration of the asteroid accompanied by hurricane-force winds will be destructive to lives and property on shore. Shock waves from airbursts are also locally destructive, as in the Chelyabinsk and Tunguska events. Because asteroid impacts produce high-amplitude but short waves, propagation into the far field is not expected to be efficient; this conclusion is supported by wavepropagation studies. Airbursts produce pressure fields over wider regions, and had been thought capable of generating propagating waves, but the amplitudes are much lower than in direct impacts, and the wavelengths are not significantly longer. 12

13 A large fraction of the impacting asteroid's kinetic energy is consumed by the vaporization of water from the transient crater. Much of this water vapor is buoyantly lofted into the stratosphere, where it may linger for months to years. Because water vapor is a potent greenhouse gas, there may be significant effects on climate. The range of impactor size considered, namely 100m to 500m diameter asteroids, is found (as expected) to bracket the threshold for danger being considered by NASA. The ongoing searches for hazardous near-earth objects should continue down to 140m diameter. The results of the simulations presented here have now been made available for unlimited release in visualization-accessible data formats (Patchett and Gisler 2017). Acknowledgments We are grateful for the assistance of the consultants at Los Alamos National Laboratory, who have helped us overcome numerous obstacles in running these enormous codes. John Patchett assisted with assimilating the vast quantities of data into manageable chunks, Boonthanome Nouanesensy computed the injection of water vapor into the stratosphere, and a team of computer graphics experts led by him and Francesca Samsel of the University of Texas have produced several of the figures for this paper and the movies for the accompanying presentation. This work is performed under an Inter-Agency Agreement between DOE-NNSA and NASA, and is part of DOE s program in Advanced Scientific Computing. References: Chesley S. R. and Ward S. N. (2006) A Quantitative Assessment of the Human and Economic Hazard from Impact-generated Tsunami. Natural Hazards 38, Dypvik H. and Jansa L. F. (2003) Sedimentary signatures and processes during marine bolide impacts: a review. Sedimentary Geology 161, Dypvik H., Smelror M., Sandbakken P. T., Salvigsen O., and Kalleson E. (2006) Traces of the marine Mjølnir impact event. Palaeogeography, Palaeoclimatology, Palaeoecology 241, Gisler G. R., Weaver R. P., Mader C. L., and Gittings M. L. (2003) Two and three dimensional simulations of asteroid ocean impacts. Science of Tsunami Hazards 21, 119. Gisler G. R. (2007) Tsunamis from asteroid impacts in deep water. Planetary Defense Conference Gisler G. R. (2008) Tsunami simulations. Annual Review of Fluid Mechanics 40, Gisler G. R., Weaver R.P., Gittings M.L., (2010). Calculations of asteroid impacts into deep and shallow water. Pure and Applied Geophysics, 168, Gittings M.L., Weaver R.P., Clover M., Betlach T., Byrne N., et al., (2008). The RAGE radiation-hydrodynamic code, Los Alamos National Laboratory Report LA-UR , Los Alamos, New Mexico, published in Computational Science and Discovery 1,

14 Gusiakov V. K. (2007) Tsunami as a destructive aftermath of oceanic impacts. In Comet/ asteroid impacts and human society (eds. P. T. Bobrowsky and H. Rickman), pp Springer, Berlin Haar, L., Gallagher, J.S., Kell, G. S., 1984, NBS/NRC Steam Tables: Thermodynamic and Transport Properties and Computer Programs for Vapor and Liquid State of Water in SI Units, McGraw-Hill, ISBN Holian K. S. (1984) T-4 Handbook of material properties data bases: Vol 1c: equations of state. In Los Alamos National Laboratory Reports, Los Alamos, NM. Lyon S. P. and Johnson J. D. (1992) Sesame: the Los Alamos National Laboratory equation of state database. In Los Alamos National Laboratory Reports, Los Alamos, NM. Masse W., Bryant E., Gusiakov V. K., Abbott D., Rambolamana G., Raza H., Courty M., Breger D., Gerard-Little P., and Burckle I. (2006) Holocene Indian Ocean cosmic impacts: the megatsunami chevron evidence from Madagascar. EOS Trans. AGU Fall Meet. Suppl. 87, Abstract PB43B Matsui T., Imamura F., Tajika E., Nakano Y., and Fujisawa Y. (2002) Generation and propagation of a tsunami from the Cretaceous/Tertiary impact event. In Catastrophic events and mass extinctions: Impact and beyond (eds. C. Koeberl and G. Macleod), pp Melosh H. J. (2003) Impact generated tsunamis: An over-rated hazard. In Lunar and Planetary Science Conference XXXIV, pp National Aeronatics and Space Administration (1976) U.S. Standard Atmosphere, U.S. Government Printing Office, Washington, D.C., Patchett, J. M., Gisler G. R. (2017) Deep Water Impact Ensemble Data Set, Los Alamos National Laboratory Tech Report LA-UR Patchett, J. M., Nouanesengsy, B., Gisler, G. R., Ahrens, J. P., Hagen, H. J., (2017), In Situ and Post Processing Workflows for Asteroid Ablation Studies (in press), Computer Graphics Forum. Pinter N. and Ishman S. E. (2008) Impacts, megatsunami, and other extraordinary claims. GSA Today 18, Popova O., Jenniskens P. et al. (2013) Chelyabinsk airburst, damage assessment, meteorite recovery, and characterization. Science, 29 Nov 2013, Samsel, F., Patchett, J. M., Rogers, D.H., Tsai, K., (2017), Employing Color Theory to Visualize Volume-rendered Multivariate Ensembles of Asteroid Impact Simulations (in press), CHI 17 Extended Abstracts. Strelitz, R. (1979). Meteorite impact in the ocean. In Lunar and Planetary Science Conference Proceedings Vol. 10, pp Tsikalas F. (2005) Mjølnir Crater as a result of oblique impact: asymmetry evidence constrains impact direction and angle. In Impact Studies (Impact Tectonism) (eds. C. Koeberl and H. Henkel), pp Springer, Berlin 14

15 Ward S. N. and Asphaug E. (2000) Asteroid impact tsunami: a probabilistic hazard assessment. Icarus 145, Wünnemann K., Weiss R., and Hoffman K. (2007) Characteristics of oceanic impact-induced large water waves reevaluation of the tsunami hazard. Meteoritics and Planetary Science 42, This document is designated LA-UR in the Los Alamos National Laboratory Report Series. 15