Impact-Induced Melting of Near-Surface Water Ice on Mars

All fresh and many older Martian craters with diameters greater than a few km are surrounded by ejecta blankets which appear fluidized, with morphologies believed to form by entrainment of liquid water. We present cratering simulations investigating the outcome of 10 km s–1 impacts onto models of the Martian crust, a mixture of basalt and ice at an average temperature of 200 K. Because of the strong impedance mismatch between basalt and ice, the peak shock pressure and the pressure decay profiles are sensitive to the mixture composition of the surface. For typical impact events, about 50% of the excavated ground ice is melted by the impact-induced shock. Pre-existing subsurface liquid water is not required to form observed fluidized ejecta morphologies, and the presence of rampart craters on different age terranes is a useful probe of ground ice on Mars over time

Shock wave induced vaporization of porous solids

Strong shock waves generated by hypervelocity impact can induce vaporization in solid materials. To pursue knowledge of the chemical species in the shock-induced vapors, one needs to design experiments that will drive the system to such thermodynamic states that sufficient vapor can be generated for investigation. It is common to use porous media to reach high entropy, vaporized states in impact experiments. We extended calculations by Ahrens [J. Appl. Phys. 43, 2443 (1972)] and Ahrens and O'Keefe [The Moon 4, 214 (1972)] to higher distentions (up to five) and improved their method with a different impedance match calculation scheme and augmented their model with recent thermodynamic and Hugoniot data of metals, minerals, and polymers. Although we reconfirmed the competing effects reported in the previous studies: (1) increase of entropy production and (2) decrease of impedance match, when impacting materials with increasing distentions, our calculations did not exhibit optimal entropy-generating distention. For different materials, very different impact velocities are needed to initiate vaporization. For aluminum at distention (m)<2.2, a minimum impact velocity of 2.7 km/s is required using tungsten projectile. For ionic solids such as NaCl at distention <2.2, 2.5 km/s is needed. For carbonate and sulfate minerals, the minimum impact velocities are much lower, ranging from less than 1 to 1.5 km/s

Complex craters: Relationship of stratigraphy and rings to impact conditions

One of the key issues associated with the understanding of large scale impacts is how the observable complex crater structural features (e.g., central peaks and pits, flat floors, ring shaped ridges and depressions, stratigraphic modifications, and faults) relate to the impactor's parameters (e.g., radius, velocity, and density) and the nonobservable transient crater measures (e.g., depth of penetration and diameter at maximum penetration). We have numerically modeled large-scale impacts on planets for a range of impactor parameters, gravity and planetary material strengths. From these we found that the collapse of the transient cavity results in the development of a tall, transient central peak that oscillates and drives surface waves that are arrested by the balance between gravitational forces and planetary strength to produce a wide range of the observed surface features. In addition, we found that the underlying stratigraphy is inverted outside of the transient cavity diameter (overturned flap region), but not inside. This change in stratigraphy is observable by remote sensing, drilling, seismic imaging and gravity mapping techniques. We used the above results to develop scaling laws and to make estimates of the impact parameters for the Chicxulub impact and also compared the calculated stratigraphic profile with the internal structure model developed by Hildebrand et. al. [1998], using gravity, seismic and other field data. For a stratigraphy rotation diameter of 90 km, the maximum depth of penetration is ∼43 km. The impactor diameter was also calculated. From the scaling relationships we get for a 2.7 g/cm^3 asteroid impacting at 20 km/s, or a 1.0 g/cm^3 comet impacting at 40 km/s, an impactor diameter of ∼13 km, and for a comet impacting at 60 km/s, an impactor diameter of ∼10 km

Impact cratering: The effect of crustal strength and planetary gravity

Upon impact of a meteorite with a planetary surface the resulting shock wave both ‘processes’ the material in the vicinity of the impact and sets a larger volume of material than was subjected to high pressure into motion. Most of the volume which is excavated by the impact leaves the crater after the shock wave has decayed. The kinetic energy which has been deposited in the planetary surface is converted into reversible and irreversible work, carried out against the planetary gravity field and against the strength of the impacted material, respectively. By using the results of compressible flow calculations prescribing the initial stages of the impact interaction (obtained with finite difference techniques) the final stages of cratering flow along the symmetry axis are described, using the incompressible flow formalism proposed by Maxwell. The fundamental assumption in this description is that the amplitude of the particle velocity field decreases with time as kinetic energy is converted into heat and gravitational potential energy. At a given time in a spherical coordinate system the radial velocity is proportional to R^(−z), where R is the radius (normalized by projectile velocity) and z is a constant shape factor for the duration of flow and a weak function of angle. The azimuthal velocity, as well as the streamlines, is prescribed by the incompressibility condition. The final crater depth (for fixed strength Y) is found to be proportional to R_0[2(z + 1)u_(or)²/g]^(1/(z+1)), where u_(or) is the initial radial particle velocity at (projectile normalized) radius R_0, g is planetary gravity, and z (which varied from 2 to 3) is the shape factor. The final crater depth (for fixed gravity) is also found to be proportional to [ρu_(or)^2/Yz]^(1/(z+1)), where ρ and Y are planetary density and yield strength, respectively. By using a Mohr-Coulomb yield criterion the effect of varying strength on transient crater depth and on crater formation time in the gravity field of the moon is investigated for 5-km/s impactors with radii in the 10- to 10^7-cm range. Comparison of crater formation time and maximum transient crater depth as a function of gravity yields dependencies proportional to g^(−0.58) and g^(−0.19), respectively, compared to g^(−0.618) and g^(−0.165) observed by Gault and Wedekind for hypervelocity impact craters in the 16- to 26-cm-diameter range in a quartz sand (with Mohr-Coulomb type behavior) carried out over an effective gravity range of 72–980 cm/s²

Planetary Cratering Mechanics

The objective of this study was to obtain a quantitative understanding of the cratering process over a broad range of conditions. Our approach was to numerically compute the evolution of impact induced flow fields and calculate the time histories of the key measures of crater geometry (e.g. depth, diameter, lip height) for variations in planetary gravity (0 to 10^9 cm/s^2), material strength (0 to 2400 kbar), and impactor radius (0.05 to 5000 km). These results were used to establish the values of the open parameters in the scaling laws of Holsapple and Schmidt (1987). We describe the impact process in terms of four regimes: (1) penetration, (2) inertial, (3) terminal and (4) relaxation. During the penetration regime, the depth of impactor penetration grows linearly for dimensionless times τ = (Ut/a) 5.1, the crater grows at a slower rate until it is arrested by either strength or gravitational forces. In this regime, the increase of crater depth, d, and diameter, D, normalized by projectile radius is given by d/a = 1.3 (Ut/a)^(0.36) and D/a = 2.0(Ut/a)^(0.36). For strength-dominated craters, growth stops at the end of the inertial regime, which occurs at τ = 0.33 (Y_(eff)/ρU^2)^(−0.78), where Y_(eff) is the effective planetary crustal strength. The effective strength can be reduced from the ambient strength by fracturing and shear band melting (e.g. formation of pseudo-tachylites). In gravity-dominated craters, growth stops when the gravitational forces dominate over the inertial forces, which occurs at τ = 0.92 (ga/U^2)^(−0.61). In the strength and gravity regimes, the maximum depth of penetration is d_p/a = 0.84 (Y/ρ U^2)^(−0.28) and d_p/a = 1.2 (ga/U^2)^(−0.22), respectively. The transition from simple bowl-shaped craters to complex-shaped craters occurs when gravity starts to dominate over strength in the cratering process. The diameter for this transition to occur is given by D_t = 9.0 Y/ρg, and thus scales as g^(−1) for planetary surfaces when strength is not strain-rate dependent. This scaling result agrees with crater-shape data for the terrestrial planets [Chapman and McKinnon, 1986]. We have related some of the calculable, but nonobservable parameters which are of interest (e.g. maximum depth of penetration, depth of excavation, and maximum crater lip height) to the crater diameter. For example, the maximum depth of penetration relative to the maximum crater diameter is 0.6, for strength dominated craters, and 0.28 for gravity dominated craters. These values imply that impactors associated with the large basin impacts penetrated relatively deeply into the planet's surface. This significantly contrasts to earlier hypotheses in which it had been erroneously inferred from structural data that the relative transient crater depth of penetration decreased with increasing diameter. Similarly, the ratio of the maximum depth of excavation relative to the final crater diameter is a constant ≃0.05, for gravity dominated craters, and ≃ 0.09 for strength dominated craters. This result implies that for impact velocities less than 25 km/s, where significant vaporization begins to take place, the excavated material comes from a maximum depth which is less than 0.1 times the crater diameter. In the gravity dominated regime, we find that the apparent final crater diameter is approximately twice the transient crater diameter and that the inner ring diameter is less than the transient crater diameter

Cometary and meteorite swarm impact on planetary surfaces

The velocity flow fields, energy partitioning, and ejecta distributions resulting from impact of porous (fragmented) icy cometary nuclei with silicate planetary surfaces at speeds from 5 to 45 km/s are different than those resulting from the impact of solid ice or silicate meteorites. The impact of 1 g/cm^3 ice spheres onto an atmosphereless anorthosite planetary surface induces cratering flows that appear similar to those induced by normal density anorthosite meteorite impact. Both of these impactors lead to deep transient crater cavities for final crater diameters less than ∼1 to ∼10 km and for escape velocities ≲10^5 cm/s. Moreover the fraction of internal energy partitioned into the planetary surface at the cratering site is 0.6 for both ice and anorthosite impactors at 15 km/s. As the assumed density of the hypothetical cometary nucleus or fragment cloud from a nucleus decreases to 0.01 g/cm^3, the fraction of the impact energy partitioned into planetary surface internal energy decreases to less than 0.01, and the flow field displays a toroidal behavior in which the apparent source of the flow appears to emanate from a disc or ringlike region rather than from a single point, as in the explosive cratering case. The edges of the crater region are in several cases depressed and flow downward, whereas the center of the crater region is uplifted. Moreover, the resultant postimpact particle velocity flow in some cases indicates the formation of concentric ridges, a central peak, and a distinct absence of a deep transient cavity. In contrast, transient cavities are a ubiquitous feature of nearly all previous hypervelocity impact calculations. The calculations of the flow fields for low density (0.01 g/cm^3) impactors exhibited surface interface (comet-planet) instabilities. These are attributed to both the Rayleigh-Taylor and Helmholtz instability conditions, and we believe that these occur in all flows involving volatile low-density (0.01 g/cm^3) projectiles. It is speculated that these hydrodynamic instabilities can give rise to concentric rings in the inner crater region in large-scale impacts on planetary surfaces, although other mechanisms for their production may also act. The ejecta mass loss versus planetary escape velocity was computed, and these results imply that the critical escape velocity, at which as much material is lost as is being accreted from a planet, ranges from 1.2 to 2.75 km/s for encounter speeds of 5 to 15 kms/s, with cometary impactors having a density of 0.01 to 1 g/cm^3. These values compare to 0.83 and 1.5 km/s for silicate impactors, thereby indicating that it is more difficult for volatiles than silicates to be accreted onto objects with escape velocities similar to the Moon, Mercury, and Mars. For objects with escape velocities in the 0.1 to 1 km/s range the accretional efficiency for silicate and various porosity ices are similar, whereas for objects with escape velocities <0.1 km/s the accretional efficiency of icy impactors becomes significantly lower than for silicate impactors

Impact of comet Shoemaker-Levy 9 on Jupiter

Three-dimensional numerical simulations of the impact of Comet Shoemaker-Levy 9 on Jupiter and the resulting vapor plume expansion were conducted using the Smoothed Particle Hydrodynamics (SPH) method. An icy body with a diameter of 2 km can penetrate to an altitude of -350 km (0 km = 1 bar) and most of the incident kinetic energy is transferred to the atmosphere between -100 km to -250 km. This energy is converted to potential energy of the resulting gas plume. The unconfined plume expands vertically and has a peak radiative power approximately equal to the total radiation from Jupiter's disc. The plume rises a few tens of atmospheric scale heights in ∼10² seconds. The rising plume reaches the altitude of ∼3000 km, but no atmospheric gas is accelerated to the escape velocity (∼60 km/s)

Impact of an asteroid or comet in the ocean and extinction of terrestrial life

Finite difference calculations describing the impact mechanics associated with a 10 to 30 km diameter silicate or water object impacting a 5 km deep ocean overlying a silicate solid planet at 30 km/sec demonstrate that from 12 to 15% of the bolide energy resides in the water. In the gravity field of the earth some 10 to 30 times the impactor mass of water is launched on trajectories which would take it to altitudes of 10 km or higher. This ejecta launched on trajectories which can achieve stratospheric heights is 10^1 to 10^2 projectile masses, similar to that resulting from impact of objects on an ocean‐free silicate half‐space (continent). As in the case of impact directly onto a continent, only the ejecta composed of impactor material, launched on trajectories which would carry it to stratospheric heights, matches the fraction (10^(−2) to 10^(−1)) of bolide (extraterrestrial) material found in the platinum‐metal‐rich Cretaceous‐Tertiary and Eocene‐Oligocene boundary layers. Oceanic impact results in impulsivelike giant tsunamis initially having amplitudes of ∼4 km, representing the solitary waterwave stability limit in the deep ocean, and containing 10^(−2) to 10^(−1) of the energy of the impact. Using the constraint of no observed turbidites in marine sediments in the Cretaceous‐Tertiary and Eocene‐Oligocene boundary materials (calculated maximum water‐sediment interface particle velocity ∼10° m/sec) implies a maximum impactor energy of ∼10^(28) to ∼10^(29) erg corresponding to a maximum diameter for a silicate impactor of ∼2 km (at 11 km/sec). Minimal global tsunami run‐up heights on the continents corresponding to impacts of this energy are 300–400 m. We speculate that such waves would inundate all low altitude continental areas and strip and silt over virtually all vegetation. As a result, the terrestrial animal food chain would be seriously perturbed. This, in turn, could have caused extinction of large terrestrial animals including the archosaurs

Machine Learning, Human Factors and Security Analysis for the Remote Command of Driving: An MCity Pilot

Conducted under the U.S. DOT Office of the Assistant Secretary for Research and Technology’s (OST-R) University Transportation Centers (UTC) program.Both human drivers and autonomous vehicles are able to drive relatively well in frequently encountered settings, but fail in exceptional cases. These exceptional cases often arise suddenly, leaving human drivers with a few seconds at best to react—exactly the setting that people perform worst in. Autonomous systems also fail in exceptional cases, because ambiguous situations preceding crashes are not effectively captured in training datasets. This work introduces new methods for leveraging groups of people to provide on-demand assistance by coordinating responses and using collective answer distributions to generate responses to ambiguous scenarios using minimal time and effort. Unlike prior approaches, we introduce collective workflows that enable groups of people to significantly outperform any of the constituent individuals in terms of time and accuracy. First, we examine the latency and accuracy of crowd workers in a future state prediction task in visual driving scenes, and find that more than 50% of workers could provide accurate answers within one second. We found that using crowd predictions is a viable approach for determining critical future states to inform rapid decision making. Additionally, we characterize different estimation techniques that can be used to efficiently create collective answer distributions from crowd workers for visual tasks containing ambiguity. Surprisingly, we discovered that the most fine-grained and time-consuming methods were not the most accurate. Instead, having annotators choose all relevant responses they thought other annotators would select led to more accurate aggregate outcomes. This approach reduced human time required by 21.4% while maintaining the same level of accuracy as the baseline approach. These research results can inform the development of hybrid intelligence systems that accurately and rapidly address sudden and rare critical events, even when they are ambiguous or subjective.United States Department of Transportation Office of the Assistant Secretary for Research and TechnologyCenter for Connected and Automated Transportationhttp://deepblue.lib.umich.edu/bitstream/2027.42/156392/4/Machine Learning Human Factors and Security Analysis for the Remote Command of Driving - An Mcity Pilot.pd