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

The Latest Achievements of Physical Geodesy

No abstract availabl

Representation of ethological events by basolateral amygdala neurons

The accurate interpretation of ethologically relevant stimuli is crucial for survival. While basolateral amygdala (BLA) neuronal responses during fear conditioning are well studied, little is known about how BLA neurons respond during naturalistic events. We recorded from the rat BLA during interaction with ethological stimuli: male or female rats, a moving toy, and rice. Forty-two percent of the cells reliably respond to at least one stimulus, with over half of these exclusively identifying one of the four stimulus classes. In addition to activation during interaction with their preferred stimulus, these cells signal micro-behavioral interactions like social contact. After stimulus removal, firing activity persists in 30% of responsive cells for several minutes. At the micro-circuit level, information flows from highly tuned event-specific neurons to less specific neurons, and connection strength increases after the event. We propose that individual BLA neurons identify specific ethological events, with event-specific neurons driving circuit-wide activity during and after salient events

Hippocampal place cells have goal-oriented vector fields during navigation

The hippocampal cognitive map supports navigation towards, or away from, salient locations in familiar environments1. Although much is known about how the hippocampus encodes location in world-centred coordinates, how it supports flexible navigation is less well understood. We recorded CA1 place cells while rats navigated to a goal on the honeycomb maze2. The maze tests navigation via direct and indirect paths to the goal and allows the directionality of place cells to be assessed at each choice point. Place fields showed strong directional polarization characterized by vector fields that converged to sinks distributed throughout the environment. The distribution of these 'convergence sinks' (ConSinks) was centred near the goal location and the population vector field converged on the goal, providing a strong navigational signal. Changing the goal location led to movement of ConSinks and vector fields towards the new goal. The honeycomb maze allows independent assessment of spatial representation and spatial action in place cell activity and shows how the latter relates to the former. The results suggest that the hippocampus creates a vector-based model to support flexible navigation, allowing animals to select optimal paths to destinations from any location in the environment

Stranger Citizens

"Stranger Citizens examines how foreign migrants who resided in the United States gave shape to citizenship in the decades after American independence in 1783. During this formative time, lawmakers attempted to shape citizenship and the place of immigrants in the new nation, while granting the national government new powers such as deportation. John McNelis O'Keefe argues that despite the challenges of public and official hostility that they faced in the late 1700s and early 1800s, migrant groups worked through lobbying, engagement with government officials, and public protest to create forms of citizenship that worked for them. This push was made not only by white men immigrating from Europe; immigrants of color were able to secure footholds of rights and citizenship, while migrant women asserted legal independence, challenging traditional notions of women's subordination. Stranger Citizens emphasizes the making of citizenship from the perspectives of migrants themselves, and demonstrates the rich varieties and understandings of citizenship and personhood exercised by foreign migrants and refugees. O'Keefe boldly reverses the top-down model wherein citizenship was constructed only by political leaders and the courts. Thanks to generous funding from the Sustainable History Monograph Pilot and the Mellon Foundation the ebook editions of this book are available as Open Access (OA) volumes from Cornell Open (cornellopen.org) and other Open Access repositories.

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