H. Jay Melosh

Magma ocean formation due to giant impacts

Current understanding of the last stages of planetary accretion suggests that mass and energy accumulation are dominated by a few large impacts. An important thermal consequence of such impacts is melting and formation of a large melt pond. If the planets isostatic adjustment time scale is short compared to the magma ponds cooling time scale, the melt may be extruded onto the surface and form a magma ocean of approximately uniform depth. Although a giant impact striking at 10–15 km s−1 deposits enough energy to melt the entire planet, the distribution of that energy is important in determining the thermal outcome of the collision. We examine the thermal effects of giant impacts by estimating the melt volume generated by the initial shock wave and corresponding magma ocean depths. Additionally, we examine the effects of the planets initial temperature on the generated melt volume. The Hugoniot curve plotted in pressure-entropy space is used to determine the shock pressure required to completely melt the material. For room temperature dunite, this pressure is about 150 GPa and the partial melt region is narrow. For dunite initially at the solidus temperature, this pressure is about 115 GPa. The partial melt region extends throughout the planet. Once the melting pressure is known, an impact melting model based on the second Hugoniot equation, the linear shock-particle velocity relationship, and the empirical particle velocity-distance relationship is used to estimate the radial distance melting occurred from the impact site. Once the distance is found, the melt regions geometry determines the associated melt volume. Partial melt volume is also estimated. The melt fraction that is not excavated during crater formation is estimated and magma ocean depths resulting from both excavated and retained melt are calculated. The model is also used to estimate the fraction of a planet melted by the initial shock wave. Nominal conditions of the Moon-forming giant impact (projectile/planet mass = 0.14, impact speed = 15 km s−1) generate melting of 30–65% of the planet depending on its initial temperature. Whole planet melting requires projectile/planet mass ratios of > 0.4 for a 15 km s−1 impact if the planet was near its solidus before the impact. Isostatic relaxation may generate a significant volume of additional melting.

Hydrocode simulation of the Chicxulub impact event and the production of climatically active gases

We constructed a numerical model of the Chicxulub impact event using the Chart-D Squared (CSQ) code coupled with the ANalytic Equation Of State (ANEOS) package. In the simulations we utilized a target stratigraphy based on borehole data and employed newly developed equations of state for the materials that are believed to play a crucial role in the impact-related extinction hypothesis: carbonates (calcite) and evaporites (anhydrite). Simulations explored the effects of different projectile sizes (10 to 30 km in diameter) and porosity (0 to 50%). The effect of impact speed is addressed by doing simulations of asteroid impacts (vi = 20 km/s) and comet impacts (vi = 50 km/s). The masses of climatically important species injected into the upper atmosphere by the impact increase with the energy of the impact event, ranging from 350 to 3500 Gt for CO2, from 40 to 560 Gt for S, and from 200 to 1400 Gt for water vapor. While our results are in good agreement with those of Ivanov et al. [1996], our estimated CO2 production is 1 to 2 orders of magnitude lower than the results of Takata and Ahrens [1994], indicating that the impact event enhanced the end-Cretaceous atmospheric CO2 inventory by, at most, 40%. Consequently, sulfur may have been the most important climatically active gas injected into the stratosphere. The amount of S released by the impact is several orders of magnitude higher than any known volcanic eruption and, with H2O, is high enough to produce a sudden and significant perturbation of Earths climate.

Hydrocode modeling of Chicxulub as an oblique impact event

Abstract Since the confirmation that the buried Chicxulub structure is the long-sought K/T boundary crater, numerous efforts have been devoted to modeling the impact event and estimating the amount of target material that underwent melting and vaporization. Previous hydrocode simulations modeled the Chicxulub event as a vertical impact. We carried out a series of three-dimensional (3D) hydrocode simulations of the Chicxulub impact event to study how the impact angle affects the results of impact events. The simulations model an asteroid, 10 km in diameter, impacting at 20 km/s on a target resembling the lithology of the Chicxulub site. The angles of impact modeled are 90° (vertical), 60°, 45°, 30°, and 15°. We find that the amount of sediments (surface layer) vaporized in the impact reaches a maximum for an impact angle of 30° from the surface, corresponding to less than two times the amount of vaporization for the vertical case. The degassing of the sedimentary layer, however, drops abruptly for a 15° impact angle. The amount of continental crust melted in the impact decreases monotonically (a consequence of the decrease in the maximum depth of melting) from the vertical impact case to the 15° impact. Melting and vaporization occur primarily in the downrange direction for oblique impacts, due to asymmetries in the strength of the shock wave with respect to the point of impact. The results can be used to scale the information from the available vertical simulations to correct for the angle of impact. A comparison of a 3D vertical impact simulation with a similar two-dimensional (2D) simulation shows good agreement between vertical 3D and 2D simulations.

Core formation by giant impacts

The timing and mechanisms of core formation remain unresolved questions in our knowledge of the Solar Systems early state. Because no known nebular processes separate iron from silicates during planetesimals formation, it is believed that the terrestrial planets formed from planetesimals containing approximately solar abundances of iron and silicates. At some stage of planetary growth, iron separated from the silicates to form cores. We propose that once a planet reaches a certain minimum mass, large impacts characteristic of late accretion can trigger core formation. Our model overcomes two major difficulties of core formation: large-scale segregation of molten iron into diapirs and displacement of the cold, elastic interior of the planet by the iron. n nLarge, high-speed impacts on relatively large planets (>∼1023 kg) result in melting beyond the transient craters dimensions, forming a large, intact melt region with a radius up to a few times the projectiles radius. Both the iron and silicates melt. Iron rapidly settles to the melt regions base because of its high density. The accumulated iron forms a large, negatively buoyant mass that stresses the underlying material. If the generated stress is larger than the maximum long term stress that the cold interior can support, the iron rapidly flows to the planets center, displacing the cold interior and forming (or adding to) a core. To investigate, we developed an analytical melting model based on the Hugoniot equations, the empirical relationship for the decline of particle velocity with distance, and the linear shock-particle velocity relationship. Using this melting model coupled with a Monte Carlo simulation of accretion, we examined the ability of giant impacts to trigger core formation. The melt forms an intact region only if the melt fraction that is not excavated from the crater is >∼0.5–0.6. This does not occur until a planet is of approximately lunar mass or greater and is struck by a planetesimal of about one-tenth its mass. If the maximum long-term stress that the planets cold interior can withstand is 2 kbar, consistent with conditions at the Earths surface, planets must be approximately the mass of Ceres before impacts induce core formation. However, core formation occurs only if an intact melt region forms at this stage and if planetesimals with impact speeds greater than about 7.25 km sec−1 exist during this stage of accretion, both of which are unlikely. If the approach velocity of the planetesimals is small, the planet must grow to ∼1024 kg (∼16−14 the Earths mass) before a giant impact will induce core formation. If a small fraction of the planetesimals have approach velocities greater than a few kilometers per second, impact-induced core formation occurs in planets with masses between 1 and 6 × 1023 kg. By the time a planet grows to Mars size, it has a nearly 100% probability that a giant impact triggered core formation. A Mercury-mass planet has about a 75% probability. Giant impacts do not explain whole-body differentiation of asteroid-sized planets.

Acoustic fluidization and the extraordinary mobility of sturzstroms

[1] Sturzstroms are a rare category of rock avalanche that travel vast horizontal distances with only a comparatively small vertical drop in height. Their extraordinary mobility appears to be a consequence of sustained fluid-like behavior during motion, which persists even for driving stresses well below those normally associated with granular flows. One mechanism that may explain this temporary increase in the mobility of rock debris is acoustic fluidization; where transient, high-frequency pressure fluctuations, generated during the initial collapse and subsequent flow of a mass of rock debris, may locally relieve overburden stresses in the rock mass and thus reduce the frictional resistance to slip between fragments. In this paper we develop the acoustic fluidization model for the mechanics of sturzstroms and discuss the conditions under which this process may sustain fluid-like flow of large rock avalanches at low driving stresses.

Constraints on the volatile distribution within Shackleton crater at the lunar south pole

Shackleton crater is nearly coincident with the Moon’s south pole. Its interior receives almost no direct sunlight and is a perennial cold trap, making Shackleton a promising candidate location in which to seek sequestered volatiles. However, previous orbital and Earth-based radar mapping and orbital optical imaging have yielded conflicting interpretations about the existence of volatiles. Here we present observations from the Lunar Orbiter Laser Altimeter on board the Lunar Reconnaissance Orbiter, revealing Shackleton to be an ancient, unusually well-preserved simple crater whose interior walls are fresher than its floor and rim. Shackleton floor deposits are nearly the same age as the rim, suggesting that little floor deposition has occurred since the crater formed more than three billion years ago. At a wavelength of 1,064 nanometres, the floor of Shackleton is brighter than the surrounding terrain and the interiors of nearby craters, but not as bright as the interior walls. The combined observations are explicable primarily by downslope movement of regolith on the walls exposing fresher underlying material. The relatively brighter crater floor is most simply explained by decreased space weathering due to shadowing, but a one-micrometre-thick layer containing about 20 per cent surficial ice is an alternative possibility.

Tectonics of mascon loading: Resolution of the strike‐slip faulting paradox

Subsidence of lunar mascon maria, impact basins partly filled with mare basalt and sites of prominent positive gravity anomalies, typically led to the formation of concentric graben (arcuate rilles) around the flanks of the basin, while compressive features (mare ridges) formed in interior regions. Although previous numerical models of the response of the lunar lithosphere to mascon loading predict that an annulus of strike-slip faulting should also have formed around mascon maria, no such faults have been observed. This “strike-slip faulting paradox,” however, arises from an oversimplification of the earlier models. Viscoelastic finite element models of lunar mascon basins that include the effects of lunar curvature, heterogeneous crustal strength, initial stress conditions, and multistage load histories show that the width of a predicted annulus of strike-slip faulting may be small. The use of Andersons criterion for predicting fault styles may also overpredict the width of strike-slip faulting. A faulting-style criterion that takes into account transitional faulting, in which both strike-slip and dip-slip components are present, predicts zones of pure strike-slip faulting that are about half of the width predicted by the Anderson criterion. Furthermore, strike-slip faulting should be observed only in regions in which flexural stresses are sufficient to induce rock failure. However, since stress patterns consistent with strike-slip faulting around mascon loads represent a transition between compressional and extensional provinces, differential stresses tend to be low in these regions and for at least part of this region are not sufficient to induce rock failure. A mix of concentric and radial thrust faulting is observed in some mascon maria, at odds with previous models that predict only radial orientations away from the basin center. This apparent discrepancy may be partly explained by the multistage emplacement of mare basalt units, a scenario that leads to a stress pattern where concentric and radial orientations of thrust faults are equally preferred. Detailed models of the Serenitatis basin indicate a 25-km-thick lunar lithosphere at the time of rille formation and a 75-km-thick lithosphere at the time of late-stage mare ridge formation. The extent of observed mare ridges and the inferred cessation of rille formation around Serenitatis prior to the time of emplacement of the youngest mare basalt units is consistent with the superposition of a global horizontal compressive stress field generated by the cooling and contraction of the lunar interior with the local stresses associated with lithospheric loading.

Crater features diagnostic of oblique impacts: The size and position of the central peak

Using Magellan data, we investigated two crater characteristics that have been cited as diagnostic of oblique impacts: an uprange offset of the central peak in complex craters, and an increasing central peak diameter relative to crater diameter with decreasing impact angle. We find that the offset distribution is random and very similar to that for high-angle impacts, and that there is no correlation between central peak diameter and impact angle. Accordingly, these two crater characteristics cannot be used to infer the impact angle or direction.

The Mechanics of Pseudotachylite Formation in Impact Events

This paper presents a discussion of the basic constraints controlling the formation of pseudotachylites in the rapidly sheared rocks in the vicinity of a large meteorite impact. The prevailing opinion among many geologists is that pseudotachylites are formed by friction melting of rocks and/or shearing associated with differential shock compression of adjacent rock types. Several physical studies of friction melting have shown that, in theory, small amounts of movement (centimeters or less) are capable of producing very thin veins of melted rock. More realistic models suggest that irregularities on the sliding surface of the order of the grain size may still create primary melt veins up to a few millimeters thick. The principal mystery of pseudotachylite formation is not that friction can cause melting, but that it seems to form thick masses of it, meters to tens of meters wide. However, such thick masses ought to preclude melting by reducing the friction between sliding rock masses. I propose that one possible solution to this conundrum is that the melt produced by sliding on narrow shear zones is extruded into the adjacent country rock, thus keeping the sliding surfaces narrow, while creating thick accumulations of melt in adjacent low pressure zones that open at the end of the shear zones. For this mechanism to operate, the melted rock must be fluid enough to extrude from the shear zone during the time available during crater collapse. This places strong constraints on the viscosity and temperature of the melt. This model may be tested by future careful investigation of the geometry of pseudotachylite occurrences.

Impact-induced perturbations of atmospheric sulfur

Abstract Asteroids and comets that are vaporized during hypervelocity impact events can inject large masses of S into the stratosphere where it can potentially affect the radiation budget of the Earth, alter the chemistry of the ozone layer, and eventually be converted to sulfuric acid rain. Relatively small carbonaceous asteroids, 0.3 km in diameter, contain 5 times more S than the entire modern stratosphere and these objects hit the Earth at an average rate of 1 per 10,000 years. Larger impact events, capable of injecting 10 15 g of S into the stratosphere, occur at an average rate of 1 per 1 million years. Calculations indicate there is sufficient O and H in the vapor plumes of most impact events to convert the S to sulfuric acid aerosols. If this conversion occurs, then the larger impact events could depress mean surface temperatures by more than 2°C for 3 years or longer.