Impact Craters on Asteroids: Does Gravity or Strength Control Their Size? (original) (raw)

Impact Fragmentation: From the Laboratory to Asteroids

Icarus, 1998

escape velocity, and thus we fail to actually form the separate bodies comprising the Eos family, and are left instead with a In this paper, we study the effect of target size on the fragsingle rubble pile conglomerate. © 1998 Academic Press mentation outcome of rock targets using a 2D numerical hy-Key Words: asteroids; impact processes. drocode. After comparing our hydrocode calculations to laboratory data (including explosive disruption experiments) to validate the results, we use the code to calculate how the critical 1. INTRODUCTION specific energy (Q*) needed to catastrophically fracture a body varies with target size in the regimes not accessible to experi-To characterize the collisional process and how it modment. Impact velocity is generally kept constant at about 2.0 km s ؊1 , although some higher velocity (ȁ5 km s ؊1) simulations ifies objects in our Solar System, various basic concepts were run to determine a velocity dependence for the fragmentaneed to be addressed. To understand asteroid collisional tion outcome. To reflect the asteroid population, target diameevolution, to accurately assign collisional lifetimes to asterters range from 10 cm to 1000 km, spanning the regimes where oids, small satellites, and meteorites, to discern how asterstrength and self-gravity (radially varying lithostatic stress) oid families are formed, and to interpret the cratered sureach dominate resistance to fragmentation. We find that there faces of asteroids, requires that we quantify a body's is a significant difference in fragmentation outcome when the resistance to fragmentation-i.e., the energy necessary to lithostatic stress is included in the computations. As expected, shatter it in an impact event. Further, for larger asteroids, surface layers fragment more easily, while the strength of the it is not just the energy necessary for fragmentation that central regions is greatly enhanced. characterizes the outcome of an impact event, but the We derive the Q* versus size relationship for three materials, energy required to fragment and disperse the body that is (basalt, strong-, and weak-cement mortar) each having different the key to its ultimate fate (Melosh and Ryan 1997). Asterstatic compressive strengths and representing a range of asteroid families are believed to be formed by disruption and oid materials. The hydrocode results showed that Q* decreased with increasing target size in the strength regime, with slopes dispersal of large parent bodies, but if the fragments do not of 0.43, 0.59, and 0.6 for basalt, strong and weak mortar, respechave enough energy to overcome their mutual gravitational tively. This decrease is directly related to the decrease in strain attraction, a single object with a rubble-pile structure is rate as target size grows. In the gravity regime, Q* increases formed instead. with increasing target size, with a slope equal to 2.6 for all This paper examines the fragmentation of target bodies three of the materials modeled. These values are much steeper of a wide range of sizes, from meter-sized and smaller than those previously derived from scaling theories. bodies (applicable to impacting planetary ring particles) Ejecta velocity distributions as a function of target size are to kilometer-sized objects representing colliding asteroids. examined as well. For large bodies, resultant ejecta speeds tend We use a two-dimensional numerical hydrocode to examto be well below escape velocity, implying that these asteroids ine impact fragmentation. Laboratory impact experiment are likely to be reaccumulated rubble piles. In simulating the data and explosive disruption experiments (Housen et al. creation of the asteroid family Eos, we find that the code-1991) are used to test hydrocode simulations of impact calculated fragment size distribution is similar in character to events at small size scales. We then apply the hydrocode the observed data, but secondary fragment sizes are signifito modeling large-scale impact events outside the reach of cantly underestimated. More importantly, the determined ejecta speeds were too low for these fragments to have achieved experimental analysis.

Dynamic tensile strength of terrestrial rocks and application to impact cratering

Meteoritics & Planetary Science, 2004

Dynamic tensile strengths and fracture strengths of 3 terrestrial rocks, San Marcos gabbro, Coconino sandstone, and Sesia eclogite were determined by carrying out flat-plate (PMMA and aluminum) impact experiments on disc-shaped samples in the 5 to 60 m/sec range. Tensile stresses of 125 to 300 MPa and 245 to 580 MPa were induced for gabbro and eclogite, respectively (with duration time of ~1 µs). For sandstone (porosity 25%), tensile stresses normal to bedding of ~13 to 55 MPa were induced (with duration times of 2.4 and ~1.4 µs). Tensile crack failure was detected by the onset of shock-induced (damage) P and S wave velocity reduction. The dynamic tensile strength of gabbro determined from P and S wave velocity deficits agrees closely with the value of previously determined values by post-impact microscopic examination (~150 MPa). Tensile strength of Coconino sandstone is 20 MPa for a 14 µs duration time and 17 MPa for a 2.4 µs duration time. For Sesia eclogite, the dynamic tensile strength is ~240 MPa. The fracture strength for gabbro is ~250 MPa, ~500 MPa for eclogite, and ~40 MPa for sandstone. Relative crackinduced reduction of S wave velocities is less than that of post-impact P wave velocity reductions for both gabbro and eclogite, indicating that the cracks were predominantly spall cracks. Impacts upon planetary surfaces induce tensile failure within shock-processed rocks beneath the resulting craters. The depth of cracking beneath impact craters can be determined both by seismic refraction methods for rocks of varying water saturation and, for dry conditions (e.g., the Moon), from gravity anomalies. In principle, depth of cracking is related to the equations-of-state of projectile and target, projectile dimension, and impact velocity. We constructed a crack-depth model applicable to Meteor Crater. For the observed 850 m depth of cracking, our preferred strength scaling model yields an impact velocity of 33 km/s and impactor radius of 9 m for an iron projectile.

Influence of Mechanical Properties Relevant to Standoff Deflection of Hazardous Asteroids

Procedia Engineering, 2013

The National Academy has recently produced reports on the potential hazard and mitigation strategies for near Earth objects (NEO) [1, 2]. The NRC reported to Congress that nuclear explosives are the only current technology to protect the Earth from impact of large asteroids. This is mainly due to difficulties in predicting the impact with high confidence leading to a short time to impact. Thus, the velocity deflection required when they are determined to be potentially hazardous asteroids (PHA) can only be achieved with nuclear explosives. A standoff explosion, without direct contact with the Near Earth Object (NEO) is a robust option for a non-destructive push, which offers several advantages. We have investigated the efficiency of energy deposition and its dependence on porosity and strength properties of NEO. An Eulerian hydrocode (GEODYN) with an interface reconstruction algorithm, wide-range equation of states and a flexible constitutive model library was used for numerical studies. The largest difficulties in predicting deflection velocity and fragmentation of asteroids are related to uncertainty in composition and mechanical properties of NEO. To reduce this uncertainty, we performed simulations of normal impact cratering of an NEO surface and related results with observables such as the crater shapes, the critical crater diameter (necessary to remove previous craters) and the maximum crater size (necessary for asteroid break-up). The velocity distribution of the material ejected from the impact crater is also germane to the asteroid deflection problem where the ablated material provides the thrust resulting in a deflection velocity. We performed parametric studies on how porosity and strength of the asteroid would affect these results.

Impact crater formation: a simple application of solid state physics

This contribution is a first step aiming to address a general question: what can be concluded on impact craters which exist on various planetary system objects, by combining astronomical data and known theoretical results from solid state physics. Assuming that the material of the target body is of crystaline structure,it is shown that a simple calculation gives the possibility of estimating the speed of the impactor responsible for the creation of a crater.A test value,calculated using observed data on the composition of some asteroids,gives a value of the speed in good agreement with results of celestial mechanics. Comment: plain LaTeX,presented at the 6 SREAC meeting Belgrade,September 2009.,and to appear in the proceedings

The approximate ratios between the diameters of terrestrial impact craters and the causative incident asteroids

When a large asteroid of diameter d hits the surface of the Earth, it produces a crater of diameter D. This paper uses the near-Earth asteroid (NEA) size and miss-distance statistics to calculate the rate at which asteroids hit the Earth. Comparison of this with the known rate at which craters have been produced on the Earth's surface indicates that E = 9.1 × 10 24 D 2.59 erg, where E is the kinetic energy of the incident NEA, and D is the diameter of the resulting crater, in km. So the ratio D/d varies from about 8 for the small 0.88-km 'Wolfe Creek type' craters, up to about 16 for craters like Chicxulub, which has a diameter of about 200 km.

Impacts on Asteroids: Fragmentation, Regolith Transport, and Disruption

Icarus, 2001

We use a numerical hydrocode model to examine the outcomes of various size impacts into targets the sizes of Asteroids 951 Gaspra and 243 Ida, which were imaged by the Galileo spacecraft. A shock wave fractures the asteroid in advance of crater excavation flow; thus, for impactors larger than 100 m impacting at 5.3 km s -1 , tensile strength is unimportant in these bodies, whether they are initially intact or are "rubble piles." Because of the shock-induced fracture, impact results are controlled by gravity. Therefore these asteroids are much more resistant to catastrophic disruption than predicted by previous estimates, which had assumed that strength was controlling these processes for rock targets.

Depth of cracking beneath impact craters: New constraint for impact velocity

Both small-scale impact craters in the laboratory and less than 5 km in diameter bowlshaped craters on the Earth are strength (of rock) controlled. In the strength regime, crater volumes are nearly proportional to impactor kinetic energy. The depth of the cracked rock zone beneath such craters depends on both impactor energy and velocity. Thus determination of the maximum zone of cracking constrains impact velocity. We show this dependency for small-scale laboratory craters where the cracked zone is delineated via ultrasonic methods. The 1 km-deep cracked zone beneath Meteor Crater is found to be consistent with the crater scaling of Schmidt (1) and previous shock attenuation calculations.

Planetary impact and shock-induced damage in target rocks

Material Science and Impact Crater Formation

2019

The surfaces of solid objects in our planetary system are dappled with craters. Some of them are due to impacts of various solid projectiles into the surfaces of the objects. A smaller part of these craters is of volcanic origin. %Some of these are of volcanic origin, but others are consequences of impacts of various projectiles into the objects concerned On the Earth,two most often mentioned such events are the "Tunguska event" of 1908. and the impact which led to the formation of the Barringer crater in Arizona. Impact craters are frequently analyzed within the "scaling theory", which is founded on dimensional analysis. The same problem can be treated by using standard laws of material science and condensed matter physics. In this chapter the two approaches will be compared and possibilities for future work indicated to some extent. Some preliminary conclusions concerning an impact into a granular target will be presented.

Numerical simulations of impact crater formation with dilatancy

Journal of Geophysical Research: Planets

Impact-induced fracturing creates porosity that is responsible for many aspects of the geophysical signature of an impact crater. This paper describes a simple model of dilatancy-the creation of porosity in a shearing geological material-and its implementation in the iSALE shock physics code. The model is used to investigate impact-induced dilatancy during simple and complex crater formation on Earth. Simulations of simple crater formation produce porosity distributions consistent with observations. Dilatancy model parameters appropriate for low-quality rock masses give the best agreement with observation; more strongly dilatant behavior would require substantial postimpact porosity reduction. The tendency for rock to dilate less when shearing under high pressure is an important property of the model. Pressure suppresses impact-induced dilatancy: in the shock wave, at depth beneath the crater floor, and in the convergent subcrater flow that forms the central uplift. Consequently, subsurface porosity distribution is a strong function of crater size, which is reflected in the inferred gravity anomaly. The Bouguer gravity anomaly for simulated craters smaller than 25 km is a broad low with a magnitude proportional to the crater radius; larger craters exhibit a central gravity high within a suppressed gravity low. Lower crustal pressures on the Moon relative to Earth imply that impact-induced dilatancy is more effective on the Moon than Earth for the same size impact in an initially nonporous target. This difference may be mitigated by the presence of porosity in the lunar crust. COLLINS ©2014. The Authors. c ≈ 1.2. We also note that bulking observed in the ejecta and the breccia lens at terrestrial craters is typically 15-20% [e.g., Pilkington and Grieve, 1992], consistent with this value representing the minimum bulk density caused by impact-related dilatancy. Measurements of dilatancy suggest that the confining pressure 3 required to suppress dilatancy is ∼10 MPa for sand and ∼100 MPa for a range of sedimentary rocks (see Figure 2); the mean stress (pressure) associated with this suppression will be somewhat higher. Simulations conducted to validate the dilatancy model as part of this work suggest p lim ≈ 50 MPa for sand and p lim ≈ 800 MPa for rock (see Appendix A). It is also noteworthy that the range of confining pressures p lim = 100-800 MPa is consistent with the depths at which fractures in the crust are expected to close under lithostatic pressure [e.g., Pilkington and Grieve, 1992]. An estimate of the maximum dilatancy coefficient max as defined here can be derived from measurements of the maximum dilatancy angle obtained from shearbox and triaxial compression tests (see equation (3)). The estimate is not perfect, however, as pressure is never zero in such experiments. Vermeer and De Borst [1984] suggest that typical dilatancy angles for a range of geologic materials including sand, concrete, and rocks are between 0 and 20 •. Alejano and Alonso [2005] show peak dilatancy angles at the onset of failure COLLINS ©2014. The Authors.

Impact Craters on Asteroids: Does Gravity or Strength Control Their Size? (original) (raw)

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