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Chapter 1
Geomaterials under Extreme Loading: The Natural Case 1
1.1. Introduction
The field of study of material under extreme loading and application to penetration and impact is classically the scope of dynamic and dynamic-derived research broadly aiming at documenting, understanding and furthermore parametrizing and predicting effects and mechanisms in either projectile, or target, or both, upon impact. The foreseen ultimate purpose is generally either the generation of damage, or the prevention from damage. Impacts thus represent a very wide range of events, and have been studied for years for various defense purposes, such as protection against projectile attack, prediction of the effects of nuclear weapons, or penetration of concrete shields. Recently, safety issues linked to the use of energetic materials addressed the problem of predicting explosions due to accidental or malevolent impacts, especially in the (very) low velocity range, i.e. below 100 m.s−1 (see [VAN 02, GRU 08] for example). The size of the craters produced in materials typically range from sub-millimeter to decameter (nuclear explosion apart) and the loading conditions (pressure and strain rates) vary considerably depending upon the properties of the material involved and the types of effects sought.
Less than half a century ago, there was another research community involved with similar cratering problematics, namely, the planetary geology community, as a result of the progressive recognition of the fundamental importance of shock, impact and collisions on the Earth and more generally on all bodies in the Solar System.
The purpose of this chapter is to address the similarities and differences between impacts in both communities. It aims to produce the matter for appraising the interest, and beyond that at establishing the means, for bridging the gap between the dynamic impact community and the meteorite impact community. So far, both have been mostly working separately. The present paper proposes an insight on some of the major characteristics of natural counterparts to the penetration and impact in geomaterials. At the same time it illustrates methodologies and approaches that may differ from those utilized in the dynamic research, in relation to difference in scales of mechanisms (and difference in background and training).
1.2. Natural impacts
1.2.1. Introduction
As mentioned in [FRE 98] in a remarkable and still unique detailed overview of the geological aspects of meteorite impacts, the picture of Earth and its place in the Solar System has considerably changed over the last decades. Explorations of the Solar System by humans and robotic spacecraft have clearly established the importance of impact cratering in shaping all the planets, including Earth. In parallel, significant progress has been made in the ability to identify impact structures on Earth. Specific petrological and geochemical criteria have been raised as “finger prints” of hypervelocity meteorite impact events. The so-called shock metamorphic evidences (see [FRE 98] and references therein) are the principal instrument utilized by geologists to recognize and parameterize meteorite impacts when all the direct morphological evidences of the phenomenon (namely the crater) have disappeared. This is typically the case in old and deeply eroded impact structure on Earth. It is even more typically the case in meteorites. “Made in impact crater” by “nature”, the crater and the parent body of meteorites are no longer available for study. Incidentally the material record provided by meteorites has been and will continue to be by far the major ground truth data source for understanding the Solar System. This record appears now as largely “biased” by impact phenomena, reinforcing the need for understanding and then deconvoluting these effects prior to further planetary interpretations of meteorites.
It is now established that impacts are responsible for the formation of all the planets and planetary objects in the Solar System including the smallest ones. For most of these objects (such as our moon), impacts are the only currently active geological process. On Earth, they were responsible for large crustal disturbances and produced huge volumes of igneous rocks, they formed major ore deposits and they are directly or indirectly involved in the major biological extinctions that have shaken life on our planet [REI 07]. Eventually the presence of complex organic compounds including the bricks of life in meteorites [AND 91, BUS 06], the capability of surviving shock and space conditions of micro-organisms [HOR 08] and the growing evidence of potentially supporting life conditions at the surface or near surface of planets and planetesimals (at least at the early stage of the Solar System [ABE 85, LAN 85]) progressively give consistency to what could be a major revolution in the history of science, i.e. the role impact in the origin of life on Earth. The transpermia/panspermia theory is gaining weight and was claimed by the late field geologist E. Shoemaker long before comet Shoemaker-Levy was recognized and later crashed on Jupiter.
1.2.2. Terrestrial record
To date, 176 confirmed impact structures have been identified on Earth. An updated record is maintained by the University of New Brunswick and is accessible at the website: www.unb.ca/passc/ImpactDatabase/.
A large concentration of impact structures is observed on the North American and European continents (Figure 1.1). This is partly due to a larger density of scientists involved in impact crater research. But this is also due to geology. These regions display continental formations that emerged several hundred millions of years ago, and thus have been exposed to impact for a much longer time than recent sedimentary or volcanic covers. The same characteristic applies at all planetary surfaces. Impact crater density is directly linked to the age of the surface exposed. Objects such as Io and icy satellites where the surface is rapidly reprocessed by endogenic (internal) processes are considerably depleted in impact craters compared to the Moon and Mercury for instance.
1.2.3. Nomenclature and general overview
The general term “impact crater” in the geological community is used to designate an hypervelocity impact crater formed by a cosmic projectile that is large enough and coherent enough to penetrate Earth’s atmosphere with little or no deceleration. It strikes the ground at virtually its original cosmic velocity (i.e. 10–70 km.s−1). Such a projectile is in the order 50 m in diameter and above for a stony object and less than half for an iron one. Smaller projectiles lose most or all of their original velocity and kinetic energy in the atmosphere through disintegration and ablation, and strike the Earth at “low velocity” (a few 100 m.s−1). The crater is only slightly larger than the projectile itself. The projectile survives, more or less intact, and much of it is found in the bottom of the pit also called penetration craters or penetration funnels. They are referred to as “small” meteorite impacts, representing less than 10% of the impact craters identified on Earth (Figure 1.1).
1.2.4. Morphological characteristics: typology of impact craters
A feature common to impact craters on all planets is the progression of morphologies with size. What differs from planet to planet is the cut-off size for the various morphologies, the latter mainly relating to the gravitational field of each planetary object and to the nature and physical state of the targeted material (rock, ice, gas). The smallest craters are bowl-shaped depressions referred as “simple” craters (Figure 1.2). All the terrestrial impact craters below about 3 km in diameter belong to this category. As the size of the crater increases, the apparent depth to diameter ratio decreases and a central peak is observed (Figure 1.3). At a larger scale the central peak shifts to a central ring (Figure 1.2). At even larger size, several concentric rings may be observed (Figure 1.2). The largest planetary impact structures appear as huge geological bullseyes, composed of multiple concentric uplifted rings and intervening down-faulted valleys. Referred to as multi-ring basins (Figure 1.2) they were produced by the impact of projectiles tens to hundreds of kilometers in diameter mainly from an early period in the Solar System (>3.9 Ga), when such large objects were more abundant and collisions more frequent. They are best observed on planets with well-preserved ancient surfaces, such as the Moon, Mercury, parts of Mars, and some of the moons of Jupiter. Mare Orientale on the Moon (D ~ 900 km), is one of the most prominent and best-known multi-ring basins but even larger features exist, such as the Valhalla Basin (D ~ 4000 km) on Jupiter’s icy moon Callisto (Figure 1.2).
The morphological-morphometrical crater distinctions presented above remain general. In the details there are variations. For instance, flat craters without typical topographic highs are commonly observed on planetary surface such as the Moon, Mars and Mercury although their diameters correspond to that of the central peak and small peak ring craters (see arrows in Figure 1.2 for instance). In addition, numerous large basins in the Solar System do not display a pronounced multiring structure such as the Caloris Basin (Mercury; D = 1300 km), the Argyre Basin (Mars; D > 900 km), and the recently identified South Pole-Aitken Basin on the Moon (D ~ 2500 km). Most exceptions may not relate to the impact phenomenon and/or target properties but to later modifications due to late geological effects. The intense “normal” geological activity on Earth including the abundance of water explains why direct evidence of the multi-ring basins on Earth is missing although Manicouagan (Canada, 100 km), Popigai (Russia, 100 km), Vredefort (South Africa, 200–300 km (see Figure 1.3), Sudbury (Canada, >200 km) and Chicxulub (Mexico, >180 km) are likely to belong to this category.
We notice that the erosion on our planet bears some advantages as it gives natural access to a cross-section deep into the structures. Also the terrestrial record is by far the easiest and most accessible data reservoir on impact. Detailed studies at old eroded craters such as at Vredefort Dome (South Africa) enable us to map the shock record within the target (e.g., Figure 1.3) or to reconstruct the detailed geometry of the crater floor such as at Haughton Dome (Canada) or Rochechouart (France) (Figure 1.4).
Although it is generally admitted that the presence of two or more interior rings in...
