Lunar Sourcebook : a User's Guide to the Moon

4 LUNAR SURFACE PROCESSES Friedrich Hörz, Richard Grieve, Grant Heiken, Paul Spudis, and Alan Binder The Moon’s surface is not affected by atmosphere, encounters with each other and with larger planets water, or life, the three major agents for altering throughout the lifetime of the solar system. These terrestrial surfaces. In addition, the lunar surface has orbital alterations are generally minor, but they not been shaped by recent geological activity, because ensure that, over geological periods, collisions with the lunar crust and mantle have been relatively cold other bodies will occur. and rigid throughout most of geological time. When such a collision happens, two outcomes are Convective internal mass transport, which dominates possible. If “target” and “projectile” are of comparable the dynamic Earth, is therefore largely absent on the size, collisional fragmentation and annihilation Moon, and so are the geological effects of such occurs, producing a large number of much smaller internal motions—volcanism, uplift, faulting, and fragments. If the target object is very large compared subduction—that both create and destroy surfaces on to the projectile, it behaves as an “infinite halfspace,” Earth. The great contrast between the ancient, stable and the result is an impact crater in the target body. Moon and the active, dynamic Earth is most clearly For collisions in the asteroid belt, many of the shown by the ages of their surfaces. Nearly 80% of the resulting collisional fragments or crater ejecta escape entire solid surface of Earth is <200 m.y. old. In the gravitational field of the impacted object; many of contrast, >99% of the lunar surface formed more than these fragments are then further perturbed into 3 b.y. ago and >70% of the lunar surface is more than Earth-crossing orbits to form the majority of the ~4 b.y. old. projectiles that impact the Earth and Moon. Despite the fact that lunar surface processes are The Moon’s population of impact craters represents less varied and dynamic than those of the Earth, a faithful record of these collisional processes over complex alterations of the lunar surface do occur; the most of the lifetime of the solar system. By most important source of such alterations, at least comparison, only the barest outlines of the cratering during the last 3 b.y., is external to the Moon. The history of Earth can be reconstructed, even for just Moon’s stable but heavily cratered surface provides the last 200 m.y., and only a few structures older evidence that planets are continuously bombarded by than 500 m.y. have been identified. external objects ranging from small dust specks to Typical impact velocities of asteroidal objects on the giant bodies tens of kilometers in diameter. Moon at present are between 15 and 25 km/sec; they The Moon and all other planets are not isolated, were apparently somewhat lower prior to 4 b.y. ago. closed systems. They are an integral part of a Such high velocities, combined with a high frequency dynamic solar system that continues to evolve under of impact events (especially before 3.8 b.y. ago), have largely gravitational forces. Orbits of predominantly expended a cumulative kinetic energy on the lunar small bodies are continually rearranged by close surface that exceeds the lunar internal energy released by volcanism and seismicity. As a 62 Lunar Sourcebook result, meteorite impact has been, and continues to returned lunar samples, the statistical number of be, the dominant lunar surface process, although lunar craters per unit area provides important volcanic and tectonic processes were also important constraints on both the relative and absolute ages of in the Moon’s distant past. various surface units and on the nature of the In this chapter, lunar surface processes are grouped projectile flux through geologic time (section 4.1.3). by decreasing importance into impact-related Such calculations permit an assessment of cumul- phenomena (section 4.1), volcanic processes (section ative cratering effects by modeling the stochastic 4.2), and tectonic activity (section 4.3). A summary of nature of repetitive impacts (section 4.1.4). lunar geologic history and associated stratigraphy is The importance of impacts on the Moon extends far presented in section 4.4. beyond the simple formation of crater-shaped landforms. The largest impact structures, which are 4.1. IMPACT PROCESSES the multiring basins that constitute major topogra- phic and structural features of the Moon, have also The projectiles that now enter the Earth/Moon served as sites for later volcanic and tectonic activity. system are derived from the asteroid belt and from Furthermore, the lunar surface materials and deeper comets. The most massive objects are entire asteroids crustal rocks that are excavated from impact craters or comets, which are generally a few kilometers in are also processed by impacts in ways that greatly diameter, but rarely a few tens of kilometers across. affect their petrographic appearance (Chapter 6), Most projectiles, however, are smaller fragments from surface debris (Chapter 7), chemical composition asteroidal collisions that vary widely in size; small (Chapter 8), and physical properties (Chapter 9). specimens reach the Earth’s surface as meteorites a few grams to a few metric tons in weight. The 4.1.1. The Morphology of Impact Structures exceedingly fine-grained “micrometeoroids” (fragments <1 mm in diameter, with masses <~ 10–2 g; see section Terminology. The fundamental shape of an impact 3.10) may be either the most fine-grained collisional feature is that of a bowl-shaped depression (a crater) debris from asteroids or small particles released from surrounded by a raised rim. Impact crater shapes comets (e.g., Gehrels, 1979; Shoemaker, 1983). vary with crater diameter (measured from rim to rim). Projectile masses impacting the lunar surface have With increasing diameter, they become ranged over 35 orders of magnitude, from microscopic proportionately shallower and develop more complex dust particles weighing 10–15 g to huge asteroids of rims and floors, including the appearance of central 1020 g; associated kinetic energies vary from a small peaks and rings. Such morphologic changes are found fraction of an erg to ~1032 ergs per individual impact. in craters on all the terrestrial planets and moons By comparison, the total internal energy released by (Pike, 1980). Some basic terms for the range of the Earth, which drives such visible processes as features associated with fresh, uneroded impact volcanism and tectonism, is estimated at 1026 to 1027 craters are illustrated by the examples in Fig. 4.1, and ergs per year (Lammlein et al., 1974). The “geologic” schematic views of various crater features are shown manifestations of impacts on the Moon range from in Fig. 4.2. microscopic craters <0.1 µm in diameter on tiny The basic morphologic subdivisions of impact grains of lunar soil to impact basins hundreds of structures are (1) simple craters, (2) complex craters, kilometers across on the lunar surface. Lunar global and (3) basins. Simple craters are generally bowl- surface evolution has thus been dominated by a small shaped with rounded or, in some cases, small, flat number of discrete but rare, large-body, basin- floors (Smith and Sanchez, 1973). They have smooth forming impacts as well as by the more continuous rims that lack terraces. With increasing diameter, pounding from numerous smaller and less energetic simple craters develop scalloped walls as large masses projectiles. The latter group produces smaller craters, of rock and regolith are slumped onto a generally together with some unique cumulative surface effects. hummocky crater floor (Figs. 4.1 c,d). Characterization of many lunar surface processes With larger diameters, simple craters evolve into therefore requires an understanding of the conse- complex craters, which are characterized by terraced quences of single hypervelocity (>3 km/sec) impacts and crenulated rims, or by zones of broad-scale of vastly different scales. Section 4.1.1 provides a (inward) slumping, and by an uplifted central peak or phenomenologic and geometric description of fresh peaks protruding from a relatively broad, flat floor. impact craters. This is followed by a summary of our On the Moon, the transition from simple to complex current understanding of impact physics and its craters takes place in the 15–20 km diameter range major geologic consequences (section 4.1.2). When (Pike, 1977). Central peaks are rarely observed in combined with isotopically-determined ages of craters <10 km, but most fresh craters >35 km and <100 km have central peaks (Wood and Lunar Surface Processes 63 64 Lunar Sourcebook Andersson, 1978). At diameters of >80 km, a Wood, 1971). Central peak basins, such as Compton, concentric zone of floor roughening with an amplitude are relatively small basins with a fragmentary ring of of several hundred meters appears around the central peaks surrounding a central peak (Fig. 4.1d). They peak or peaks (Hale and Grieve, 1982). At larger occur in the 140–175 km diameter range and are diameters (>100 km) this zone is replaced by a transitional to peak-ring basins. Peak-ring basins, fragmentary ring of peaks in approximately the same which have a well-developed ring but lack a central region (Figs. 4.1 d,e). peak (Fig. 4.1 e), are found in the 175–450 km The appearance of this inner ring marks the diameter range; a well-known example is the transition from craters to so-called basins (Hartmann Schrödinger Basin. The largest basins are multiring and Kuiper, 1962). Impact basins have been subdi- basins, which have as many as six concentric rings. vided into three morphologic types (Hartmann and Multiring basins are generally more than 400 km in Fig. 4.2. Schematic views of lunar impact structures, indicating principal morphologic elements.

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