Solar System impact history

both of which have glassy areas that were melted in a shock event, but then quickly cooled. In each case, detailed °Ar- Ar studies, comparing multiple samples, have shown that some samples have retained ( inherited ) a fraction of the °Ar that they contained before the impact, and yield age spectra that would be very difficult to interpret without multiple samples. Hence, in trying to find the age of a crater, one would like to 

Lunar impact history a cataclysm Given its proximity to the Earth, the Moon s impact history presumably is indicative of the impact history of the Earth as well. The majority of published Ar- Ar ages of lunar rocks come from the 1970s, most of them published in the annual Proceedings of the Lunar Science Conference. However, these early studies raised questions that are still being sorted out by detailed studies with more advanced technology. 

The most notable aspect of Ar- Ar ages of lunar impact melts is that the majority of them cluster at an age just younger than 4.0 Ga (Fig. 1), first noted by Turner et al. 

Two studies that appeared less than a year apart led to detailed experimental studies that have lent strong support to the idea of a cataclysm. Although the idea still cannot be considered verified, some of the earlier questions have been answered, and means have been found to address a new set of questions. 

8 to 4.0 Ga old. They suggested that most of the visible basins may have formed in as little as 55 to 60 Ma, implying a very high flux of very large objects for a very short period of time. They found several other samples that were older than 4.0 Ga, but none of the old samples were impact melts. Since some older samples were observed, they argued that the lack of old impact melts was not a result of all old rocks having been reset, but rather the result of a cataclysm. 

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In this chapter, we review what is known about the chronology of the solar system, based on the radioisotope systems described in Chapter 8. We start by discussing the age of materials that formed the solar system. Short-lived radionuclides also provide information about the galactic environment in which the solar system formed. We then consider how the age of the solar system is estimated from its oldest surviving materials - the refractory inclusions in chondrites. We discuss constraints on the accretion of chondritic asteroids and their subsequent metamorphism and alteration. Next, we discuss the chronology of differentiated asteroids, and of the Earth, Moon, and Mars. Finally, we consider the impact histories of the solar system bodies, the timescales for the transport of meteorites from their parent bodies to the Earth, and the residence time of meteorites on the Earth s surface before they disintegrate due to weathering. 

The early history of Earth is greatly influenced by the probable impact of a Marssized body to form the Moon. Core-formation models suggest both Earth and the impactor were already differentiated by the time of the impact (Tonks Melosh 1992). The lack of a clear182W excess in uncontaminated lunar samples implies that the Moon-forming impact took place >50 Myr after the start of the Solar System (Touboul et al. 2007). The oldest known lunar samples are 150 Myr younger than CAIs, based on Sm-Nd dating (Touboul etal. 2007), which provides a lower limit on the Moon s age. 

The cratered surfaces of asteroids and terrestrial planets underscore the importance of impacts for the formation and evolution of the solar system. Early in the history of the solar system such collisions were the mechanism for accretion of planetesimals and finally the planets themselves [1], The effects of these still ongoing collisions are visible from the megascopic down to the submicroscopic length scale, i.e., they range from large impact craters and their ejecta blankets down to shock-metamorphic effects in minerals [2-4]. These effects form as a result of the interaction of strong shock waves with the affected solid matter. 

Meteorites, which come mainly from various asteroids but also from Mars and our Moon, provide otherwise unobtainable information about objects in the inner Solar System in both space and time. The presence of meteorites on Earth allows application of the full gamut of instrumentation able to analyze materials from the asteroids (minor planets). Mars and the Moon with state of the art sensitivity and accuracy. Many meteorites include material that condensed and accreted in the primitive Solar nebula and were subsequently unaltered some contain evidence for pre-Solar nuclear processes. Information in meteorites tracks the evolution of their parent bodies, both interiors and surfaces, the impacts that ejected them and the nuclear radiation history that occurred as they travelled Earthward. Meteoritic material also allows the dating of all of these episodes and determination of the composition of the Sun s surface and the particles streaming from it. This chapter is a brief tutorial outlining this information and indicating how it is obtained. 

The Earth and planetary system were formed 4.6 billion years ago. Certain meteorites as fragments from small planets have preserved a primitive cosmic composition and contain records of the early history of the solar system. Because of the lack of an atmosphere, the lunar surface has not been reworked and still exhibits the craters from the impact of large planetesimals which were abundant in space at the stage of planet formation. The oldest rocks on earth have an age of 3.5 to 4 billion years. 

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