Age of the solar system

In order to determine the age of the solar system, we must first decide what is really meant by its age. Astronomers and theoreticians tell us that the process of forming a star from a dense core of gas and dust in a molecular cloud takes on the order of a few million years. The process can be divided into four stages that are defined by the characteristics of four classes 

Artistic rendering of four observed stages of star formation, (a) Class 0 object a deeply embedded hydrostatic core surrounded by a dense accretion disk. Strong bipolar jets remove angular momentum, (b) Class I object protostar in the later part of the main accretion phase, (c) Class II object or T Tauri star pre-main-sequence star with optically thick protoplanetary disk, (d) Class III object or naked T Tauri star star has an optically thin disk and thus can be directly observed. Some may have planets. 

Chronology of the solar system from radioactive isotopes 

Class I obj ects also have bipolar outflows, but they are less powerful and less well collimated than those of Class 0 objects. This stage lasts 100 000 to 200 000 years. Class //objects, also known as classical T Tauri stars, are pre-main-sequence stars with optically thick proto-planetary disks. They are no longer embedded in their parent cloud, and they are observed in optical and infrared wavelengths. They still exhibit bipolar outflows and strong stellar winds. This stage lasts from 1-10 million years. Class ///objects are the so-called weak line or naked T-Tauri stars. They have optically thin disks, perhaps debris disks in some cases, and there are no outflows or other evidence of accretion. They are observed in the visible and near infrared and have strong X-ray emission. These stars may have planets around them, although they cannot be observed. 

The tools that we have to determine the age of the solar system and the chronology of early solar system materials are the long-lived and short-lived radiochronometers discussed in Chapter 8. The long-lived radionuclides tell us that the oldest objects in the solar system formed at -4.5-4.6 Ga (Fig. 9.8). But the precision of most of these measurements is not sufficient to investigate the details of those early times. In addition, there are uncertainties in the half-lives of the different radionuclides that translate into absolute uncertainties of several million to tens of millions of years. Short-lived radionuclides provide the time resolution necessary to unravel the details of early solar system history. 

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Samples that are 4.6 X 10 years old have been found in meteorites. This is the best present estimate for the age of the solar system. Example illustrates this type of calculation for rock from the Earth s moon. 

The half-life is independent of the initial number of nuclei. For 14C decay the half-life is 5717 years, whereas the 238U decay half-life is 4.5 billion years. Carbon dating works well for timescales in the recent past and is used for dating objects such as the Turin shroud, but 238U is better for timescales of the age of the solar system. 

The value of the decay constant (1) must have remained constant over the age of the solar system and the galaxy, and it must be accurately known. As we discussed in Chapter 2, this third assumption is well founded for conditions relevant to cosmochemistry. The concordance of dates given by systems using a variety of decay paths and astronomical observations of decay rates of newly synthesized elements over billions of years provides strong evidence that the decay rates have remained constant. In addition, detailed experiments and theoretical models have identified the extreme conditions (e.g. centers of stars) under which this assumption breaks down for certain isotopes, thereby identifying the exceptions that prove the rule. (4) It must be possible to assign a realistic value to the initial abundance of the... 

Equation (8.47), with t = 0 and the composition of lead from meteoritic troilite used for the initial isotopic ratio of lead, was used by Clair Patterson (1955,1956) to determine the age of the Earth. In the 1950s, the largest uncertainty in determining the age of the Earth was the composition of primordial lead. In 1953, Patterson solved this problem by using state-of-the-art analytical techniques to measure the composition of lead from troilite (FeS) in iron meteorites. Troilite has an extremely low U/Pb ratio because uranium was separated from the lead in troilite at near the time of solar-system formation. Patterson (1955) then measured the composition of lead from stony meteorites. In 1956, he demonstrated that the data from stony meteorites, iron meteorites, and terrestrial oceanic sediments all fell on the same isochron (Fig. 8.20). He interpreted the isochron age (4.55+0.07 Ga) as the age of the Earth and of the meteorites. The value for the age of the Earth has remained essentially unchanged since Patterson s determination, although the age of the solar system has been pushed back by —20 Myr. 

Burkhardt, C., Kleine, T., Bourdon, B. et al. (2008) Hf-W mineral isochron for Ca, Al-rich inclusions age of the solar system and the timing of core formation in planetesimals. Geochimica et Cosmochimica Acta, 72, 6177—6197. 

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. 

Several isotope systems indicate that CAIs are the oldest objects to have formed in the solar system. Cosmochemists have adopted their formation age as the age of the solar system. Based on a combination of data from the Pb-Pb and 182H-182W systems (see below), we assign an age for the CAIs of 4568.2 0.5 Ma. To this precise age, we must add an absolute uncertainty of -0.2% (9 Myr at the age of the solar system) because of... 

How accurately do we know the age of the solar system What is the largest uncertainty in the age Why can we determine the sequence and timing of events in the early solar system with higher precision than we know the absolute age ... 

In contrast to the terrestrial planets, the giant planets are massive enough to have captured and retained nebular gases directly. However, concentrations of argon, krypton, and xenon measured in Jupiter s atmosphere by the Galileo spacecraft are 2.5 times solar, which may imply that its atmosphere preferentially lost hydrogen and helium over the age of the solar system. 

A synthesis of the radiometric age of the solar system and the ages of its constituents... 

These questions are of prime interest. As we said in the introduction, the study of organic matter in carbonaceous chondrites is interesting because it may give some insight into the origin of our solar system. It is out of the question to discuss here in detail the train of events that is described as the formation of the solar system. Nevertheless, it is important to remember that the age of the solar system is about 4.5 Gyr (4.5 x 109 yr). 

Anders, E., and C. M. Stevens Thallium-205 and the Age of the Solar System. Presented at the 1960 Spring Meeting of the American Geophysical Union, Washington. Zit. nach [281). 

Having pointed out that viscosity is the key quantity for the overall behavior and structure of the nebula, we must ask ourselves what is its origin. The first possibility is evidently particle viscosity. It can however be seen that in that case, the timescale for mass redistribution would be much longer than the age of the solar system itself. Lin (1981) has also pointed out that density perturbations could not grow into protoplanets because they could not accrete material beyond their immediate vicinity. 

A practical application of eqs. (16.6), (16.7) and (16.8) is the calculation of the age of the solar system. MS analysis of meteorites containing negligible amounts of U gives the following values for the isotope ratios of the Pb isotopes 2 Pb Pb = 9.4 and 207pb 204pb 2Q 3 jf these values are assumed to be the initial isotope ratios at the time of formation of the solar system, the age is obtained from the present isotope ratios of the Pb isotopes in the solar system and the ratios of the present abundances of U and Pb, for example by application of eq. (16.6) ... 

The chronology of aubrites is poorly constrained. Bogard et al. (1967) presented Rb-Sr and K-Ar ages for Norton County that are compatible with the age of the solar system, while Compston et al. (1965) determined an age of 3.7 Ga for Bishopville. Hohenberg (1967) determined that Shallowater began retaining Xe from decay at roughly the same time as chondritic meteorites. 

Comets are surviving members of a formerly vast distribution of solid bodies that formed in the cold regions of the solar nebula. Cometary bodies escaped incorporation into planets and ejection from the solar system and they have been stored in two distant reservoirs, the Oort cloud and the Kuiper Belt, for most of the age of the solar system. Observed comets appear to have formed between 5 AU and 55 AU. From a cosmochemical viewpoint, comets are particularly interesting bodies because they are preserved samples of the solar nebula s cold ice-bearing regions that occupied 99% of the areal extent of the solar nebula disk. All comets formed beyond the snow line of the nebula, where the conditions were... 

As accurately as these calculations can be made, however, the behavior of celestial bodies over long periods of time cannot always be determined. For example, the perturbation method has so far been unable to determine the stability either of the orbits of individual bodies or of the solar system as a whole for the estimated age of the solar system. Studies of the evolution of the Earth-Moon system indicate that the Moon s orbit may become unstable, which will make it possible for the Moon to escape into an independent orbit around the Sun. Recent astronomers have also used the theory of chaos to explain irregular orbits. 

The decay process of a different radioisotope, uranium-238 to lead-206, is commonly used to date objects such as rocks. Because the half-life of uranium-238 is 4.5 X 10 years, it can be used to estimate the age of objects that are too old to be dated using carbon-14. By radiochemical dating of meteorites, the age of the solar system has been estimated at 4.6 X 10 years of age. 

There are 280 naturally occurring nuclides that make up the 83 stable and long-lived elements. These are all the elements up to Bi with Z = 83, except for unstable Tc (Z = 43) and Pm (Z = 61) that only have short-lived isotopes, but the long-lived Th and U bring the total back to 83. Here long-lived or short-lived is with respect to the half-life of an isotope against radioactive decay and the age of the solar system. Long-lived means then an element is still present in measurable quantities since the solar system formed 4.6 Gyr ago, and radioactive isotopes with half-lives above 0.6 Gyr usually qualify... 

To determine the ages of more ancient objects or of objects that do not contain carbon, different radioisotopes are measured. For example, by comparing the ratio of to its final decay product, ° Pb, geochemists found that the oldest known surface rocks on Earth—granite in western Greenland—are about 3.7 billion years old, and determining this ratio in samples from meteorites gives 4.65 billion years for the age of the Solar System, and thus Earth. 

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