Proto planets

Therefore, there exist several of lines of evidence to support the view that impact erosion may have had a significant effect on Earth s composition. However, in most cases the evidence is suggestive rather than strongly compelling. Furthermore, we have a very poor idea of how this is possible without fractionating potassium isotopes (Humayun and Clayton, 1995), unless the entire inventory of potassium is vaporized (O Neill, 1991a,b Halliday et al, 1996 Halliday and Porcelli, 2001). We also do not understand how to lose heavy elements except via hydrodynamic escape of a large protoatmosphere (Hunten et al, 1987 Walker, 1986). Some of the loss may have been from the proto-planets that built the Earth. 

Tajika, E. and Matsui, T. (1990) Evolution of terrestrial proto-C02 atmosphere with thermal coupled history of the Earth. Earth Planet. Sci. Lett., 113, 251-266. 

A vital event in the further development of the Earth was its collision with a smaller planet, possibly as big as Mars. It is assumed that this gigantic collision took place between four and four and a half billion years ago (Sleep et al., 2001), and that it also resulted in the birth of our moon (Luna), which was formed from partially vaporized matter from the Earth. It is likely that not all of the proto-Earth was melted by the energy set free in the collision, but that sections of it remained in their original form. However, more exact information is not yet available. 

The Sun formed some 4.5 Gyr ago (Gyr is a Gigayear or 109 years) from its own gas cloud called the solar nebula, which consisted of mainly hydrogen but also all of the heavier elements that are observed in the spectrum of the Sun. Similarly, the elemental abundance on the Earth and all of the planets was defined by the composition of the solar nebula and so was ultimately responsible for the molecular inventory necessary for life. The solar system formed from a slowly rotating nebula that contracted around the proto-sun, forming the system of planets called the solar system. Astronomers have recently discovered solar systems around... 

The formation of the planets around the proto-sun initially started as a simple accretion process, aggregating small particles to form larger particles. This process was common to all planets, even the gas giants Jupiter and Saturn and to a lesser extent Neptune and Uranus. The planetesimals form at different rates and as soon as Jupiter and Saturn had reached a critical mass they were able to trap large amounts of hydrogen and helium from the solar nebula. The centres of Jupiter... 

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. 

While the amount of dust and small particles that underwent thermal processing remains difficult to constrain both in the entire proto-solar nebula and in protoplanetary disks around other stars, in the Asteroid Belt over 80% of the pre-chondritic components have been melted. These heating events may play a crucial role in defining the bulk composition of planetesimals and planets by reprocessing much or all... 

In this chapter we compare the evolution of protoplanetary disks to that of the proto-solar nebula. We start by summarizing the observational constraints on the lifetime of protoplanetary disks and discuss four major disk-dispersal mechanisms. Then, we seek constraints on the clearing of gas and dust in the proto-solar nebula from the properties of meteorites, asteroids, and planets. Finally, we try to anchor the evolution of protoplanetary disks to the Solar System chronology and discuss what observations and experiments are needed to understand how common is the history of the Solar System. 

Planet formation followed the planetesimal and protoplanet formation in the final stage of the disk evolution (Chapter 10). Gas giants, Jupiter and Saturn, captured disk gas due to their large gravities, and other planets, including Earth, may also have some evidence of disk-gas capture. In the second part of this section, we will seek constraints on the timing of dust and gas dispersal in the proto-solar disk from planets (Section 9.3.2). 

Beckwith S. V. W., Henning T., and Nakagawa Y. (2000) Dust properties and assembly of large particles in proto-planetary disks. In Protostars and Planets IV (eds. V. Mannings, A. P. Boss, and S. S. Russell). University of Arizona Press, Tucson, pp. 533-558. 

Models of planetary evolution assume that at the time of planetary formation the solar system had a single universal and well-mixed composition from which aU parts of the solar system were derived (see Podosek, 1978). Information as to the elemental and isotopic characteristics of this primordial composition is presently available from the Sun, meteorites, and the atmospheres of the giant planets (Wider, 2002). In the case of the Sun, distinction is usually made between the present-day composition, which is available via spectral analysis of the solar atmosphere and capture of the solar wind, either directly in space or by using metallic foU targets, and the proto-Sun (the composition at the time of planetary accretion) whereby the lunar regolith and/or meteorites are utilized as archives of ancient solar wind. As discussed below, the distinction is only really important for helium due to production of He by deuterium burning. 

A problem with adopting the solar wind He/" He ratio as representative of the solar nebula is the production of He from deuterium very early in solar system history consequently, the solar wind value (—4.4 X 10 ) is too high by a factor between —2.5 and —3 relative to the proto-Sun (Geiss and Reeves, 1972). To circumvent this difficulty, recourse has been made to analyzing the giant planets whose atmospheres are expected to reflect proto-solar values (Wieler, 2002). Jupiter is the only giant planet whose atmospheric He/" He ratio has been determined (Mahaffy et al., 1998). 

So when does the first evidence of improbable, information-containing, metabolic replication occur in the fossil record The Earth is 4,500 million years old, as judged by several corroborating radionuclide studies of the oldest rocks on the planet show. Meteoric bombardment of the proto-Earth continued heavily until 4,000 MY A, probably precluding life during this period. The majority of the oldest rocks on Earth are 3,500 million years old, and the earliest microfossils are from 3,000+ MYA, hence we only have a window of about 500 million years from the end of the meteoric bombardment to the first signs of microbial life. This means we are either very lucky, or life is a high-on certainty ... 

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