Planetesimal

After planetary accretion was complete there remained two groups of surviving planetesimals, the comets and asteroids. These populations still exist and play an important role in the Earth s history. Asteroids from the belt between Mars and Jupiter and comets from reservoirs beyond the outer planets are stochastically perturbed into Earth-crossing orbits and they have collided with Earth throughout its entire history. The impact rate for 1 km diameter bodies is approximately three per million years and impacts of 10 km size bodies occur on a... 

Tiny microparticles came together to form microagglomerates, and these in turn formed larger clots, which then formed larger bodies, the diameter of which was initially measured in centimetres but later increased to metres such planetary building blocks are known as planetesimals . Computer simulations indicate that these existed around four and a half billion years ago (Wetherhill, 1981). Planetesimals grew to form bodies which were several kilometres across, and there were often collisions in which larger bodies were swallowed up by smaller ones a process which is not unknown in modern economics ... 

In the region of the terrestrial planets, there may have been several thousand planetesimals of up to several hundred kilometres in diameter. During about ten million years, these united to form the four planets—Mercury, Venus, Earth and Mars—which are close to the sun. Far outside the orbit of the planet Mars, the heavier planets were formed, in particular Jupiter and Saturn, the huge masses of which attracted all the hydrogen and helium around them. Apart from their cores, these planets have a similar composition to that of the sun. Between the planets Mars and Jupiter, there is a large zone which should really contain another planet. It... 

Because of their similar history, the four terrestrial planets have similar layer structures. However, their surfaces and atmospheres show enormous physical and chemical differences. The development of the primeval Earth via the agglomeration of planetesimals was accompanied by a vast temperature increase caused by contributions from three different phenomena ... 

The kinetic energy set free in collisions with planetesimals was proportional to the square of the velocity of the body which hit the Earth. Thus, if a planetesimal hit the Earth s surface with a velocity of 11 km/s, the amount of energy set free would correspond to the explosion of the corresponding amount of the explosive TNT (trinitrotoluene). The increased compression due to the increase in mass led to pressure increases in the interior of the planet and thus to temperature increases up to around 1,270 K (Press and Siever, 1994). 

This planet, the nearest to the sun, has almost no atmosphere its surface is covered with craters. During the formation of Mercury, planetesimals were able to impact the planet s surface without any resistance. Thus, the lack of erosion processes (due to... 

The paramount importance of carbon in the cosmos is shown by the fact that more than 75% of the approximately 120 interstellar and circumstellar molecules so far identified are carbon containing (Henning and Salama, 1998). Molecules apparently travel from the ISM via protoplanetary discs to the planetesimals and from there, via accretion, to the heavenly bodies formed. The molecules so far identified in ISM come from quite different types of compounds ... 

The central importance of phosphorus and its derivatives in our world leads to the question as to where the element came from phosphorus compounds must have been present on the Earth after its formation, but we do not know their source. Were they already present in planetesimals, or was phosphorus brought to Earth from space (in an elemental form or as compounds) ... 

The volatile materials would have vaporised from the surface of the planetesimals once the temperature reached 160 K below this temperature water sticks to silicate surfaces and condenses, ultimately freezing into ice. The new gaseous material is swept away from the planetesimals by the solar wind of particles, leaving bare planetesimals too small to acquire and maintain an atmosphere. The temperature gradient and location within the solar nebula are then important to the ultimate nature and composition of the planets themselves and interplanetary debris. 

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... 

The early period of the Earth s history, known geologically as the Hadean, is associated with huge bombardment of the surface of the planet by meteors and comets. The sequence of events in the Earth s formation is shown in Figure 7.3, starting with the Hadean. Any volatile materials on the surface of dust grains or planetesimals deposited on the Earth will be removed and become part of the atmosphere, or more generally the volatile component inventory of the planet. 

Figure 7.3 Formation of the Earth (a) the Hadean (b) melting of the planetesimals (c) core fractionation (d) mature continental structure. (Reproduced from Starosta and Zelik, 2002, with permission from Cambridge University Press)...

The capture of the moon by collision between the Earth and another large co-orbiting planetesimal with all of the consequences of a close moon larger tidal rise and fall... 

Cosmochemistry is the study of the chemical composition of the universe and the processes that produced those compositions. This is a tall order, to be sure. Understandably, cosmochemistry focuses primarily on the objects in our own solar system, because that is where we have direct access to the most chemical information. That part of cosmochemistry encompasses the compositions of the Sun, its retinue of planets and their satellites, the almost innumerable asteroids and comets, and the smaller samples (meteorites, interplanetary dust particles or IDPs, returned lunar samples) derived from them. From their chemistry, determined by laboratory measurements of samples or by various remote-sensing techniques, cosmochemists try to unravel the processes that formed or affected them and to fix the chronology of these events. Meteorites offer a unique window on the solar nebula - the disk-shaped cocoon of gas and dust that enveloped the early Sun some 4.57 billion years ago, and from which planetesimals and planets accreted (Fig. 1.1). 

Cosmochemistry is the study of the chemical compositions of various solar system materials. Chondrites are the most abundant primitive samples. They are essentially sedimentary rocks composed of mechanical mixtures of materials with different origins (chondrules, refractory inclusions, metal, sulfide, matrix), which we will call components. Chondrites formed by the accretion of solid particles within the solar nebula or onto the surfaces of growing planetesimals. They are very old (>4.5 billion years, as measured by radioactive chronometers) and contain some of the earliest formed objects in the solar system. Chondrites have bulk chemical compositions very similar to the solar photosphere, except... 

Now that we have gained some appreciation of the value and limitations of the various kinds of samples available for cosmochemical analysis, we will begin to consider what these materials really tell us. In Chapter 4, we saw how solar system abundances of elements and isotopes are determined using the most primitive chondrites. In subsequent chapters, we will see how elements are fractionated in space, planetesimals, and planets. 

The Shallowater aubrite evidence for origin by planetesimal impact. Geochimica et Cosmochimica Acta, 53, 3291-3307. 

Several kinds of physical processes have been hypothesized to have affected planetesimals prior to their accretion into planets. These processes could conceivably explain some chemical fractionations observed among asteroids and planets. 

Planetary differentiation is a fractionation event of the first order, and it involves both chemical fractionation and physical fractionation processes. Planetary crusts are enriched in elements that occur in silicate minerals that melt at relatively low temperatures. Recall from Chapter 4 that the high solar system abundances of magnesium, silicon, and iron mean that the silicate portions of planetesimals and planets will be dominated by olivine and pyroxenes. Partial melting of sources dominated by olivine and pyroxene ( ultramafic rocks ) produces basaltic liquids that ascend buoyantly and erupt on the surface. It is thus no surprise that most crusts are made of basalts. Remelting of basaltic crust produces magmas richer in silica, eventually resulting in granites, as on the Earth. 

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. 

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