Using the heat capacity, the temperature change created by an impact of a given mass can be calculated:
How many kilograms of the larger body are going to be heated by the impact? Once this quantity is known, it can be divided by the number of kilograms being heated to determine how many degrees they will be heated. How widespread is the influence of this impact? How deeply does it heat and how widely? Of course, the material closest to the impact will receive most of the energy, and the energy input will go down with distance from the impact, until finally the material is completely unheated. What is the pattern of energy dispersal? Energy dispersal is not well understood, even by scientists who study impactors. Here is a simpler question: If all the energies were put into melting the impacted material evenly, how much could they melt? To melt a silicate completely requires that its temperature be raised to about 2,700°F (1,500°C), as a rough estimate, so here is the mass of material that can be completely melted by this example impact:
This means that the impactor can melt about 25 times its own mass (4.5 X 1014/1.7 X 1013 = 26).This is a rough calculation, but it does show how effective accretion can be in heating a growing body and how it can therefore help the body to attain a spherical shape and to internally differentiate into different compositional shells.
Meteorites contain minerals that hold clues about how long it took them to crystallize and therefore how rapidly the material cooled.The original meteorite material was apparently quite hot in many cases, well above 1,800°F (1,000°C), since in some cases it started as a complete melt. In other cases, temperatures this high are required to allow certain trace atoms to diffuse and move through crystals as they apparently have in these bodies. Since the original bodies were hot and stayed hot for some time, they were necessarily large, from the arguments that can be made about heat of accretion of the bodies.This reasoning leads to the inevitable conclusion that the meteorites started as parts of larger bodies, planetesimals of some sort, which were then broken up by catastrophic impacts and the smaller pieces then cooled rapidly in the freezer of space. The large original body that accreted from the solar nebula is known as the parent body of the smaller pieces that fall into the Earth's atmosphere today.
In many cases the size of the original body, the time of breakup, and the speed of cooling can be calculated from the meteorite fragments found on Earth. Dynamical studies of asteroids show that large asteroids, those 120 miles (200 km) in diameter or larger, are likely to be broken by impact within tens of billions of years. Right now, 21 asteroids this large are known in the solar system. In the early solar system, there may have been a thousand times more of these large asteroids. Four hundred of these original 2,000 or so bodies should have been destroyed by collisions in the first million years of the solar system.The meteorite record shows that many of the parent bodies of the meteorites that land on Earth broke up very early in the solar system, consistent with these estimates for sizes and populations of asteroids. And though evidence shows that the parent bodies of meteorites were in some cases hundreds of kilometers in diameter, there is no evidence that bodies even as large as the Moon existed in the early asteroid belt.
When minerals in a meteorite indicate that the material cooled slowly inside the parent body before it was broken up, there is another clever way to determine how large the parent body was. Plutonium 244 (244Pu) is a radioactive isotope that ejects a large particle with a lot of energy when it decays. The flying particle breaks through the crystal structure of the mineral of which the plutonium is a part.The path of the particle can be seen as a curving track if the mineral is polished and etched with acid; these curving paths are called fission tracks. If the mineral grain is reheated sufficiently after fission tracks are formed, the crystal reorganizes itself into a clean crystal structure, and the fission track is erased. This process is called annealing. Each mineral has a different temperature at which annealing can occur. Pyroxene loses its fission tracks, or anneals, at about 570°F (300°C), while a calcium mineral called whitlockite anneals at 210°F (100°C). By counting the density of fission tracks in different minerals, the speed of cooling between the annealing temperatures of the different minerals can be calculated.
The size of the parent body of the meteorite can be calculated using this cooling rate. Rock is an insulating material that holds its heat for a long time.The larger the body, the more slowly the interior cools.The mass of rock needed to transfer heat at a given rate can be calculated. In this way the likely size of a chondrite parent body has been calculated to be a couple of hundred miles (several hundred km).
The idea that meteorites were fragments of material from the asteroid belt came long before the radiogenic heating and cooling evidence discussed above. In the early 1800s, scientists were already suggesting that meteorites came from asteroids. Now, there is proof: In some cases the compositions of meteorites that have fallen to Earth can be shown to match the compositions of asteroids. In some cases these asteroids are thought to be the large remnants of the even larger parent bodies of both asteroids and meteorites; in other cases the meteorites that fell to Earth may simply have been smashed off the asteroid by smaller impacts.
Because meteorites originate with asteroids that formed in different parts of the solar system and through different processes, meteorites themselves have a wide range of compositions and appearances. In the next section, the types of meteorites and what can be learned from them are described, beginning with the most primitive type, the chondrites.
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