Asteroid chemical compositions

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). 

Many asteroids are dry, as evidenced by meteorites in which water is virtually absent. These samples include many classes of chondrites, as well as melted chunks of the crusts, mantles, and cores of differentiated objects. Anhydrous bodies were important building blocks of the rocky terrestrial planets, and their chemical compositions reveal details of processes that occurred within our own planet on a larger scale. The distributions of these asteroids within the solar system also provide insights into their formation and evolution. 

Asteroids have been a focus of spectroscopic studies for decades. Spectra obtained from telescopes on the Earth can identify some of the minerals that make up asteroids, but do not measure asteroid chemistry. Nevertheless, spectroscopic matches can be used to link some meteorite classes to then probable parent bodies, and thus allow indirect assessments of then chemical compositions. A few asteroids have been visited and analyzed by spacecraft. Chemical analyses require long data integrations from orbit or actually landing on the surface, and analyses of only two small near-Earth asteroids have been reported. 

There is evidence from chondrites that the solar nebula was well mixed between 0.1 and 10 AU during its first several million years of the evolution, as shown by the homogeneity in concentrations of many isotopes of refractory elements (Boss 2004 Chapter 9). This is likely caused by the evaporation and recondensation of solids in the very hot inner nebula, followed by outward transport due to turbulent diffusion and angular momentum removal. Materials out of which terrestrial planets and asteroids are built have been heated to temperatures above 1300 K and are thus depleted in volatile elements. The inner solar nebula, with some exceptions, does not retain memories of the pristine interstellar medium (ISM) chemical composition (Palme 2001 Trieloff Palme 2006). 

In contrast to the small number of differentiated parent bodies represented by evolved achondritic meteorites, the number of parent bodies inferred from the chemical compositions of iron meteorites may be as large as 50 (Wasson, 1990). Of the 13 major iron meteorite groups, 10 appear to be from cores of differentiated meteorites. Many additional cores are inferred from the ungrouped irons, which make up —15% of iron meteorites. It is a puzzle why we appear to sample many more cores than mantles of these asteroids (see Chapter 1.12 for further discussion). 

Chemists have long been fascinated by the composition of the planets, asteroids, comets, meteors, and other objects that make up the solar system. Are these bodies made of the same elements and compounds, in the same proportions, as the Earth itself How do their surfaces and atmospheres differ, if at all, from those of Earth If chemical differences among solar system bodies do exist, how can they be explained What does the chemical composition of solar system bodies tell scientists about their possible origins ... 

Data about the chemical composition of asteroids comes primarily from three sources (1) the spectrographic analysis of light reflected off an asteroid by Earth-based observatories and the Hubble Space Telescope (2) the laboratory analysis of meteorites, which in most cases are known or presumed to have originated in the asteroid belt and (3) observations made by the spacecraft Galileo on its... 

Modern space technology has produced data about comets, meteors, asteroids, and the Moon that answer questions people have been asking for centuries about these bodies. Today scientists know a great deal about the chemical composition of these objects, their orbits through the solar system, and some mechanisms by which they may have been produced. Studies that produced these data are ongoing, and even more detailed understandings of these objects can be expected in the future. 

The nature of specific minerals found in meteorites, and their chemical compositions, as well as the cooling rates determined for irons and stony irons indicate that high hydrostatic pressures did not exist. The temperature profiles are consistent with those that asteroids (<1000 km diameter) would have had over the 4.57 Ga history of the Solar System (7). These minerals include those formed during primary nebular condensation and accretion into primitive parent bodies and those formed by secondary processes -thermal metamorphism and/or differentiation - during alteration of primitive bodies into evolved ones. 

The only in situ chemical data for asteroids are from the NEAR Shoemaker spacecraft, which orbited 433 Eros in 2000-1, and from the Japanese Hayabusa spacecraft, which visited 25143 Itokawa in 2003. NEAR obtained numerous measurements of the surface composition using X-ray fluorescence and gamma-ray spectrometers, and Hayabusa carried an XRF. The magnesium/silicon and aluminum/siUcon ratios for both asteroids are consistent with the compositions of chondrites. However, sulfur is depleted in Eros relative to chondritic compositions, possibly due to devolatilization by impacts or small degrees of melting. 

When water, the universal solvent, is present on a planet, an asteroid, or in a meteorite, a wide variety of chemical reactions take place that can completely alter the mineralogy and chemistry of an object. Some meteorites show extensive evidence of aqueous alteration. To understand the conditions under which the alteration occurred, one must be able to infer the amount, composition, and temperature of the fluids from the minerals that they produced. 

Some of these processes also cause measurable isotopic effects. Evaporation into empty space can cause the residual liquid or solid to become enriched in heavy isotopes. Processes that do not necessarily produce chemical fractionations can also produce isotopic effects. Diffusion is an example of such a process. Also, if the various constituents that go into making asteroids and planets have different isotopic compositions, the formation of these bodies can result in bulk compositions that are isotopically fractionated. Oxygen isotopes... 

Crystallization of melts provides another way to fractionate chemical elements. Most crystals are denser than the melt from which they form, so crystal settling can separate the phases. Convection currents in hot magma can also entrain crystals and carry them to the bottom of a magma chamber. Accumulations of crystals are a common occurrence on the floors of terrestrial plutons, and many achondrites are cumulates formed in an analogous way in asteroids. Cumulates and the complementary melt have different elemental compositions. 

It is very difficult to reconstruct parent body compositions from differentiated meteorites. In this section, we will describe the chemical characteristics of the meteorites themselves, but the compositions of the bulk asteroids from which they were derived can only be inferred in the broadest terms and are usually assumed to have been chondritic. 

---