Extraterrestrial materials

Most atoms on earth came from the interstellar dust and gas from which the sun and solar system formed. However, in the space science community "extraterrestrial materials" generally refers to objects now on earth that were solidified prior to arriving on earth.

To date these fall into a small number of broad categories, namely: (i) Meteorites too large to vaporize on atmospheric entry but small enough to leave fragments lying on the ground, (ii) moon rocks brought back by the Apollo missions, (iii) unmelted micrometeorites typically less than 100 microns in diameter collected on earth and in the earth's stratosphere, and (iv) specimens returned from space-borne collection missions like NASA's Long Duration Exposure Facility (LDEF) and the recent Stardust sample return mission.

A minor but notable subset of the materials that make up the foregoing collections were also solidified "outside of our solar system". These are often referred to as interstellar materials, some of which are also presolar i.e. they predate the formation of our solar system.

Each of these types of material is (or should) be treated elsewhere. This entry, therefore, is here primarily to consider the relationship between types of extraterrestrial materials on earth, as well as the types of extraterrestrial material we'd like to have a closer look at.

Special Characteristics

Oxidation State

Thanks to the "romance" between carbon and oxygen atoms, which form gases when they combine, the solid material in a given solar system probably depends on whether carbon abundances were greater, or less, than those of oxygen. What's left over goes into solids. There was more oxygen than carbon in our solar system, so solid planetary surfaces (as well as primitive meteorites) are largely made up of oxides like SiO₂. If the converse was true, carbides might be the dominant form of "rock".

Larger planetary bodies in our early solar system were hot enough to experience melting and differentiation, with heavier elements (like nickel) finding their way into the core. The lighter silicate rocks presumably float to the surface. Thus this might explain why iron on earth's surface is depleted in nickel, while iron meteorites (from the center of a differentiated planetoid) are rich in nickel. Since it's not a very active rock-forming element, most of our nickel is in the earth's core. A key feature of extraterrestrial materials, in comparison to naturally occurring terrestrial materials, is therefore Ni/Fe ratios above several percent.

Another consequence of the leftover oxygen in our early solar system is the fact that presolar carbides from star systems with higher ratios of C/O are easier to recognize in primitive meteorites, than are presolar silicates. Hence presolar carbon and carbide grains were discovered first.

Elemental Abundances

Present day elemental abundances are superimposed on an (evolving) galactic-average set of elemental abundances that was inherited by our solar system, along with some atoms from local nucleosynthesis sources, at the time of the sun's formation [H. E. Suess and H. C. Urey (1956) Abundances of the elements, "Rev Mod Phys" 28:53-74.] [A. G. W. Cameron (1973) Abundances of the elements in the solar system, "Space Sci Rev" 15:121-146.] [E. Anders and M. Ebihara (1982) Solar-system abundances of the elements, "Geochim. Cosmochim. Acta" 46:2363-2380.] . Knowledge of these average solar system elemental abundances is serving as a powerful tool for tracking chemical and physical processes involved in the formation of planets, and the evolution of their surfaces. Awaiting more on this here cf. cosmic abundance.

Impact and irradiation effects

The atmosphere of planet earth, and the earth's magnetic field, protect us from impact and irradiation by a wide range of objects that fly around in space. Those who know about these effects can often learn from them a lot about the history of extraterrestrial materials.

Microcraters etc.

Microcraters, "pancakes", and "splashes" were first seen on the surface of lunar rocks and soil grains. They tell us about the direct exposure of an object's surface to space in the absence of a vacuum. These structures are quite unlike features found on the surface of terrestrial rocks and soil. They have also been identified on grains found in meteorites, and on man-made objects exposed to the micrometeorite flux in space.

Nuclear particle tracks

Nuclear track damage trails from the passage of heavy ions in non-conducting solids were first reported by E. C. H. Silk and R. S. Barnes in 1959 [E. C. H. Silk and R. S. Barnes (1959) Examination of fission fragment tracks with an electron microscope, "Phil. Mag." 4:970-971] . Their etchability along with many subsequent applications were established by Fleischer, Price and Walker, starting with their work at General Electric Laboratories in Schenectady, New York [R. L. Fleischer, P. Buford Price, and Robert M. Walker (1975) "Nuclear Tracks in Solids" (U. California Press, Berkeley).] . See also solid state nuclear track detector.

Nuclear particle tracks subsequently found many uses in extraterrestrial materials, thanks both to the exposure of those materials to radiation in space, and to their sometimes ancient origins. These applications included (i) determining the exposure of mineral surfaces to solar flare particles from the sun, (ii) determining the spectrum of solar flare particle energies from the early sun, and (iii) the discovery of fission tracks from extinct isotopes of Plutonium 244 in primitive meteorites.

Nuclear spallation effects

Particles subject to bombardment by sufficiently energetic particles, like those found in cosmic rays, also experience the transmutation of atoms of one kind into another. These spallation effects can alter the trace element isotopic composition of specimens in ways which allow researchers in the laboratory to fingerprint the nature of their exposure in space.

These techniques have been used, for example, to look for (and determine the date of) events in the pre-earth history of a meteorite's parent body (like a major collision) that drastically altered the space exposure of the material in that meteorite. For example the Murchison meteorite landed in Australia in 1967, but its parent body apparently underwent a collision event about 800,000 years ago [M. W. Caffee, J. N. Goswami, C. M. Hohenberg, K. Marti and R. C. Reedy (1988) in "Meteorites and the early solar system" (ed. J. F. Kerridge and M. S. Matthews, U Ariz. Press, Tuscon AZ) 205-245.] which broke it into meter-sized pieces.

Isotopic abundances

The isotopic homogeneity of our planet and our solar system provides a blank slate on which to detect the effect of nuclear processes on earth, in our solar system [Robert N. Clayton (1978) Isotopic anomalies in the early solar system, "Annual Review of Nuclear and Particle Science" 28:501-522.] , and in objects coming to us from outside the solar system. Awaiting more here, see natural abundance.

Noble gases

Noble gases are particularly interesting from an isotopic perspective, first because they avoid chemical interactions, secondly because many of them have more than one isotope on which to carry the signature of nuclear processes (xenon has many), and finally because they are relatively easy to extract from solid materials by simple heating. As a result, they play a pivotal role in the unfolding drama of extraterrestrial materials study.

Some clues to moving further might be found in this book [Minoru Ozima and Frank A. Podosek (2002) "Noble gas geochemistry" (Cambridge U. Press, NY, second edition) ISBN 0521803667] and this article [Charles M. Hohenberg, Noble gas mass spectrometry in the 21st century, "Geochimica et Cosmochimica Acta" 70:18, A258.] .

Radiometric dating in general

Isotopic abundances provide important clues to the date of events that allowed a material to begin accumulating gaseous decay byproducts, or that allowed incident radiation to begin transmuting elements. Because of the harsh radiations, unusual starting compositions, and long delays between events sometimes experienced, isotopic dating techniques are of special importance in the study of extraterrestrial materials.

Until more perspective is added here on the importance of these tools to extraterrestrial materials research, you can find clues to some strategies not mentioned here under radiometric dating.

Other isotopic studies

The categories of study mentioned before this are classic isotopic applications. However, extraterrestrial materials also carry information on a wide range of other nuclear processes. These include for example: (i) the decay of now-extinct radionuclides, like those supernova byproducts introduced into solar system materials shortly before the collapse of our solar nebula [Ernst Zinner (2003) [http://www.sciencemag.org/cgi/content/summary/300/5617/265 An isotopic view of the early solar system] , "Science" 300:5617, 265-267.] , and (ii) the products of stellar and explosive nucleosynthesis found in almost undiluted form in presolar grains [Ernst Zinner (1998) Stellar nucleosynthesis and the isotopic composition of presolar grains from primitive meteorites, "Annual Review of Earth and Planetary Sciences" 26:147-188.] . The latter are providing astronomers with up-close information on the state of the whole periodic table in exotic environments scattered all across the early Milky Way.

See also

* meteorites
* moon rock
* cosmic dust
* interplanetary dust cloud
* presolar grains
* cosmochemistry

References and Footnotes

External links

* [http://www/psrd.hawaii.edu/ Planetary Science Research Discoveries] Educational journal with articles about extraterrestrial materials.