Small is beautiful: The analysis of nanogram-sized astromaterials

The capability of modem methods to characterize ultra-small samples is well established from analysis of interplanetary dust particles (IDPs), interst ellar grains recovered from meteorites, and other materials requiring ultra -sensitive analytical capabilities. Powerful analytical techniques are avai lable that require, under favorable circumstances, single particles of only a few nanograms for entire suites of fairly comprehensive characterization s. A returned sample of > 1000 particles with total mass of just 1 mu g per mits comprehensive quantitative geochemical measurements that are impractic al to carry out in situ by flight instruments. The main goal of this paper is to describe the state-of-the-art in microanalysis of astromaterials. Given that we can analyze fantastically small quantities of asteroids and c omets, etc., we have to ask ourselves, how representative are microscopic s amples of bodies that measure a few to many kilometers across? With the Gal ileo flybys of Gaspra and Ida, it is now recognized that even very small ai rless bodies have indeed developed a particulate regolith. Acquiring a samp le of the bulk regolith, a simple sampling strategy, provides two critical pieces of information about the body. Regolith samples are excellent bulk s amples because they normally contain all the key components of the local en vironment, albeit in particulate form. Furthermore, because this fine fract ion dominates remote measurements, regolith samples also provide informatio n about surface alteration processes and are a key link to remote sensing o f other bodies. Studies indicate that a statistically significant number of nanogram-sized particles should be able to characterize the regolith of a primitive asteroid, although the presence of larger components (e.g., chond rules, calcium-aluminum-rich inclusions, large crystal fragments, etc.) wit hin even primitive meteorites (e.g., Murchison) points out the limitations of using data obtained from nanogram-sized samples to characterize entire p rimitive asteroids. However, the most important asteroidal geological proce sses have left their mark on the matrix, because this is the finest-grained portion and therefore most sensitive to chemical and physical changes. Thu s, the following information can be learned from this fine grain size fract ion alone: (1) mineral paragenesis; (2) regolith processes; (3) bulk compos ition; (4) conditions of thermal and aqueous alteration (if any); (5) relat ionships to planets, comets, meteorites (via isotopic analyses, including O ); (6) abundance of water and hydrated material; (7) abundance of organics; (8) history of volatile mobility; (9) presence and origin of presolar and/ or interstellar material. Most of this information can be obtained even fro m dust samples from bodies for which nanogram-sized samples are not truly r epresentative. Future advances in sensitivity and accuracy of laboratory analytical techni ques can be expected to enhance the science value of nano- to microgram-siz ed samples even further. This highlights a key advantage of sample returns- that the most advanced analysis techniques can always be applied in the lab oratory and that well-preserved samples are available for future investigat ions.