Small is beautiful: The analysis of nanogram‐sized astromaterials
Authors: ZOLENSKY M. E. , PIETERS C. , CLARK B. , PAPIKE J. J. 
Published: February 2010

Abstract— The capability of modern methods to characterize ultra‐small samples is well established from analysis of interplanetary dust particles (IDPs), interstellar grains recovered from meteorites, and other materials requiring ultra‐sensitive analytical capabilities. Powerful analytical techniques are available that require, under favorable circumstances, single particles of only a few nanograms for entire suites of fairly comprehensive characterizations. A returned sample of > 1000 particles with total mass of just 1 μg permits comprehensive quantitative geochemical measurements that are impractical 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 comets, etc., we have to ask ourselves, how representative are microscopic samples of bodies that measure a few to many kilometers across? With the Galileo flybys of Gaspra and Ida, it is now recognized that even very small airless bodies have indeed developed a particulate regolith. Acquiring a sample of the bulk regolith, a simple sampling strategy, provides two critical pieces of information about the body. Regolith samples are excellent bulk samples because they normally contain all the key components of the local environment, albeit in particulate form. Furthermore, because this fine fraction dominates remote measurements, regolith samples also provide information about surface alteration processes and are a key link to remote sensing of 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., chondrules, calcium‐aluminum‐rich inclusions, large crystal fragments, etc.) within even primitive meteorites (e.g., Murchison) points out the limitations of using data obtained from nanogram‐sized samples to characterize entire primitive asteroids. However, the most important asteroidal geological processes have left their mark on the matrix, because this is the finest‐grained portion and therefore most sensitive to chemical and physical changes. Thus, the following information can be learned from this fine grain size fraction alone: (1) mineral paragenesis; (2) regolith processes; (3) bulk composition; (4) conditions of thermal and aqueous alteration (if any); (5) relationships 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 from dust samples from bodies for which nanogram‐sized samples are not truly representative.

Future advances in sensitivity and accuracy of laboratory analytical techniques can be expected to enhance the science value of nano‐ to microgram‐sized samples even further. This highlights a key advantage of sample returns—that the most advanced analysis techniques can always be applied in the laboratory and that well‐preserved samples are available for future investigations.