Other
Scientific paper
Sep 1995
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1995metic..30r.500c&link_type=abstract
Meteoritics, vol. 30, no. 5, page 500
Other
1
Asteroids, S, Densities, Meteorites, Dalgaranga, Estherville, Lundsgard, Marmande, Mincy, Nerft, Rakovka, Shelburne, Tolvca, Uberuba
Scientific paper
The Galileo flyby of Ida has highlighted the S-asteroid controversy, posed new questions about asteroidal structure, and pointed out a major gap in our studies of meteorites. The density of Ida determined by Galileo is 2.6 +/- 0.5 g/cm3; that of Phobos measured by the Phobos mission is 1.9 +/- 0.05 g/cm3. Both are substantially below measured ordinary chondrite densities. But in fact very few density measurements of meteorites can be found in the literature, and many do not include the effects of porosity. Only 46 meteorite porosity measurements have been published in the last 35 years, obtained by six different methods and covering only a small fraction of meteorite types. Beginning a systematic, consistent, and statistically significant data base of meteorite density and porosity measurements, we have started measuring the bulk density of meteorites in the Vatican collection. Here we report on one technique, of several under consideration. Previous methods of measuring the volume (including pore spaces) of small, irregular meteorite samples have included cutting the sample into a cube (destructive and potentially altering the porosity) or wrapping the sample in plastic and immersing it in water (though a less-than-perfect fit of a rough the surface can exaggerate inferred pore space). Here, we place the sample in a container of known volume which then is filled with a powder of known bulk density. (For now, we simply used sugar in an ordinary measuring cup; other substances, such as fine silica sand, will also be tested.) After measuring the mass of the meteorite plus powder, the meteorite mass itself, the container volume, and the powder density, the meteorite density can be derived. This method is non-destructive, if the powder is carefully chosen, and follows irregularities the size of the powder itself on the meteorite surface. There are several sources of error, however. The effective density of the powder can change significantly depending on how it is packed in the container; and its density may also vary with humidity. To minimize the first problem, we pack the powder in a way as repeatable as possible, and average several measurements (5 to 10 per sample) for a statistical error of < 0.1 g/cm3. To minimize systematic error, the density of the powder itself is constantly re-measured during each run. (In addition, two samples listed below are polished slabs whose volumes were measured directly.) Our preliminary measurements are intended only as an exploration of the procedure. Nonetheless they are suggestive. Note that the bulk density of a given meteorite varies from sample to sample, and even more among meteorites within a class (see mesosiderites; but even within an ordinary chondrite class, bulk density can vary by 0.5 g/cm3) and that, to date, all meteorites measured are significantly more dense than Ida or Phobos. With more data we can begin to look for significant and systematic variations in porosity within meteorite types. Eventually, this knowledge can directly support the interpretation of spacecraft data on the amount of void space in asteroids.
Britt Daniel T.
Consolmagno Guy J.
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