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T. V. Johnson (Jet Propulsion Laboratory/Caltech)
It is becoming evident that many small bodies (comets, asteroids, and satellites) must be highly porous if their compositions have been correctly inferred. Porosities were calculated for eleven small bodies whose compositional identifications are generally accepted (6 "C-objects", 4 "ice", 1 "basalt"). For the seven bodies with volumes < 107 km3 calculated mean porosities are between 0.3 and 0.7. When the error bars are taken into account, only one of these, Phobos, has a porosity below 0.2 and two objects, Prometheus and Mathilda have porosities constrained to be above 0.5 and 0.4 respectively. The composition of S-class asteroids remains a subject of debate partially because of the uncertainty in the porosity to be assumed. Densities for two S-class asteroids have been reported, Ida (2.55±0.65 gm cm-3, Belton et al., Nature, 374, 785, 1995) and Eros (2.5±0.8 gm cm-3, Yeomans et al.; Veverka et al., Sci. in press, 1999). Belton et al. suggested, based on analogy to lunar regolith studies that porosities greater than 0.4 were unlikely and concluded that "the results…point away from sub-classes of stony-irons …. and a stony bulk composition with moderate NiFe content appears favored". The arguments above suggest that considerably larger porosities may be appropriate. Ida and Eros are both somewhat smaller than Mathilde. If we assume Mathilde's relatively well constrained porosity (0.52±0.07), then their nominal sample densities are both ~5 gm cm-3, equivalent to the stony-iron composition given in Belton et al., although the errors permit a stony or chondritic interpretation at one extreme. I suggest that the Belton et al. conclusion for Ida should be modified (and also applied to Eros) to "the results point away from stony classes and a stony-iron bulk composition with considerable NiFe appears favored". This work was carried out at Caltech's Jet Propulsion Laboratory under a contract from NASA.