THE DENSITY AND POROSITY OF MARTIAN METEORITES

We measured via glass bean immersion the bulk density of the Martian meteorites Nakhla, Chassigny, and shergottites Dar al Gani 476 and Sayh al Uhamyr 005. To compute model porosities we compared these values with grain density values calculated from their known mineralogy. As previously reported, our 153g Nakhla has a 3.10 (g/cc) bulk density, 3.29 grain density, measured porosity 5.7%, model 6.9%. We find our 15.6g Chassigny has a bulk density of 3.48, model porosity 3.2%. The values for the shergottites are: DaG 476, 18.3g, density 3.21, porosity 2.7%; SaU 005, 19.2g, density 3.23, porosity 2.1%. All densities are ± 0.07 or better.

In addition, we are examining thin sections of these meteorites to see if microcrack areas visible in SEM images can be related to the measured or inferred porosity; our first result shows that a section of DaG 476 made from the sample measured here has a microcrack porosity of 3.4 ± 2.2%, consistent with our hand-sample measurements.

Few literature values exist SNCs. Terho found a bulk 3.32g density for Chassigny, significantly less than ours, and 3.2 for Nakhla, larger but not statistically different from ours. He also found densities for the shergottites Zagami (3.07) and EET 79001 (3.12) significantly less than our shergottites. (The Flynn et al. grain density of Zagami is 3.43.) Different compositions among the shergottites may account for these densities.

These densities and porosities have significance both for Mars and the processes that transport meteorites to Earth. They are directly applicable to Martian geophysical models, which in the past have sometimes used unrealistically low ?round numbers? (e.g. 3) for crustal densities. But the low porosities also imply that the process of breaking these meteorites from Mars and landing them on Earth did not significantly add microcracks to their structure, implying that the higher porosities seen in ordinary chondrites may well be characteristic of their in situ (asteroidal) state.