Talk:M-type asteroid
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[edit] Porosity
It's odd with these large M-type asteroids in that they seem to have densities far below anything expected of solid metallic bodies. (I'm thinking of two particular examples 22 Kalliope and 16 Psyche). They have densities of 2.4 and ~2 g/cm³ which, if they are made of Ni-Fe metal, indicates a huge porosity of about 70% or more! That's more porous than plausible! This is discussed in e.g. D.T. Britt et al, Asteroids III, p. 485. Off the top of my head possible but completely unchecked explanations would be e.g.
- The metallic component seen in the M-type spectra accounts for only some part of the surface and overall composition but is what makes these asteroids have their distinctive spectra. E.g if they are 50% metal and 50% silicate, the observed density is more reasonable.
- The metallic spectra are due to extensive, preferentially metallic, surface melt on these objects from impacts (see for comparison suppositions about the surface of 6 Hebe, where something similar is proposed), but do not correspond to a metallic composition deep inside the body.
- Several different compositions have similar M-type spectra, not all of them metallic.
- A couple of coincidental errors in the mass estimates (after all there aren't that many big M-type asteroids, and the mass estimates based on close approach perturbations are not particularly accurate). Presumably the density of 22 Kalliope is pretty well known, though, since it has a moon.
These are just suppositions, though. Does anyone have any fresh data, references, or analysis of this issue? Michaelbusch? Deuar 21:18, 9 May 2006 (UTC)
Current scientific consensus, or at least as close as there is to consensus, is that different compositions can give the M-type spectral class. Objects that are spectrally the same show very different radar albedos. There really is no 'metallic spectrum' in the visible and near-IR. Rather, metallics and many other things are featureless except for a spectral slope towards the red. Note: 16 Psyche is metallic. Its density is estimated at ~3.5 g/cm^3. Michaelbusch 22:41, 9 May 2006 (UTC)
