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Asteroids: Mantles were battered to bits
News and Views commentary by Clark R. Chapman
Nature385, 293-295, (1997)
Astronomers once regarded asteroids as monotonously grey boulders and planetoids, the 'vermin of the skies' that interfered with their photographs of galaxies. But by the mid-1980s, astronomers had collected spectral reflectance data for hundreds of asteroids, and thousands of meteorites had been collected in Antarctica. The taxonomy of asteroids agreed with the mineralogy of meteorites: both include diverse materials ranging from iron-nickel alloy to the coal-black carbonaceous chondrites.
Some asteroids and meteorites are thought to be primitive, undifferentiated materials. But others reflect extensive chemical fractionation due to thermal heating and partial melting, yielding models of asteroidal parent bodies resembling the Earth, with a metallic core, a thick overlying olivine mantle (about three-quarters of the volume), topped with a thin basaltic crust.
Clusterings in trace-element chemistry imply that iron meteorites must come from the cores of more than 60 differentiated parent bodies. Yet there are practically no olivine-rich asteroids that might be fragments of the mantles of those collisionally stripped cores -- there are only seven so-called A types among about 1000 asteroids now spectrally sampled. Similarly, of thousands of collected meteorites, only a handful (including some now thought to be Martian rocks) are nearly pure olivine.
Where is the missing olivine? It is a common component of most S-type asteroids (the second most common type) and such meteorites as the undifferentiated ordinary chondrites. But from astronomers' distant perspective, it is impossible to distinguish the spatial scale of olivine, whether homogeneously mixed as fine grains as in undifferentiated meteorites, or as huge chunks of mantle fragments mixed in a rubble pile formed when a differentiated parent body broke up and gravitationally reassembled. The latter possibility would resolve one astronomical underpinning of the olivine shortage, but it is denied by the absence of olivine-rich meteorites from such asteroids.
Meteoriticists have preferred instead to bury the missing olivine. For example, the so-called HED meteorites are basaltic and inferred to come from the crust of a single differentiated parent body, probably the 500-km Vesta, the only large asteroid to have the spectral signature of basalt. Indeed, despite slight color variations revealed by recent Hubble Space Telescope images , Vesta's thin crust remains more or less intact. Therefore, it is reasonable that Vesta's olivine-rich mantle, protected beneath its intact crust, has not been sampled. Some other meteorite types, previously thought to come from deep within the HED parent body (stony-irons and IIIAB irons), must come from some other, now disrupted parent body or bodies -- they can hardly be derived from the deep interior of still-intact Vesta!
Burbine et al. propose a new collisional history for the main asteroid belt that may resolve the great olivine shortage. They have invoked the latest understanding of what it takes to destroy an asteroid, including revised, lower estimates [Ahrens, T.J. & Love, S.G. Lunar Planet. Sci. 27, 1-2, 1996] of the fraction of projectile kinetic energy that can go into high-velocity fragments -- those moving fast enough to escape from the parent body. The authors conclude that collisions among known asteroids are much too infrequent to break up the 60 or more parent bodies required to explain the variety of iron meteorites. But if the current asteroids are just the remnant of the collisional comminution of 50 times as many asteroids, then plenty of metallic cores could have been liberated by catastrophic collisions. And the mantles and crusts associated with them, made of rocks much weaker than the metallic cores, would have been 'battered to bits.' Long ago, the olivine would have been comminuted into dust and swept into the Sun.
Indeed, we see some remaining metallic cores, sandblasted clean of their once- overlying mantle rocks. An example is the 260-km asteroid Psyche, whose extreme radar reflectivity [Ostro, S.J., Campbell, D. & Shapiro, I.I. Science 229, 442-446, 1985] confirms the metallic composition, tentatively inferred from reflectance spectra. In the Burbine et al. scenario Psyche is a lucky core, and most others have been fragmented. Even luckier are those weaker, rocky asteroids like the dark C-type asteroid Mathilde, which will be studied by the Near Earth Asteroid Rendezvous spacecraft in June. They must be like lottery winners, the rare ones that just happened to avoid any catastrophic collisions.
One would also expect some moderately lucky differentiated rubble piles to remain (not destroyed but not wholly intact). As we get no olivine-mantle meteorites from them, they can't be common near the dynamical escape hatches from the asteroid belt through which we get meteorites, but some S-type asteroids far from such resonances could be such differentiated rubble piles.
Vesta poses an even greater puzzle. Burbine et al. calculate that if the asteroids had been more than 100 times as populous as the present belt, there would be little chance that even one differentiated object could have survived catastrophic fragmentation and disruption. They fail to note, however, that just several times the current impact rate in the asteroid belt should have destroyed Vesta's thin basaltic crust, unless it was formed recently. Yet the HED meteorites, thought to be derived from Vesta's crust, are among the oldest known meteorites! I like much of the 'battered to bits' scenario. But then Vesta is a bigger mystery than ever.
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