Meteorites look different from Near-Earth Asteroids
1er août 2008
An international team of scientists including astronomers from the European Space Agency (ESTEC, Netherlands), MIT (Cambridge, USA) and the Paris Observatory (LESIA) has discovered that meteorites differ in composition from near-Earth asteroids. They explored the mineralogical composition of near-Earth asteroids (NEAs) and their most closely intersecting subset (potentially hazardous asteroids, PHAs) via visible and near-infrared spectroscopy. A priori these km-sized asteroids should have similar compositional distributions to meteorites delivered by more frequent (and less dangerous) smaller impacts. Surprisingly, they do not. Nearly 2/3 of the km-sized Earth-crossing asteroids match a single meteorite class (LL chondrites) that comprises only 8% of all falls. The rather specific spectral signature of NEA’s allows them to trace their origin to a specific source at the inner edge of the asteroid belt (Flora family). The much broader compositional distribution of smaller meter-sized bodies (sampled as meteorites) implies a broader range of source regions (likely throughout the asteroid belt). One possible explanation is the role of a size dependent process, such as the Yarkovsky effect, in transporting material from the main belt.
Ordinary chondrites (OCs) are the most common class of meteorites, accounting for 80% of all recovered falls. While numerous factors (e.g. meteoroid size and strength prior to atmospheric impact) can bias the statistics for recovering samples, the most commonly falling meteorites should reasonably correspond to the most commonly observed asteroids in the vicinity of Earth. Their asteroidal analogs are the so-called S- and Q-type asteroids.
These asteroids were observed in the near infrared by means of IRTF/SpeX as part of a regularly scheduled program of near-Earth object characterization conducted jointly between the IRTF, University of Hawaii, and MIT. These NIR spectroscopic measurements (0.8-2.5 µm) complemented already acquired visible spectra recorded during a survey of 2000 asteroids (SMASS). Using a radiative transfer model, the team performed a detailed mineralogical comparison of these telescopically measured asteroid spectra with analogous wavelength laboratory measurements of ordinary chondrite meteorites (Fig 1).
Figure 1 : Rapport olivine sur olivine+orthopyroxene [ol/(ol+opx)] pour 57 chondrites ordinaires et 38 NEAs de types S et Q. L’intervalle de composition pour les membres de la famille de Flora est indiqué ainsi que la composition des géocroiseurs potentiellement dangereux (PHAs ; 8 des 12 PHAs observés ont une composition dans l’intervalle 73-79%, la composition des 4 autres est reportée avec un symbole’+’). 95% (±4%) des OCs de type H et L sont à gauche de la ligne pointilléé (x=70) et 95% (±4%) des OCs de type LL sont à droite de cette ligne. L’abscisse des labels H, L and LL indique leur ol/(ol+opx) moyen. Il apparaît que la plupart des NEAs (63%) ont une composition compatible avec celle des chondrites LL qui ne représentent que 8% des chutes (Vernazza et al.). Cliquer sur l’image pour l’agrandir
It appears that meteorites (OCs) and NEAs (S and Q-types) do not have the same compositional distribution. In particular, S and Q-types NEAs best match LL chondrites but these are a minority among all OCs (10%). The immediate implication is that the NEAs sampled telescopically (in the size range having radii between 300m to 10km) are not the immediate parent bodies of smaller objects that fall to Earth as meteorites (i.e. preatmospheric meteorite parent bodies having radii on the order of meters). Instead, both populations are directly delivered from the main-belt (MB) to the near-Earth space but must have different source regions within the MB.
Interestingly, a mineralogical compatibility is established between most NEAs and the Flora family members (this family lies in the inner belt and accounts for 15-20% of all inner main belt asteroids) ; this family had been predicted (by dynamical simulations) to be the source for a substantial fraction of km-sized NEAs. While theory and observations agree on this latter point, it was also expected that most meteorites should originate from this inner region and the new results imply that this can not be. Further dynamical modelling should help solving this paradox. For now, it is suggested that the Yarkovsky effect (Fig. 2) could well be the cause for this surprising result.
Figure 2 : Cette figure illustre la dynamique des petits et grands objets au sein de la ceinture principale interne. Les résonances nu-6 (1) et 3:1 apparaissent en gris foncé et leur efficacité relative ’d’envoi de matériel’ vers la Terre est mentionnée. Le carré délimité par des pointillés indique la zone (a,e) occupée par la famille de Flora. La variation moyenne du demi grand axe (en UA) due à l’effet Yarkovsky (2) est représentée par une flèche placée devant chaque objet. Pour un objet d’1km : Delta a 0.01-0.02 UA en 0.5 milliard d’années. Les zones gris claires autour des résonances indiquent les régions sources attendues pour ces NEAs. Pour un objet de 5m : Delta a 0.2 UA en moins de 80 millions d’années. L’objet peut ainsi atteindre une des 2 résonances sans aucune ’difficulté’. Enfin, la densité surfacique d’astéroïdes en fonction de la distance au Soleil est mentionnée au bas de la figure. La densité de surface plus importante près de la résonance 3 :1 permet de compenser partiellement le manque d’efficacité de cette résonance (par comparaison directe avec nu-6). En prenant en compte la densité surfacique et l’efficacité de chaque résonance, la résonance nu-6 (1) devrait être 1.5x plus efficace que la résonance 3 :1. En conclusion, les NEAs devraient majoritairement venir de la région qui borde la résonance nu-6 alors que les météorites peuvent provenir de la ceinture interne toute entière voire au delà de 2.5 UA. Cliquer sur l’image pour l’agrandir
Reference Compositional differences between meteorites and near-Earth asteroids, Nature 454, 2008. P. Vernazza, R P. Binzel, C. A. Thomas, F. E. DeMeo, S. J. Bus, A.S. Rivkin, A. T. Tokunaga
Contact
Pierre Vernazza (Docteur de l’Observatoire de Paris en postdoc à l’ESA, pierre.vernazza chez esa.int)
