Meteorites look different from Near-Earth Asteroids
1 August 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: Model results for the ratio of olivine to olivine+orthopyroxene for 57 ordinary chondrites and 38 S- and Q-type NEAs. The inferred compositional ranges for the Flora family and PHAs (8 among 12 observed PHAs have ol/(ol+opx) ratios in the 73-79% range; the 4 others are displayed with ’+’ symbols) are also shown. 95% (±4%) of H and L OCs are on the left of the dotted line and 95% (±4%) of LL OCs are on the right. The x position of the H, L and LL labels indicates their average ol/(ol+opx) value. The compatibility between an NEA and an LL chondrite occurs when the ol/(ol+opx) ratio for an NEA falls into the LL compositional range (70-85%). It appears that most OCs and large S- and Q-type NEAs do not have compatible mineralogy. Most NEAs ( 63%) are compatible with an LL mineralogy (ol/ol+opx in the 70-85% range) which account for 10% of the OCs falls (from Vernazza et al.). Click on the image to enlarge it
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: Here we illustrate the dynamics of small and large bodies in the inner asteroid belt. We show the zones of the nu-6 and 3:1 resonances in dark grey and note their relative efficiencies for delivery to Earth (Bottke et al. 2002). The square delimited by dotted lines shows the domain in the (a,e) space of the Flora family. We show the mean drift (in AU) due to the Yarkovsky effect of kilometer- and meter-sized bodies over a given timespan. For a 1km object: Delta a: 0.01-0.02 AU in 0.5 Gyr, as indicated by the small arrow next to the body. The light gray shading at both the nu-6 and 3:1 resonances shows the expected source regions for most NEAs. For a 5m object: Yarkovsky drift in Delta a is substantially greater 0.2AU in a much shorter timescale < 80 Myrs), enabling either resonance to be reached (bi-directional arrows at center). At the bottom, we show the relative surface density of asteroids as a function of heliocentric distance. The higher surface density near the 3:1 resonance partly overcomes the lower relative efficiency of this resonance. Incorporating both surface density and delivery efficiency, the nu-6 resonance remains 1.5X more likely to deliver km-sized NEAs to Earth (Bottke et al. 2002). The net outcome is that km-sized NEAs should be favored from the inner belt (Flora family) while meter-sized meteorite parent bodies can originate from the entire inner asteroid belt and from beyond 2.5 AU (not shown). Click on the image to enlarge it
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 at esa.int)
