Re-assessing the ordinary chondrites paradox

¹ Universidade Federal do Rio de Janeiro / Observatório do Valongo Lad.Pedro Antônio, 43 - 20080-090 Rio de Janeiro, Brazil e-mail: thais.mothe@astro.ufrj.br
² Observatório Nacional, Rua Gal. José Cristino, 77 20921-400 Rio de Janeiro, Brazil e-mail: lazzaro@on.br,flavialuzia@on.br,carvano@on.br
³ Southwest Research Institute, 1050 Walnut St., Suite 300 Boulder, Colorado 80302, USA e-mail: davidn@boulder.swri.edu
⁴ Universidad de la Serena Av. Cisternas 1200, La Serena, Chile e-mail: aramirez@dfuls.cl

Received: 9 December 2009
Accepted: 23 February 2010

Abstract

Context. The so-called ordinary chondrite paradox has been a recurring topic in planetary science in the past twenty years. This paradox originated from the first comparisons between spectrophotometric measurements of meteorites and asteroids. Basically it is based on the lack of spectral analogs of the ordinay chondrites (OC) among the Main Belt asteroids, although present among the NEAs. Several hypothesis to account for the paradox have been published, all considering the effect of space weathering and different degrees of resurfacing on the asteroids surfaces.

Aims. The aim of the present paper is to further investigate this intriguing problem considering that the small-sized population of Main Belt asteroids has not yet been analyzed.

Methods. Spectroscopic observations of Main Belt asteroids with a size smaller than 5 km – similar to that of NEAs – were performed with the 8-m Gemini telescope. Furthermore, spectra obtained in large spectroscopic surveys were used to perform a statistical analysis of the fraction of spectral analogous to the OCs in the Main Belt and NEA populations.

Results. The main result of this work is that the vast majority of the Main Belt Sk- and Sq-class asteroids can be matched to OC meteorites in the visible part of the spectrum.

Conclusions. Our results suggest that a considerable fraction of the OC material in the Main Belt is presently unweathered enough to be comparable to meteorite laboratory spectra.