A radar survey of M- and X-class asteroids II. Summary and synthesis

Michael K. Shepard, Beth Ellen Clark, Maureen Ockert-Bell, Michael C. Nolan, Ellen S. Howell, Christopher Magri, Jon D. Giorgini, Lance A.M. Benner, Steven J. Ostro, Alan W. Harris, Brian D. Warner, Robert D. Stephens, Michael Mueller

Research output: Contribution to journalArticlepeer-review

70 Scopus citations


Using the S-band radar at Arecibo Observatory, we observed six new M-class main-belt asteroids (MBAs), and re-observed one, bringing the total number of Tholen M-class asteroids observed with radar to 19. The mean radar albedo for all our targets is σ̂OC=0.28±0.13, significantly higher than the mean radar albedo of every other class (Magri, C., Nolan, M.C., Ostro, S.J., Giorgini, J.D. [2007]. Icarus 186, 126-151). Seven of these objects (Asteroids 16 Psyche, 129 Antigone, 216 Kleopatra, 347 Pariana, 758 Mancunia, 779 Nina, 785 Zwetana) have radar albedos indicative of a very high metal content (meanσ̂OC=0.41±0.13), and consistent with a remnant iron/nickel core interpretation (irons) or exotic high metal meteorite types such as CB. We propose designating these high radar albedo objects as Mm. Two asteroids, 110 Lydia and 678 Fredegundis, have more moderate radar albedos (meanσ̂OC=0.22), but exhibit high values (σ̂OC~0.35) at some rotation phases suggesting a significant metal content. The remaining 10 objects have moderate radar albedos (σ̂OC=0.20±0.06) at all rotation phases. Most of our targets have visible/near-infrared spectra (Hardersen, P.S., Gaffey, M.J., Abell, P.A. [2005]. Icarus 175, 141-158; Fornasier, S., Clark, B.E., Dotto, E., Migliorini, A., Ockert-Bell, M., Barucci, M.A. [2009]. Icarus, submitted for publication) that indicate the presence of at least some silicate phases. All of the non-Mm asteroids show a positive correlation between visual and radar albedo but the reasons for this are not clear. All of the higher radar albedo targets (the 7. Mm asteroids, Lydia, and Fredegundis) show moderate to large variations in radar albedo with rotation phase. We suggest that their high radar reflectivity exaggerates irregularities in the asteroid shape to cause this behavior. One-third of our targets show evidence for asteroid-scale concavities or bifurcation. Based on all the evidence available, we suggest that most Tholen M-class asteroids are not remnant iron cores or enstatite chondrites, but rather collisional composites of silicates and irons with compositions more analogous to stony-iron meteorites and high-iron carbonaceous chondrites.

Original languageEnglish (US)
Pages (from-to)221-237
Number of pages17
Issue number1
StatePublished - Jul 2010
Externally publishedYes


  • Asteroids
  • Asteroids, Composition

ASJC Scopus subject areas

  • Astronomy and Astrophysics
  • Space and Planetary Science


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