M-type asteroid

M-type asteroids are asteroids of unknown composition; they are moderately bright (albedo 0.1–0.2). Some, but not all, are made of nickel-iron, either pure or mixed with small amounts of stone. These are thought to be pieces of the metallic core of differentiated asteroids that were fragmented by impacts. They are thought to be the source of iron meteorites.

There are also M-types whose composition is uncertain. For example, 22 Kalliope has an accurately known density that is far too low for a solid metallic object or even a metal rubble pile: a rubble pile of iron-nickel metal would need about 70% porosity which is inconsistent with packing considerations. Kalliope and 21 Lutetia have features in their spectra which appear to indicate the presence of hydrated mineralscite journal
author= A.S. Rivkin "et al"
title=The nature of M-class asteroids from 3-micron observations
journal= Icarus | year= 2000 | volume= 145 | pages= 351
url= http://adsabs.harvard.edu/cgi-bin/nph-bib_query?bibcode=2000Icar..145..351R
accessdate = 2007-07-23 | doi= 10.1006/icar.2000.6354] and silicates,cite journal
author=D.F. Lupishko "et al"
title=UBV photometry of the M-type asteroids 16 Psyche and 22 Kalliope
journal= Solar System Research | year= 1982
volume= 16 | pages= 75
url=http://adsabs.harvard.edu/abs/1982AVest..16..101L
accessdate = 2007-07-23 ] anomalously low radar albedos inconsistent with a metallic surface,cite journal
author=C. Magri "et al"
title= Radar constraints on asteroid regolith compositions using 433 Eros as ground truth
journal= Meteoritics & Planetary Science | year=2001
volume=36 | pages=1697
url=http://adsabs.harvard.edu/abs/2001M&PS...36.1697M
accessdate = 2007-07-23 ] as well as characteristics more in common with C-type asteroids. A variety of other M-type asteroids do not fit well into a metallic body picture.

M-type spectra are flat to reddish and usually devoid of large features, although subtle absorption features longward of 0.75 µm and shortward of 0.55 µm are sometimes present.cite journal
author= S.J. Bus and R.P. Binzel
title=Phase II of the Small Main-belt Asteroid Spectroscopy Survey: A feature-based taxonomy
journal=Icarus | year=2002 | volume=158 | pages=146
doi=10.1006/icar.2002.6856 ]

16 Psyche is the largest M-type asteroid, and does appear to be metallic. 21 Lutetia, an anomalous, probably nometallic body, will be the first M-type asteroid to be imaged by a spacecraft when the Rosetta space probe visits it on July 10, 2010. Another M-type, 216 Kleopatra, was imaged by radar by the Arecibo Observatory in Puerto Rico and has a dog bone-like shape.

M type was one of three basic asteroid types in early classifications (the others being the S and C types), and was thought to indicate a metallic body.

References