21 Lutetia: a missing link between Earth-like worlds and primitive space rocks

The asteroid 21 Lutetia was discovered in Paris (and hence the name Lutetia) by Hermann Goldschmidt in November 1852.  The origin of 21 Lutetia  has intrigued astronomers. It has been classified as an M-type asteroid, which are generally bright, metallic bodies which are sometimes mixed with rocky materials and have mostly featureless spectra. They are thought to be the metallic fragments of the cores of differentiated planetesimals and the parent bodies of the iron meteorites.  Lutetia, however, also has spectral features resembling the C-type asteroids, which are darker carbonaceous bodies that are thought to derive from primitive undifferentiated bodies. Lutetia’s spectra suggests the presence of hydrated (meaning water-containing) minerals, which also points to a more primitive surface.

On 10 July 2010 Lutetia was approached at a distance of about 3200 km by the Rosetta spacecraft, which is on its way to the comet 67P/Churyumov–Gerasimenko. The aim of the asteroid fly-by were to try to determine the asteroid’s true composition and decide whether it is an M-type or C-type asteroid; to accurately determines its mass and size and hence its bulk density; and of course to test Rosetta’s scientific instruments as it continues on to the comet.  Rosetta accurately mapped the entire surface of the knobbly 121 km x 101 km x 75 km asteroid, as well as measure its gravity. Results of the Rosetts analysis of Lutetia have been published in this week’s edition of the journal Science.

High resolution map of 21 Lutetia (Coutesy ESA 2010 MPS for OSIRIS Team)

Observations made with the Rosetta OSIRIS (Optical, spectroscopic and Infrared Remote Imaging System), with the help of ground-based observations and models, have derived a volume of 5.0×1014 m3. Doppler shift of radio signals from the spacecraft suggest a gravitational perturbation mass of 1.700 ± 0.017 × 1018 kg, which is much lower than expected from ground-based observations. Thus Pätzold et al. derived a bulk density of 3.4 x 103 kg/m3. Compared with the low value of the total mass, this high density indicates a non-chondritic bulk composition enriched in heavy elements like iron.  Sierks et al. show that the OSIRIS images provide evidence of a complex geology and an ancient surface which, together with the high bulk density, suggests partial differentiation of the asteroid and hint that Lutetia may be a primordial planetesimal.

Coradini et al. used the Rosetta VIRTIS (Visible, InfraRed, and Thermal Imaging Spectrometer) to investigate the surface composition and temperature of Lutetia.  They found no absorption features from either silicates or hydrated minerals across the spectral range 0.4–3.5 μm, and nor did they find any spectral signatures of surface alterations resulting from space weathering. They suggest that Lutetia is likely a remnant of the primordial planetesimal population, unaltered by differentiation processes and composed of chondritic materials of enstatitic or carbonaceous origin. The absence of absorption features coupled to the large bulk density is a strong indication that Lutetia has a metal-rich composition.

It is through that the interior of Lutetia melted, likely due to heating by short-live radioisotopes, causing partial differentiation which allowed a core to form but which did not effect the primordial surface. This is a surprise, as it was previously thought that only two types of asteroids existed: fully differentiated or totally unmelted.  21 Lutetia appear to be a new type of asteroid with a melted interior but an unmelted crust.

For more details, see

[Posted by Sarah Maddison & Francesco Pignatale]

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