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Astron. Astrophys. 321, 652-659 (1997)

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5. Conclusions

The fairly high values of the ejection velocity required to inject fragments from Iris into the 3/1 resonance (Sect. 2), could suggest that this object should not be considered as a major source of meteoritic material. Nevertheless, as quoted in Sect. 2, some kind of heterogeneity seems likely to exist on the basis of the results of observations based on different techniques (photometry, polarimetry, radar) suggesting that some energetic impacts actually occurred. From spectroscopy we have not any further proof of the occurrence of energetic impact events in the past. The lack of any spectroscopic evidence of surface heterogeneity resulting from our observations, however, should not be overemphasised, since this simply suggests that we have not any proof of an evident mineralogic heterogeneity on the Iris' surface.

It has been shown by observations of the Vesta family and its surrounding objects (Binzel & Xu, 1993a) that very high ejection velocities (of the order of 1 km/sec) can be imparted to fragments from collisional events involving large asteroids. This is also confirmed by typical values of ejection velocities that can be derived from an analysis of the other families presently known (Zappalà et al., 1996). On the basis of these evidences we should be cautious in ruling out a possible role of objects like (7) Iris as important sources of meteorites. In particular, we should take into account that the size of Iris is favourable for a high-delivery efficiency (Farinella et al. 1993).

Therefore, the main conclusions of the present paper can be summarised as follows:

  • i. from a dynamical point of view (7) Iris is a possible source of debris through the 3:1 mean motion resonance with Jupiter. The required ejection velocity has been shown to be not implausible, although fairly high.
  • ii. Spectroscopically Iris is a typical S(IV)-type asteroid. This makes this object a good candidate for being a supplier of OCs (Gaffey et al. 1993). No rotationally-related spectral features have been found, but this does not rule out the possibility that a real surface texture heterogeneity, possibly related to the presence of large impact craters, could actually exist. Rather, the lack of spectroscopically observable features on the surface, does not support the idea that Iris might be a differentiated object. In turn, this fact could strengthen the conjecture that this object could be a supplier of OC s.
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© European Southern Observatory (ESO) 1997

Online publication: June 30, 1998