M399_6b 43..46

evaporation. Taking the mean density and Bond albedo of a typical Vulcanoid to be the same as that of Mercury, we ®nd that objects with radii z satisfying 0:1 ( z ( 50 km can evade both drag and evaporation in the Vulcanoid belt. This is one of the most dynamically stable regimes in the entire Solar System. If further searches do not detect any intra-mercurial objects, this is a strong indicator that other processesÐsuch as planetary migrationsÐmay have disrupted the population. Although there are no known intra-mercurial bodies, we can ®nd candidate objects for the Earth±Mars belt. Suppose we search an asteroidal database for objects with inclinations i , 108 and eccentricities e , 0:2 between the semimajor axes of Earth and Mars, then we ®nd that there are ten objects. Of these, seven lie within our suggested Earth±Mars belt (1.08±1.28 AU), which is evidence for an enhancement of nearly circular orbits in this region. An even more striking test is to search through the objects between the Earth and Mars for asteroids of low eccentricity and inclination that are not planet-crossing. There are only three such objects (1996 XB27, 1998 HG49 and 1998 KG3) among all the asteroids in this database, and all three lie between 1.08 and 1.28 AU. Most of the ,50 asteroids with semimajor axes at present located in our Earth±Mars belt are moving on orbits with large eccentricities and inclinations. They are not dynamically stable, and will evolve on timescales of the order of a few million years. Most of these objects are believed to be asteroids ejected from resonance locations in the main belt, although a handful may even be comets whose surfaces have become denuded of volatiles. However, the seeming enhancement of circular orbits in this region hints at a primordial population whose orbits are very slightly eccentric and slightly inclined. Ejection from the main belt will tend to increase the eccentricity of an asteroid. So the slightly eccentric objects may well be remnant planetesimals, the original denizens of the region before it was colonized by asteroids from the resonance locations in the main belt. M