reaches of the disk, far from the sun’s 
heat. 
At this point of development the 
earthy lumps were still random mix- 
tures of those chemical elements that 
had been present in the interstellar 
dust grains that were drawn into the 
solar system. Actually, various com- 
plicated and interesting things had 
happened to the interstellar dust 
grains by now, but there had been 
no major sorting-out, or fractionation, 
of the chemical elements. 
As their orbits carried these lumps 
of primordial material slowly past one 
another, they tended to clump together 
and gradually build up into larger 
masses. The biggest of these aggre- 
gations, or planetesimals, attained 
diameters of several hundred kilome- 
ters. Planetesimals represent a stage 
halfway between lumps and planets. 
Some of the planetesimals, probably 
the largest ones, grew very hot inside. 
The source of the heat is poorly un- 
derstood, but there is no doubt the 
heating occurred since we can still see 
the effects it had by examining me- 
teorites. The planetesimals in question 
melted inside, lava erupted on their 
surfaces, and the nickel-iron metal they 
contained melted and sank to their cen- 
ters, collecting into dense cores. 
The aggregation of lumps and the 
growth of planetesimals continued, 
but the orbital motions of these objects 
became more disorderly. Instead of 
merging into a single, larger body 
when they chanced to meet, the plane- 
tesimals sometimes struck one another 
at such high speeds that they were 
smashed and the fragments were dis- 
persed. The planetary disk orbiting 
the sun now contained a wide diversity 
of bodies: some were still growing, 
others were fragments of once larger 
bodies; some still had their primordial, 
unfractionated composition, others 
were fragments of the lava layers and 
metal cores from planetesimals that 
had melted and fractionated. 
Eventually, all the lumps and frag- 
ments and planetesimals in the inner- 
most solar system were swept up into 
five large bodies: Mercury, Venus, 
Earth, Earth’s moon, and Mars. Over 
the ages, these objects, acting like gi- 
ant vacuum cleaners, have totally 
cleansed their zone of the solar system 
of primordial crumbs. The materials 
that were swept up into the planets 
have been assimilated and repeatedly 
fractionated by geologic processes — 
