In the cooling protoplanetary nebula dust settles at the mid-plane and the grains grow to become larger objects through collision and adhesion. After reaching a size of about 1 km, the accretion proceeds dominated by the mutual gravitational attraction of those bodies, growing possibly to moon-sized protoplanets at the end of the accretion. The accretional growth time scale is still under debate. It could have taken several million years for bodies with a radius of 500 km to accrete to their final size, although much shorter time scales have been discussed. Also, runaway growth up to planetary embryos can take place in less than 0.1 Ma. Recent estimates of the Hf/W ratio in Mars' mantle based on the correlation between the Th/Hf and 176Hf/177Hf ratios in chondrites suggest that it took Mars less than 3 Ma to reach half of its present radius. This means that Mars could be a planetary embryo that escaped collision and merging with other embryos, and thus did not develop into a planet like Earth.
Figure 1: The internal temperature in a growing (accreting) Vesta-sized planetesimal (DLR).