Well. Water would be needed I guess. Water and winds, tides, well, like here on Earth, or was that Area 51?

Blessings,
Mikromarius

Another possibility is that the 'Exploded Planet Hypothesis' is correct...as Mars was once a moon of the former planet reduced to Astroid belt

all that 'buckshot' of rocks was laid on the surface of Mars...hot lava while cooling will tend to solidify in spheres as it travels thru a low gravity enviro. ??

Google up for more info
this post is only a conjecture

[Edited on 10-2-2004 by riffraffalunas]