I can’t wait until MESSENGER gets in orbit. The high resolution images should be able to give some decent dating values to these ridges. I’m curious how old these ridges are. I would guess that decent dating of these features would give a pretty good understanding of the interior dynamics of the planet.
It is interesting that the Moon doesn’t have these features and that Mercury doesn’t have maria. It would appear that Mercury cooled much faster than the Moon (so that the maria didn’t have time to form on Mercury and so that the crust on the Moon had time to relax). I would bet that this would be due to a much higher accretion energy per unit mass. This should be easy to disprove if the dating on these features is younger than ~3 billion years. We’ll know in a couple of years.