And since several comet missions have observed comets close up as bone-dry rocks with little or no volatiles, the idea of comets giving Earth its oceans (or using comets to Terraform Mars) is untenable.
Well, that's categorically false. Of course some
comets would've exhausted their volatiles after enough passes near the Sun evaporated them all, but comet missions and other astronomical observations have confirmed the presence of water and volatiles on comets. In fact, this was just in the news
. Other findings of water and volatiles in comets:
True, there is some legitimate debate about whether comets were the primary source of Earth's water -- some findings suggest it may have been meteorites from the inner portions of the system instead
. But nobody is claiming that comets are bone-dry -- except for one crackpot page I found with lots of exclamation points and misspelled rantings about scientist conspiracies and extended tracts on politics and philosophy.