Slitherine

I tried looking it up on wiki (no dice), but was wondering why the Messari portals seem impervious to damage. Is their a turn/time limit before you can take them down? My units would not attack them for several turns, then I just walked in and took possession. Is there some kind of mechanic involved that I can't seem to figure out? Just askin'...

I bet you tried to step into them with naval-units. Did the very same thing back then.
While those can capture actual cities, they cannot do the same with hives and messari-portals.

With normal land-units you should always be able to conquer them.

Actually I tried to use ranged fire on them with artillery (both land based and naval) but it either wouldn't fire when I right clicked or the unit moved with a mind of its own. Aircraft were useless too. Now that I think about it the land units captured without damage as long as there was no messari troops or tanks or mother ship present. Ranged fire by units not trying or unable to capture was treated as nil. Thanks!

Ground bases ranged-units actually should be able to walk into the portal as well, given they are empty of course, otherwise they would fire.
But they cannot be destroyed by naval- and air-units regardless of their weapon.

A black hole will destroy a portal. Also a few nukes will do (dunno how much exactly).