Off the top of my head...
The location of any other space faring civilizations within the galaxy.
The location of the nearest earth-like planet. (land, oceans, atmosphere)
Definitive answer to the ice on the moon question (the Lunar Orbiter may have this one soon.)
The existence and nature of lifeforms in Europa's ocean.
Are there any small locations on Mercury, maybe around the poles, where the temperatures could stay within a safe 0 to 43 C range?
Is there sufficient nitrogen on Mars for terraforming?
Does the Sun have a brown dwarf companion way out there we cannot see?
Can Ganymede's magnetic field provide any meaningful protection against Jupiter's radiation?
Is it possible that there is a vast ocean layer on Jupiter far beneath the clouds, as depicted in Ben Bova's Jupiter novel?
Are there any large quantities of gold and platinum in any NEAs that could be reached by Orion. (greed could spark a new gold rush into space).
Is Amalthea's mass so low because it is loose ruble or because it is hollow? (Lol, Yeah I know just kidding on this one but it’s a cool idea. Amalthea should not be that close to Jupiter if it is made of ruble, unless it was captured fairly recently.)
Space/Science » in reply to 10 things about planets
Cool list! Here's mine