Last year, NASA blasted a hole in the moon looking for water. Some water sprayed up, but just how much hasn't been known until now. Turns out, it was 41 gallons, way more than anyone was expecting.
Sure, 41 gallons isn't a lot by Earth standards, but when you're talking about an atmosphereless satellite in the cold vacuum of space, it's a big deal. And they think that there could be a full billion gallons of water in the crater that they hit, which is certainly a serious amount of water on any planet.