You've pretty much nailed it. In the high latitudes, sunlight strikes the ground at a steep angle, so there's less of it per square inch. If that leads to the t

You've pretty much nailed it. In the high latitudes, sunlight strikes the ground at a steep angle, so there's less of it per square inch. If that leads to the temperature staying below freezing year-round, you get permanent ice caps. (I think there's an extra twist on the Moon; the lack of atmosphere means that direct sunlight is blazingly hot even at the poles, but also that shadowed areas stay much much cooler since there's no moving air to distribute the heat evenly. Lunar ice doesn't form a continuous cap, but nestles in the bottoms of polar craters where the light never reaches.)