Why are some moons of Jupiter brighter than others?
The moons of Jupiter vary greatly in their size and colours. Imagine. Nip outside and put these things at the bottom of your garden; a white marble, a white bowling ball and a small cherry tomato Wait till night falls, and now shine a torch at your strange collection of objects. They’re all the same distance from you, and so each square unit of area down at your garden end receives the same amount of light. But more light is bounced back to your eye from the bowling ball than from the marble – simply because the bowling ball is bigger. It intercepts more light from your torch, and throws more light backward. Also, the tomato and the marble are about the same size, but they are different colours – the marble reflects all the colours of light, the tomato reflects mostly red. So the marble seems brighter – more light comes back towards your eye. Now, brightness is just a measure of the total light that comes back. So, Europa and Ganymede are bright because they’re big and mostly covered i
