Why does uranus glow?

18 July 2013


Why does the outer gas planets reflect light at nighttime?


Chris - I think what he's getting at is, why should something which is a giant ball of gas reflect light? The reason the planets reflect light is that light is shining on them from the sun and that they have an atmosphere and that atmosphere has particles in it and those particles in the same way that snow and tiny ice crystals will reflect light, that bounces light back off and it comes back towards your eye. Things like Saturn have a very dense atmosphere. They are very big bodies and so, they have an atmosphere which is extremely dense and they have things like ammonia in their atmosphere which forms tiny crystals. Those tiny crystals do actually have a very reflective surface. And so, the albedo, the reflection of these gas giants is really quite high. It's maybe 59%, 60% of the light that goes on to them comes straight back at you. So, they do actually appear quite bright. And the fact is there are tiny dot in the sky. So, all that light that is coming back is centred on one tiny little speck in the sky, so it makes it look much brighter than it really actually is.

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