Question: @ Ciarán , why is there so much water on our planet?

Ciarán O'Brien answered on 18 Nov 2014:

There are a combination of reasons why we have so much water.

The planet is a good size for holding on to water, as its gravity helps prevent it floating away.

The earth’s atmosphere is another big reason. The air pressure keeps our water liquid and stops it just boiling away into space (with the help of gravity)

Distance from the sun helps too. Mercury would have had water, but it’s so close to the sun that the radiation just blasted it away, along with its atmosphere.

Mars has a lot of water too, but it’s locked away in different chemicals on the surface (and frozen at the poles). There are some reasons as to why Mars is much drier than Earth. Its atmosphere is quite thin, so the air pressure wouldn’t be quite as good at keeping the water liquid. Mars also has less gravity than Earth, so it’s been leaking more water vapor and other gases out into space than Earth

Earth leaks like this too, but has lots of earthquakes and volcanic activity that spews new gas out to replace it. Mars doesn’t seem to have any volcanoes any more, so it has nothing to replace lost gas and water.