Question: Why is the world mostly water?

Melissa Ladyman answered on 13 Jun 2016:

This is a really good question! Water is around us all of the time, but we just don’t think about it. But, without water life as we know it would not be possible.

I had to really think about this because it’s not something that I just know. In fact, no one really knows how that water even got here! We know why it stays here- conditions on planet Earth are perfect for water. The temperature is just right- water can exist in liquid form (in the oceans), in gas form (in the air) and in solid form (ice). The Earth’s gravity makes sure that the water (and we) doesn’t float away. But how did all that water get here?

One explanation is that some of it arrived on icy asteroids when they crashed into the planet millions of years ago. But it might have been trapped here when the Earth was first formed. Do you have any ideas? The answer is still waiting out there to be found!