Earth is the 'Goldilocks' planet. It is neither too hot nor too cold. As a result, our world is the only place we know where water can take the form of a gas, a liquid or a solid (ice).
We usually come across ice in the winter, but ice exists all year at the poles and on the tops of high mountains.
The largest ice sheets on land cover Antarctica and Greenland. There is also a huge area of ice in the Arctic, but this is quite thin and floats on the ocean. Ice caps and mountain glaciers often grow or shrink. Today, many scientists think global warming is causing the ice to melt and sea level to rise, increasing the threat of floods.
Over a long period, data from satellites such as ESA’s ERS and Envisat can tell us what is happening.
Images from space show the break-up of ice sheets and movement of icebergs. They also detect changes in ocean temperature and colour. Radar instruments show changes in ice thickness and global sea level. And they help ships to sail safely through ice sheets in winter.