Water on Mars, 4 billion years ago. Artwork of the surface of Mars, showing an impact crater (right) and a nearby river (running from lower left to centre right) filled with water and mist. The sun is at upper right. Four billion years ago, Mars was similar to Earth but, for reasons not understood, there was a sudden change and Mars became a global desert. In 2004, Mars Express, the European Space Agency (ESA) spacecraft, identified water ice trapped amid frozen carbon dioxide over the Martian south pole. In 2006, NASA took images using their Mars Global Surveyor that showed sediment deposits thought to have been laid down by flowing liquid. The presence of water indicates that Mars may have supported life.