We knew that there was water on Mars. There is frozen water near the north and south poles of Mars. We also knew that billions of years ago there was water on Mars. There may even have been an ocean. Now, Mars is a dry planet.
Scientists wonder where the water went.
Scientist thought that if there was still water on Mars, it would be deep inside the planet. Mars is so cold that any water near the surface would have to be frozen. The average temperature on Mars is colder than freezing. In the new pictures of Mars, it looks like water has been flowing not long ago. There are pictures of gullies that flow across old craters.