Was there an ocean on Mars?

Scientists have thought for a long time that Mars used to have water on it like lakes and rivers.

But new research shows that a long time ago there might have been an ocean there too.

A Nasa probe which orbited the planet for 10 years has collected data that shows there might be a Martian equivalent of a continental shelf - which is what defines the boundaries of Earth's oceans.

Scientists compared their findings to a huge "bathtub ring" where water may have once met land on Mars.

The scientists used data gathered from Nasa's Mars Global Surveyor.

Planetary scientist Michael Lamb says "interpreting ancient landforms is not easy", their study shows "telltale signs of a coastal shelf".

There are also areas on the Red Planet where rivers may have flowed into a larger body of water.

Which means that Mars may once have looked much more like Earth than it does now.

Like Earth and our solar system's other planets, Mars formed roughly 4.5 billion years ago.

Today, it's known as the Red Planet - a dusty, cold, desert world but it's thought that early in its history, Mars was warmer and wetter.

Where did the all the water go? Lead author of the research Abdallah Zaki says that "remains highly debated", with different scientists having different theories about it.