However, in order for liquid water to have existed on the surface of the red planet billions of years ago the conditions would have to have been warm enough, and the models that climate researchers rely on suggest it simply wouldn't have been.

However, new research from NASA has thrown further confusion into theories about water on Mars after the Curiosity rover uncovered rocks indicating the existence of a lake - but no signs of the carbon levels required to keep that water unfrozen.

"The same Martian bedrock in which Curiosity found sediments from an ancient lake where microbes could have thrived is the source of the evidence adding to the quandary about how such a lake could have existed".

For the past few decades, scientists had believed that Mars' carbon dioxide-filled atmosphere - chemically memorialized in its sedimentary outcrops - helped melt the planet's bountiful ice into flowing rivers, streams and ponds billions of years ago.

"We've been particularly struck with the absence of carbonate minerals in sedimentary rock the rover has examined", Thomas Bristow of NASA's Ames Research Center in Moffett Field, California, said in a statement.

Other greenhouse gases such as sulfur-dioxide could have been responsible for maintaining liquid water but would be hard to detect, as Bristow explained to Space.com: "The downside of all these other greenhouse gases is that they tend to be quite reactive, so when you put them in the atmosphere, they don't hang out an especially long time". When carbon dioxide interacts with water on the surface of the Earth, it undergoes chemical reactions to produce carbonate, which can then interact with other elements and compounds to produce carbonate minerals at the bottom of bodies of water. But maybe the planet was always a cold place - too cold, in fact, to explain the presence of liquid water on its surface even at some point in the ancient past.

But, NASA scientists are struggling to prove that mars had liquid water billions of years before.

The scientists detailed their findings online today (Feb. 6) in the journal Proceedings of the National Academy of Sciences.

It doesn't mean that carbonates are absent from Mars. An atmosphere with plenty of carbon dioxide would be the simplest answer, Dr. Bristow said, but "it doesn't seem that easy solution will work in this case". Or at least they weren't always arid, since we now know liquid water once existed on the surface of the red planet.

The researchers determined that for no carbonate to be detected by the rover today, the level of Carbon dioxide in the atmosphere could only have been in the tens of millibars, a unit of atmospheric pressure, when the lake still existed.

In its early days, the Gale Crater could be simply described as a glacial lake surrounded by masses of ice. However, "the nature of the minerals in the samples we focused on don't support that conclusion", Bristow said.

"The downside of all these other greenhouse gases is that they tend to be quite reactive, so when you put them in the atmosphere, they don't hang out an especially long time", Bristow said.

If the lakes were not frozen, the puzzle is made more challenging by the new analysis of what the lack of a carbonate detection by Curiosity implies about the ancient Martian atmosphere. Researchers have certainly found the mineral on the Red Planet before.