Scientists were wrong about discovering water on Mars

In 2015, scientists announced something incredible: They had strong evidence that there was liquid water flowing on Mars today. The implications were huge for the possibility of alien life and resources for future crewed missions to the Martian surface. Now, all of those hopes have been put on hold by a new study published in Nature Geoscience. Following is a transcript of the video.

These mysterious marks on Mars are at the heart of a giant controversy. They're called recurring slope lineae (RSL).

Scientists first discovered them in 2011 from images taken by NASA's Mars Reconnaissance Orbiter. Almost immediately, scientists noticed something puzzling.

RSL have regular seasonal changes. During warm seasons they grow darker and longer. But in cooler seasons they fade away.

In 2015, scientists announced they had a solution. RSL were likely the result of liquid water flowing downhill on Mars. Later reports speculated that RSL may even be a good place for life.

Now, all of that has been called into question.

New evidence adds another mystery to the mix most RSL only occur on slopes that are steeper than 27º. The researchers argue that if RSL were from water then they should appear on shallow slopes as well as steep ones.

They report that instead of water RSL are probably made from falling sand and dust. However, their idea doesn't explain why RSL change with the seasons.

Until we visit these unusual Martian features this mystery will remain one of Mars' best-kept secrets.