NASA has a Mars explorer about 50 kilometres from a site that shows promise it holds Martian water, but the US space agency can't send the rover there to find out.
Why not? The rover, which has travelled 225 million kilometres from Earth to get there, is not sterile enough.
In a revealing response during an "ask me anything" session on website Reddit, NASA's chief scientist for its Mars program, Rich Zurek, said: "Because liquid water appears to be present ... we have to take extra precautions to prevent contamination by Earth life. Our current rovers have not been sterilised to the degree needed to go to an area where liquid water may be present."