This week, NASA scientists announced that they had found chemical evidence of liquid water on the surface of Mars. While they make a compelling case, the existence of seasonal rivers of briney water will never be 100 percent confirmed until we can see it and touch it and analyse it, and if it’s not actual humans on Mars doing that, we’ll have to study it vicariously through our far-flung robots.

Except we can’t. Not as long as those far-flung robots originated on Earth, anyway. Right now, NASA’s Curiosity rover is about 50 kilometres from the site that scientists suspect holds liquid Martian water, but thanks to an international treaty signed in 1967, it’s not allowed to go anywhere near it.

This is because to get where it is on the surface of Mars, Curiosity had to travel 225 million kilometres from Earth through space, and along the way it could have picked up dirt and dust and all kinds of mysterious microbes that make it far from sterile. As Marcus Strom points out at The Sydney Morning Herald, "That's a long way to go without having the right sort of shower."

Gross, Curiosity. Just go stand over there, will you? No, no, keep going, further. Yep, keep going. Okay that's good, thank you!

And while scientists do their best to sterilise their space-faring equipment once it arrives at its destination via what Swinburne University astronomer Alan Duffy describes as "a very intense ultraviolet tanning salon", if they can’t guarantee sterilisation, there’s no going near that water.

"Because liquid water appears to be present ... we have to take extra precautions to prevent contamination by Earth life," Rich Zurek, the chief scientist for NASA’s Mars program, explained during a Reddit AMA yesterday. "Our current rovers have not been sterilised to the degree needed to go to an area where liquid water may be present."

As Akshat Rathi writes for Quartz, every country on Earth is bound by the stipulations of the 1967 Outer Space Treaty, which forbids "anyone from sending a mission, robot or human, close to a water source in the fear of contaminating it with life from Earth".

Not that NASA couldn't sterilise the crap out of its rovers if it wanted to. As UNSW astrobiologist Malcolm Walter told The Sydney Morning Herald, they could blast Curiosity with crazy amounts of heat and radiation that would wipe out anything and everything that managed to survive the journey from Earth without a shadow of a doubt, but then they'd be wiping out the rover's internal electronics in the process. Not exactly practical.

"In order to be completely sterile, they'd have to use really powerful ionising radiation or heat, both of which would damage the electronics," says Walter. "So they go as far as they dare."

What's the solution? We all know that NASA is planning on sending humans to Mars for the first time in mid-2030, so maybe some lucky astronauts will get to see liquid Martian water with their own eyes. Another option would be to send robots to Mars that are capable of building other robots that can investigate that water with little risk of contamination. Last year, NASA announced that it's developing robots that can 3D-print infrastructure on Mars, so this could well be a possibility.

Until then, Curiosity and its rover buddy Opportunity will just have to revel in the ambigious state of their cleanliness, and stay the hell away from the water.