Is there liquid water flowing on Mars? The short answer is "we think so, but we don’t know." It's true that the Mars Curiosity rover has found areas on the planet’s surface that look like streambeds, and it's also found that Mars' soil contains a small percentage of water. However, it’s still anyone’s guess if actual flowing water exists on the planet.
Back in 2011, a University of Arizona student, Lujendra Ojha, discovered slopes on the surface of Mars that could have been created by running water. He called these slopes RSL (Recurring Slope Linae). Ojha partnered with Georgia Tech professor James Wray to search for other RSL locations on the planet: particularly focusing on southern rocky areas. They found about a dozen RSL locations. Most importantly, these RSL’s are weakest when the planet is coldest and strongest during its warmer seasons, suggesting ice melting and becoming flowing water in Mars' warmer months. Makes sense, right?
However, they didn't find proof of H2O in those RSLs, because the slopes' spectral signatures didn’t show any association with water, not even salt. Instead, the signatures showed iron. It also doesn’t help that these RSLs vary significantly from year to year, regardless of Mars' current temperatures. A slope might appear one year and be gone the next. The planet’s dust levels also change regularly and affect the study of these slopes, making it even more difficult to find definitive evidence of running water there. Finally, if there is running water on the planet, where is it coming from? We have so many questions, but so little proof.
So how do we actually prove that water exists on Mars? More than likely, the only real way to find proof is by putting humans on the planet to study it first-hand. Of course, we’re still a decade away from the first manned Mars mission, so at this point, the proof behind the question about water on Mars still remains undiscovered.
Via Georgia Tech