There is a deep and watery mystery on Mars. Whether or not the Red Planet ever had water on it has long been solved. Plenty of evidence exists for ancient water flows and Mars still contains frozen ice at its poles. But a more tantalizing puzzle remains: Does liquid water still flow on the Martian surface?

Since 2011, NASA's Mars Reconnaissance Orbiter (MRO) has been taking pictures of mysterious dark features that appear during warm seasons, flow downhill, and then vanish as the weather grows colder. Known as "recurring slope lineae" these odd streaks have become one of the hottest topics in Martian research. While its been years since their discovery, the features just keep growing more mysterious. Just recently, scientists determined that the flows contain iron-bearing minerals, though what this means is still a head-scratcher. Above, you can see some of the best images and gifs of these features (Open the gallery to fullscreen for the best view).

Viking orbiter observations in the 1970s first recorded dark material that would spontaneously appear on dust-covered slopes on Mars. Scientists long ago concluded that these features were caused by tiny avalanches cascading down crater walls and other steep terrain. But in 2011, MRO captured images of something completely new and different. Thin finger-like streaks appeared in equatorial regions on Mars, traveling hundreds of meters and branching out like tiny rivers. Unlike the previously recorded dark slope material, which appeared at any time during the Martian year, these new features were only seen at the warmest periods on Mars, late spring and summer, and subsequently disappeared in the winter. The new features were also only a few meters across, whereas the previous dark slopes lines were generally 20 to 200 meters across.

Scientists at NASA suggested that these new features were coming from frozen water in the soil that was heating up and flowing downhill. Though the temperatures on the slopes remained below water's freezing point, the addition of brines could lower the freezing point enough to turn the water liquid. Essentially, they were saying that Mars was weeping salty tears. But when researchers used MRO's spectrometer, they found no evidence for water in the features.

Scientists have no idea why the features turn mysteriously dark. One might assume they are darker than surrounding material because they're wet. But the features aren't wet (MRO's spectroscope showed no water). One suggestion is that underground water flows are rearranging Martian sand grains in a way that makes them turn dark. If that's the case, why the streaks turn bright again in the winter is unknown.

Salty water remains the best potential explanation for recurring slope lineae, though the scientists are yet to detect any water where they occur. Because MRO captures images during Martian mid-day, it's possible that its spectrometer is missing early-morning moisture, which is evaporating by noon. More recent observations have shown that the features contain iron-bearing minerals, though it's not entirely clear why.