The revelation that dark streaks flowing downhill on Mars are signs of present-day liquid water has sparked debate on how best to investigate the Red Planet features.

These dark streaks, called Recurring Slope Lineae (RSL), flow down steep slopes during warm seasons and fade when the weather is cooler. The streaks are caused by liquid water laden with salts, scientists announced last month.

The new RSL findings may well bolster the odds that life exists today on Mars. Moreover, RSL might be a draw for future human explorers, as those sites could lead to underground aquifers on the planet, NASA officials have said. [Flowing Water on Mars: The Discovery in Pictures]

However, exploring RSL sites further is a matter of some controversy. One contentious issue has to do with "planetary protection" — how best to safely survey RSL sites up close, without contaminating these features with Earth-transported microbes.