"It is not as simple as driving a rover to a potential site and taking a scoop of soil," said Jim Green, Nasa's director of planetary science.
"Not only are these on steep slopes, we need to ensure that planetary protection concerns are met. In other words, how can we search for evidence of life without contaminating the sites with bugs from Earth?," he added in a Nasa statement.
After approval of mission extension, Nasa's Curiosity Mars rover would continue to climb to higher and younger strata on Mount Sharp - mountain in Gale crater - to investigate how the ancient, water-rich environments found till now persisted as the red planet dried out.
A stroll on these destinations would help the one-tonne rover closer to locations where dark streaks are present on slopes and allow it to capture images of the potential water sites from miles away and see if any are the seasonally changing type.
Nasa's High-Resolution Imaging Science Experiment (HiRISE) camera on the Mars Reconnaissance Orbiter (MRO) has observed many features of interest. They appear as dark lines that appear to ebb and flow over time.
"Planetary scientists think these gullies or recurring slope lineae (RSLs) may appear seasonally as a form of briny water at or near the surface of the Red Planet under warmer conditions," Nasa said.
Nasa is also worried about how close could the rover safely get to an RSL?
"In terms of coming much closer, we need to understand well in advance the potential for Earth organisms to come off the rover and that will tell us how far away the rover should stay," said Catharine Conley, Nasa's planetary protection officer.
The darkish streaks are considered "special regions" on Mars, where extra precautions must be taken to prevent contamination because of the suspected presence of liquid water, considered a prerequisite for life.