The Curiosity rover, drilling at the base of a mountain on Mars, has uncovered more evidence that ancient water existed on the Red Planet.

The NASA rover has successfully drilled rock samples from two sites at Mount Sharp inside Gale Crater since arriving there 5 months ago.

The recent sample, from a rock dubbed Mojave 2, tested as more acidic than the first sample, suggesting the rock could have been laid down by ancient water that was more acidic than that which created the first sample, NASA says.

Chemical analysis by the rover's onboard instruments found a higher level of jarosite -- an oxidized mineral made up of iron and sulfur that forms in an acidic environment -- than what was seen in a sample from a site dubbed Confidence Hills that was drilled in September.

NASA scientists say they believe Mount Sharp began forming from sediments in a number of lakes that continuously filled and then dried in the Red Planet's ancient past.

"Open questions include whether the more acidic water evident at Mojave 2 was part of environmental conditions when sediments building the mountain were first deposited, or fluid that soaked the site later," NASA scientists said in a statement.

A first attempt to obtain a sample at the "Mojave" site failed when the sample broke up and could not be delivered to Curiosity's onboard analytical instruments.

Concern over the fragile character of finely layered rocks at the site led the rover team to try a slightly more gentle drilling technique, tested extensively on Earth before its use by Curiosity.

"This was our first use of low-percussion drilling on Mars, designed to reduce the energy we impart to the rock," said John Michael Morookian of NASA's Jet Propulsion Laboratory in Pasadena, Calif. "Curiosity's drill is essentially a hammer and chisel, and this gives us a way not to hammer as hard."

The drill hammers 30 times a second, and can utilize six percussion settings ranging from a gentle tapping to all-out vigorous banging.

Beginning at a low percussion level and using a new algorithm sent from Earth, Curiosity spent 10 minutes slowly drilling around 2-1/2 inches in the soft sedimentary Mojave 2 rock to obtain the sample.

After landing in the Gale Crater on the Red Planet in 2012, Curiosity took a slow, 2-year trek to arrive at Mount Sharp, stopping a number of times to take samples from the Martian surface.