At its current location for inspecting an active sand dune, Nasa's Curiosity Mars rover is adding some sample-processing moves not previously used on the Red Planet.

Sand from the second and third samples the rover is scooping from "Namib Dune" will be sorted by grain size with two sieves.

The coarser sieve is making its debut, and using it also changes the way the treated sample is used for laboratory analysis inside the rover.

"It was pretty challenging to drive into the sloping sand and then turn on the sand into the position that was the best to study the dunes," said Michael McHenry from Nasa's Jet Propulsion Laboratory in California.

Curiosity has scooped up sample material at only one other site since it landed on Mars in August 2012.

The mission's current work is the first close-up study of active sand dunes anywhere other than the Earth.

Investigation of the dunes is providing information about how wind moves and sorts sand particles in conditions with much less atmosphere and less gravity than on the Earth.

Curiosity scooped its first dune sample on January 14 but the rover probed the dune first by scuffing it with a wheel.

Curiosity collected its second scoop on January 19. This is when the coarser sieve came into play.

"What you have left is predominantly grains that are smaller than one mm and larger than 150 microns," said John Michael Morookian, rover planning team lead for Curiosity.

Curiosity reached the base of Mount Sharp in 2014 after fruitfully investigating outcrops closer to its landing site and then trekking to the layered mountain.