The rovers are testing a "smart" software upgrade

Space agency scientists have begun testing four new skills included in flight software that has been uploaded to the robots' onboard computers.

The two American rovers, Spirit and Opportunity, are approaching their third year on the Martian surface.

One of the new capabilities is designed to allow the craft to make "intelligent" decisions in the study of Martian clouds and dust devils.

Spirit has photographed dozens of dusty whirlwinds in action, and both rovers have photographed clouds.

Until now, however, scientists on Earth have had to sift through many transmitted images from Mars to find the few shots that have relevant detail.

With the new software boost, the rovers can recognize dust devils or clouds and select only the relevant parts of those images to send back to Earth.

Decisions, decisions

Another novel skill, called "visual target tracking", enables a rover to keep recognising a particular landscape feature as it moves.

"The rover keeps updating its template of what the feature looks like. It may be a rock that looks bigger as the rover approaches it, or maybe the shape looks different from a different angle, but the rover still knows it's the same rock," said Khaled Ali, of Nasa's Jet Propulsion Laboratory (JPL) in Pasadena, US.

The software might allow a rover to pick out dust devils by itself

The combination gives Spirit and Opportunity a capability which engineers call "go and touch". This has yet to be tested on Mars.

So far in the mission, whenever a rover has driven to a new location, team members on Earth have had to wait for images of the site to arrive on the ground in order to decide where the rover should place its contact instruments.

After the new software has been tested and validated, the crew will have the option of letting a rover choose a science target for itself the same day it drives at a new location.

The new software also improves the autonomy of each rover for navigating away from hazards by building better maps of their surroundings than they have done previously.